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Master's Theses and Doctoral Dissertations Master's Theses, and Doctoral Dissertations, andGraduate Capstone Projects

2008

Laser surface thermal lensing (STL): An opticalmethod for surface analysis of glass transitioneffects in transparent polymersNandita Prasad

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Recommended CitationPrasad, Nandita, "Laser surface thermal lensing (STL): An optical method for surface analysis of glass transition effects in transparentpolymers" (2008). Master's Theses and Doctoral Dissertations. 193.http://commons.emich.edu/theses/193

LASER SURFACE THERMAL LENSING (STL): AN OPTICAL METHOD FOR

SURFACE ANALYSIS OF GLASS TRANSITION EFFECTS IN TRANSPARENT

POLYMERS

By

Nandita Prasad

Submitted to the Department of Chemistry

Eastern Michigan University

in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

In

Chemistry

Thesis Committee:

Dr. Donald Snyder, Chair

Dr. Marshall Thomsen

Dr. Timothy Brewer

June 3rd, 2008

Ypsilanti, Michigan

ii

APPROVAL

LASER SURFACE THERMAL LENSING (STL): AN OPTICAL METHOD FOR

SURFACE ANALYSIS OF GLASS TRANSITION EFFECTS IN TRANSPARENT

POLYMERS

By

Nandita Prasad

Dr. Donald M. Snyder ________________________________ ______________Thesis Advisor Date

Dr. J. Marshall Thomsen ________________________________ ______________Committee Member Date

Dr. Timothy Brewer ________________________________ ______________Committee Member Date

Dr. Ross Nord ________________________________ ______________Department Head Date

Dr. Deborah deLaski-Smith ________________________________ ______________Interim Dean of the Graduate School Date

iii

DEDICATION

This thesis is dedicated to my entire family, who has offered me unconditional support and

love throughout the course of this thesis.

iv

ACKNOWLEDGEMENTS

I would like to thank my thesis advisor, Dr. Donald Snyder for his support,

encouragement, and guidance throughout the length of this project. I would also like to

thank Dr. Marshall Thomsen, who collaborated with Dr. Donald Snyder, for his support and

guidance whenever I needed it.

I also thank the Department of Chemistry for giving me the opportunity to pursue my

graduate studies. My heartfelt gratitude to Dr. Krish Rengan for his timely guidance and

support throughout the admission process and during my entire stay at Eastern Michigan

University. I also thank Dr. Kevin Kuehn (Biology Department) for the use of the

microscope in his laboratory and the National Science Foundation for its partial funding to

support the project. I am also thankful to all the committee members for taking the time to

read and edit my thesis. I’m very grateful to my family, especially my husband, Steve M.

Fernandes, for their guidance and moral support.

Last but not least I would like to thank my co-worker, Bushra Hussain, for her

assistance and cooperation in the initial set-up of the experiments.

v

ABSTRACT

Absorption of energy from a modulated laser beam by a transparent substrate leads to cyclic

thermal expansion and contraction of the surface at the pump beam focus. This minute

distortion of the surface forms a transient optical lens structure, which can be monitored by

the changes it induces in a second laser beam reflected at an angle off the same focal point.

The Surface Thermal Lensing (STL) technique is based on detecting the modulated

interference pattern that results from interaction of the probe beam wavefront as it reflects

from both the flat surface surrounding the transient lens and the curved lens surface itself.

Since a variety of physical properties for a polymer change sharply as the solid is heated

through the glass transition temperature, changes in the STL signal can be used to detect Tg

in spatially resolved surface regions as small as the focal point of the pump laser. This report

will examine the relationship between the magnitude of the STL signal and such instrument

parameters as pump beam intensity and chopping frequency, and the thermo-mechanical

properties of several transparent polymers.

