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Three-Dimensional Bit Optical Data Storage in a Photorefractive Polymer

A thesis submitted

by

Daniel John Day

for the degree of

DOCTOR OF PHILOSOPHY

Centre for Micro-Photonics School of Biophysical Sciences and Electrical Engineering

Swinburne University of Technology

This PhD thesis is dedicated to my family

i

Imagination is more important than knowledge. - Albert Einstein

ii

Abstract As the computer industry grows, so will the requirements for data storage. Magnetic

memory has been the most stable method in terms of capacity and recording/reading

speed. However, we have reached the point where a substantial increase in the

capacity cannot be produced without increasing the size of the system. When

compact discs (CDs) were introduced in the 1980’s they revolutionized the concept

of data storage. While the initial force behind compact discs could easily be said to

be the music industry, once recordable and rewritable discs became available they

quickly found more use in the computer industry as backup devices. Since their

inception, the capacity requirements have far exceeded what is available on a

compact disc, and they are now following the same path as magnetic memories.

Following this trend, it could be assumed that digital versatile discs or digital video

discs (DVDs) have a limited lifetime as a storage medium. In fact it has been noted

(Higuchi et al., 1999) that the maximum capacity of digital video discs will be

reached in 3 – 5 years. The question then is, what comes next?

The efficiency of conventional optical data storage is extremely poor. For an

optically thick recording medium, both CDs and DVDs use less than 0.01% of the

total volume to store the information. Three-dimensional bit optical data storage

endeavors to increase the efficiency by recording information in a volume that is

greater than 90% of the total volume.

The concept of three-dimensional bit optical data storage was first proposed by

Parthenopoulos and Rentzepis in 1989, where they demonstrated that capacities far

exceeding that of compact discs could be achieved.

Three-dimensional bit optical data storage relies on creating a highly localised

chemical or physical change within a recording medium, such that further layers can

be recorded without causing interference. Ideally the chemical/physical change in

the material should be reversible to enable erasable/rewritable data storage. In order

to create a highly localised effect nonlinear excitation can be used; whereby the

excitation is limited to a small region around the focal spot. Depending on the

iii

Abstract

material and recording method there are several techniques for reading the

information such as transmission imaging or reflection confocal microscopy.

However, all the recording and reading methods require focusing to a deep position

within a recording medium, such focusing encounters spherical aberration as a result

of the difference in the refractive indices between the immersion and recording

media.

This thesis has concentrated on several areas to understand and develop the concept

of three-dimensional bit optical data storage.

The photorefractive effect in crystals has been studied for many years and is now

widely used in optoelectronic devices. The use of photorefractive polymers is a

relatively new and exciting development in optical data storage. Until now they have

been used solely in the area of holographic data storage. The research in this thesis

was conducted using photorefractive materials that were fabricated in two polymer

matrices, poly(N-vinylcarbazole) (PVK) and poly(Methyl Methacrylate) (PMMA).

The recording samples also consisted of the following compounds in various

proportions, 2,5-dimethyl-4-(p-nirtophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-

fluorenone (TNF) and N-ethylcarbazole (ECZ).

In this project two-photon excitation was used as the recording mechanism to

achieve erasable/rewritable data storage in a photorefractive polymer. As a result of

two-photon excitation, the quadratic dependence of excitation on the incident

intensity produces an excitation volume that is confined to the focal region in both

the transverse and axial directions. Therefore, focusing the laser beam above or

below its previous position provides a method by which layers of information can be

recorded in the depth direction of a material, without causing interference from

neighbouring layers. The feasibility of two-photon excitation in photorefractive

polymers is demonstrated in this thesis.

The quadratic relationship between excitation and incident light in two-photon

excitation requires high photon density to ensure efficient excitation. The use of

iv

Abstract

ultra-short pulsed lasers, while effective, is not a practical solution for an optical data

storage system. This thesis demonstrates the ability to produce three-dimensional

erasable/rewritable data storage in a photorefractive polymer using continuous wave

illumination.

Using this technology it has been possible to achieve a density of 88 Gbits/cm3,

which corresponds to a capacity of 670 Gbytes on a compact disc sized recording

medium. This is an increase of 1000 times the capacity of a CD and 130 times the

capacity of current DVDs.

While erasable optical data storage is an exciting prospect there are problems

associated with the deterioration of the information. For long term information

storage a permanent recording process would be more practical. It is demonstrated

that there is a point after which further increases in the recording power result in the

formation of a micro-cavity. While two-photon excitation is the recording method

for erasable data storage, the increase in power results in an increase in ultra-violet

absorption such that multi-photon excitation may occur. This thesis demonstrates the

ability to record multi-layered arrays of micro-cavities.

The change in refractive index associated with an erasable bit is less than 1%. As a

result only phase sensitive reading methods (transmission imaging or differential

interference contrast (DIC) microscopy) can be used to image a recorded bit. Both

transmission and DIC imaging systems have poor axial resolution and therefore limit

the density of the recording system, as well as being large optical systems. The

introduction of a split or quadrant detector reduces the size of the optical reading

system and is demonstrated to be sensitive enough to detect the phase changes of a

recorded bit. However, the change in refractive index across a micro-cavity is large

enough that reflection confocal microscopy can be used to detect a bit. It is

demonstrated in this thesis that multi-layered micro-cavity arrays can be read using

reflection confocal microscopy.

v

Abstract

Focusing of light to deep positions within an optical thick recording medium has the

effect of increasing spherical aberration resulting from the refractive index

mismatching between the immersion and recording media. The work in this thesis

illustrates the effect of spherical aberration on the performance of both the recording

and reading systems.

The work conducted in this thesis shows the ability to record multi-layered

erasable/rewritable information in a photorefractive polymer using pulsed and

continuous wave two-photon excitation. It has also been demonstrated that through

multi-photon excitation multi-layered micro-cavity arrays can be fabricated. It has

also been illustrated that while spherical aberration deteriorates the performance of

the recording and reading systems it is possible to achieve a density of greater than

88 Gbits/cm3.

vi

Acknowledgements During the course of my PhD there have been many people involved in helping to

make this a success. I would like to thank them for their assistance and patience.

First and foremost I would like to thank my supervisor Professor Min Gu, whose

advice and guidance were invaluable throughout the course of my research.

Many thanks go to Dr. Xiaosong Gan whose knowledge and humour helped keep me

sane. I would also like to thank Dr. Steven Schilders for his patience in teaching me

the finer points of experimental confocal microscopy.

I would like to thank Associate Professor Andrew Smallridge for taking on the

impossible job of trying to teach me organic chemistry, and Mr. Rad Bak who was

there to help me when I just didn’t get it.

I would like to acknowledge Victoria University, as the initial research was

conducted with its ultra-short pulsed laser facility.

I would also like to thank the students from the Centre for Micro-Photonics Mr.

Damian Bird, Mr. Dru Morrish, Mr. Djenan Ganic, Dr. PuChun Ke, Mr. Dennis

McPhail and Ms. Nina Rimac for their discussions, support and many lunch breaks.

Thanks also go to the three Electrical Engineering students Mr. Simon Siemin, Mr.

Javier Martinez and Mr. Mujahid Ashraf who designed the electronics for the split

and quadrant detectors.

The assistance of the technical staff Mr. Donald Ermel, Mr. Mark Kivanen, Mr.

Hayrettin Arisoy and Mr. Abdurrahman Kuzucu enabled me to complete my PhD

with minimal problems, although I think that they would disagree.

I would like to extend thanks to Professor David Booth for his discussions which

helped me continue towards my goal.

vii

Acknowledgement

Thanks also go to Olympus Australia for the loan of the FluoView microscope.

I would also like to thank the Australian Research Council that supported me through

an Australian Postgraduate Award Scholarship.

Last, but certainly not least I would like to thank my friends and family for putting

up with my nonsense. Their support and understanding have made light the

challenges that had lain before me.

viii

Declaration

I, Daniel John Day, declare that this thesis entitled

Three-Dimensional Bit Optical Data Storage in a Photorefractive Polymer

is my own work and has not been submitted previously, in whole or in part, in

respect of any other academic award.

Daniel John Day

Centre for Micro-Photonics

School of Biophysical Sciences and Electrical Engineering

Swinburne University of Technology

Australia

Dated the 9th of March 2001

ix

Table of contents Abstract……………………………………………………………………………… iii

Acknowledgements………………………………………………………………….vii

Declaration……………………………………………………………………………ix

Table of contents……………………………………………………………………... x

List of figures………………………………………………………………………..xiv

List of tables…………………………………………………………………...……xxi

Chapter One

Introduction to optical data storage 1.1 Introduction………………………………………………………………………. 1

1.2 Optical data storage……………………………………………………………….2

1.2.1 Compact discs/digital video discs………………………………………...3

1.2.2 Magneto-optical discs……………………………………………………. 8

1.2.3 Solid immersion lens……………………………………………………...9

1.3 Three-dimensional data storage…………………………………………….……11

1.3.1 Holographic storage……………………………………………………..11

1.3.2 Three-dimensional bit optical data storage……………………………...13

1.4 Objectives of this thesis………………………………………………………… 13

1.5 Preview of the thesis…..………………………………………………………... 15

Chapter Two

Review of three-dimensional bit optical data storage 2.1 Introduction……………………………………………………………………... 18

2.2 Three-dimensional bit optical data storage……………………………………... 18

2.2.1 Single-photon versus two-photon excitation…………………………… 20

2.2.2 Photopolymerization effect……………………………………………...23

2.2.3 Photobleaching effect..…………………………………………………. 24

2.2.4 Photochromic effect……………………………………………………..26

2.3 Photorefractive effect…………………..……………………………………….. 29

2.3.1 Photorefractive material………………………..………………………..29

x

Table of contents

2.3.1.1 Photorefractive crystals……………………………….………... 29

2.3.1.2 Photorefractive polymer………………………………………... 32

2.3.2 Localised photorefractive effect………………………………………... 32

2.3.3 Three-dimensional bit photorefractive data storage……….…….……... 35

2.4 Formation of micro-cavities…………………………………………………….. 36

2.5 Reflection confocal microscopy………………………………………………... 39

2.6 Spherical aberration resulting from refractive index mismatching……………... 40

2.7 Summary………………………………………………………………………... 43

Chapter Three

Photorefractive polymer material 3.1 Introduction……………………………………………………………………... 45

3.2 Fundamentals of photorefractivity……………………………………………… 46

3.2.1 Optical nonlinearity in photorefractive polymers………………………. 47

3.2.1.1 Linear electro-optic effect………………………………….…... 50

3.2.1.2 Orientational enhancement mechanism…………………………51

3.2.2 Required elements for photorefraction in organic photorefractive polymer

samples……………………………………………………………………………… 51

3.2.3 Special properties of organic photorefractive polymers………………... 53

3.3 Polymer sample preparation……………………………………………………. 54

3.3.1 Nonlinear optical chromophore preparation…………………………… 54

3.3.2 Photosensitive compound preparation……………………………….…. 55

3.3.3 Plasticizer compound……………………………………………………56

3.3.4 Polymer compounds………………………………………………….… 57

3.3.5 Recording sample preparation……………………………………….…. 58

3.4 Summary………………………………………………………………………... 60

Chapter Four

Three-dimensional bit optical data storage 4.1 Introduction……………………………………………………………………... 62

4.2 Experimental recording system………………………………………………….63

4.3 Experimental reading system…………………………………………………… 66

xi

Table of contents

4.3.1 Transmission reading……………………………………………………66

4.3.2 Differential interference contrast reading………………………………. 69

4.4 Pulsed beam illumination……………………………………………………….. 72

4.4.1 Multi-layered data storage……………………………………………… 72

4.4.2 Erasable/rewritable data storage………………………………………... 74

4.5 Bit characterisation………………………………………………………………76

4.6 Continuous wave illumination………………………………………………….. 80

4.6.1 Requirements for two-photon excitation with continuous wave

illumination…………………………………………………………………………. 81

4.6.2 Continuous wave multilayered data storage……………………………. 82

4.6.3 Continuous wave erasable/rewritable data storage……………………... 84

4.7 Alternative detection techniques………………………………………………... 85

4.7.1 Split detector……………………………………………………………. 86

4.7.2 Quadrant detector………………………………………………….…… 87

4.8 Summary………………………………………………………………………... 88

Chapter Five

Formation of micro-cavities 5.1 Introduction……………………………………………………………………... 90

5.2 Formation of micro-cavities…………………………………………………….. 90

5.2.1 Experimental recording and reading system…………………………… 92

5.2.2 Single cavity………………………………………………………….… 92

5.2.3 Multi-layered cavity arrays……………...……………………………... 93

5.3 Refractive index mismatch ……………………………………………………...95

5.3.1 Intensity point spread function…………………………………………. 96

5.4 Summary…………………………………………………………………….… 100

Chapter Six

Reflection confocal microscopy reading of micro-cavities 6.1 Introduction……………………………………………………………………. 101

6.2 Reading micro-cavities…………………………………………………………102

6.2.1 Reflection confocal reading system……………………………………102

xii

Table of contents

6.2.2 Single cavity…………………………………………………………... 102

6.2.3Multi-layered cavity arrays……………………………………………. 104

6.3 Theoretical evaluation of reflection confocal microscopy for three-dimensional

data storage………………………………………………………………………....105

6.3.1 Three-dimensional transfer function with spherical aberration…….…. 105

6.3.2 Readout efficiency of reflection confocal microscopy………………... 111

6.4 Summary………………………………………………………………………. 115

Chapter Seven

Conclusion 7.1 Thesis conclusion……………………………………………………………… 117

7.2 Future work……………………………………………………………………. 119

7.3 3DCD technology…………………………………….……………………….. 121

References…………………………………………………………………………. 123

Glossary…………………………………………………………………………… 134

List of publications………………………………………………………………... 137

xiii

List of figures Chapter One Fig. 1.1: The relationship between the recorded digital information and the

way that it is represented on the CD/DVD…………………………………….. 3

Fig. 1.2: Illustration of the pits and land of a CD/DVD……………………….. 3

Fig. 1.3: Illustration of the properties of a recordable CD (CD-R)………….… 4

Fig. 1.4: The schematic of the optical system used in CDs and DVDs……….. 5

Fig. 1.5: A comparison of the minimum pit length and track pitch between

CDs and DVDs (Encyclopedia Britannica, 2000)……………………………... 6

Fig. 1.6: Illustration of single and double layer DVDs………………………... 7

Fig. 1.7: Recording mechanism in magneto-optical discs…………………….. 8

Fig. 1.8: Schematic diagram of a solid immersion lens recording system…….. 9

Fig. 1.9: Schematic diagram of a holographic recording and reading system

(Wang et al., 1997)…………………………………………………………….. 12

Chapter Two Fig. 2.1: Schematic diagram for (a) 2-D and (b) 3-D optical data storage…….. 19

Fig. 2.2: Energy level diagram for (a) single-photon and (b) two-photon

excited fluorescence…………..…………………………………….…………. 21

Fig. 2.3: Fluorescence from (a) single-photon and (b) two-photon excitation

(Tatterson, 1997)………………………………………………………………. 22

xiv

List of figures

Fig. 2.4: Multi-layered information recorded in a photobleaching polymer.….. 25

Fig. 2.5: Photochromic material 1,3,3-trimethylindolino-6’-nitrobenzopyrylo-

spiran (NSP), indicating (a) isomer 1 and (b) isomer 2 (Toriumi et al.,

1997)…………………………………………………………….…………….. 26

Fig. 2.6: Absorption curve for NSP for (a) isomer 1 and (b) isomer 2 (Toriumi

et al., 1997)……………………………………………………………………. 27

Fig. 2.7: Energy level diagram of NSP for (a) isomer 1 and (b) isomer 2. (c)

thermal relaxation can occur from the ground state of isomer 2 direct to the

ground state of isomer 1. (d) two-photon excitation of isomer 1 using two

laser beams of wavelengths 1064 nm and 532 nm. (e) two-photon

fluorescence reading of isomer 2 using two laser beams of wavelength 1064

nm (Parthenopoulos and Rentzepis, 1989)……………………………….……. 28

Fig. 2.8: Band transport model for charge transport in Fe doped LiNO3 (Saleh

and Teich, 1991)………………………………….…………….……………… 30

Fig. 2.9: Interference pattern produced by two intersecting waves…………… 33

Fig. 2.10: Diffraction pattern of an objective, which corresponds to the

interference pattern produced from multiple beams intersecting in a circularly

symmetric fashion……………………………………….…………………….. 33

Fig. 2.11: Photorefractive mechanism………………………………………… 34

Fig. 2.12: Reflection mode confocal microscope……………………………… 39

Fig. 2.13: Converging rays of an objective for (a) nI < n2 and (b) n1 > n2…….. 41

xv

List of figures

Chapter Three Fig. 3.1: Demonstration of electron donors and acceptors on the molecule 4-

(N,N-dimethylamino)-4’-nitrostilbene (DANS) (Marder et al., 1997)………... 49

Fig. 3.2: Illustration of poling of a photorefractive polymer sample using an

applied electric field across two ITO coated glass slides……………………… 50

Fig. 3.3: Chemical structure of the nonlinear optical chromophore

DMNPAA……………………………………………………………………... 55

Fig. 3.4: Chemical structure of the photosensitive compound TNF……….….. 56

Fig. 3.5: Chemical structure of the plasticizer ECZ…………………………… 56

Fig. 3.6: Chemical structure of the polymer compound PVK…………………. 57

Fig. 3.7: Chemical structure of the polymer compound PMMA……………… 58

Fig. 3.8: Absorption curve of (a) PVK:DMNPAA:ECZ:TNF and (b)

PMMA:DMNPAA:ECZ:TNF…………………………………………………. 59

Chapter Four Fig. 4.1: Schematic diagram of the recording system…………………………. 63

Fig. 4.2: Picture of the recording system in the laboratory……………………. 64

Fig. 4.3: Schematic diagram of a transmission reading system……………….. 67

Fig. 4.4: Olympus FluoView microscope for reading the recorded

photorefractive bits using transmission or DIC imaging……………………… 68

Fig. 4.5: Schematic diagram of a differential interference contrast imaging

system………………………………………………………………………….. 69

xvi

List of figures

xvii

Fig. 4.6: Relationship between the phase difference between the two laterally

displaced beams and the image intensity as a result of different phase bias

(Cogswell and Sheppard, 1992)……………………………………………… 71

Fig. 4.7: Recorded 24x24 bit patterns at different depths in the photorefractive

polymer under two-photon excitation. The spacing between adjacent layers is

20 µm, and the bit separation is 3.2 µm. (a) the first layer including the letter

A, (b) the second layer including the letter B and (c) the third layer including

the letter C………………………………………..………………………….. 73

Fig. 4.8: Demonstration of writing, erasing and rewriting in the same area.

(a) letter A is recorded, (b) letter A is erased after beingexposed to UV

illumination for 1-2 s, and (c) letter B is recorded in the same area. The

marked artifacts 1 and 2 indicate that the images are in the same area…….….. 75

Fig. 4.9: Images of 24x24 bit patterns recorded by two-photon excitation in a

photorefractive polymer. (a) letter A after first reading, and (b) letter A after

reading 1000 times…………………………………………………………….. 76

Fig. 4.10: Relationship between bit size and (a) power, (b) exposure time and

(c) recording depth, for a recording objective of numerical aperture 0.8. The

points marked by a diamond (♦) indicate erasable data storage, and the points

marked by a circle (•) are conditions under which micro-cavities are formed

(nonerasable data storage as discussed in Chapter Five)…………………...…. 77

Fig. 4.11: Relationship between bit size and (a) power, (b) exposure time and

(c) recording depth, for a recording objective of numerical aperture 1.3. The

points marked by a diamond (♦) indicate erasable data storage, and the points

marked by a circle (•) are conditions under which micro-cavities are formed

(nonerasable data storage as discussed in Chapter Five)…………………….... 79

List of figures

xviii

Fig. 4.12: Three-dimensional bit optical data storage in a photorefractive

polymer under continuous wave two-photon excitation. (a) the first layer

including the letter A, (b) the second layer including the letter B, and third

layer including the letter C…………………………………………………….. 83

Fig. 4.13: Erasable/rewritable bit optical data storage in a photorefractive

polymer under continuous wave two-photon excitation. (a) the letter E is

recorded. (b) the letter E is erased after illuminating the same region with UV

light. (c) the letter F is recorded into the same region as indicated by the

artifacts 1 and 2………………………………………………………………... 84

Fig. 4.14: Optical setup for a phase sensitive microscope with a

detector………………………………………………………………………… 86

Fig. 4.15: Recorded pattern in a photorefractive polymer read using a split

detector………………………………………………………………………… 87

Fig. 4.16: Quadrant detector configuration……………………………………. 87

Fig. 4.17: Recorded pattern in a photorefractive polymer read using a

quadrant detector………………………………………………………………. 88

Chapter Five Fig. 5.1: A single cavity formed in a photorefractive polymer under multi-

photon excitation. (a) transverse and (b) axial images of the cavity in

transmission microscopy.……………….……………………………………... 93

Fig. 5.2: Multi-layered micro-cavity arrays in a photorefractive polymer

under multi-photon excitation. (a) the first layer with the letter A recorded

near the surface and (b) the second layer with the letter B recorded with a

separation of 20 µm in the depth direction.……………………………………. 94

List of figures

xix

Fig. 5.3: (a) Transverse and (b) axial cross sections of the intensity point

spread function at different depths in the photorefractive polymer. The

objective is a dry objective with numerical aperture of 0.7.…...……………… 97

Fig. 5.4: (a) Transverse and (b) axial FWHMs of the intensity point spread

function as a function of the focal depth in the photorefractive

polymer.……………………………………………………………………….. 98

Fig. 5.5: Normalised maximum value of I P

nP(r,z) at the focus as a function of

the recording depth in a photorefractive polymer for n = 1, 2, 4,

corresponding to single-photon, two-photon and four-photon excitation,

respectively………………....…………………………………………………. 99

Fig. 5.6: Recording intensity required to create a micro-cavity versus

recording depth under multi-photon excitation.……………………………….. 100

Chapter Six Fig. 6.1: A single cavity formed in a photorefractive polymer using multi-

photon excitation. (a) transverse and (b) axial images of the cavity in

reflection confocal microscopy.……………………………………………….. 103

Fig. 6.2: Multi-layered micro-cavity arrays in a photorefractive polymer

recorded under multi-photon excitation and read with reflection confocal

microscopy. (a) the first layer with the letter A recorded near the surface and

(b) the second layer with the letter B recorded with a separation of 20 µm in

the depth direction.……………………………………….…………………... 104

Fig. 6.3: The spatial frequency component of the light collected by an

objective after diffraction by a grating of spatial frequency m (Gu,

1996)………………………………………….……………………………….. 106

List of figures

xx

Fig. 6.4: Dependence of the modulus of the 3-D CTF on the focal depth d

when a plane wave at wavelength 800 nm is focused by an objective (NA =

0.7) from air to a medium of refractive index 1.49: (a) d = 0; (b) d = 50 µm;

(c) d = 100 µm, (d) d = 200 µm.………………………………………………. 109

Fig. 6.5: Dependence of the modulus of the 3-D CTF on the focal depth d

when a plane wave at wavelength 800 nm is focused by an objective (NA =

1.4) from oil (nB2B = 1.513) to a medium of refractive index 1.49: (a) d = 0; (b)

d = 50 µm; (c) d = 100 µm, (d) d = 200 µm……………………..…….………. 110

Fig. 6.6: Readout efficiency as a function of the numerical aperture of a

reading objective at different recording depths for (a) air and (b) oil

immersion media………………………….…………….……………………... 112

Fig. 6.7: Readout efficiency as a function of focal depth for different

numerical aperture reading objectives for (a) air and (b) oil immersion

media…………………………………………………………………………... 113

Fig. 6.8: Intensity of the reflected signal from micro-cavities for different oil

immersion numerical aperture reading objectives.……………………..……... 115

Chapter Seven Fig. 7.1: Projected capacity for DVDs compared to current 3DCD technology. 121

Fig. 7.2: Projected capacity for 3DCD technology……………………………. 122

List of tables Chapter One Table 1.1: Comparison of the optical parameters between CDs and DVDs.... 6

Chapter Two Table 2.1: Three-dimensional optical data storage using

photopolymerisation…...………………………………………………………. 24

Table 2.2 Three-dimensional optical data storage using photobleaching

polymers.………………………………………………………………………. 25

Table 2.3 Three-dimensional optical data storage using a photochromic

polymer.…………………………………………………………………….…. 29

Table 2.4 Three-dimensional optical data storage using photorefractive

materials.………………………………………………………………………. 36

Table 2.5: Three-dimensional optical data storage with micro-cavities………. 35

Chapter Three Table 3.1 Figure-of-merit for inorganic and organic photorefractive materials

(Moerner and Silence, 1994).………………………………………………….. 47

xxi

Chapter One

Introduction to optical data

storage

1.1 Introduction Since the invention of the first computer there has always existed the need for some

form of information storage systems other than printed hardcopies. One of the first

of these such systems was computer ribbon; although somewhat awkward it freed the

user from having to input the required information at the beginning of every session.

At the time this was one of the greatest advances in computer technology. Several

other data storage systems have followed over the years, and each time there has

been a limit to the amount of information that could be stored.

In the last few years we have witnessed the use of audio cassettes, floppy disks

(magnetic mediums), compact discs (CDs), digital versatile discs or digital video

discs (DVDs) (optical mediums) and now the slow emergence of a hybrid technology

magneto-optical discs (MO). Throughout this period there have also been

improvements in the way that the information has been encoded and transmitted.

New techniques for compressing data has lead to another format of storing

information, that is fast becoming popular for audio tracks. Moving Picture Experts

Group (MPEG) files are compressed to such an extent that the capacity of a compact

disc is now available on a small memory chip, using the latest compression MP3 (3rd

generation).

However, all of the above systems have or will reach a finite limit beyond which

they cannot increase the storage capacity of the devices. In the case of MPEG

systems, they can only increase the capacity of the memory chip by increasing the

1

Chapter One Introduction to optical data storage

size of the chip or decreasing the size of the components used in designing the chip.

Increasing the size of the chip will work up to a point beyond which the system

becomes impractical. The size of the components is bound by a limit below which

the components become physically impossible, and economically unfeasible to

manufacture. For the optical systems (CDs and DVDs) the finite limit is imposed by

the wave nature of light: the size of a minimum resolvable point is usually no less

than half of the wavelength of the light used to image it. Therefore the amount of

information that can be stored on an optical disk is limited by the wavelength of the

light used to record or read the information.

A new technology that has emerged that is based on optical recording, but is not

restricted by the wave nature of light, is near-field optical data storage. In the near-

field, diffraction is not a dominant effect and so the size of the recorded bit is limited

by the optical system used. Typically there are two devices used for recording and

reading in the near-field region, a fibre probe or a solid immersion lens (SIL). Both

systems are capable of producing a recorded bit 10 times smaller than that in CDs or

DVDs, but again both the fibre probe and SIL systems are limited to recording one

layer of information near the surface of the recording material.

All of the above mentioned recording systems could be classified as two-dimensional

recording systems, where they only record one layer near the surface of the material.

The recording materials that are used in CDs and DVDs are manufactured to be 1.2

mm thick, in which case the recording systems are using 0.01% of the volume of the

material. In terms of the storage capacity per device, such two-dimensional systems

are very inefficient. To effectively utilize the other 99.99% of the volume we need to

investigate three-dimensional recording and reading systems.

1.2 Optical data storage This section will provide an introduction to the different types of optical data storage

systems. A review of compact discs, digital video discs and magneto-optical discs is

2


covered, as well as the areas with emerging technologies, holographic and solid

immersion lens.

1.2.1 Compact discs/digital video discs

In 1983 a collaboration between Phillips and Sony saw the introduction of compact

discs into the consumer market (Encyclopedia Britannica, 2000). Within three years

CDs were selling at over one million per year. At the time the capacity of a CD was

no more than what was currently available on magnetic cassettes tapes; however it

introduced the ability to record and replay audio tracks in digital quality sound.

0001000010001000000000100001000100

Figure 1.1: The relationship between the recorded digital information and the way

that it is represented on the CD/DVD.

Recording information in digital reduces any interference that would corrupt the

quality of the information, but as a drawback requires a tremendous amount of

storage space; one second of digital audio requires over one million bits.

Pit Land

Figure 1.2: Illustration of the pits and land of a CD/DVD.

3


The information is stored on a disc in a helical pattern of pits and land as represented

in figure 1.1. The edge of each pit corresponds to the 1’s in binary notation, and the

land, the area between the two pits, corresponds to the 0’s (see figure 1.2).

The optical setup in the reading system is based upon the interference of the light

reflected from the pit and the land. The discs are fabricated such that the light

reflected from the land has traveled half a wavelength more than that from the pits

and therefore destructively interferes producing no reflection from the pits. The

structure of a recordable CD is illustrated in figure 1.3.

Metal layer Dye layer

Polymer layer

Substrate

Figure 1.3: Illustration of the properties of a recordable CD (CD-R).

The standard design for optical systems incorporates a laser diode operating at an

appropriate wavelength for reading either a CD or a DVD. A diffraction grating

follows the laser diode, that has the effect of producing a main peak and two side

lobes which are used in the tracking mechanism. The three peaks then pass through

a polarizing beam splitter which transmits only parallel polarized light, followed by a

collimator. A quarter waveplate is used to convert the light to circular polarization

before being focused down onto the CD/DVD. If the light strikes land then it is

4


reflected back through the objective and converted by the quarter waveplate back

into linearly polarized light; however this time with vertical polarization. The

polarizing beam splitter then reflects the vertical polarized light through a focusing

lens and a cylindrical lens onto a quadrant detector. The cylindrical lens is used in

the auto-focussing mechanism. A schematic diagram of the optical system used in

CDs and DVDs is illustrated in figure 1.4.

Laser diode

Objective

Collimating lens Photodetector array

Concave singlet lens Cylindrical lens

¼ wave plate

Disc (with pit)

Polarizing beamsplitter

Diffraction grating

Figure 1.4: The schematic of the optical system used in CDs and DVDs.

Within ten years the capacity of a CD fell behind what was required of storage

devices, and the DVD emerged on the market. Early versions of the DVD were able

to store 4.7 Gigabytes (1 byte is equal to 8 bits) of information almost 7.5 times more

information than a CD. Current predictions have the DVD limited to 25 Gigabytes

per disc, if double layer, double sided technology is used, within the next five years

(Higuchi et al., 1999).

5


6

Table 1.1 illustrates the changes that were made to the optical system for CDs and

DVDs to increase the capacity of the system.

Table 1.1: Comparison of the optical parameters between CDs and DVDs.

Parameters CD DVD

Diameter 120 mm 120 mm

Thickness 1.2 mm 1.2 mm

Laser Wavelength 780 nm 640 nm

Numerical Aperture 0.45 0.60

Minimum Pit Length 0.834 µm 0.40 µm

Track Pitch 1.6 µm 0.74 µm

Data Capacity (per layer) 0.68 Gbytes 4.7 Gbytes

Layers 1 1,2,4

By using a shorter wavelength laser and a higher numerical aperture (NA) objective

the DVD optical system is able to produce a smaller focused spot, therefore reducing

the minimum pit length and track pitch (see figure 1.5). This reduction produces an

increase in the storage capacity per layer.

Figure 1.5: A comparison of the minimum pit length and track pitch between CDs

and DVDs (Encyclopedia Britannica, 2000).


As well as increasing the density per layer in DVDs, further development has been

conducted into producing double layer, double sided DVDs which would increase

the capacity by 2 and 4 times respectively. Figure 1.6 illustrates the structures of

single and double layer DVDs.

Dummy substrate

Reflective layer Semi-transmissive layer

Figure 1.6: Illustration of single and double layer DVDs.

The technology involved in DVDs is such that further increases beyond 25 Gbytes in

storage capacity are unlikely. Due to the multi-layered material structuring involved

in one recording layer, the signal from the second layer is significantly degraded, and

therefore further layers on the same side are impossible. The high tolerances on the

thickness of the layers in DVDs increases the cost of manufacturing a disc. It has

been estimated that the cost of one recordable DVD will equal the cost of twenty

CDs. At this point in time the capacity of twenty CDs is greater than one DVD.

7


1.2.2 Magneto-optical discs

Recordable magneto-optical discs are quite different to CDs in the way that data is

recorded and retrieved. On a conventional CD, microscopic pits reflect light from a

laser beam. Their presence or absence makes up the digital code that is converted

into a music signal. On a MO disc it is the polarity of a magnetic field that makes up

the digital code. During recording, a laser beam heats a minute portion of the disc

while a recording head on the opposite side writes the code by changing the polarity

of the magnetic field. Then for playback the laser reads the disc by detecting

differences in light reflected by the coded magnetic layer. Figure 1.7 illustrates the

optical and magnetic setup of a MO system.

Magneto-optical discs are immune to adverse magnetic influences (unlike standard

cassettes) as they need to be heated to around 180° Celsius for the polarity to be

altered.

Detector

Laser

Objective

Disc rotation

Recording head

Old New MO disc Cross-sectional view

0 1 0 1 0

Writing signal

Figure 1.7: Recording mechanism in magneto-optical discs.

8


This method of data storage has been used successfully in computer applications for

some time and is extremely reliable and durable. In fact, it allows a disc to be re-

recorded up to a million times with no loss of quality. The longevity of MO memory

far surpasses any tape format, and has been estimated by Sony at well over thirty

years with no loss of quality (Encyclopedia Britannica, 2000).

1.2.3 Solid immersion lens

All of the different systems described above work in the far-field region where the

maximum resolution, and therefore the data density, is defined by the wave nature of

light. By introducing a specially designed high-refractive index medium between the

objective and the recording medium, the effective numerical aperture of the system is

increased. Figure 1.8 illustrates the typical configuration of a solid immersion lens

recording system.

Collimating lens

Laser diode

SIL Air gap 100 nm

Objective

Photodetector array

Figure 1.8: Schematic diagram of a solid immersion lens recording system.

9


The SIL lens is normally designed to be a hemisphere or super-hemisphere, with a

refractive index greater than 1.9. Materials such as GaP with a refractive index of

3.3 have been used (Hirota et al., 1999). An appropriate combination will result in a

recording system with an effective numerical aperture as high as 1.9. Such a high

numerical aperture allows the SIL lens to work using total internal reflection.

Solid immersion technology has been demonstrated with both MO (Yeh and

Mansuripur, 1999) and phase change (Hirota et al., 1999) recording media.

However, a limitation of the system is the tolerance on the 100 nm air gap between

the SIL and the recording medium. Changes in the size of the gap dramatically

affect the signal contrast of the readout system (Milster et al., 1999). The presence

of dust on the recording surface will easily destroy the performance of the recording

system.

1.3 Three-dimensional storage

1.3.1 Holographic storage

The concept of holography is accredited to Dennis Gabor who was attempting to

improve the image quality of electron microscopy in 1947. Since the 1970’s

holography has been applied to optical data storage (d’Auria et al., 1974). While the

density of this method of recording is expected to reach the limit of Tbit/cm3, the

data transfer rates are far superior to that of conventional storage systems. Due to its

ability to record and readout a whole plane at a time, a transfer rate somewhere

between 1 and 100 Gbits/s is predicted (Wang et al., 1997). A comparison of the

achievable recording densities between holographic and multi-layered bit recording

by Tanaka and Kawata in 1996 summarized that for holographic to reach Tbits/cm3 it

has to employ angle multiplexing.

Two techniques used by holographic storage to record multiple pages of information

within the same region are angle and wavelength multiplexing. By slightly changing

11


the angle or wavelength of the reference beam multiple holograms can be recorded

on top of each other without interference. An advantage of holographic storage is

that the information can be randomly accessed if the recording conditions (i.e.

reference angle) are known.

There are several very similar techniques for holographic recording and the method

described below is just one example. For two-photon holographic recording the

sample is placed at the spatial and temporal intersection of two beams (see figure

1.9). The first beam (probe beam) is focused to a thin sheet of light in the recording

medium. The second beam (pump beam) is passed through a spatial light modulator

(SLM) after being expanded and collimated. The SLM is computer controlled and

can impart a desired recording pattern onto the collimated pump beam.

Pulse delay stage

Nd:YAG laser

He:Ne laser

Green path

IR path

Beam expander

CCD camera Recording

medium Vacuum chamber

Telescope

Anamorphic telescope

SLM

Figure 1.9: Schematic diagram of a holographic recording and reading system (Wang

et al., 1997).

After passing through the SLM the pump beam is then recollimated and imaged onto

the plane illuminated by the probe beam. For reading the fluorescence signal, the

12


wavelength of the probe beam (Helium:Neon) is changed to provide single-photon

excitation of each of the previously recorded planes. A cooled charge coupled

device (CCD) camera then captures the resulting fluorescence.

A problem with most holographic data storage systems is that quite often the reading

system used erases the recorded information. Researchers have been working on

different methods to solve this problem such as thermal fixing or periodic rewriting

of the recorded information.

1.3.2 Three-dimensional bit optical storage

Three-dimensional bit data storage (Parthenopoulos and Rentzepis, 1989) is another

technique whereby information can be recorded within the volume of a recording

medium. The pits described in CDs and DVDs are a result of a stamping process that

produces the discs, whereas the bits created in three-dimensional storage are a

chemical/physical change in the material (not necessarily a recessed region). The

materials and methods of three-dimensional bit optical data storage are the subject of

this thesis and as such are reviewed and discussed in Chapter Two.

1.4 Objectives of this thesis

As seen in sections 1.1 and 1.2, there will be a need for increased storage capacity in

data storage systems. While there are proposed methods (e.g. SIL) to increase the

density of two-dimensional recording techniques, the surface area of the recording

medium ultimately limits the capacity of conventional two-dimensional devices.

Three-dimensional optical data storage as introduced by Parthenopoulos and

Rentzepis (1989), demonstrated the ability of achieving a density 4000 times that

which is currently available from CDs. The material used for that work was a gel

based solution that polymerized upon illumination, which is not a practical method

for storing information.

13


The photorefractive effect has been studied for a long time in crystals and is thought

to be well understood. The nature of the photorefractive effect is such that it is

reversible, and therefore provides an excellent recording medium for

erasable/rewritable optical data storage. However, photorefractive crystals

themselves are expensive and difficult to manufacture into a large volume recording

medium. A new class of photorefractive materials is photorefractive polymers.

Polymer based materials are relatively inexpensive to manufacture and can be

fabricated into large recording media. The fabrication of a photorefractive polymer

consisting of 2,5-dimethyl-4-(p-nirtophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-

fluorenone (TNF) and N-ethylcarbazole (ECZ) in either poly(N-vinylcarbazole)

(PVK) or poly(Methyl Methacrylate) (PMMA) as the host matrix, will form the

recording medium of this work.

Recording multiple layers of information within a medium, without physically

fabricating the layers, requires a method of recording whereby only material within

the focal spot is excited. Parthenopoulos and Rentzepis (1989) and Strickler and

Webb (1991) have demonstrated the use of two-photon excitation; where the

quadratic dependence of the excitation on the incident intensity produces an

excitation volume that is confined to the focal region. This thesis will investigate the

recording of multi-layered information in a photorefractive polymer using two-

photon and multi-photon excitation.

To produce efficient two-photon excitation an ultra-short pulsed laser beam is

required. Several authors have demonstrated continuous wave illumination two-

photon excitation in biological imaging (Hänninen et al., 1994; Hell et al., 1998).

This thesis will explore certain conditions under which continuous wave illumination

can produce two-photon excitation and therefore erasable/rewritable three-

dimensional bit optical data storage in a photorefractive polymer.

Through an investigation of the recording parameters of erasable/rewritable bit data

storage it was discovered that there exists a condition under which the formation of a

14


micro-cavity occurs. This thesis will investigate a method of permanent data storage

through the formation of micro-cavities.

The change in refractive index associated with an erasable bit is approximately 0.1%

variation. Such a small change can only be detected using an imaging technique that

is phase sensitive. While both transmission imaging and DIC can successfully detect

a bit, they have poor axial resolution and therefore limit the storage capacity of the

system. Reflection confocal microscopy has better axial and transverse resolution

with a simpler optical setup, making the complete optical system more practical.

This thesis will demonstrate the possibility of using a reflection confocal microscope

for reading a three-dimensional micro-cavity array.

Spherical aberration is a result of the difference in refractive indices between the

immersion and recording media. The effect of spherical aberration becomes more

pronounced as the focus beam penetrates deeper into the recording medium. The

work in this thesis will include a study of the effects of spherical aberration on the

performance of the recording and reading systems.

1.5 Preview of the thesis A review of current research into three-dimensional bit optical data storage is carried

out in Chapter Two. An introduction to photorefractive materials, both crystals and

polymers, is presented along with a comparison of recording using either multiple

intersecting beams, or a single focused beam. Three-dimensional recording is

achieved using a nonlinear excitation method, two-photon excitation. An

explanation of two-photon excitation and its advantages over single-photon

excitation is given in section 2.3.1. The different materials and recording

mechanisms used by other researchers for three-dimensional bit optical data storage

are discussed in the remainder of section 2.3. Section 2.4 describes the formation of

micro-cavities, which can be used as a permanent form of bit optical data storage. A

review of reflection confocal microscopy is covered in section 2.5 as it is discovered

to be an excellent method for imaging the micro-cavities. Finally section 2.6

15


illustrates the concept of spherical aberration as a result of the difference in the

refractive indices of the immersion and recording media.

Photorefractive polymers are relatively new and it is therefore a challenge to

fabricate them. Chapter Three discusses the elements required to produce a

photorefractive effect in a polymer matrix. A description of the processes used to

make each of the individual compounds and the final polymer sample for this work is

provided.

Chapter Four demonstrates the recording of three-dimensional bits in a

photorefractive polymer under two-photon excitation. A description of both the

recording and reading systems is covered. In particular a reading method based on

differential interference contrast microscopy is described as it is more efficient than

transmission imaging at detecting the small refractive-index changes associated with

a recorded bit. Section 4.4 and 4.6 demonstrate both erasable/rewritable and three-

dimensional recording of bits in a photorefractive polymer under pulsed and

continuous wave two-photon excitation, respectively. The use of continuous wave

illumination is important as it removes the requirement that an ultra-short pulsed

laser is needed to produce efficient two-photon excitation. Consequently, a high

power laser diode could be used. In order to determine the performance of the

recording system a characterization of the recorded bit size under different recording

conditions (e.g. power, exposure time) is conducted. In an attempt to reduce the size

of the optical reading system, a detection system based on a split or quadrant detector

was tested.

The formation of micro-cavities is based on the nonlinear absorption of light;

however in this case the energy is absorbed before it has time to dissipate to the

surrounding medium, which results in the strong modulation of the refractive-index.

Section 5.2 shows a micro-cavity that is formed as a result of a micro-explosion

created by the high temperature and pressure present in the focal region of the

recording beam. The use of a point spread function to simulate the effect of the

spherical aberration on the focal region is conducted in section 5.3.

16


Chapter Six introduces reflection confocal microscopy as a method for reading the

large change in refractive-index associated with a micro-cavity. Reflection confocal

microscopy is unable to detect the small phase changes of an erasable bit; however, it

produces strong reflected signals from both top and bottom surfaces of the cavity.

Section 6.2.1 discusses the advantages of using reflection confocal microscopy

compared with the previous imaging techniques used, transmission and differential

interference contrast microscopy. As mentioned above, spherical aberration affects

the recording system It also reduces the performance of any reading system that is

used to focus deep within a medium. A coherent transfer function is used to

calculate the deterioration of the frequency response of reflection confocal imaging

in the presence of spherical aberration, in section 6.3.2, and it is further expanded to

determine the readout efficiency of the system.

Chapter Seven is a conclusion of the work presented in this thesis as well as

discussion on possible future work that can be conducted to improve the performance

of a three-dimensional bit optical data storage system in a photorefractive polymer.

17

Chapter Two

Review of three-dimensional bit

optical data storage

2.1 Introduction This chapter reviews the advances made in three-dimensional bit optical data storage.

The materials used are of particular importance as they determine the type of

recording and reading methods that can be used.

This chapter is divided into the following sections: section 2.2 reviews the different

recording methods and materials for three-dimensional bit optical data storage.

Section 2.3 discusses the photorefractive effect as well as describes the use of

photorefractive crystals and polymers. A short summary of the work conducted in

photorefractive materials using three-dimensional bit storage is included in section

2.3.3. Section 2.4 discusses the formation of micro-cavities in three-dimensional

space, while section 2.5 looks at reflection confocal microscopy, which can be used

to image the cavities. Section 2.6 reviews spherical aberration that results from the

mismatch in refractive indices between the immersion and recording media. Finally,

section 2.7 will summarize and discuss three-dimensional bit optical data storage.

2.2 Three-dimensional bit optical data storage According to the diffraction theory of light in a far-field region, the light distribution

in the focus spot has a certain size, which is primarily dictated by the wavelength of

the light and the numerical aperture of the objective. The shorter the wavelength and

the higher the numerical aperture, the smaller is the resulting diffraction pattern in

the focal region of the objective. It is this property which limits the capacity of

18

Chapter Two Review of three-dimensional bit optical data storage

optical data storage systems. Current optical data storage systems only record

information within a two-dimensional plane near the surface of the material, using

approximately 0.01% of the available volume in a CD or a DVD. If the third spatial

dimension is used to record information there is an instant increase in storage

capacity without increasing the volume of the storage medium. As optically thick

recording media are currently being used, a future system would benefit from being

able to record in an identical volume medium, which would provide the basis for a

next generation backwards compatible system.

In three-dimensional bit data storage, information is stored in three dimensions by

recording a layer of bits (information) in the transverse (x-y) plane near the surface,

and then successive layers are recorded at different depths into the material (see

figure 2.1). By focusing a laser beam into specific materials, different types of

physical and chemical changes are created. The number of layers that can be

recorded within the volume of the material is dependent on the axial resolution of the

recording and reading methods. Higher axial resolution will allow the distance

between layers to be reduced and therefore increase the storage capacity.

2-D data bits 3-D data bits

(a) (b)

Figure 2.1: Schematic diagram for (a) 2-D and (b) 3-D optical data storage.

19


However, when a recording beam is focused into a volume medium, scattering

caused by the medium occurs; the shorter the wavelength the stronger the scattering

process. As a result, the energy carried by the recording beam cannot be efficiently

transferred into a deep position in the recording medium (Bohern and Huffman,

1983). To overcome this problem, a two-photon excitation process has been

employed (Strickler and Webb, 1991; Blanca and Saloma, 1998).

2.2.1 Single-photon versus two-photon excitation

As indicated in figure 2.2 the principle difference between single- and two-photon

excitation is the absorption of one or two incident photons, respectively. In single-

photon excitation, the absorption of a photon (typically in the ultra-violet (UV) to

visible region) promotes an electron from the ground state to an excited state. The

energy of the absorbed photon is given by,

E = hv, (2.1)

where h is Planck’s constant and v is the frequency of the incident photon.

The process of two-photon excitation requires that two photons, each having energy

of E/2, be absorbed simultaneously to excite an electron from the ground state to an

excited state (Denk et al., 1990). For this transition to happen there is the

requirement that both photons are spatially and temporally coincident which is a

second order nonlinear effect (Shen, 1984).

The energy required to promote an electron from the ground state to an excited state

typically corresponds to the energy of a photon with a wavelength in the UV-visible

region of the electromagnetic spectrum. Most compounds manufactured to increase

the photosensitivity of the recording materials have absorption bands in this region.

This therefore requires the use of a laser and optics designed to work at the shorter

wavelengths. A disadvantage with this is the high UV absorption in glass used for

the optics.

20


(a)

Figure 2.2: Energy level diagram

fluorescence.

In two-photon excitation, the n

intensity and excitation, means

lower than that for single-photon

pulsed laser, with a pulse widt

increase the excitation efficiency

illumination for two-photon exc

(see section 4.6). Continuous w

high powered laser diode to be u

more practical.

The probability of two-photon

excitation because two-photon ex

1984). Due to the quadratic d

intensity (second order nonlinea

volume within the focal regio

fluorescence produced from (a)

linear absorption probability of

fluorescence as seen in figure 2

0υh 1υh
Excited state
2υh

2υh

1υh
Ground state
21

(b)

for (a) single-photon and (b) two-photon excited

onlinear relationship between the incident light

that the probability for excitation is significantly

excitation (Shen, 1984). Therefore an ultra-short

h of a few hundreds of femtoseconds is used to

. It should be noted that the use of continuous wave

itation in biological tissue has been demonstrated

ave two-photon excitation would allow for a small

sed as the laser source making a recording system

excitation is lower than that for single-photon

citation is a second order nonlinear process (Shen,

ependence of the excitation on the illumination

r process), the excitation is confined to a small

n of the objective. Figure 2.3 illustrates the

single-photon and (b) two-photon excitation. The

single-photon excitation means that excitation (or

.3(a)) occurs almost along the entire illumination


path. The highly localised excitation (or fluorescence as seen in figure 2.3(b)) of

two-photon excitation is a direct result of the nonlinear absorption.

(a) (b)

Figure 2.3: Fluorescence from (a) single-photon and (b) two-photon excitation

(Tatterson, 1997).

The localised excitation (shown in figure 2.3 (b)) results in a property known as

optical sectioning. Optical sectioning provides the ability to record a layer of

information above or below a previous layer without an overlap of information,

otherwise known as crosstalk.

Another advantage of two-photon excitation is the use of a near-infrared wavelength

to excite the materials in the UV-visible region. According to Mie scattering theory

(Bohern and Huffman, 1983), the shorter the wavelength, the larger the scattering

cross section. Therefore when focusing deep into the medium the photons are likely

to scatter more than for focusing near the surface. This becomes a significant

problem when bit and layer spacing is reduced to near the diffraction limit.

Denk et al. (1990) for the first time, reported on the use of two-photon fluorescence

in conjuction with a laser scanning fluorescence microscopy. At the time this was

22

HeWolff

Rectangle

HeWolff

Text Box

Image not available-See printed version


23

deemed a breakthrough in imaging, as it was possible to excite UV dyes with a near-

infrared wavelength while achieving high resolution with less probability of

photobleaching living cells.

2.2.2 Photopolymerisation effect

Strickler and Webb (1991) were the first to demonstrate the ability to produce high-

density optical data storage using two-photon excitation. They achieved a density as

high as 0.3 Tbits/cmP

3P, with a bit spacing of 1 µm and a layer spacing of 3 µm using a

photopolymerisable solution.

In photopolymerisation, a gel solution consisting of a monomer and a photoinitiator

are combined in a cell. Upon illumination the photoinitiator produces free radicals

that start the polymerisation of the monomer. Using two-photon excitation, the

polymerisation can be confined to within the excitation region of the focus spot. It is

ideal to irradiate the sample with UV light before recording, so as to gel the sample

to prevent distortion of the recorded planes from shrinkage or flow.

As the sample polymerises, a change in material density occurs at the recorded bit.

This change corresponds to a change in refractive index of 0.8% for Cibatool

XR5081 (Strickler and Webb, 1991), a change from 1.541 for the monomer to 1.554

for the polymer.

Such a large change in the refractive index for the recorded bit can then be read using

a phase sensitive microscope. Differential interference contrast (DIC) microscopy

can be used to produce a phase/intensity map of the recorded pattern, thereby

effectively reading the pattern of recorded bits. Further discussions on DIC are

continued in Chapter Four.

Table 2.1 covers the different materials and equipment that have been used to record

information within three dimensions using photopolymerisation.


24

Table 2.1: Three-dimensional optical data storage using photopolymerisation.

Author Material Objective λ (nm)

Strickler et al. (1991) Cibatool 60x 1.4 620

Wang et al. (2000) (see reference) 40x 0.6 488

Cumpston et al. (1999) (see reference) N/A N/A

Sun et al. (1999) Nopocure 800 100x 1.35 400

Maruo et al. (1997) SCR 500 60x 0.85 790

It should be noted that the ability to fabricate structures with resolution that is close

to the diffraction limit could be useful for creating micro structures for a wide range

of applications including, for example, photonic crystal structures.

2.2.3 Photobleaching effect

Bhawalker et al. (1996) reported on the abilities of high-efficiency two-photon

excitation in a new fluorescent material. A large two-photon absorption cross-

section fluorophore is required to generate efficient fluorescence with a given

wavelength of light. In the case of two-photon photobleaching data storage, the

fluorophore is doped into a polymer block. Illuminating the sample with an

appropriate laser wavelength and average power (typically < 1 mW) from an ultra-

short pulsed laser will produce two-photon fluorescence. Increasing the power above

the bleaching threshold will cause the fluorophore to breakdown (bleach) and stop

fluorescing.

Using this method a series of bleached patterns can be recorded in the material as

illustrated in figure 2.4. The high localisation of the fluorescence in the transverse

and axial directions, known as optical sectioning (section 2.2.1), enables multiple

layers to be recorded in the depth direction with a small layer spacing, resulting in a

high capacity. The recorded information is read back using a two-photon

fluorescence scanning microscope with the illumination power reduced to below the

bleaching threshold.


25

Figure 2.4: Multi-layered information recorded in a photobleaching polymer.

Unfortunately, the information recorded using this method is permanent. Also,

subsequent reading may photobleach the background, reducing the contrast of the

recorded information, ultimately leading to an ineffective recording material.

Table 2.2 covers the different photobleaching polymers and equipment that have

been used to record information in three dimensions.

Table 2.2: Three-dimensional optical data storage using photobleaching polymers.


Shih et al. (1997) APSS 40x 1.3 800

Pan et al. (1997) APSS 40x 1.3 800

Day et al. (1998) APSS 40x 0.75 800

Pudavar et al. (1999) AF240 60x 1.4 800


2.2.4 Photochromic effect

Photochromism is the change of the molecular structure with a corresponding change

in absorption upon illumination of an appropriate wavelength of light. The original

lower energy state of the material is termed isomer 1, and the slightly higher energy

state is referred to as isomer 2.

Parthenopoulos and Rentzepis (1989; 1990) reported the three-dimensional recording

of information in Spirobenzopyran (SP) using “virtual” two-photon excitation. To

achieve the energy required for “virtual” two-photon excitation they used two

orthogonal beams at wavelengths of 1064 nm and 532 nm, which when overlapped

both spatially and temporally, excite at 400 nm (see figure 2.5). This differs slightly

from the two-photon excitation process described in section 2.2.1, where a single

beam with a wavelength of half the energy required is focused into the sample. This

second method for recording has been demonstrated in photochromic materials by

Toriumi et al. (1998).

442 nm

S1

S0

x

612 nm

(c)

1064 nm

1064 nm

532 nm

1064 nm (e) (d)

S1

S0

(a) (b)

Figure 2.5: Energy level diagram of SP for (a) isomer 1 and (b) isomer 2. (c) thermal

relaxation can occur from the ground state of isomer 2 direct to the ground state of

isomer 1. (d) two-photon excitation of isomer 1 using two laser beams of

wavelengths 1064 nm and 532 nm. (e) two-photon fluorescence reading of isomer 2

using two laser beams of wavelength 1064 nm (Parthenopoulos and Rentzepis,

1989).

Figure 2.6 illustrates both isomer 1 and isomer 2 for the compound 1,3,3-

trimethylindolino-6’-nitrobenzopyrylospiran (NSP). Most photochromic compounds

26


can easily be introduced into a polymer matrix, thereby creating a photochromic

polymer (Toriumi et al., 1997).

CH3CH3

N

CH3

O NO2N

CH3CH3

O

NO2CH3

612 nm

442 nm

(b) (a)

Figure 2.6: Photochromic material 1,3,3-trimethylindolino-6’-nitrobenzopyrylospiran

(NSP), indicating (a) isomer 1 and (b) isomer 2 (Toriumi et al., 1997).

If the photochromic compound NSP is in isomer state 1 and is illuminated with 442

nm light it will convert to isomer state 2. Illuminating the material while it is in state

2 with 612 nm light will convert it back to the original isomer state 1. The

absorption band of both isomers 1 and 2 are shown in figure 2.7 for the compound

NSP, which shows the possibility of two-photon excitation using a laser at

approximately 800 nm (Toriumi et al., 1998).

The ability of photochromic compounds to transfer between isomer states makes it

ideal for use in optical data storage. However, as can be seen with figure 2.5, the

ground state energy of the second isomer is typically slightly higher than the ground

state energy of isomer 1. As a result of the difference in energy between the two

ground states, there is a probability that molecules can thermally relax back to the

ground state of isomer 1, thereby destroying the recorded information.

There exists a couple of methods by which the information can be read from

photochromic materials. The first is detecting the fluorescence signal from isomer 2

(Parthenopoulos and Rentzepis, 1989), and the second is to detect the change in

refractive index of the recorded bits (Kawata et al., 1996).

27


1.5

1.0

0.5

0 400

Abs

orpt

ion

(a.u

.)

Figure 2.7: Absorption curve o

al., 1997).

As seen from figure 2.7, there

isomer 2. Using two-photon e

bits (isomer 2) producing flu

system described in section

fluorescence from the backgrou

that there is erasure of the recor

The second reading method in

recorded bit. Changing the che

slight change in the refractive

that a wavelength can be used

isomer 1 nor isomer 2. Th

deterioration, and improves the

Further information on the det

index are discussed in Chapters

(reflection confocal microscopy

(a)

600

Wavelength (n

f NSP for (a) isomer

is a region of wavele

xcitation to excite th

orescing. This diff

2.2.3 for photob

nd is read. A proble

ded information, as it

volves detecting the c

mical structure of the

index. The advanta

to read the informa

is method reduces

stability of the optica

ection methods for r

Four (transmission, D

).

(b)

800

m)

1 and (b) isomer 2 (Toriumi et

ngths that are only absorbed by

is region, results in the recorded

ers to the fluorescence reading

leaching materials where the

m with this method of reading is

can convert back to isomer 1.

hange in refractive index of the

compound means that there is a

ge of this method for reading is

tion that is absorbed by neither

the probability of information

l data bits.

eading the changes in refractive

IC and split detectors) and Five

28


29


information within three dimensions using a photochromic polymer.

Table 2.3: Three-dimensional optical data storage using a photochromic polymer.


Parthenopoulos et al. (1989) SP N/A 1064/532

Parthenopoulos et al. (1990) SP N/A 1064/532

Toriumi et al. (1997) NSP 40x 0.85 441.6

Toriumi et al. (1998) B1536 100x 1.4 760

2.3 Photorefractive effect A photorefractive material has the ability to detect and store the spatial distribution

of an optical intensity pattern as a change in the refractive index. The

photogenerated charges create a space-charge distribution, which produces an

internal electric field that alters the refractive index by Pockel’s effect (electro-optic

effect) (Saleh and Teich, 1991).

2.3.1 Photorefractive materials

2.3.1.1 Photorefractive crystals

The mechanism outlined in this section is used to describe the photorefractive effect

in crystals, although the internal behavior differs slightly to that of the polymers.

Figure 2.8 demonstrates the band transport model, which explains charge transport in

highly ordered structures like crystals.

The band transport model for the photorefractive crystal Fe:LiNbOB3 B shows that upon

absorption of a photon an electron is excited from the donor level to the conduction

band (figure 2.8(a)). The free electrons then diffuse through the material (figure.

2.8(b)), where they eventually recombine (figure 2.8(c)), with an FeP

3+P trap. As a


30

result of the position dependent space-charge distribution, an electric field is formed

(figure 2.8(d)), which in turn modulates the refractive index.

Figure 2.8: Band transport model for charge transport in Fe doped LiNbOB3 B (Saleh

and Teich, 1991).

When a photorefractive material is illuminated by an intensity distribution I(r),

which varies in the r direction, it is accompanied by a change in refractive index

∆n(r). As illustrated in figure 2.8, there are several processes that take place. The

following discussion of the photorefractive effect is representative of the

fundamental photorefractive effect described by Saleh and Teich (1991).

First, the absorption of a photon creates a free charge. For the case of

photorefractive crystals, an electron is excited from the donor level to the conduction

band. The rate of photogeneration G(r) is proportional to the intensity of the

illumination and the number density of the noninoized donors.

),()()( rINNsrG DD+−= (2.1)

where NBDB and +DN are the number density of the donors and ionized donors

respectively, and the photogeneration constant is s.

The next step involves the diffusion and recombination of the free charges. Given

that the illumination I(r) is non-uniform, the number density of the free charges, n(r),

Conduction band

Donor level

Valence band

Electric field

Fe P

2+P

Fe P

3+P

(a)

(b)

(c)

(d)


31

is also non-uniform. This produces regions of high concentrations of like charges.

As a result the charges diffuse to areas of low concentrations. They then recombine

at a rate R(r) that is proportional to the number density of free charges and ionized

donors, n(r) and +DN respectively, where

.)()( += DR NrnrR γ (2.2)

Here Rγ is a constant. At equilibrium the rate of recombination R(r) and

photogeneration G(r) are equal, and so we have

,)()()( ++ =− DRDD NrnrINNs γ (2.3)

from which we get a space-charge distribution given by

).()( rIN

NNsrnD

DD

R+

+−=γ

(2.4)

The non-uniform distribution of charges creates a position dependent electric field

E(r). By observing the steady-state condition, it can be determined that the

magnitudes of the drift and diffusion electric current densities are equal and of

opposite sign, to give the following:

,0)()( =−=drdnTkrErneJ eBe µµ (2.5)

where eµ is the electron mobility, k BBB is the Boltzmann constant and T is the

temperature. The position dependent electric field becomes

.)(

1)(drdn

rneTkrE B= (2.6)

As the material is electro-optic, the internal electric field modifies the refractive

index according to Pockel’s effect.

),(21)( 3 rErnrn e−=∆ (2.7)

where n and r BeB correspond to the refractive index and electro-optic coefficient of the

material.

If we assume that the ratio 1−+DD NN is constant and independent of r, then n(r) is

proportional to I(r) and the electric field can be written as


32

.)(

1)(drdI

rIeTkrE B= (2.8)

Substituting the electric field into the expression for Pockel’s effect (equation 2.7)

gives an expression for the position dependent refractive index change as a function

of the illumination.

.)(

121)( 3

drdI

rIeTkrnrn B

e−=∆ (2.9)

This simple theory assumes that no external DC electric field is applied, as is

consistent with our experimental setup (see Chapter Four).

2.3.1.2 Photorefractive polymers

Unlike photorefractive crystals, the photorefractive polymers do not yet have a

defined charge transport mechanism. While the band transport model can work

under certain conditions (i.e. no applied DC field), it cannot be used explicitly as a

general model of the behavior of organic photorefractive polymer compounds

(Moerner and Silence, 1994; Moerner et al., 1997).

For non-crystalline polymers the mobility of charges is severely limited as a result of

the disordered structure of the compound compared with crystals. Consequently, the

accuracy of the band transport model is reduced when the photorefractive effect in

polymer based materials is considered. A charge hopping model, where charges

travel by hopping through side-chains or guest molecules appears to be the best

model to describe a polymer based photorefractive material (Bosshard et al., 1995).

Further understanding of the model and the different materials required to produce a

photorefractive effect in a polymer are discussed in detail in Chapter Three.

2.3.2 Localised photorefractive effect

The photorefractive effect is based on the non-uniform space-charge distribution

produced, for example, by two intersecting beams as illustrated in figure 2.9.


The sinusoidal interference pattern produced by the two beams is a result of the sum

of the two intensity patterns (Saleh and Teich, 1991).

Beam 2

Beam 1

Figure 2.9: Interference pattern produced by two intersecting waves.

Multiple beams

Figure 2.10: Diffraction pattern of an objective, which corresponds to the

interference pattern produced from multiple beams intersecting in a circularly

symmetric fashion.

33


This concept can be applied to the interference pattern produced from multiple

beams. Increasing the number of intersecting beams in a circularly symmetric

fashion creates an Airy Pattern, corresponding to the diffraction pattern of an

objective (see figure 2.10).

Modulated refractive index

Internal space-charge field

Diffusion and recombination

Photogeneration

Objective

Figure 2.11: Photorefractive mechanism for a focusing beam.

As described in section 2.3.1.1 there are four main processes that must take place in

order to have a modulation of the refractive index as a function of the incident light.

Figure 2.11 illustrates the four processes, photogeneration, diffusion and

recombination, internal space-charge distribution and modulation of the refractive

index.

34


The first physical process required for the photorefractive effect is the generation of

mobile charges in response to the non-uniform spatial illumination. The second

element for the photorefractive effect is the transport and recombination of the

generated charges. The resulting non-uniform distribution of charges creates an

internal space-charge field. Finally, the presence of a nonlinear chromophore in a

non-uniform space-charge field modulates the refractive index via Pockel’s effect

(Saleh and Teich, 1991). Further discussion on the materials required to produce a

photorefractive effect in a polymer is covered in Chapter Three.

2.3.3 Three-dimensional photorefractive data storage

As described before, a photorefractive material is one that undergoes a change in

refractive index as a result of a non-uniform illumination. As the change in

refractive index is caused by an internal electric field, the information is easily erased

by irradiating the sample with uniform illumination to produce a uniform distribution

of the charges, and therefore remove any change in the refractive index associated

with a recorded bit.

Previous work by Kawata et al. (1995; 1998) have used single- and two-photon

excitation to record information within the photorefractive LiNbO3 crystal. Hisaka et

al. (2000) was also successful at recording in a Ce-doped SBN:75 crystal using

domain reversal single-photon excitation.

As yet, photorefractive polymers have only been used in holographic data storage

and two beam coupling experiments (Meerholz et al., 1994; Birabassov et al., 1998;

Bittner et al., 1998; Stankus et al., 1994; Matsushita et al., 1999; Pham et al., 1995a;

1995b). There are several advantages to using polymer based materials for data

storage. First, manufacturing of polymers into large recording samples is relatively

easy and fast compared to growing highly doped crystals. Large scale fabrication

facilities already exist around the world to produce todays CDs and DVDs. It seems

logical then that the next generation of optical data storage should be compatible

with these technologies. Second, as a result of the difficulties in manufacturing high

35


36

quality inorganic crystals there exists a large difference in the production cost of a

sample, not only making it expensive but impractical as a commercial product. The

final advantage is that there is room for improving the performance of the

photorefractive polymers, by tailoring the dopant compounds to suit the recording

and reading systems. One such change would be to increase the absorption within a

specific wavelength region used for recording, thereby reducing the laser power

required to record information.


information within three dimensions using photorefractive materials.

Table 2.4: Three-dimensional optical data storage using photorefractive materials.


Ueki et al. (1996) Fe:LiNbOB3 B 40x 1.0 476.5

Kawata et al. (1995) LiNbOB3 B 40x 1.0 476.5

Kawata et al. (1998) LiNbOB3 B 40x 0.85 760

Hisaka et al. (2000) Ce:SBN:75 40x 0.75 488

It has been illustrated that there are several distinct advantages for using a

photorefractive polymer for three-dimensional bit optical data storage (e.g.

erasable/rewritable). Therefore Chapter Three will discuss in detail the different

processes required to achieve a photorefractive effect in a polymer and the

fabrication of the materials used in this thesis. Chapter Four will demonstrate the

ability to record multi-layered information using the photorefractive effect in a

polymer under two-photon excitation. Further, it will also demonstrate

erasable/rewritable bit optical data storage which can be achieved using the

photorefractive effect.

2.4 Formation of micro-cavities Another method for three-dimensional optical data storage is the formation of micro-

cavities. Glezer et al. (1996) were the first to report the use of micro-cavities in


37

three-dimensional bit optical data storage, and with such a system achieved a density

of 17 Gbits/cmP

3P. The major disadvantage of this method of data storage is that the

creation of micro-cavities is a permanent process, but proves to have some

advantages (e.g. reading with reflection confocal microscopy) over other methods

(i.e. photobleaching). Some of these advantages will be discussed later in this

section and in more detail in Chapters Five and Six.

Again the nonlinear absorption of multi-photon excitation is the basis for the ability

to record information using micro-cavities; however, this time the energy is

increased beyond the damage threshold of the recording material. To achieve this,

the output from an ultra-short pulsed laser is regeneratively amplified to produce µJ

of energy (Glezer et al., 1996; Glezer and Mazur, 1997; Watanabe et al., 1999).

Previous work using photopolymerising, photobleaching, photochromic and

photorefractive effects used energies on the order of nJ.

Using atomic force microscopy (AFM) Glezer and Mazur (1997) were able to

determine that the size of the micro-cavities created by multi-photon excitation

within silica was 200 - 250 nm in diameter. They believe that the effects of self

focusing cannot be ignored when using femtosecond pulses; this would account for

the size of the cavities being much smaller than full width at half maximum

(FWHM) of 0.9 µm for the objective that they used to record. Further, analysis of

the diffraction pattern of an array of cavities reveals that the change in refractive

index (∆n) of a cavity could quite possibly be as large as 0.45, which is consistent

with the belief that a void is indeed created.

Although permanent data storage is not as exciting as erasable/rewritable data

storage, the formation of the cavities allows the use of reading systems that have a

far superior resolution than transmission imaging which is used to read the change in

refractive index associated with the photorefractive effect (Watanabe, 2000). Of

particular interest is the detection of photoluminescence from the cavities created in

silica, as demonstrated by Watanabe et al. (1999; 2000), using single-photon and

two-photon excitation, respectively. The photoluminescence signal is produced by


38

the high density area immediately surrounding the cavity. It is in this region of the

sample that they discovered an absorption band (centered at 250 nm) that is

associated with the absorption of the oxygen vacancy defect VB0 B.

Another process that can be used to read the micro-cavities in three dimensions with

high transverse and axial resolution is confocal microscopy (Gu, 1996). The change

in refractive index in the photorefractive polymer under conditions where cavities are

not formed is approximately 1%. The resulting small change in phase is not able to

be detected using reflection confocal microscopy (Gu, 1996). However, when

cavities are created the change in refractive index can be as large as that encountered

at a material air interface. For the polymer material used in these experiments, that

corresponds to a ∆n of approximately 0.49. A further discussion on confocal

microscopy for reading micro-cavity arrays is given in Chapter Six.


information within three dimensions based on the formation of micro-cavities.

Table 2.5: Three-dimensional optical data storage with micro-cavities.


Glezer et al. (1996) Fused silica 20x 0.65 780

Glezer and Mazur (1997) Fused silica 20x 0.65 780

Watanabe et al. (1999) Vitreous silica 100x 1.3 400

Watanabe et al. (2000) Vitreous silica 100x 1.3 800

It should be noted that the formation of micro-cavities has applications in the area of

photonic crystals. By creating a lattice of micro-cavities the desired photonic crystal

behavior may be achieved. This reversed engineering technique, compared to the

formation of the periodic structures through photopolymerisation, can be used to

create optical components analogous to electrical transistors.


2.5 Reflection confocal microscopy The idea for a confocal geometry was first proposed by Minsky (1961; 1988), when

he attempted to reduce the scattered light from a sample under a conventional

microscope. The concept of confocal microscopy is that the sample is illuminated by

a point source of light, which is then collected by a point detector. The principles of

confocal microscopy have been studied by many researchers (Wilson and Sheppard,

1984; Gu, 1996), and are thought to be well understood. Figure 2.12 illustrates the

principle of confocal microscopy in a reflection mode.

Beam splitter

Pinhole

Detector

Point source

Objective

z scan

Object

Axial response from mirror

Figure 2.12: Reflection mode confocal microscope.

When an object is located in the focal plane, the reflected light is focused onto a

point detector. However, as the object moves away from the focal plane (along the

optical axis), the reflected signal is focused either before or after the point detector

(depending on the direction of movement), and as a result less light is detected. This

ability to discriminate in the depth direction is known as the optical sectioning

property. A three-dimensional image of a thick object can be formed by creating a

stack of images produced at different depths through the object. It was discovered by

Sheppard and Choudhury (1977) that there is also an improvement in the transverse

resolution of 1.4 compared to conventional microscopy.

39


A theoretical analysis on the ability to use reflection confocal microscopy as a

reading method for three-dimensional data storage was conducted by Wilson et al.

(1996), in which they stated that it would not work for photopolymer based memory.

However, Ishikawa et al. (1998) and Toriumi et al. (1998) showed that under strict

recording and reading conditions it was possible to use reflection confocal

microscopy to read the change in refractive index from a recorded bit in a

photochromic material.

As a result of the optical sectioning property, reflection confocal microscopy has

high axial resolution. This enables layers to be recorded with a spacing of only a

couple of micrometers, vastly increasing the storage capacity of the system.

2.6 Spherical aberration resulting from refractive

index mismatching. The currently achievable three-dimensional bit optical recording density is far below

the possible limit of tera bits per cubic centimeter. One of the main reasons for this

low density is the mismatch of the refractive indices between the immersion medium

and the recording material, resulting in spherical aberration. It has been

demonstrated by Day and Gu (1998) that this aberration source can drastically alter

the distribution of the light intensity in the focal region of a high numerical aperture

objective and reduce the intensity at the focus.

Richards and Wolf (1959) investigated the electromagnetic field near the focus of an

aplanatic system, which was later used by Török et al. (1995; 1997) to describe the

focusing of light by a high numerical aperture system into a stratified medium.

When the refractive index of a recording material does not match that of its

immersion medium, the diffraction pattern in the focal region of an objective is

distorted compared with the diffraction-limited pattern made by an objective used in

a uniform medium. Figure 2.13 illustrates the refraction of the converging rays of an

objective when n1 < n2 (figure 2.13(a)) and n1 > n2 (figure 2.13(b)). The distortion

40


41

derives from the fact that the refraction of a converging ray depends on the angle of

convergence. As a result, a high numerical aperture objective suffers from more

distortion than a low numerical aperture objective.

(a)

(b)

Figure 2.13: Converging rays of an objective for (a) nB1 B < nB2 B and (b) nB1 B > nB2 B.

According to results obtained by Török et al. (1995), the electric field of the

diffraction pattern by an objective that satisfies the Sine condition (Gu, 1999) can be

expressed if an incident plane wave is focused from the first medium of refractive

index nB1 B, into the second medium of refractive index nB2B, as

Refracted beams

nB1 B < nB2 B

Refracted beams

nB1 B > nB2 B

θB1 B

θ’B1 B

θB2 B

θ’B2 B

d


42

,cos2

,2sin,2cos

1

2

20

φ

φφ

iIE

IEIIE

z

y

x

−=

=+=

(2.10)

where

( ) ( ) ( ) ( ) [ ] ,cosexpsincossincos,0

122110212

110 ∫ +Φ+=

α

θθθθττθθ dikznikrnJzrI ps (2.11)

( ) ( ) ( ) [ ] ,cosexpsinsinsincos,0

122111212

111 ∫ +Φ=

α

θθθθτθθ dikznikrnJzrI p (2.12)


122112212

112 ∫ +Φ−=

α


where θB1 B and θB2 B are the angles of a ray of convergence in the first and second media,

respectively. J B0 B, J B1B and J B2 B are Bessel functions of the first kind of order zero, one and

two respectively. k is the wave number in vacuum and r and z are radial and axial

coordinates, respectively, with an origin at the focus that would occur if there was no

second medium. The term α is the maximum angle of convergence of the objective

that is determined by the numerical aperture of the objective, and τBs B and τ BpB are the

Fresnel transmission coefficients for the s- and p-polarization states respectively. In

equations 2.11 to 2.13, Φ is given by

( ),coscos 2211 θθ nnkd −−=Φ (2.14)

where d, called the focal depth, is the distance from the interface of the two media to

the diffraction-limited focus. It is clear that the function Φ acts as a spherical

aberration source because of its dependence on the angle , which leads to a distortion

of the diffraction pattern. For a given value of d the larger the difference in the

refractive indices between the two media, the stronger the effect of the spherical

aberration.

Spherical aberration can also be caused if an objective is operated at a non-designed

tube length (Sheppard and Gu, 1991). It was shown by Day and Gu (1998) that

operating the objective at a carefully selected tube length, other than the designed

tube length, can introduce spherical aberration into the system, with a sign opposite


to that produced from the refractive index mismatch. As a result the net effect of the

two aberration sources is minimized.

The effect of spherical aberration on the performance of the recording and reading

systems will be discussed in Chapters Five and Six, respectively.

2.7 Summary This chapter has been used to illustrate the current directions in three-dimensional bit

optical data storage. The future of three-dimensional optical data storage is based on

the nonlinear interaction of light with the recording material, because it appears that

multi-photon excitation offers the most advantages.

Three-dimensional bit optical data storage can be broken down into two areas,

nonerasable and erasable/rewritable. The first type, nonerasable, covers

photopolymerisation, photobleaching and micro-cavities. While these systems are

permanent they have demonstrated the ability to record and read information in a

volume medium at densities of the order of Terabits/cm3. Although promising there

are difficulties in using some of these systems. For example, photopolymerisation

requires the recording material be completely contained as it is a solution, which

therefore makes it impractical as a recording medium for an optical data storage

system.

The second type of systems, erasable/rewritable, in photochromic and

photorefractive materials are proving to be more promising in terms of their

application in a data storage system.

Polymer based recording materials are fast becoming superior to their crystal

counterparts. There is more flexibility in the addition of different compounds to

modify the performance of the recording sample, whether it is for photorefractive,

photochromic or photobleaching effects.

43


Reflection confocal microscopy is an ideal imaging system for reading three-

dimensional optical data storage, as it has many advantages over transmission

imaging systems. As mentioned above, there is an improvement of 1.4 times in the

transverse resolution over conventional imaging, and there is also the optical

sectioning property, both of which will increase the performance of the reading

system, and therefore lead to an increase in the storage capacity.

All optical systems that focus deep into a second medium with a refractive index

different from that at the immersion medium are affected by spherical aberration.

Therefore for a three-dimensional optical data storage system, it is important to

understand the effect of spherical aberration on the recording and reading systems.

44

Chapter Three

Photorefractive polymer material

3.1 Introduction Initial experiments into three-dimensional bit and holographic data storage using the

photorefractive effect were in photorefractive crystals (e.g. LiNbO3). However

photorefractive crystals are expensive and time consuming to manufacture into large

volume samples. Photorefractive polymers provide the opportunity to create

inexpensive materials with easily tailored properties very similar to those of the

crystals.

This chapter presents the materials and preparation of the photorefractive polymer

samples that were used as the recording medium for three-dimensional optical data

storage. This chapter is divided into the following sections: section 3.2 covers the

fundamentals of photorefractivity. As illustrated in figure 2.3, there are several

processes involved in creating a refractive index change as a result of a non-uniform

illumination. A figure-of-merit is described, which is used to compare the

performance of inorganic, organic, crystalline and polymer based materials. The

experimental work in this thesis was carried out on a photorefractive polymer that

was manufactured by the author. The steps involved in manufacturing the nonlinear

optical (NLO) chromophore and photosensitive compounds as well as the

preparation of the final recording sample are described in section 3.3. Section 3.4

provides a summary of photorefractive polymers.

45

Chapter Three Photorefractive polymer material

46

3.2 Fundamentals of photorefractivity Since its first observation by Chen in LiNbOB3 B (Chen, 1967), the photorefractive

effect has been defined as the spatial modulation of the index of refraction due to the

charge redistribution in an optically nonlinear medium. This effect arises as a result

of a non-uniform space-charge distribution generated by the drift and diffusion of

photogenerated charges from a spatially modulated light source. The resulting

internal space-charge electric field modulates the refractive index based on the linear

electro-optic (EO) effect. For the past thirty years this definition has been used to

describe the nonlinear optical process of photorefraction in inorganic crystals.

However in 1990 the first observation of the photorefractive effect in an organic

crystal 2-(cyclooctylamino)-5-nitropyridine (COANP) doped with 7,7,8,8-

tretracyanoquinodimethane (TCNQ) paved the way for an improvement in

performance from photorefractive materials (Sutter and Günter, 1990). However the

growth of high-quality inorganic or organic crystals is very difficult as dopants are

expelled during crystallization. Polymeric materials can however be doped, with

relative ease, using high concentrations of molecules with varying sizes. Polymer

based materials can also be molded into a variety of configurations depending on the

application.

To compare the performance of inorganic and organic photorefractive materials we

employ a figure-of-merit that compares the possible change in refractive index for

different materials as a result of the internal space-charge distribution (assuming

equal densities of trapped charges). The figure-of-merit Q may be defined as

(Moerner and Silence, 1994)

r

ernQ

ε

3

= , (3.1)

where n is the refractive index of the material, rBeB is the effective electro-optic

coefficient and εBr B is the dc dielectric constant relative to the permitivity of free space

ε B0B. The figure-of-merit for different materials including inorganic, organic, crystal

and polymer based materials can be seen in Table 3.1.


47

Table 3.1: Figure-of-merit for inorganic and organic photorefractive materials

(Moerner and Silence, 1994).

Material r BeB (pm/V) n ε Br B

Q (pm/V)

BiB12BSiO B20B 5 2.54 56 1.5

GaAs 1.43 3.4 12 4.7

BaTiO B3B 1640 2.4 3600 6.3

LiNbOB3B 31 2.2 32 10.3

KNbOB3B 380 2.3 240 19.3

(Sr,Ba)NbB2BOB6B 216 2.3 750 3.5

Organic crystal 67 2.0 3.2 168

Organic polymer 30 1.6 4.0 31

PR polymer to date 3.1 1.7 7.0 2.2

It is well known that for inorganics the figure-of-merit does not change significantly

from material to material, which is due to the fact that the optical nonlinearity is

controlled by the large ionic polarizability. In organic materials the nonlinearity

comes from the asymmetry of the electronic charge distributions of the ground and

excited states. It is for this reason that organics with large electro-optic coefficients

do not have a correspondingly large dc dielectric constant. From this it can be seen

that there is the potential for an increase in the performance by a factor of 10 when

using organic photorefractive polymeric materials.

The rest of this section will detail optical nonlinearity in subsection 3.2.1 as well as

necessary elements and special properties of photorefractive polymer based materials

in subsections 3.2.2 and 3.2.3, respectively.

3.2.1 Optical nonlinearity in photorefractive polymers

The nonlinear response of a material to the strength of an applied optical field is

termed optical nonlinearity (e.g. second-harmonic generation). The ability of the

charges in a material to be displaced by an electrical or optical field is called

polarizability P(t), where the polarization of the material depends upon the strength


48

of the applied field. The polarizability can be expressed as a power series of the field

strength E(t) as

( ) ( ) ( ) ( ) ...332211 +++= tEtEtEtP χχχ (3.2)

The quantities χP

1P, χP

2P and χ P

3P are the linear, second-order nonlinear and third-order

nonlinear optical susceptibilities respectively. Typically in low intensity applications

the induced polarizability of a molecule is a linear function of the strength of the

applied optical field. However, in high intensity applications the polarizability is no

longer a linear function of the applied field.

Nonlinear optical phenomena in organic materials originates in the virtual electron

excitations of the individual molecules or polymer side chains compared with

inorganic systems where nonlinear phenomena arise from the energy-gap structure

(Bosshard et al., 1995). This property of organic materials allows researchers to

design new materials with improved nonlinear optical performance by controlling the

placement of individual substituent groups, while maintaining the existing thermal,

chemical and mechanical properties. The excellent property of nonlinear organic

materials is the ease with which the applied fields affect the motions of the electrons.

The π electrons, characteristic of unsaturated organic compounds, are not tightly

bound to the individual positive nuclear sites and therefore their paths or orbitals may

extend over long distances, scanning an entire molecule or even a macroscopic solid

(Garito et al., 1994).

In order for an organic material to exhibit second-order nonlinear optical effects the

molecule or polymer side-chain structures must be noncentrosymmetric, as the even-

order electric dipole coefficients are zero for centrosymmetric molecules. Third-

order nonlinear effects can occur in both noncentrosymmetric and centrosymmetric

materials. Two methods exist for imparting noncentrosymmetry to non-crystalline

systems.

The first method of producing noncentrosymmetric molecules is by chemically

attaching electron donor and acceptor groups at diametrically opposed positions in


the molecule (Bosshard et al., 1995), as illustrated in figure 3.1. This situation is a

molecular analog to p- and n- type doping in semiconductors.

N

CH3 CH3

NO2

Electron donor

Electron acceptor

Conjugation length

Figure 3.1: Demonstration of electron donors and acceptors on the molecule 4-(N, N–

dimethylamino)-4’-nitrostilbene (DANS) (Marder et al., 1997).

The second method is the poled-polymer approach where a large electric field is

applied across the sample, as illustrated in figure 3.2 (Moerner and Silence, 1994;

Zhang et al., 1996). If dipolar nonlinear optical molecules are doped into a polymer

based material and are subjected to a large electrical field at or above the polymer’s

glass transition temperature, Tg, then the dipolar molecules will orient themselves in

the direction of the electric field. If the polymer is then cooled back to its glassy

state with the electric field still applied, the field-induced noncentrosymmetric

orientation of the NLO molecules will be frozen in place resulting in a material with

a second-order optical nonlinearity.

One example of poling (see figure 3.2) is to produce a photorefractive polymer

sample sandwiched between two indium tinoxide (ITO) coated glass slides, which

are separated by a rubber spacer to give the desired sample thickness (Meerholz et

al., 1994; Moerner et al., 1994). A voltage is then applied across the ITO coated

49


slides creating an electric field inside the sample. The temperature can then be raised

above Tg by placing the sample in an oven.

Photorefractive polymer sample

ITO coated glass slides

Spacer

-

+

V

Figure 3.2: Illustration of poling of a photorefractive polymer sample using an

applied electric field across two ITO coated glass slides.

However, there exists a situation where if the glass transition temperature of the

material is sufficiently low enough the NLO species can orient themselves at a room

temperature producing an orientational enhancement mechanism (OEM), which will

be presented in subsection 3.2.1.2.

3.2.1.1 Linear electro-optic effect

The linear electro-optic effect is a second-order nonlinear optical process where an

applied field modulates the refractive index of the polymer material. In this case the

applied field is a combination of external electrical and optical fields.

The large electro-optic coefficient and low dielectric constants, which are

characteristic of organic materials, lead to a figure-of-merit that typically ranges

from 20 to 80, compared with that of inorganic materials like LiNbO3 which has a

figure-of-merit of 10.3 (see Table 3.1).

Polymers with a low glass transition temperature can have a loss of alignment and

consequently a decrease in the electro-optic coefficient in a device at operating

temperatures.

50


3.2.1.2 Orientational enhancement mechanism

The orientational enhancement is a mechanism (Moerner et al., 1994) inherent in

systems that use a polymer as the host material with a Tg that is sufficiently low so as

to allow the NLO chromophores to be orientated at the device’s operating

temperature (typically at a room temperature). For samples with an external electric

field the OEM will produce a modulation in the birefringence of the material and a

modulation in the electro-optic response which combine constructively to increase

the diffraction efficiency of the material. This mechanism also allows for the NLO

chromophores to be oriented by the space-charge field alone provided that they have

sufficient mobility and dipole moment. For a focussed laser beam, the local

electromagnetic (EM) field produced in the focus of a high numerical aperture (NA)

objective is five orders of magnitude larger than the field produced in the objective

aperture. It is most likely that the combination of the increased electromagnetic field

produced in the focus of a high numerical aperture objective and the OEM produces

photorefractive bits, which is demonstrated in the following chapters of this thesis.

For polymer hosts that have a high Tg, the addition of a plasticizing agent or high

concentrations, approximately 30%, of dopants will reduce the effective Tg of the

sample.

3.2.2 Required elements for photorefractivity in organic

photorefractive polymer samples

There are four processes that must take place in order to produce a change in

refractive index in an organic photorefractive polymer material; generation of charge

carriers by illumination, transport of the charge carriers, trapping of the charge

carriers in the dark regions and a linear electro-optic response.

The first physical process is the generation of mobile charges in response to the

focused illumination profile (see figure 2.3). This may be viewed as the separation

of electrons and holes (Moerner and Silence, 1994) induced by the absorption of

51


light in organic materials. A photosensitive compound is normally doped into the

polymer host to provide the charge generation.

The second process is the transportation of one type of the charges, which in the case

of organic materials is more likely to be the holes (see figure 2.11). If the mobility of

the electrons and the holes was the same then the net effect would be to cancel the

internal space-charge field created by this process. The two physical processes

giving rise to charge transport are diffusion due to density gradients or drift due to an

applied field. As most polymer materials with reasonable optical transparency are

relatively good insulators the ability of the charges to move by diffusion alone is

limited. The dominant mechanism for charge transportation would be drift, which

stimulates the charges to hop from one transport molecule to another. Typically the

polymer is used as the charge transportation medium, and a couple of different

polymers will be discussed later in this chapter.

The third effect required for photorefraction is the trapping of the mobile charges

(see figure 2.11). This effect is especially important when the lifetime of the

recorded information is considered. A trapping site can be considered to be a region

in the material where the charge is no longer able to move. The time that the charge

is immobile is determined by the depth of the trap compared to the energy that the

charge gains through absorption of incident illumination or from heating of the

sample. The thermal energy of the charges at room temperature is one of the main

causes of erasure of the recorded photorefractive pattern with time.

The final element necessary for photorefraction is the linear electro-optic effect. As

defined in Section 3.2.1.1, the electro-optic effect modulates the refractive index as a

result of the internal space-charge field (see figure 2.11). A NLO chromophore is

normally doped into the polymer host to provide the electro-optic effect. The

chromophore used throughout the experiments in this thesis is presented later in

section 3.3.1.

52


53

3.2.3 Special properties of organic photorefractive

polymers

There are several physical effects in organic materials that are not observed in

inorganic materials. Such effects can be modified by changing the molecular

properties of the dopant molecules in the polymer host.

The quantum efficiency of charge generation, φBq B, is expected to be highly dependent

on the strength of the applied field. Through the absorption of a photon the

generation of a bound electron-hole pair (Frenkel exciton) and the separation of this

pair to create a free hole competes with geminate recombination which results in an

applied field dependence of φBq B.

The second extra effect present in doped organic polymers is the dependence of the

mobility of the charges on the applied field and molecular spacing. The mobility of

the charges increases as a function ( ) 21

log E of the applied field, compared with

inorganics where the mobility is relatively independent of the applied field. The

mobility is also strongly dependent on the distance between the charge transport

molecules, which requires that they be in high enough concentration to form a

connected network to facilitate the hopping of the charges. The high concentration

of the NLO chromophores required for the photorefractive effect may ultimately

reduce the mobility of the charges in photorefractive polymers (Bittner et al., 1998).

The third physical effect is that the second-order optical nonlinearity of the material

is dependent on the noncentrosymmetry of the doped chromophores. This can be

controlled by adding certain substituents to the chromophores to produce strongly

dipolar molecules or by poling the polymer sample, both of which have been

discussed previously.


3.3 Polymer sample preparation This section will deal with the individual materials that were used to create the

photorefractive polymeric sample in this thesis. These particular compounds have

been previously reported (Meerholz et. al., 1994; Volodin et al., 1996; Bittner et al.,

1998; Lundquist et al., 1996) for their use in two-beam coupling experiments, which

produced diffraction efficiencies near 100%. The use of these compounds in this

thesis was the first time in the sense that they had been used for three-dimensional bit

optical data storage.

Section 3.3.1 will introduce the NLO chromophore used to generate the electro-optic

effect. The chromophore and the photosensitive compound, described in section

3.3.2, were unable to be purchased and so the methods used to produce these

compounds are described. The plasticizing agent and the polymers are presented in

sections 3.3.3 and 3.3.4, respectively.

3.3.1 Nonlinear optical chromophore preparation

The chromophore is designed to produce a physical change in the recording material

after the creation of an internal space-charge field. In these experiments the

chromophore 2,5-dimethyl-4-(p-nirtophenylazo)anisole (DMNPAA), as seen in

figure 3.3, was chosen as a result of its high diffraction efficiency in two-beam

coupling experiments. A high diffraction efficiency indicates that the chromophore

produces a large change in the refractive index as a result of the electro-optic effect.

The process for manufacturing DMNPAA is outlined as follows: 4-cyanoaniline

(1.38 g) was dissolved in aqueous hydrochloric acid (18%, 10ml) and cooled to 0°C.

While stirring a solution of sodium nitrite (0.7 g) in water (10ml) was cooled to 0°C

and added dropwise over 30 min. 2,5-dimethylanisole (1.36 g) was cooled and

added to the stirring solution over 30 min. The final solution was allowed to return

to room temperature and continue stirring for 7 days during which the product would

form as a red precipitate. Filtration in chloroform, drying (Na2S04), and

recrystalization from methanol gave a 99% pure product of 2,5-dimethyl-4-(p-

54


nirtophenylazo)anisole. The resulting compound was confirmed to be DMNPAA

using nuclear magnetic resonance (NMR) scanning.

NN

OCH3

CH3

CH3

CH3

Figure 3.3: Chemical structure of the nonlinear optical chromophore DMNPAA.

3.3.2 Photosensitive compound preparation

The photosensitive compound 2,4,7-trinitro-9-fluorenone (TNF), as seen in figure

3.4, is used to generate charges upon illumination. By introducing a photosensitive

material into the polymer sample the absorption spectra can be tailored to increase

the absorption in the region used for two-photon excitation. Tailoring of the

absorption spectra can also reduce erasure from the reading process or from

unwanted light (i.e. sunlight). Absorption by any compound other than the

photosensitive compound will increase the background absorption and not contribute

to the photorefractive effect. Increasing background absorption will decrease the

number of trapping sites with energies low enough to promote quasi-permanent

storage charge storage.

The process for manufacturing TNF is as follows: to a cold mixture of concentrated

nitric acid (20 ml) and concentrated sulfuric acid (10 ml) there was added

portionwise 9-fluorenone (1 g). The solution was refluxed for six hours and poured

onto ice. The precipitate was filtered, dried and then recrystalized from glacial acetic

55


acid to produce 99% pure 2,4,7-trinitrofluorenone. The resulting compound was

confrmed to be TNF using NMR scanning.

ONO2

NO2

NO2

Figure 3.4: Chemical structure of the photosensitive compound TNF.

3.3.3 Plasticizer compound

The plasticizer N-ethylcarbazole (ECZ), as seen in figure 3.5, is added to reduce the

glass transition temperature of the sample. As discussed in section 3.2.1.2 an

increase in the refractive index modulation by the orientational enhancement

mechanism can be achieved when the glass transition temperature approaches the

operating temperature of the device (typically at a room temperature). It was

discovered by Bolink et al. (1996) that the addition of ECZ increased the electro-

optic coefficient of a polymer sample. However, by replacing large quantities of the

polymer with the plasticizer there is a decrease in the polymeric binding, which leads

to the NLO crystallizing (Bittner et al., 1998). The result is an opaque sample that is

not usable for optical data storage.

NCH2CH3

Figure 3.5: Chemical structure of the plasticizer ECZ.

56


57

The glass transition temperature of one of the recording samples used in this thesis

with the concentrations DMNPAA:PVK:ECZ:TNF, 30:53:16:1 respectively, is

approximately 17°C (Bittner, 1998). The plasticizer ECZ was commercially

available.

3.3.4 Polymer compounds

Two polymers were adopted as the host material for the photorefractive polymer

recording samples used in this thesis. The first is poly(N-vinylcarbazole) (PVK), the

chemical structure can be seen in figure 3.6. Poly(N-vinylcarbazole) belongs to a

class of polymers known as charge-transporting polymers: a substantial increase in

the electrical conductivity occurs when the polymer is exposed to light.

Figure 3.6: Chemical structure of the polymer compound PVK.

Poly(N-vinylcarbazole) was the polymer used by Meerholz et al. (1994) when they

reported a diffraction efficiency near 100%. While the charge-transport properties of

PVK promote a large modulation of the refractive index, there are a few

disadvantages regarding the use of PVK. From a health and safety point of view,

PVK is known to cause an eczema-type rash on the people handling it.

Experimentally, the refractive index of PVK is 1.69. Such a high refractive index

increases the spherical aberration that results from the refractive index mismatch

when a beam is focused deep into the material (see section 2.6). Further, it was

difficult to fabricate recording samples with high concentrations of dopants. Several

methods were used, but most resulted in recording samples where the NLO

chromophore had crystallized out. It was later discovered that the high average laser

NCH* CH2 *n


58

powers required for continuous wave two-photon excitation (see section 4.6) of PVK

based samples resulted in damage. Another polymer, poly(Methyl Methacrylate)

(PMMA), was then chosen as the host as it has a higher melting point, and was less

likely to be destroyed. The chemical structure of PMMA can be seen in figure 3.7.

Figure 3.7: Chemical structure of the polymer compound PMMA.

Poly(Methyl Methacrylate) is a widely used plastic that has found many areas of

applications in the automobile industry alone. As such there exists a considerable

amount of knowledge regarding the preparation and use of the polymer.

Poly(Methyl Methacrylate) is a polar compound and so has a high dielectric constant

at temperatures below its glass transition temperature (104°C). The lower refractive

index of PMMA (n = 1.49) means that as a recording sample there is less spherical

aberration than that encountered when PVK is used, as a result the optical

performance of the recording and reading systems is improved (see Chapters Five

and Six).

3.3.5 Recording sample preparation

The recording media based on both PVK and PMMA were made in a very similar

fashion. First, the correct weight of each of the compounds was measured out. All

of the compounds were solids except for the monomer Methyl Methacrylate, which

was measured out by volume based on a density of 0.936 g/cmP

3P. The compounds

were then combined in a small teflon vial that had a diameter of 15 mm. Before

adding the monomers, they were first distilled to remove the small quantity of

inhibitor that was added to prevent polymerisation during storage. A small magnetic

stirrer was then added to the mixtures as they were placed in a waterbath. The

* CH2C *n

CH3

COOCH3


59

0

0.5

1

1.5

2

378 428 478 528 578 628 678

Wavelength (nm)

Abs

orba

nce

(a.u

.)

0

0.5

1

1.5

2

378 428 478 528 578 628 678

Wavelength (nm)

Abs

orba

nce

(a.u

.)

temperature of the waterbath was maintained at 80°C and 60°C for the PVK and

PMMA samples, respectively. A higher temperature was required for dissolving all

of the compounds into the monomer Vinyl Carbazole.

(a)

(b)

Figure 3.8 : Absorption curve of (a) PVK:DMNPAA:ECZ:TNF and (b)

PMMA:DMNPAA:ECZ:TNF.


The samples were then left to stir for several hours to increase the homogeneity of

the mixed compounds. A small amount of the initiator Azodi-isobutylonitrile

(AiBN) was then included into the sample to start the free radical polymerisation. At

this point the air in the vial was replaced with nitrogen, as the oxygen in air can

terminate the polymerisation process, resulting in a sample that was not completely

polymerised or a sample with very short polymer chains, poor optical quality and

reduced electro-optic performance. As the sample started to become viscous the

stirrer was removed, and manual agitation of the vial was used instead. The vial was

once again filled with nitrogen. Once the sample appeared to have become solid it

was removed from the waterbath and placed in the dark for a couple of days. When

the polymerisation process was complete the sample had shrunk enough so that it

was easily removed from the teflon vial. Figure 3.8 shows the absorption curves for

both the PVK and PMMA based recording samples.

The absorption curves of both samples are very similar, and neither show significant

changes to the absorption band as the quantity of the NLO chromophore, DMNPAA,

is changed. The samples that were produced in the teflon vials did not require extra

processing (e.g. polishing), and could be used for recording immediately.

3.4 Summary

This chapter has illustrated the different processes that are required to produce a

photorefractive effect in a polymer based material. In addition a comparison of the

figure-of-merit for different inorganic crystals, organic crystals and polymer

materials is shown in Table 3.1. From this table it can be seen that the refractive

index of polymer based materials is lower and therefore more desirable (see Chapters

Five and Six) than that for crystals. More importantly the figure-of-merit for the

polymers is theoretically better than the crystals, even though the number proposed is

yet to be reached.

The preparation of two of the compounds, DMNPAA and TNF is described in

section 3.3. After both the compounds were manufactured, they were used to

60


fabricate the recording samples as described in section 3.3.5. It can be seen from

figure 3.8 that the absorption curves for both the PVK and PMMA based recording

samples are very similar. In both cases, the absorption band extends through the

visible region of the spectrum into the red. This large absorption band is not ideal as

the absorption of unwanted light may excite the material, and therefore erase the

recorded information (see Chapter Four).

61

Chapter Four

Three-dimensional bit optical

data storage

4.1 Introduction Three-dimensional bit optical data storage has the ability to reach a density of

Tbits/cm3 (Strickler and Webb, 1991). This can be achieved using nonlinear two-

photon excitation of the recording material. The aim of this chapter is to demonstrate

the feasibility of the photorefractive polymer fabricated in the last chapter for three-

dimensional bit optical data storage.

This chapter is separated into the following sections: section 4.2 will illustrate the

recording system used for both pulsed beam and continuous wave illumination. The

two methods used for reading, transmission and differential interference contrast, are

detailed in section 4.3. Section 4.4 will demonstrate the ability to record multi-

layered and erasable/rewritable information within the volume of a photorefractive

polymer using pulsed illumination. In an attempt to determine the performance of

the recording system, section 4.5 characterises the recorded bits. Section 4.6

discusses the relationship between two-photon excitation and the pulse width of the

illumination beam. To demonstrate the relationship, multi-layered and

erasable/rewritable information is recorded in a photorefractive polymer under

continuous wave illumination. Section 4.7 will discuss the use of alternative

detection techniques for reading the bits and finally section 4.8 will summarise

erasable/rewritable three-dimensional bit optical data storage.

62

Chapter Four Three-dimensional bit optical data storage

4.2 Experimental recording system A schematic diagram of the experimental recording system used to record

information within the three dimensions of a volume recording material is illustrated

in figure 4.1. Figure 4.2 is a picture of the system in the laboratory.

x

z

F1

L3

O2

Laser

M2

Mechanical shutter

O1

L2BS2 P2

PMT

CCD

F2

White light source

Recording sample

x-y-z translation stage

y

BS1

A

L1

P1

M1

Variable neutral density filter

Figure 4.1: Schematic diagram of the recording system.

To control the logic state of the recorded information a mechanical shutter was used

to block the beam. In these experiments a NM Laser shutter with a minimum rise

and fall time of 2 ms was used. This limited the speed of the recording to 500 bits

63


64

per second (bps). Typically the shutter was operated with a rise and fall time of 15

ms, to avoid any inaccuracy in the exposure time. After the shutter, the beam was

spatially filtered and collimated. Using a 10x objective (OB1B) with numerical aperture

(NA) of 0.25, the laser beam was focused through a pinhole (PB1 B), typically 10 – 30

µm in diameter. The light collected from the pinhole was then collimated with lens

(LB1B). The focal length of LB1B, and therefore the collimated beam diameter were

dictated by the size of the back aperture of the recording objective (OB2 B). Following

LB1 B was a variable aperture (A) and a 50/50 beam splitter (BSB1 B). The initial beam

steering mirrors (MB1 B, M B2B) and the two beam splitters were coated for ultra-short

pulses, to try to reduce the amount of pulse broadening throughout the system.

Figure 4.2: Picture of the recording system in the laboratory.

An increase in the pulse width decreases the peak power within the focus of the

objective, which therefore decreases the efficiency of two-photon excitation. The


65

relationship between the rate of two-photon excitation and the pulse width will be

discussed in section 4.6.1 which considers continuous wave two-photon excitation in

the recording process.

The recording sample was mounted in a computer controlled translation stage. Two

recording stages were used through this work. The first was a Melles Griot

Nanoblock system, and the second was a Melles Griot NanoMover. The Nanoblock

consisted of a 200 x 200 µm x-y piezo-electric stage and a 200 µm z piezo-electric

stage. The displacement of the stage was controlled by an analog voltage signal from

a computer sent to the drive box. The resolution of each axis in the Nanoblock was

50 nm. The NanoMover translation stage has three individual axes, each with a

travel of 25 mm. To achieve the longer travel, stepper actuators were used in the

NanoMover, which results in a slightly reduced accuracy of 100 nm and a

repeatability of 200 nm, compared to the Nanoblock. GPIB communication was

used to control the NanoMover.

The recording program for both systems was written using the software LabVIEW.

The program was designed to accept an input file made up of 1’s (bit) and 0’s (no

bit) corresponding to where a bit was to be recorded. Later versions of the program

included delays to reduce any vibrations in the stage as can be seen in the images in

section 4.6.3.

In order to determine the position of the focus before recording the sample was

scanned in the z (axial direction). Scanning the sample in the z direction produced

an axial response (see Chapter Six; reflection confocal microscopy). The scanned

signal from the surface of the sample was collected by the objective (OB2 B) and

reflected from beam splitter 1 through another lens (LB2 B) which focused the light into

a pinhole (PB2 B) mounted in front of a detector, typically a photomultiplier tube (PMT).

Once the surface of the sample was determined, the focus position could be

accurately translated throughout the sample to record multiple layers.


To view the recording process in real-time, a white light source was focused by lens

L3 onto the back of the recording sample. The light was then collected by the

objective, O2, and reflected by BS1 towards the PMT. Between L2 and the detection

pinhole another beam splitter was placed to reflect the light onto a Charge Coupled

Device (CCD). The image from the CCD was collected by a frame grabber and

displayed real time on a computer monitor. A long-pass filter (F1), with a cutoff

wavelength of 600 nm, was placed after the white light source to prevent any

unnecessary erasure of the recorded information as the recording sample has strong

absorption in the ultra-violet to visible wavelength region. A short-pass filter (F2),

with a cutoff wavelength of 750 nm was placed in front of the CCD, to reduce the

chances of damaging the CCD array from the focused laser light.

4.3 Experimental reading system The change in refractive index created by the recording process in the

photorefractive polymer is typically less than a 1% change. Such a small change can

only be measured by an optical system that is sensitive to phase of a reading beam.

The two methods that were used in this project were transmission and differential

interference contrast (DIC).

4.3.1 Transmission reading

The transmission system is the simplest method to detect phase changes along the

optical axis of a material. A schematic of a transmission system is illustrated in

figure 4.3.

The use of a light source illuminating the sample and being collected by the CCD

represents a transmission system. However, the image collected by the CCD camera

does not have high enough resolution to read the recorded information for the

purposes of this work.

66


For this project an Olympus FluoView scanning microscope was used to read the

recorded information. Figure 4.4 is an upright Olympus FluoView scanning

microscope that can be used to for transmission or DIC imaging. The FluoView

microscope is designed for single-photon fluorescence, and as such has a

Krypton:Argon laser coupled to it for ultra-violet (UV) - blue excitation at

wavelengths of 488 and 564 nm.

x

z

y

Objective

Mercury lamp

Recording sample

Transmitted light detector

Olympus FluoView sc

Emission filters

PMT2

PMT1

x-y galvanometer mirrors

Olympus microscope

Figure 4.3: Schematic diagram of a transmission reading system

Dichroic beamsplitter

anning box

Laser

.

67


As these wavelengths are within the absorption band of the photorefractive polymer

(see figure 3.8), they cannot be used as they will erase the recorded information.

Instead a Helium:Neon laser (632.8 nm) is coupled to the microscope. To improve

the performance, the dichroic beamsplitter designed to reflect the laser and transmit

the fluorescence (for normal operation) is replaced with a standard 50/50

beamsplitter.

Figure 4.4: Olympus FluoView microscope for reading the recorded photorefractive

bits using transmission or DIC imaging.

68


69

Although able to detect the changes in refractive index of a recorded bit, the

transmission system has poor axial resolution, and therefore determines the minimum

distance between recorded layers.

4.3.2 Differential interference contrast reading

Nomarski differential interference contrast (DIC) imaging (Nomarski, 1955) has

become a widely used tool in microscopy for retrieving phase information from a

sample. A couple of key features of DIC microscopy result in its popularity

compared with the older methods of phase imaging, based on Zernike phase contrast

systems. The first is the ability to obtain both phase and intensity and display them

in the form of a shadowed boundary at the point of increasing or decreasing phase.

The amount of highlighting produced at the boundary can be varied by changing the

phase delay introduced into the system. The second feature is the apparent optical-

sectioning property compared with other conventional microscope imaging

techniques (Cogswell, 1991; Preza, 1999). However, it should be noted that this only

applies to the higher spatial frequency components of the object and cannot be

compared with the optical-sectioning property of confocal microscopes (see Chapter

Six).

Figure 4.5: Schematic diagram of a differential interference contrast imaging system.

φ∆

Detector

Objective Collection lens

Recording sample

Wollaston prism

Polarizer

Analyser


70

Figure 4.5 illustrates the schematic of a DIC imaging system, while figure 4.4 shows

the Olympus FluoView microscope that can be used to produce images of a sample

using both transmission and DIC microscopy.

The image from DIC microscopy is produced by the difference in the amplitudes of

two images, which are displaced laterally and have been phase shifted with respect to

each other. A plane polarized laser beam is split into two components by a

Wollaston prism. The two waves, e- and o- waves become spatially separated and

are focused into the sample. While passing through the sample, the two beams will

experience different amounts of phase shift if they pass through different regions of

refractive index. The beams are then collected and recombined with another

Wollaston prism before passing through an analyser and being detected. An image is

formed by scanning the sample through the focus position of both beams.

To optimize the sensitivity of the system and the contrast of the resulting image,

control over the amount of lateral shear and phase shift of the two components is

required. The lateral shear is a function that is designed into the Wollaston prism,

while the phase shift is adjusted by lateral displacement of the second Wollaston

prism analyser. It is also possible to alter the relative strength of the two beams by

rotating the two polarizers.

The relationship between the phase shift introduced by the Wollaston prism and the

relative image intensity can be described by the subtraction of the two illumination

beams. The complex amplitude of an object can be written as (Cogswell and

Sheppard, 1992)

.θiaet = (4.1)

If the amplitudes of the two component beams with phases ± φ, are subtracted,

( ) ( ) ,sin2 φθφθφθ iii iaeaeaeU =−= −+ (4.2)

the image intensity is given by the square of the amplitude, which becomes,

,sin4 22 φaI = (4.3)

where φ is the retardation bias.


71

Equation 4.3 indicates that the image intensity is independent of the absolute value of

the object phase. However, if the phase in the object changes by ∆θ between the

two laterally displaced beams, then this produces a change in the value of φ and

results in a variation in the image intensity (∆I), as seen in figure 4.6.

Figure 4.6: Relationship between the phase difference between the two laterally

displaced beams and the image intensity as a result of different phase bias (Cogswell

and Sheppard, 1992).

As will be shown in section 4.4 and 4.6, using DIC microscopy to image the small

changes in refractive index associated with a recorded bit is an effective method,

though there are a number of disadvantages. The poor resolution in the axial

direction is a limiting factor in the density of recorded information, and the optical

system required is not easily incorporated into a compact device.

φ π/2-π/2

I

∆θ

∆ I

I

φ π/2-π/2 ∆θ

∆ I

I

φ π/2-π/2

∆θ

∆ I


72

In section 4.7 a brief investigation of alternative detection methods using split and

quadrant detectors was conducted to determine their sensitivity to the change in

refractive index while simplifying the optical system.

4.4 Pulsed beam illumination To achieve high efficiency two-photon excitation, an ultra-short pulsed laser is

needed (as described in section 2.2.1). In these experiments the laser used was a

Spectra Physics Tsunami, pumped by a 10 W Millennia. The Tsunami is an ultra-

short pulsed Ti:Sapphire, which has a wavelength range from 700 – 1000 nm.

Within the wavelength range it produces ultra-short pulses of ~70 fs at a repetition

rate of 82 MHz. The laser can also be operated in continuous wave (CW) mode,

which is demonstrated in section 4.6.

4.4.1 Multi-layered data storage

With two-photon excitation for recording, the first demonstration of three-

dimensional bit optical data storage in photorefractive polymers was achieved (Day

et al., 1999). The photorefractive material used consisted of nonlinear chromophore

2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA), the photosensitive compound

2,4,7-trinitro-9-fluorenone (TNF) and the plasiticizer N-ethylcarbazole (ECZ) all

doped into the polymer host poly(N-vinylcarbazole) (PVK). The concentration of

the materials DMNPAA:PVK:ECZ:TNF was 10:73:16:1 as a percentage of the total

weight. Owing to difficulties in manufacturing a homogenous sample with high

overall concentration of dopants, the samples were manufactured with a thickness

ranging from 100 - 200 µm.

Figure 4.7 demonstrates the ability to record multiple layers of information within

the volume of a photorefractive polymer under pulsed two-photon excitation. The

wavelength used for excitation was 800 nm, with an average power of 7.5 mW in the

focus of a Zeiss Fluar objective with numerical aperture and a magnification factor of

0.75 and 20, respectively. The objective operated at an infinite tube length and was


73

corrected for a 170 µm thick cover slip. The exposure time for each bit was 20 ms.

It can be seen from the absorption curve of this material (see figure 3.8(a)) that there

is no absorption beyond 630 nm, and therefore the recording process was two-photon

excitation. The spacing between bits in each plane is 3.2 µm, while the spacing

between layers is 20 µm. With this bit and layer spacing the density of the recorded

information is 5 Gbits/cmP

3P. In this case the layer spacing was large enough to

prevent cross talk from the neighbouring layers.

(a) (b)

(c)

Figure 4.7: Recorded 24x24 bit patterns at different depths in the photorefractive

polymer under two-photon excitation. The spacing between adjacent layers is 20

µm, and the bit separation is 3.2 µm. (a) the first layer including the letter A, (b) the

second layer including the letter B and (c) the third layer including the letter C.


74

For reading the change in refractive index caused by the two-photon photorefractive

process we employed an Olympus FluoView scanning microscope and used it in a

DIC mode (see section 4.3.2). A Helium:Neon laser of wavelength 632.8 nm was

coupled in the FluoView microscope for reading the recorded information, as the

wavelength of 632.8 nm is on the edge of the absorption band and causes minimal

erasure to the recorded information. The power within the focal region of an

Olympus UPlanApo objective was less than 2 mW. The numerical aperture and

magnification factor of the objective are 0.7 and 20, respectively. The objective

operated at an infinite tube length and was corrected for a 170 µm thick cover slip.

The values for recording power and exposure time were chosen so as to create a large

enough change in refractive index to provide good contrast in the DIC image and to

maintain the ability to erase, while a high density of information was maintained. A

further discussion on the relationship between recording power, exposure time and

numerical aperture is given in section 4.5.

4.4.2 Erasable/rewritable data storage

The principle of the photorefractive effect is that a non-uniform illumination will

lead to a position dependent space-charge field, which in turn modulates the

refractive index of an electro-optic material. If the charges are redistributed

uniformly then the information based on the change in refractive index is erased.

Figure 4.8 demonstrates that due to the photorefractive effect, an array of bits can be

written, erased and rewritten under two-photon excitation for recording and ultra-

violet illumination for erasing.

The parameters for the recording process are the same as used for figure 4.8. The

background in figure 4.8 has increased shadowing as a result of an increase in the

lateral shear. To erase the information the sample was illuminated with the ultra-

violet line of the mercury lamp in the FluoView microscope. A typical time for

complete erasure of the information was 1 – 2 seconds. The rewritten pattern (see

figure 4.8(c)) was obtained using the same recording parameters as mentioned above.


75

(a) (b)

(c)

Figure 4.8: Demonstration of writing, erasing and rewriting in the same area. (a)

letter A is recorded, (b) letter A is erased after being exposed to UV illumination for

1-2 s, and (c) letter B is recorded in the same area. The marked artifacts 1 and 2

indicate that the images are in the same area.

Using erasable materials may lead to an issue regarding the stability of the recorded

information. Figure 4.9 shows the deterioration of the recorded information after

being read 1000 times. The contrast of the recorded bits in figure 4.9(b) are 50% of

that in figure 4.9(a). This result illustrates that there is weak erasure of the

information due to the slight absorption of the light beam used for reading.

1

2

20 µm

1

2

1

2


76

There is also strong absorption of sunlight and fluorescent lights, which result in

erasing the information. Typically the recorded sample has to be read within 30

minutes; otherwise the information will have deteriorated enough to prevent reading.

If kept in the dark, the information will maintain its contrast for several hours, at

which point thermal relaxation destroys the quality of the information. Thermal

relaxation in this particular material is strong due to the reduction in the glass

transition TBg B temperature by the introduction of the plasticiser. A better stability

could be achieved by reducing the absorption band and increasing the glass transition

temperature of the sample.

(a) (b)

Figure 4.9: Images of 24x24 bit patterns recorded by two-photon excitation in a

photorefractive polymer. (a) letter A after first reading, and (b) letter A after reading

1000 times.

4.5 Bit characterisation In order to understand the relationship between the recording parameters and the

resulting change in refractive index, a characterisation of the recorded bits was

completed. From a practical point-of-view a low numerical aperture objective was

used in the recording process, as there is less spherical aberration involved with the

smaller focusing angles. This was then compared with the performance of an oil

immersion objective with high numerical aperture.

20 µm


77

(a)

(b)

(c)

Figure 4.10: Relationship between bit size and (a) power, (b) exposure time and (c)

recording depth, for a recording objective of numerical aperture 0.8. The points

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40

Recording depth (µm)

Tran

sver

se b

it di

amet

er ( µ

m)

Micro-cavity

Micro-cavity

00.20.40.60.8

11.21.41.61.8

17 27 37 47 57

Recording power (mW)

Tran

sver

se b

it di

amet

er (µ

m)

0

0.5

1

1.5

2

2.5

5 105 205 305 405 505

Exposure time (ms)

Tr

ansv

erse

bit

diam

eter

(µm

)


78

marked by a diamond (♦) indicate erasable data storage, and the points marked by a

circle (•) are conditions under which micro-cavities are formed (nonerasable data

storage as discussed in Chapter Five).

Figure 4.10 shows the relationship of the bit size to the recording power, exposure

time and recording depth. In recording, an Olympus ultra-long working distance

(ULWD) objective with numerical aperture and a magnification factor of 0.8 and 20,

respectively, was used. For these experiments the recording laser was operated in a

pulsed mode at a wavelength of 800 nm.

The bits were read using the Olympus FluoView microscope operated in

transmission, with an Olympus PlanApo oil immersion objective that had numerical

aperture and a magnification factor of 1.4 and 60, respectively. The bit spacing was

increased to 7.8 µm to prevent any interference on the bit size from neighbouring

bits. The exposure time was maintained at 25 ms while the recording power was

varied, or the power was kept at 25 mW while the exposure time was altered. For

recording at different depths the power and exposure time were 25 mW and 25 ms,

respectively.

From figure 4.10, it can be seen that an increase in power, exposure time and depth

results in an increase in the bit size. In figures 4.10(a) and (b) there is a region at the

highest power and longest exposure time where the bit size decreases. It is at this

point where a micro-cavity is formed. The formation of micro-cavities will be

discussed in Chapter Five. However, the changing in bit size as a function of depth

is a result of spherical aberration introduced from the mismatch in refractive indices

(Day and Gu, 1998) between the immersion (air n = 1.0 ) and recording (polymer n =

1.49) media, which will be further discussed in section 5.3.


79

(a)

(b)

(c)

Figure 4.11: Relationship between bit size and (a) power, (b) exposure time and (c)

recording depth, for a recording objective of numerical aperture 1.3. The points

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60

Recording depth (µm)

Tran

sver

se b

it di

amet

er ( µ

m)

Micro-cavity

Micro-cavity

0.50.60.70.80.9

11.11.21.31.4

6.5 8.5 10.5 12.5 14.5 16.5

Recording power (mW)

Tr

ansv

erse

bit

diam

eter

(µm

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

5 15 25 35 45 55 65 75

Exposure time (ms)

Tr

ansv

erse

bit

diam

eter

(µm

)


80

marked by a diamond (♦) indicate erasable data storage, and the points marked by a

circle (•) are conditions under which micro-cavities are formed (nonerasable data

storage as discussed in Chapter Five).

Figure 4.11 shows the relationship of the spot size to the recording power, exposure

time and recording depth for an oil immersion high numerical aperture objective.

The objective used was a Zeiss Fluar objective with numerical aperture and a

magnification factor of 1.3 and 40, respectively. To demonstrate the relationship of

the bit size to power the exposure time was maintained at 25 ms. To illustrate the

relationship between bit size and exposure time the power was kept at 14 mW. The

power and exposure time were kept at 14 mW and 25 ms, respectively, when the bit

size versus depth relation was measured. The reading conditions are the same as

described previously in this section.

The same characteristics, as those shown in figure 4.10, can be seen in figure 4.11

using the high numerical aperture objective. However, a comparison of the

corresponding graphs shows that using the higher numerical aperture objective

results in a smaller bit size under almost all recording conditions except for those in

figures 4.10(c) and 4.11(c). This feature results from the smaller focal region

produced by the higher numerical aperture objective.

Figure 4.11(c) further illustrates the effect of spherical aberration as a result of the

mismatched refractive indices. Although the immersion oil has a refractive index of

1.513, which is close to that of the polymer (1.49), there is still a pronounced effect

of spherical aberration due to the use of a high numerical aperture objective.

4.6 Continuous wave illumination Two-photon excitation is the important recording method for three-dimensional

optical data storage. However, it requires the use of an ultra-short pulsed laser for

efficient excitation as described in section 2.2.1, which is an impractical method. A

better alternative would be to use an inexpensive laser diode.


81

4.6.1 Requirements for two-photon excitation with

continuous wave illumination

An early report on continuous wave illumination for two-photon excitation by

Hänninen et al. (1994) demonstrated the ability to produce fluorescence from

biological samples. This report also demonstrated a relationship between

fluorescence intensity and pulse width. Although they considered a square pulse

function, they found considerable agreement between their expression and the

experimental results. It was soon discovered (König et al., 1995) that the high

average powers required to achieve an efficient two-photon excitation rate was quite

likely damaging the biological tissue.

According to Denk et al. (1990) the following is a more accurate relationship

between the illumination parameters and the two-photon absorption rate, φ BflB:

( )

, 1675.0 2

22

AP

fAVE

fl ωσ

τφ

h⎟⎟⎠

⎞⎜⎜⎝

⎛= (4.4)

where the factor 0.675 holds for a secant-squared pulse shape with a pulse width and

a repetition rate τ and f, respectively. σB2 B is the two-photon absorption cross section,

ωh is the photon energy and A is the focal area. The relationship in equation 4.4

leads to

, 675.0

fPP AVECW τ

= (4.5)

for achieving the same two-photon absorption rate under pulsed and continuous wave

illumination. For a repetition rate of 82 MHz and a pulse width of 200 fs in the focus

the average power for recording needs to be increased approximately by a factor of

200.


82

4.6.2 Continuous wave multi-layered data storage

Continuous wave two-photon excitation has been previously demonstrated for bit

optical data storage in a photobleaching polymer (Gu and Day, 1999) as well as

holographic data storage in a photorefractive crystal (Imbrock et al., 1999).

The material used in section 4.4 for multi-layered data storage was based on the

polymer host PVK. However it was found that under the high average powers

required for continuous wave two-photon excitation PVK can be melted. The

polymer poly(Methyl Methacrylate) (PMMA) was chosen due to its higher melting

point. The recording sample consisted of DMNPAA:PMMA:TNF:ECZ in the

following percentages of total weight 30:53:16:1. The absorption curve is slightly

different from the PVK based sample and is shown in figure 3.8(b). Due to the high

concentration of dopants the sample can only be fabricated to 100 – 200 µm in

thickness without reducing significantly the optical quality of the sample.

Figure 4.12 demonstrates the ability to recorded three-dimensional optical data

storage in the photorefractive polymer under continuous wave two-photon excitation.

For these experiments a Zeiss NeoFluar oil immersion objective with numerical

aperture of 1.3 and a magnification factor of 40 was used. The higher numerical

aperture results in an increased power density of 3 – 4 times that of the lower

numerical aperture objective used under pulsed excitation.

The wavelength used for recording was 800 nm with an average power of 77 mW in

the focus and exposure time of 10 ms. A bit spacing of 6.75 µm and a layer spacing

of 20 µm produce a density of 1.1 Gbits/cmP

3P. Owing to the difficulty in

manufacturing thick recording samples and the large layer spacing only a few layers

can be recorded.

The bits in figures 4.12 and 4.13 appear to be bumps rather than pits as shown in

figures 4.7 and 4.8, the reason for the difference is due to the position of the focus

above or below the bits and the amount of shearing introduced by DIC microscopy.


(a) (b)

(c)

Figure 4.12: Three-dimensional bit optical data storage in a photorefractive polymer

under continuous wave two-photon excitation. (a) the first layer including the letter

A, (b) the second layer including the letter B, and (c) the third layer including the

letter C.

The images in figure 4.12 were produced by scanning the sample in the Olympus

FluoView microscope in a DIC mode. The laser source used was a Helium:Neon

laser of a wavelength at 632.8 nm and was focused through an Olympus UPlanApo

objective with numerical aperture of 0.7 and a magnification factor 20.

83


84

4.6.3 Continuous wave erasable/rewritable data storage

To confirm that using the high average power for recording under continuous wave

two-photon excitation creates a change in refractive index via the photorefractive

effect, the erasable/rewritable feature is demonstrated.

Figure 4.13 shows the ability to record, erase and record information within the same

region of the material under continuous wave two-photon excitation for recording

and ultra-violet illumination for erasing. The wavelength for recording was 800 nm,

with a recording average power of 300 mW in the focal region of an ULWD

objective with numerical aperture of 0.75 and a magnification factor of 40. An

exposure time of 2 ms was used. The bit spacing was 5.6 µm.

(a) (b)

(c)

Figure 4.13: Erasable/rewritable bit optical data storage in a photorefractive polymer

under continuous wave two-photon excitation. (a) the letter E is recorded. (b) the

1

2

1

2

1

2


85

letter E is erased after illuminating the same region with UV light. (c) the letter F is

recorded into the same region as indicated by the artifacts 1 and 2.

The images were read using the Olympus FluoView microscope in a DIC mode. The

reading wavelength was 632.8 nm and was focused by an Olympus UPlanApo

objective with numerical aperture and a magnification factor of 0.7 and 20,

respectively.

The recorded bits in figure 4.13 are significantly stronger than that observed in figure

4.12 for the multi-layered recording. This is due to the proximity of the surface. It

was discovered that it was easy to damage the material when recording within 10 µm

of the surface with high power. When a laser beam is focused near the surface under

high recording powers, there is sufficient heating to ablate the recording medium.

The stability of the recorded information under continuous wave two-photon

excitation is approximately the same as that under pulsed illumination, as they both

use the photorefractive effect to create the recorded bit.

4.7 Alternative detection techniques DIC microscopy requires the insertion of optics into the light path for reading that

are not required in the recording process, and would reduce the performance of the

recording system significantly. Further, its transmission imaging nature limits its

applicability in a practical system. It would be advantageous then to investigate

alternative methods for reading the change in refractive index of a bit, which would

allow the same system for recording to be used for reading the data without

introducing new optical components. Noda et al. (1990) reported the detection of

phase information using an annular illumination microscope. However, computer

reconstruction is required to retrieve the phase information. A more promising

method is to use a split detector (Hamilton, 1983; Kawata et al., 1996). In this

section the applicability of split detection and quadrant detection is explored in

photorefractive polymers


4.7.1 Split detector

Kawata et al. (1996) demonstrated the ability to read multi-layered optical memory

from a photopolymerisable material, where the change in refractive index approaches

10%. This is substantially larger than the value in photorefractive polymers.

Objective

Recording sample

Detector

y

x

z

Figure 4.14: Optical setup for a phase sensitive microscope with a split detector.

The optical system required to use a split detector is extremely simple, and does not

require optics to be introduced into the recording system, as illustrated in figure 4.14.

The sensitivity of the detector to the phase change requires that it is mounted within a

couple of millimeters behind the sample. A larger separation will result in a loss of

phase information. Figure 4.15 is an image of a recorded pattern read using the split

detector. The pattern was recorded under pulsed two-photon excitation with

recording parameters similar to that used in section 4.4.

The split detector is mounted with the two sensitive regions aligned along the x axis.

When reading the information, the sample is raster scanned, with the fast direction

along the x axis. The signals from both detectors are then subtracted, amplified and

feed to the computer which displays the resulting image. For this experiment a

Hamamatsu split detector S4204 was used.

86


Figure 4.15: Recorded pattern in a photorefractive polymer read using a split

detector.

As can be seen from figure 4.15, there are fast fluctuations in the signal intensity in

the slow scanning direction (y axis), which results in the streaked image. The cause

for these fluctuations is not known, but it may result from the amplification of the

subtracted signal, as they do not appear in figure 4.17 for the quadrant detector.

4.7.2 Quadrant detector

To improve the stability and sensitivity of the detector the split detector was replaced

with a quadrant detector. The quadrant detector was used by Sasaki et al. (1997) to

measure the radiation pressure on a trapped microparticle in real time.

D

C A

B

2.64 mm

y

x

z

Figure 4.16: Quadrant detector configuration.

The following equations detail the signal processing used to produce a phase image

of the recorded bits and are based on the detector configuration shown in figure 4.16.

87


Z=A+B+C+D, (4.6)

Y=((A+C)-(B+D))/Z, (4.7)

X=((A+B)-(C+D))/Z. (4.8)

The detector used in these experiments was supplied by RS components (652-027).

The pattern of bits was recorded using the same recording parameters as detailed in

section 4.4.

Figure 4.17: Recorded pattern in a photorefractive polymer read using a quadrant

detector.

As can be seen from figure 4.17 the signal detected by the quadrant detector is more

stable than the corresponding signal from the split detector. It was observed that at

low recording powers, the split and quadrant detectors could detect the changes in

refractive index that were not detectable using DIC microscopy.

4.8 Summary It has been shown in this chapter that two-photon excitation under pulsed and

continuous wave illumination can be used to record information via the

photorefractive effect in a polymer based material. As a result of localised

cooperative feature of two-photon excitation it is possible to record multiple layers of

information within a volume. Further, the process of a charge distribution resulting

88


in a change in refractive index provides the ability for the recorded information to be

erased, and new information written into the same region.

According to the relationship between the bit size and the recording power, a density

of 88 Gbits/cm3 could be achieved if an objective with numerical aperture of 1.3 was

used to record with an average power of 7 mW. A density of 88 Gbits/cm3

corresponds to a capacity of 670 Gbytes on a CD, a factor of 1000 times greater than

the capacity of a CD.

89

Chapter Five

Formation of micro-cavities

5.1 Introduction From Chapter Four, it can be seen that erasable/rewritable three-dimensional bit

optical data storage has advantages over conventional optical storage systems. In

particular, the results in section 4.5 show that the erasable/rewritable nature occurs

only within a certain range of illumination power. Beyond this range, micro-cavities

may be formed in the photorefractive polymer. The creation of permanent data bits

through the formation of micro-cavities prevents information from being destroyed.

The large change in refractive index associated with a micro-cavity provides the

opportunity to use other methods of reading the information (i.e. reflection confocal

microscopy, see Chapter Six) which have higher transverse and axial resolution, and

therefore do not limit the density of the recorded information, as seen in Chapter

Four.

This chapter is divided into the following sections: section 5.2 will illustrate the

recording system used to create micro-cavities. Section 5.3 investigates the effect of

spherical aberration resulting from the mismatch in refractive indices, on the

performance of the recording system particularly under multi-photon excitation.

Finally, in section 5.4, a summary of the formation of three-dimensional micro-

cavity arrays will be presented.

5.2 Formation of cavities The formation of cavities is a result of the nonlinear interaction between the

illumination light and the transparent medium. Here a transparent medium refers to

the medium having no absorption at the illumination wavelength. Therefore,

90

Chapter Five Formation of micro-cavities

91

excitation of the material relies upon multi-photon absorption, rather than impurities

or defects (Glezer and Mazur, 1997). As has been demonstrated in Chapter Four,

multi-photon excitation can be used to deliver energy not only to the surface of the

material, but also within the depth. The use of the femtosecond pulses to create a

cavity under multi-photon excitation means that the energy is transferred to the

material, and the excitation ends before excess energy is transferred away from the

excitation region.

A micro-explosion in the material is the method by which micro-cavities are formed.

Increasing the recording power increases absorption in the ultra-violet region to a

point where multi-photon excitation takes place. The absorption of high energy

ionizes the polymer and produces a plasma. Under the high temperature and

pressure as a result of the plasma generation, material is forced out of the focal

region. The resulting structure is a central volume of less dense material (typically a

void), surrounded by a region of higher density material (Glezer et al., 1996).

Previous demonstrations of cavity formations by Glezer et al. (1996), Glezer and

Mazur (1997) and Watanabe et al. (1999, 2000), all used the output from a

regeneratively amplified Ti:Sapphire pulsed laser, which can produce high energy

(µJ) single shot pulses. This is compared to our low energy (nJ), high repetition rate

(82 MHz) pulsed laser. The advantage of the previously used system is that the

energy can be delivered to the material in a single shot rather than ~10PPP

6P shots. If we

assume that the electrons dissipate their energy in picoseconds (Glezer and Mazur,

1997), then in the system used for our experiments, there is some time for the

electrons to lose energy before each of the pulses. This results in some instability in

the formation of the cavities, again due to the high concentration of dopant

molecules.

From figures 4.10 and 4.11, it can be seen that for both the high and low numerical

aperture recording systems, there is a threshold point, beyond which cavity formation

begins.


5.2.1 Experimental recording and reading system

The experimental systems for recording under two-photon excitation and reading in

transmission have already been discussed in Chapter Four. The only change in the

recording system is that it is operated at recording powers and exposure times above

the threshold point indicated by figures 4.10 and 4.11. However, what is not shown

in figures 4.10 and 4.11 is that there is a very short range of powers and exposure

times that can be used to fabricate a cavity. In the case of the high numerical

aperture objective, extending the exposure time by another 10 – 20 ms beyond the

point at which a cavity is formed, results in a large deformation of the material. With

such a high concentration of dopants, poor sample preparation can lead to areas with

increased quantities of the photosensitive material, which upon illumination absorb

too much energy and reduce the quality of the cavity.

5.2.2 Single cavity

Figure 5.1 illustrates a single cavity formed using a low numerical aperture objective

(NA = 0.8). The illumination wavelength was 800 nm, with an average power of 33

mW and an exposure time of 250 ms. The material used in this case had a high

concentration of dopants, 30:53:16:1 of DMNPAA:PMMA:ECZ:TNF respectively.

The image of the single cavity is formed using an Olympus FluoView microscope

operated in transmission (see section 4.3.1). The objective used for reading was an

Olympus PlanApo oil immersion objective with numerical aperture of 1.4 and a

magnification factor of 60. The reading wavelength was 632.8 nm from a

Helium:Neon laser with an average power in the focus less than 2 mW.

From figure 5.1(b), it is clear that the cavity is well defined in the axial direction, and

almost completely spherical in shape in the transverse direction. The axial image

(figure 5.1(b)) of the micro-cavity appears to be slightly asymmetric; this is due to

the lateral shear that is introduced in the transverse plane by DIC microscopy. It can

also be seen that there is a region of different density material surrounding the edge

of the cavity, which is consistent with the described mechanism which suggests that

92


93

a void is surrounded by denser material, created by the high temperature and pressure

associated with plasma formation and a micro-explosion.

(a) (b)

Figure 5.1: A single cavity formed in a photorefractive polymer under multi-photon

excitation. (a) transverse and (b) axial images of the cavity in transmission

microscopy.

5.2.3 Multi-layered cavity arrays

The nonlinear absorption confines the excitation to a small volume within the focal

region, as seen in sections 4.4.1 and 4.6.2 which showed erasable multi-layered data

storage. Under the slightly different conditions required for the formation of

cavities, the same property holds. Figure 5.2 illustrates the ability to produce multi-

layered arrays of micro-cavities in a photorefractive polymer under multi-photon

excitation.

In recording, an Olympus ULWD objective with numerical aperture and a

magnification factor of 0.8 and 100, respectively, was used. The recording power for

both layers was 33 mW; and the exposure time for the two layers was 250 ms and

290 ms, respectively. The reason for the increased exposure time is due to the effect

of spherical aberration resulting from the mismatch in refractive indices between the

immersion and recording media, and will be further discussed in section 5.3. The

separation between points is 7.81 µm.

3.5 µm 3.5 µm

x

z


94

(a) (b)

Figure 5.2: Multi-layered micro-cavity arrays in a photorefractive polymer under

multi-photon excitation. (a) the first layer with the letter A recorded near the surface

and (b) the second layer with the letter B recorded with a separation of 20 µm in the

depth direction.

Reading of the two layers was done using an Olympus FluoView microscope

operated in transmission. The objective used was an Olympus PlanApo oil

immersion objective with numerical aperture and a magnification factor of 1.4 and

60, respectively. The wavelength used for the reading was 632.8 nm from a

Helium:Neon laser. Subsequent reading of the sample using 542 nm and 488 nm

from a green Helium:Neon laser and an Argon ion laser respectively, produced no

images, as both wavelengths are within the absorption curve (see figure 3.8) and are

strongly absorbed.

The formation of micro-cavities in a photorefractive polymer provides the ability for

an optical data storage device to record permanent as well as the erasable/rewritable

(see Chapter Four) information. The simplicity of the system allows the same

method for recording erasable bits to be used in recording permanent bits just by

increasing the recording power.


95

5.3 Refractive index mismatch Figures 4.10(c) and 4.11(c) for erasable data storage show that there is an increase in

the size of the recorded bit as the recording depth is increased. Although it is

predicted that three-dimensional bit optical data storage can theoretically achieve

terabits per cubic centimeter, so far it has only been able to demonstrate ~10 – 100

Gigabits per cubic centimeter depending on which material is used. One of the

reasons for this low density is the mismatch of the refractive indices between the

recording material and its immersion medium, resulting in spherical aberration

(Török et al., 1996). It has been demonstrated that this aberration source can

dramatically alter the distribution of the light intensity in the focal region of a high

numerical aperture objective and reduce the intensity at the focus.

Early work by Sheppard and Gu (1991a), as well as Sheppard and Török (1997)

considered the refractive index mismatch when focusing into water, as water

approximately represents the refractive index of a biological sample. In the other

report by Sheppard et al. (1994a), the effect of the objective aperture size on the first

three orders of spherical aberration was considered, and methods for compensating

for the spherical aberration using a variable tube length were introduced (Sheppard

and Gu, 1991a). All of these papers considered small refractive index mismatches

(∆n ~ 0.2). However, when a beam is focused from air into a polymer recording

material, the refractive index mismatch is approximately 0.5 (Day and Gu, 1998).

The use of an oil immersion objective to record or read information significantly

reduces the spherical aberration as the difference in refractive indices between the oil

and polymer are approximately 0.015. Using oil as an immersion medium is not a

practical method for reducing the spherical aberration.

While the effect of the spherical aberration was not apparent in the erasable three-

dimensional data storage, its effect is pronounced in the formation of micro-cavities

due to a higher order nonlinear excitation process. As described in section 5.2.3 the

exposure time was increased to record a successive layer at a deeper position. This is

due to the reduction of the intensity when the laser beam is focused deeper in to the

material, which is shown in this section.


96

5.3.1 Intensity point spread function

A method for calculating the performance of an optical system under the influence of

spherical aberrations is the point spread function (PSF) analysis. The point spread

function can be used to describe the behavior of the focal spot under different

experimental conditions. Ignoring the cross-polarization terms (equations 2.12 and

2.13), we can express the PSF function according to equation 2.11, as


012211021

21

1∫ +Φ+=α


From equation 5.1, which describes the intensity point spread function (IPSF) of

light focused from the first medium of refractive index nBBB1 B into the second medium of

refractive index nB2 B, we get the relationships between the focus spot size in the

transverse and axial directions, as well as the intensity change, to the recording

depth, d.

For our experiments the refractive index of the photorefractive polymer (nB2B) was

determined to be 1.49. A numerical aperture of 0.7 and recording wavelength of 800

nm was used in the calculations, as early experiments into three-dimensional data

storage were conducted using an objective with numerical aperture 0.7. Figure 5.3

illustrates the effect of spherical aberration on the IPSF as the objective is focused

deeper into the second medium.

The effect of the refractive index mismatch leads to a broadening of the point spread

function in both transverse and axial directions as the recording depth is increased.

This is confirmed in figure 5.4, where the full width at half maximum (FWHM) of

the intensity point spread function in the transverse and axial directions is plotted.

According to figure 5.4, the volume of the focal region increases by a factor of 1.5 at

a depth of 40 µm, compared with that at the surface.


97

d = 0 µm d = 20 µm

d = 50 µm d = 100 µm

d = 0 µm

d = 20 µm

d = 50 µm

d = 100 µm

(a)

(b)

Figure 5.3: (a) Transverse and (b) axial cross sections of the intensity point spread

function at different depths in the photorefractive polymer. The objective is a dry

objective with numerical aperture of 0.7.

As the focal region increases in both the transverse and axial directions, the intensity

within the focal region will also be reduced. This can be seen in figure 5.5, where


98

the intensity of the light in the focal region decreases with the recording depth (see

the curve for n = 1 in figure 5.5).

(a)

(b)

Figure 5.4: (a) Transverse and (b) axial FWHMs of the intensity point spread

function as a function of the focal depth in the photorefractive polymer.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100

Focal depth (µm)

∆r ( µ

m)

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

Focal depth (µm)

∆z ( µ

m)


99

The probability for multi-photon excitation is proportional to I P

nP(r,z), therefore, the

relationship between the maximum excitation probability in the focus and the focal

depth is calculated for single-photon (n = 1), two-photon (n = 2), and four-photon (n

= 4) excitation. Investigation of the transverse and axial bit FWHMs indicates that

there is only a slight difference between single-photon and multi-photon excitation.

What can be seen from figure 5.5 is that the probability for higher order excitation

decreases faster than that for lower order excitation when the focal depth d is

increased. This result indicates that the effect of the refractive index mismatch is

stronger in the former case than that in the latter case, which is confirmed by our

observation that the exposure time was increased to record a second layer at a deeper

position in the material. Further investigation finds that while keeping the exposure

time constant the intensity of the recording beam has to be increased to create a

cavity, as shown in figure 5.6. An increase in the incident power by a factor of 1.8

(as seen in figure 5.6) is required to record at a depth of 40 µm.

Figure 5.5: Normalised maximum value of I P

nP(r,z) at the focus as a function of the

recording depth in a photorefractive polymer for n = 1, 2, 4, corresponding to single-

photon, two-photon and four-photon excitation, respectively.

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

Focal depth (µm)

Max

imum

exc

itatio

n pr

obab

ility

(a.u

.)

n = 1

n = 2 n = 4


100

Figure 5.6: Recording intensity required to create a micro-cavity versus recording

depth under multi-photon excitation.

5.4 Summary The formation of micro-cavities in a photorefractive polymer can be achieved under

multi-photon excitation. The ability to create micro-cavities allows for a versatile

optical data storage system that can be used for rewritable or permanent information

storage in a given photorefractive polymer.

It has been shown that spherical aberration arising from the mismatch in the

refractive indices between the immersion and recording media results in a reduction

in the recording performance of the optical system. The effect of spherical

aberration is stronger in the formation of micro-cavities, as the micro-cavities are

created via multi-photon excitation (n > 2).

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40

Focal depth (µm)

Rec

ordi

ng p

ower

(mW

)

Chapter Six

Reflection confocal microscopy

reading of micro-cavities

6.1 Introduction The modulation of the refractive index in a photorefractive polymer upon two-photon

excitation provides an excellent method for three-dimensional bit optical data

storage. However, as has been demonstrated in Chapter Four, one of the major

limitations to the recording density is the resolution of the transmission or DIC

reading systems. Another imaging technique with high resolution in both the

transverse and axial directions is reflection confocal microscopy. While providing

an improvement in the resolution of the reading system, reflection confocal

microscopy is not sensitive enough to the phase change produced in the

photorefractive polymer under the erasable/rewritable recording conditions.

Ishikawa et al. (1998) demonstrated reflection confocal microscopy reading of three-

dimensional photochromic memory for a stacked recording medium that extended

the spatial frequency distribution enough to read the information. Toriumi et al.

(1998) provided the first evidence of reflection confocal microscopy reading of an

unmodified photochromic recording medium.

This chapter will demonstrate the ability to read the large change in refractive index

associated with a micro-cavity by reflection confocal microscopy. Further

investigation will illustrate the performance of reflection confocal microscopy in the

presence of spherical aberration and therefore its efficiency in reading three-

dimensional bit optical data bits formed by micro-cavities.

101

Chapter Six Reflection confocal microscopy reading of micro-cavities

This chapter is divided into the following sections. Section 6.2 covers the

experimental reflection confocal reading system, and reflection confocal images of

single and multi-layered micro-cavities. Section 6.3 presents a theoretical evaluation

of reflection confocal microscopy using the concept of three-dimensional coherent

transfer functions with spherical aberration included, as well as the readout

efficiency of confocal microscopy for three-dimensional data storage.

6.2 Reading micro-cavities According to Glezer and Mazur (1997), the micro-explosion that creates the cavity

produces a void at the position where the light was focused. They confirmed this by

analyzing the diffraction pattern produced by an array of cavities, which indicated

that the change in refractive index could be as large as 0.45. Such a large refractive

index change is consistent with the difference in the refractive indices of a vacuum

and the recording material that they used. A reflection confocal microscope should

then be able to detect the light reflected from both the front and back surfaces of the

cavity. The light reflected from the bottom surface should be significantly stronger

than the signal from the top surface, which will be highly aberrated as a result of the

polymer – vacuum interface.

6.2.1 Reflection confocal reading system

The reflection confocal microscope used in these experiments was an Olympus

FluoView 500 microscope. The microscope has three lasers (488, 542 and 632.8

nm) fibre coupled to it for both fluorescence and nonfluorescence imaging. As

mentioned in section 5.2, imaging with 488 nm and 542 nm is not possible due to the

strong absorption at both wavelengths.

6.2.2 Single cavity detection

The same cavity featured in figure 5.1 is read in the reflection confocal microscope

(see figure 6.1). The cavity was read with an Olympus PlanApo oil immersion

102


103

objective with numerical aperture and a magnification factor of 1.4 and 60

respectively. The illumination wavelength was 632.8 nm with an average power of

less than 2 mW.

The axial scan of the cavity (see figure 6.1(b)) clearly shows both the top and bottom

surfaces of the cavity. In this case both surfaces suffer from severe spherical

aberration and exhibit strong side lobes associated with confocal axial scans. There

is no signal from the sides of the cavity as the reflection signal does not come back to

the collection aperture of the microscope.

(a) (b)

Figure 6.1: A single cavity formed in a photorefractive polymer using multi-photon

excitation. (a) transverse and (b) axial images of the cavity in reflection confocal

microscopy.

The transverse confocal image shows the reflection signal from the bottom surface

(air – polymer interface) of the cavity. Comparing the width of the cavity from

figure 6.1(a) with that in the corresponding transmission image in figure 5.1(a), one

can see that the reflection image shows a cavity that is smaller in diameter. This is

due to the optical sectioning of confocal microscopy, where only a small section of

the sphere on the bottom surface is in the focal region, and therefore imaged on the

detector. If the optical resolution of the reflection confocal microscope is 0.5 µm

then the diameter of the image in figure 6.1(a) produced from a 3.5 µm micro-cavity

3.5 µm

x

z

3.5 µm


104

would approximately be 1 µm. A reflection image through the middle of the cavity,

which would represent the true diameter, is not possible, as the reflection signal does

not return to the collection aperture of the microscope.

6.2.3 Multi-layered cavity arrays

It has been shown that it is possible to record multiple layers of micro-cavity arrays,

and that reading of the cavities with reflection confocal is possible. Given high axial

resolution of confocal imaging, it should therefore be possible to read the same

multi-layered array shown in figure 5.2, without the cross talk from the neighboring

layer, as seen in figure 5.2(a). This feature is illustrated in figure 6.2.

The cavity arrays were read using an Olympus PlanApo oil immersion objective with

numerical aperture and a magnification factor of 1.4 and 60, respectively. The

wavelength used for reading was 632.8 nm. The spacing between bits in the plane

was 7.81 µm with a layer spacing of 20 µm.

(a) (b)

Figure 6.2: Multi-layered micro-cavity arrays in a photorefractive polymer recorded

under multi-photon excitation and read with reflection confocal microscopy. (a) the

first layer with the letter A recorded near the surface and (b) the second layer with

the letter B recorded with a separation of 20 µm in the depth direction.


105

With the ability to read the cavities using reflection confocal microscopy, the layer

separation could be reduced considerably without risking cross talk between any

layers. At this point, the limitation on the density of the system is now the recording

process, not the reading process. As indicated in section 5.2, the use of ~10 P

6P pulses

to create a high temperature and pressure plasma required for photoionization and

therefore cavity formation is not a stable process. Future experiments would benefit

from using the output from a regeneratively amplified Ti:Sapphire ultra-short pulsed

laser.

6.3 Theoretical evaluation of reflection confocal

microscopy for three-dimensional data storage The performance of reflection confocal microscopy can be evaluated using the

concept of three-dimensional coherent transfer functions (CTF) (Gu, 1996).

However, the effect of spherical aberration resulting from the refractive index

mismatch between the immersion and recording media is not included in the present

theory. To effectively evaluate the reading ability of reflection confocal microscopy

in three-dimensional optical data storage, the inclusion of spherical aberration is

necessary.

6.3.1 Three-dimensional transfer function with spherical

aberration

An optical system acts as a low-pass spatial frequency filter. Typically, the low

spatial frequency information is transmitted efficiently and the fine details of the

object, represented by the high frequency components, are not transmitted. Consider

the diffraction of a plane wave incident upon a grating of spatial frequency m. After

passing through the grating there are three beams: one that was transmitted straight

through, and two that were diffracted and now travel at an angle of θ± with respect

to the incident beam. The angle of diffraction θ is determined by the spatial

frequency of the grating m and the incident wavelength λ.


106

θ = mλ. (6.1)

From this equation it is obvious that the larger the spatial frequency, the larger the

angle becomes. The largest angle able to be transmitted by an imaging system is α,

which is determined by the numerical aperture of the imaging objective. Therefore

the spatial frequency limit is determined by

mBo B = α/λ, (6.2)

where mBo B is the cut-off frequency, as illustrated in figure 6.3.

Figure 6.3: The spatial frequency component of the light collected by an objective

after diffraction by a grating of spatial frequency m (Gu, 1996).

The fine details of an object which are represented by spatial frequencies greater than

the cut-off frequency mBo B cannot be imaged. The more spatial frequencies that can be

imaged will result in a higher quality image. The transfer function analysis is a

method that describes the efficiency of an optical system in imaging different spatial

frequencies.

The three-dimensional coherent transfer function for an objective inside the second

medium can be obtained by performing the three-dimensional Fourier transform of

m

Cut-off mB0 B = α/λ

Transfer function

Lens

Grating

Plane wave λ

θ = m λ θ

Maximum convergence angle: α

Spatial frequency: m


107

the three-dimensional amplitude point spread function (APSF) for an objective (Gu,

1999). Equation 5.1, gives the three-dimensional intensity point spread function

which is the modulus squared of the amplitude point spread function. Therefore the

three-dimensional amplitude point spread function is

( ) ( ) ( ) ( )1100

211 sincossin, θθττθθα

krnJPAzrA ps∫ += (6.3)

[ ] .cosexp 122 θθ dikzni +Φ

The three-dimensional coherent transfer function for an objective, when a beam is

focused inside the second medium, becomes (Gu et al., 2000; Sheppard et al., 1994b)

( ) ( ) ( ) ,1

1,2

2

⎥⎥⎦

⎤

⎢⎢⎣

⎡

−

−−′=l

lslPslc δ (6.4)

where

( ) ( ) ( ) ( ),

coscosexpcos

1

221 θ

θθττθ

Φ+=′

iPlP ps (6.5)

and

l = sinθB2 B. (6.6)

Here l and s are the radial and axial spatial frequencies, respectively, and have been

normalized by n B2 B/λ.

The three-dimensional coherent transfer function for reflection confocal microscopy

is given by the three-dimensional auto-convolution of equation 6.4 and is given by

(Gu et al., 2000; Sheppard et al., 1994b)

( )

( ) ( ) ( )

( ) ( ) ( )

⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

≥+≤+′′

≤+≤+′′

+

=

∫

∫

−

−+

−+

.,0

,cos2,cossin2,

,4cossin2,

4

,

222

2

22

222

022

220

otherwise

sslsldPP

slsldPP

sl

slcr

αααβθθ

ααβθθ

π

π

βπ

π

(6.7)


108

Here

,cos21

412

sin22

221

0

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−

+−

+= −

ssll

sls αβ (6.8)

and

.4

12cos12

cos22

222 ⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ +−

+=± sl

slsls

βθ m (6.9)

The value of α is given by

,sin2

1⎟⎟⎠

⎞⎜⎜⎝

⎛= −

nNAα (6.10)

where NA is the numerical aperture of the objective. In the following calculations it

is assumed that the objective obeys the sine condition, and therefore

( ) .cos 11 θθ =P (6.11)

When the numerical aperture becomes small, the result is equation 6.7 reduces to

those given by the small angle theory (Jackson et al., 1994; Sheppard and Hole,

1995; Wang and Frieden, 1990).

First, consider the case of focusing the light from air (nB1 B = 1) into the recording

polymer (nB2 B = 1.49). Figure 6.4 shows the three-dimensional coherent transfer

function for reflection confocal microscopy at different depths, focused by an

objective of numerical aperture 0.7.

It can be seen that at the interface between the two media the three-dimensional

coherent transfer function looks like the case for an aberration free system (Sheppard

et al., 1994b). As the focus position is moved into the second medium the transfer

function becomes distorted by axial modulations (Gu and Sheppard, 1992). At a

focal depth of 200 µm (see figure 6.4(d)), the value of the transfer function within

the passband is almost zero, except along l = 0.


109

(a) (b)

(c) (d)

Figure 6.4: Dependence of the modulus of the 3-D CTF on the focal depth d when a

plane wave at wavelength 800 nm is focused by an objective (NA = 0.7) from air to a

medium of refractive index 1.49: (a) d = 0; (b) d = 50 µm; (c) d = 100 µm, (d) d =

200 µm.

Now consider the case of focusing into the second medium of refractive index 1.49

using an oil immersion objective with numerical aperture of 1.4 (see figure 6.6).

This situation corresponds to the condition in the experiments in section 6.2.

From figure 6.5 it can be seen that the high numerical aperture oil immersion

objective increases the passband of the transfer function in both the transverse and

axial directions. The matching of the refractive indices, reduces the spherical

aberration, which can be seen when comparing the different focus depths calculated

- 2

- 1

0

1

2

l

1.5

1.75

2

s

- 2

- 1

0

1

2

l

- 2

- 1

0

1

2

l

1.5

1.75

2

s

- 2

- 1

0

1

2

l

- 2

- 1

0

1

2

l

1.5

1.75

2

s

- 2

- 1

0

1

2

l

- 2

- 1

0

1

2

l

1.5

1.75

2

s

- 2

- 1

0

1

2

l


110

- 2

- 1

0

1

2

l

00.5

11.5

2

s

- 2

- 1

0

1

2

l

in figures 6.4 and 6.5. There is still a substantial amount of aberration present in the

oil immersion case at a depth of 200 µm due to the large focusing angles in a high

numerical aperture objective.

(a) (b)

(c) (d)

Figure 6.5: Dependence of the modulus of the 3-D CTF on the focal depth d when a

plane wave at wavelength 800 nm is focused by an objective (NA = 1.4) from oil (nB2 B

= 1.513) to a medium of refractive index 1.49: (a) d = 0; (b) d = 50 µm; (c) d = 100

µm, (d) d = 200 µm.

The application of a variable tube length to compensate for the spherical aberration

resulting from the refractive index mismatching has been successfully demonstrated

- 2

- 1

0

1

2

l

00.5

11.5

2

s

- 2

- 1

0

1

2

l

- 2

- 1

0

1

2

l

00.5

11.5

2

s

- 2

- 1

0

1

2

l

- 2

- 1

0

1

2

l

00.5

11.5

2

s

- 2

- 1

0

1

2

l


111

for three-dimensional bit optical data storage both theoretically (Gu et al., 2000) and

experimentally (Day and Gu, 1998).

6.3.2 Readout efficiency of reflection confocal microscopy

The readout efficiency is a measure of how well the reflection confocal microscopy

reading system can detect the recorded information. To be able to read the data bits

using reflection confocal, the support region (passband) of spatial frequencies of the

three-dimensional CTF must overlap with the support region of the spatial

frequencies of the recorded bits.

To determine the readout efficiency, the volume under the modulus of the CTF, was

calculated. The physical meaning of this efficiency represents the amplitude at the

focus. Figures 6.4 and 6.5 illustrate that as the effect of spherical aberration

increases, there is a reduction in the volume of the three-dimensional CTF, which

means that less spatial frequencies can be detected.

Figures 6.6(a) and (b) show the readout efficiency versus numerical aperture at

different depths, for air and oil immersion media, respectively. From figure 6.6 it

can be seen that the readout efficiency increases with numerical aperture for a given

focal depth. This agrees with the definition of the transfer function; increasing the

region of detected spatial frequencies increases the quality of the image. It is

interesting to note that in the air immersion case at a large recording depth (d = 400

µm) the efficiency at the largest numerical aperture is not worse than the efficiency

at the lowest numerical aperture.

For a given numerical aperture there is a decrease in the readout efficiency with

depth resulting from the spherical aberration induced by the mismatch in refractive

indices. At a depth of 50 µm the readout efficiency using air immersion drops by

80% when using a 0.95 numerical aperture. However, for an oil immersion objective

of 1.4 numerical aperture, the readout efficiency at a depth of 50 µm, drops by only

30%, indicating that the matching of the refractive indices reduces the effect of

spherical aberration.


112

(a)

(b)

Figure 6.6: Readout efficiency as a function of the numerical aperture of a reading

objective at different recording depths for (a) air and (b) oil immersion media.

0

2

4

6

8

10

12

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

Numerical aperture

Rea

dout

eff

icie

ncy

(a.u

.)d = 0 µm

d = 10 µm d = 50 µm

d = 100 µm

d = 200 µm

d = 400 µm

Rea

dout

eff

icie

ncy

(a.u

.)

0

5

10

15

20

25

30

35

40

45

50

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Numerical aperture

Rea

dout

eff

icie

ncy

(a.u

.)

d = 0 µm d = 10 µm

d = 50 µm d = 100 µm

d = 200 µm

Rea

dout

eff

icie

ncy

(a.u

.)


113

(a)

(b)

Figure 6.7: Readout efficiency as a function of focal depth for different numerical

aperture reading objectives for (a) air and (b) oil immersion media.

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400

Focal depth (µm)

Nor

mal

ised

read

out e

ffic

ienc

y (a

.u.

NA = 0.5 NA = 0.65

NA = 0.75 NA = 0.85

NA = 0.95

Nor

mal

ised

read

out e

ffic

ienc

y (a

.u.)

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200

Focal depth (µm)

Nor

mal

ised

read

out e

ffic

ienc

y (a

.u.

NA = 0.5 NA = 0.85

NA = 0.95NA = 1.2

NA = 1.3 NA = 1.4

Nor

mal

ised

read

out e

ffic

ienc

y (a

.u.)


114

Comparing the d = 0 µm curves in both figures 6.6(a) and (b) we can see that they

are very similar between the numerical apertures 0.5 and 0.95. Regardless of the

immersion medium, at the interface between the two media (d = 0 µm) there is no

spherical aberration, and so the curves should be very similar. However, for the

same range of numerical apertures, it is clear that at a given depth the air immersion

objectives experience more spherical aberration, and therefore a decrease in

performance.

Figure 6.7 shows the readout efficiency versus depth, calculated for different

numerical aperture reading objectives for both the dry and oil immersion cases, and

normalised by the value at d = 0 µm. The low numerical aperture calculations in

figure 6.7(b) for oil immersion show different readout efficiencies compared with the

same numerical apertures in figure 6.7(a).

The curves in figure 6.7 illustrate that for a given reading objective there is a

decrease in the readout efficiency as the objective is focused deeper into the

recording medium. As the reading system focuses deeper into the recording medium

there is an increase in the spherical aberration resulting from the mismatch in the

refractive indices between the immersion and recording media (see section 5.3),

thereby reducing the readout efficiency. Figure 6.7(b) indicates that the readout

efficiency does not change significantly when using a low numerical aperture

objective with oil immersion. However, when the numerical aperture becomes large,

the readout efficiency decreases with the focal depth appreciably. It should be

pointed out that the efficiency for a high numerical aperture readout objective is

greater than that for a low numerical aperture objective at a given depth (see figure

6.6(b)).

Figure 6.8 provides an experimental confirmation of the relationship between the

readout efficiency and the numerical aperture used to image the cavities. Only

numerical apertures larger than 1.0 were used, as no reflection signal could be

detected using a numerical aperture smaller than 1.0. The curves in figure 6.8

indicate that as the numerical aperture is increased the intensity of the reflected


115

signal is also increased, which agrees with the readout efficiency curves in figure

6.6(b).

Figure 6.8: Intensity of the reflected signal from micro-cavities for different oil

immersion numerical aperture reading objectives.

6.4 Summary The void created as a result of the formation of a micro-cavity is a large enough

change in refractive index for successfully useing reflection confocal microscopy to

detect the micro-cavities. As a result reflection confocal microscopy can be used for

a permanent three-dimensional optical data storage readout system in a

photorefractive polymer. The high axial resolution of confocal microscopy will

allow the system to reduce the layer spacing considerably without risking the

possibility of cross talk between neighboring layers.

The effect of the spherical aberration resulting from the mismatch in refractive

indices between the immersion and recording media has been demonstrated to have

an impact on the performance of reflection confocal microscopy to read the recorded

information. It has been illustrated that there is a reduction in the efficiency of a

0

50

100

150

200

250

300

0 5 10 15 20

Position (µm)

Mic

ro-c

avity

inte

nsity

(a.u

.) NA = 1.4

NA = 1.3

NA = 1.25

Mic

ro-c

avity

inte

nsity

(a.u

.)


reflection confocal reading system for a particular objective when focusing to a

deeper position within the recording medium. However, the readout efficiency with

depth increases when the numerical aperture of the readout objective is increased.

116

Chapter Seven

Conclusion

7.1 Thesis conclusion The work in this thesis represents a detailed investigation into three-dimensional bit

optical data storage in a photorefractive polymer. Key areas of research include; the

fabrication of a photorefractive polymer, multi-layered erasable/rewritable under

two-photon excitation with pulsed and continuous wave illumination, multi-layered

micro-cavity formation under multi-photon excitation, reading multi-layered micro-

cavity arrays using reflection confocal microscopy, investigation into the effect of

spherical aberration as a result of the refractive index mismatch between the

immersion and recording media.

A photorefractive polymer based on the following compounds 2,5-dimethyl-4-(p-

nirtophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-fluorenone (TNF) and N-

ethylcarbazole (ECZ) in either poly(N-vinylcarbazole) (PVK) or poly(Methyl

Methacrylate) (PMMA) as the host matrix, was fabricated and used as the recording

medium for this work.

The creation of a highly localized modulation of the refractive index under two-

photon excitation in the photorefractive polymer provided the ability to record multi-

layered information. As the photorefractive effect is a reversible process,

erasable/rewritable bit optical data storage was also demonstrated. While two-

photon excitation provided the means to record multi-layered information, it requires

a high photon density with the focal spot for efficient excitation. Typically an ultra-

short pulsed laser is used for this process; however, such a laser is impractical.

Continuous wave two-photon excitation was applied to record erasable/rewritable

multi-layered information in a photorefractive polymer.

117

Chapter Seven Conclusion

It was discovered that at a certain power range ultra-short pulsed illumination in the

photorefractive polymer leads to a new recording process. At high recording powers

photoionization within the focal region occurs. The resulting plasma leads to the

formation of a micro-cavity through the rapid heating and expansion of the material

during this process. The highly localized nature of a micro-cavity allows multi-

layered arrays to be fabricated and thereby creating three-dimensional permanent bit

optical data storage in a photorefractive polymer.

The effect of the spherical aberration as a result of the refractive index mismatch

between the immersion and recording media reduces the efficiency of the recording

system. It was illustrated that as the recording depth is increased, changes in the

intensity distribution in the focus reduce the recording efficiency. It was

demonstrated that the higher order nonlinear excitation required for the fabrication of

micro-cavities exhibits a stronger influence from spherical aberration than two-

photon erasable bit recording.

The large change in refractive index associated with the formation of a micro-cavity

provided the ability to use reflection confocal microscopy to detect the cavities. It

was demonstrated that a reflection signal from the top and bottom surfaces of a

micro-cavity can be imaged. Therefore reflection confocal microscopy may be used

to read permanent three-dimensional bit optical data storage in a photorefractive

polymer. Investigation into the readout efficiency of reflection confocal microscopy

under spherical aberration was conducted. It was discovered that the readout

efficiency of any objective deteriorates as the focal depth is increased. However, an

objective with a high numerical aperture under oil immersion conditions (reduced

spherical aberration) has an increased readout efficiency compared with a low

numerical aperture objective.

In conclusion, the work conducted in this thesis shows the ability to record multi-

layered erasable/rewritable and permanent information in a photorefractive polymer

under two-photon pulsed and continuous wave illumination and multi-photon

excitation. It has also been demonstrated that while spherical aberration deteriorates

118


119

the performance of the recording and reading systems it is possible to achieve a

density of greater than 88 Gbits/cmPPPPP

3P.

7.2 Future work There are several key areas where further research can be conducted to improve the

performance of the work carried out in this thesis.

The recording medium has a significant role in a data storage system, and further

research into the three following areas should be pursued:

• The absorption band of the current photorefractive polymer extends from the

ultra-violet into the visible region of the electromagnetic spectral range. Such

a large absorption spectrum results in information erasure due to the

absorption of unwanted light (e.g. sunlight). Reducing the absorption spectral

range will have the effect of increasing the lifetime of the recorded

information. Consideration should also be given to increasing the two-photon

absorption efficiency. This would allow µW of pulsed light or mW of

continuous wave illumination to be used for recording, making the recording

system safer and more practical.

• The glass transition temperature of the material should be increased. The

photorefractive polymer used in this thesis has a glass transition temperature

at 17°C, at room temperature. It is known that heating of photorefractive

materials to near the glass transition temperature erases the stored

information. While a low glass transition temperature has been used to

promote a modulation of the refractive index for holographic storage, it may

not be required for bit optical data storage. Increasing the glass transition

temperature should also significantly reduce the effect of thermal relaxation

and increase the lifetime of the recorded information.


120

• The nonlinear chromophore used in this thesis DMNPAA could be modified

to provide an increase in the modulation of the refractive index of a recorded

bit. As demonstrated in Chapter Six, a large refractive index change

associated with a cavity allows reflection confocal microscopy to be used as

the reading system. Attaching the chromophore to the polymer backbone

should increase the buildup of the space-charge field, as the polymer matrix

acts as an antenna for electrons (Moerner and Silence, 1994). As a result the

use of reflection confocal microscopy to read the erasable/rewritable

information will simplify the optical setup and help in the miniaturization of

the system to a working prototype.

Further advances in the optical system will improve the performance of the

recording system and could simplify the transition to a compact design.

• The application of variable tube length compensation, as described by Day

and Gu (1998), to an optical data storage system will reduce the effects of

spherical aberration. From the work conducted in this thesis (see Chapters

Five and Six) the effect of spherical aberration degrades the recording

performance when a beam is focused deep into the recording medium. If the

efficiency of the recording system is to be increased to include more than

90% of the volume of the recording medium then this would appear to be an

ideal form of compensation. However, the method described above requires

inserting a lens into the recording system making it hard to reduce the size of

the overall system. A better method would be to attach a liquid crystal phase

modulator to the back aperture of the recording objective.

• Further advances in the optical system to reduce losses due to the optical

components would allow the introduction of a high power continuous wave

laser diode. This would be a final step towards the miniaturization of the

recording and reading systems. In doing so, the system would come close to

becoming a next generation optical data storage system that could be used

one day, to replace compact discs and digital video discs.


121

Another application for three-dimensional micro-cavity arrays is in photonic

crystal devices, where periodical structures can be tailored to modify the

propagation of electromagnetic waves. Such a device could possible lead to the

creation of optical circuits.

7.3 3DCD technology With so much attention being focused on DVD technology there appears to be little

room for any competing data storage systems; this will change as fast as information

technology industry roles over. The capacity of DVD storage systems will peak at

25 Gbytes per disk within five years (see figure 7.1), at which time there will be a

need for a next generation system that is compatible with CD and DVD technology,

as well as has the potential for continued growth in storage capacity. The next

generation system is the three-dimensional compact disc (3DCD).

Figure 7.1: Projected capacity for DVDs compared to current 3DCD technology.

The technology involved in this system provides the ability to record, read and erase

information from anywhere within a volume recording material. This allows

multiple layers to be recorded within a disk, and thereby increasing the storage

capacity of the system.

0

5

10

15

20

25

30

Cap

acity

/dis

c (G

Byt

es)

Current CD CurrentDVD

DVD in 1-2years

Best DVDin 5 years

Current3DCD-RW


The current state of three-dimensional (3D) optical data storage is a table-top based

research system that can record, erase and rewrite on a recording sample up to a

capacity of 30 Gbytes. With some further developments the system could reach 500

Gbytes per disk within three years. However, there remains available the ability for

continued growth to produce an optical data storage system with a capacity of 2.5

Tbytes per disk (see figure 7.2).

0

500

1000

1500

2000

2500

Cap

acity

/dis

c (G

Byt

es)

1st Year 2nd Year 3rd Year Future growth

Figure 7.2: Projected capacity for 3DCD technology.

122

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134

Glossary

I(r) Intensity distribution in the radial direction.

r Radial coordinate.

∆n Change in refractive index.

n General refractive index.

nB1 B Refractive index of the immersion medium.

nB2 B Refractive index of the recording medium.

G(r) Rate of photogeneration of electrons in a photorefractive crystal.

NBDB Number density of donors. +DN Number density of ionized donors.

s Constant of photgeneration.

n(r) Number density of free charges.

R(r) Recombination rate of free charges.

Rγ Constant of recombination.

E(r) Position dependent electric field.

E(t) Time dependent electric field.

µBeB Electron mobility.

k BBB Boltzmann constant.

T Temperature.

e Charge of an electron.

r BeB Electro-optic coefficient.

E Energy of absorbed photon.

h Planck’s constant.

h Planck’s constant.

v Frequency of a photon.

x x-axis spatial coordinate.

y y-axis spatial coordinate.

z z-axis spatial coordinate.

Q Figure-of-merit.

Glossary

135

rε DC dielectric constant.

0ε Permitivity of free space.

P(t) Polarizability of a material. 1χ Linear optical susceptibility. 2χ Second-order nonlinear susceptibility. 3χ Third-order nonlinear susceptibility.

TBg B Glass transition temperature.

φ Bq B Quantum efficiency of charge generation.

t Complex amplitude of an object.

θ Angle of wavefront.

φ Phase of a wave front.

a Constant of the complex amplitude of an object.

i Imaginary component.

∆θ Change in phase between two laterally displaced beams.

∆I Variation in image intensity.

φ BflB Two-photon absorption rate.

τ Pulse width.

f Laser repetition rate.

PBAVEB Average power of a pulsed laser.

PBCWB Average power of a continuous wave laser.

A Focal area.

σ B2B Two-photon absorption cross-section.

ω Frequency of a photon.

m Spatial frequency of a grating.

mBo B Cut-off spatial frequency of an imaging system.

λ Wavelength of light.

α Largest acceptance angle of an objective.

I(r,z) Intensity point spread function.

P(θB1 B) Pupil function of lens 1.

Glossary

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τ Bs B Fresnel diffraction coefficient for s-polarized light.

τ BpB Fresnel diffraction coefficient for p-polarized light.

J B0B Zero order Bessel function of the first kind.

Φ Aberration function.

k Wavenumber.

c(l,s) Three-dimensional coherent transfer function inside the second medium.

c P

rP(l,s) Three-dimensional coherent transfer function for reflection confocal microscopy.

l Radial spatial frequency.

s Axial spatial frequency.

d Focus depth of the recording objective.

List of author’s publications

Journals 1. “Effect of refractive-index mismatch on three-dimensional optical data storage

density in a two-photon bleaching polymer”, Day D. and Gu M., Appl. Opt. 37, 6299-

6304 (1998).

2. “Use of two-photon excitation for erasable-rewritable three-dimensional bit optical

data storage in a photorefractive polymer”, Day D., Gu M. and Smallridge A., Opt.

Lett. 24, 948-950 (1999).

3. “Use of continuous wave illumination for two-photon three-dimensional optical data

storage in a photobleaching polymer”, Gu M. and Day D., Opt. Lett. 24, 288-290

(1999).

4. “Three-dimensional coherent transfer function for reflection confocal microscopy in

the presence of refractive-index mismatch”, Gu M., Day D., Nakamura O. and

Kawata S., J. Opt. Soc. Am. A 18, 2002-2008 (2001).

5. “Rewritable three-dimensional bit optical data storage in a PMMA-based

photorefractive polymer with continuous wave illumination”, Day D., Gu M. and

Smallridge A., Advd. Mat. 13, 1005-1007 (2001).

6. “Three-dimensional micro-cavity formation in a photorefractive polymer”, Day D.

and Gu M., to be submitted, Appl. Phys. Lett. (2001).

Conference proceedings 1. “Increase of the three-dimensional storage density in two-photon polymers”, Gu M. and

Day D., J. Optoelectronics. Laser, Topical Meeting of the International Commission for

Optics: Optics for Information Infrastructure, 9, 299-301, (Tianjin, P. R. China, 1998).

137


2. “Two-photon multi-layer bit data storage by use of continuous wave illumination”, Gu M.

and Day D., Proc. SPIE, 18th Congress of the International Commission for Optics, 3740,

444-445, (San Francisco, U.S.A., 1999).

3. “High-density erasable three-dimensional optical bit data storage in a photorefractive

polymer using two-photon excitation”, Day D., Gu M. and Smallridge A., Proc. SPIE,

International Symposium on Optical Memory and Optical Data Storage, 3864, 103-105,

(Hawaii, U.S.A.,1999).

4. “Two-photon three-dimensional bit optical data storage in photorefractive polymers”, Gu

M., Day D. and Smallridge A., Technical Digest of CLEO2000, 188, (San Francisco,

U.S.A., 2000).

Conference papers 1. “Three-dimensional optical memory in two-photon bleaching material”, Day D., Schilders

S. P. and Gu M., 11th Conference of the Australian Optical Society, (Adelaide, Australia,

1997).

2. “Spherical aberration in three-dimensional optical data storage using two-photon

microscopy”, Day D. and Gu M., 11th International Conference on 3D image processing

in microscopy and the 10th International Conference on confocal microscopy, (Sydney,

Australia, 1998).

3. “Digital three-dimensional optical in a two-photon bleaching polymer using continuous

illumination”, Day D. and Gu M., 4th Australasian Conference on Optics Lasers and

Spectroscopy 1998, (Christchurch, New Zealand, 1998).

138


4. “Two-photon three-dimensional optical bit data storage under continuous wave

illumination”, Gu M. and Day D., Focus on Microscopy 99, (Heidelberg, Germany, 1999).

5. “Erasable/rewritable three-dimensional bit optical data storage in photorefractive

polymers”, Gu M., Day D. and Smallridge A., Australian Technology Week in Taipei,

(Taipei, Taiwan, 1999).

6. “Three-dimensional erasable/rewritable bit optical data storage in a photorefractive

polymer using two-photon excitation”, Day D., Gu M. and Smallridge A., Australian

Optical Society Conference 99, (Sydney, Australia, 1999).

7. “High-density optical data storage based on grey level recording in photobleaching

polymers using two-photon excitation under ultra-short pulse and continuous wave

illumination”, Ganic D., Day D. and Gu M., Australian Optical Society Conference 99,

(Sydney, Australia, 1999).

8. “Two-photon excitation for optical data storage in photorefractive polymers”, Gu M. and

Day D., 2000 European Materials Research Society Spring Meeting, (Strasbourg, France,

2000).

9. “Multi-layered bit optical memory in photorefractive polymer”, Gu M. and Day D.,

International Photonics Conference 2000, (Hsinchu, Taiwan, 2000).

139


Articles 1. “Three-dimensional optical data storage”, Gu M., Day D. and Schilders S. P., Australian

Optical Society News, 11, 1-4 (1997).

2. “Joint international symposium on optical memory and optical data storage 1999”, Day

D., Australian Optical Society News, 13, 5-7 (1999).

3. “Pump up the Volume”, The Economist, July 31 (1999).

Patents 1. “Erasable/rewritable optical data storage with photorefractive polymers”, Gu M., Day

D. and Smallridge A., PCT/AU00/00117 (1999).

2. “Method and device”, Gu M., Day D., PR4965 (2001).

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