vi

TABLE OF CONTENTS

DEDICATION ………………………………………………………………………… iii

ACKNOWLEDGEMENTS ……………………………………………………………. iv

ABSTRACT ……………………………………………………………………………. v

TABLE OF CONTENTS..……………………………………………………………. vi

LIST OF FIGURES ……………………………………………………………………. vii

CHAPTER 1: INTRODUCTION ………………………………………………………. 1

Detection techniques based on thermodynamic properties…..………………….….7

Photothermal Interferometry ……………………………………………………. 7

Photothermal Reflectance ………………………………………………………. 8

Photothermal Diffraction ……………………………………………………….. 9

Photothermal Deformation …………………………………………………….. 10

Transverse Photothermal Deflection ………………………………………….. 11

Collinear Thermal Lensing …………………………………………………….. 12

CHAPTER 2: SURFACE THERMAL LENSING…………………………………… 14

Glass Transition Temperature ……………………………………………………. 17

CHAPTER 3: EQUIPMENT SET-UP ………………………………………………… 19

CHAPTER 4: EXPERIMENTAL PROCEDURE ……………………………………... 22

CHAPTER 5: RESULTS AND DISCUSSION ………………………………………... 25

CHAPTER 6: CONCLUSION ………………………………………………………….. 37

REFERENCES ………………………………………………………………………… 39

vii

LIST OF FIGURES

Figure Page

1.1 Basic processes responsible for photothermal spectroscopy signal generation ………… 3

1.2 Thermal waves for characterizing thin films…………………………………………….. 4

1.3 Experimental set-up of photothermal interferometry……………………………………. 8

1.4 Schematic of photothermal reflectance………………………………………………….. 9

1.5 Experimental set-up of photothermal diffraction……………………………………… 10

1.6 Schematic of photothermal deformation…………………………………………………10

1.7 Schematic of the transverse photothermal technique…………………………………….11

1.8 Schematic of the collinear thermal lensing………………………………………………12

2.1 Schematic of STL signal…………………………………………………………………15

2.2 Schematic of STL signal generation……………………………………………………..16

2.3 Expected STL signal behavior at Tg with gradual increase in temperature…………… 18

3.1 Equipment set up for STL………………………………………………………………..19

4.1 Structure of polycarbonate……………………………………………………………… 24

4.2 Structure of Cyclic Olefin Polymer…………………………………………………… 24

5.1 Probe beam power calibration………………………………………………………… 25

5.2 STL signal as a function of time…………………………………………………………26

5.3 STL signal as a function of time…………………………………………………………27

5.4 Pictures of the damaged surface…………………………………………………………28

5.5 STL signal as a function of pump power………………………………………………...29

5.6 STL signal as a function of probe power………………………………………………...30

5.7 STL signal as a function of time…………………………………………………………31

viii

5.8 STL signal as a function of pump power for zeonex…………………………………….32

5.9 STL signal as a function of probe power………………………………………………...33

5.10 STL signal as a function of pump frequency…………………………………………...34

5.11 STL signal as a function of temperature………………………………………………..35

5.12 Phase as a function of temperature……………………………………………………..36

CHAPTER 1

Introduction

Photothermal effects in solid materials have been studied for a number of applications

since the 1880s, but the initial work in this field was limited by the weak responses of the

solid structures to the low intensity light sources then available. Tremendous advances have

been achieved in these studies since the development of lasers, which have better beam

quality and greater power densities. The applications of these studies are varied.

Photothermal spectroscopic methods have been applied in many scientific and technical

fields such as life sciences, material sciences, manufacturing, medical research,

environmental sciences, instrumentation for analysis, and non-destructive evaluation (1).

Photothermal spectroscopy is a set of highly sensitive analytical methods used to

monitor changes caused by absorption of radiation. Absorption of radiation causes heating in

the sample, which induces changes in temperature, pressure, and refractive index of the

sample. Local optical and thermal properties can be probed using this technique. The merit

of this technique is that it is a non-contact and non-destructive method. Photothermal

methods can be applied to a broad range of materials (gas, liquid, liquid crystal, and solid),

transparent or opaque. Samples of arbitrary shapes can be analyzed using this method in air

or vacuum (1-8).

Photothermal spectroscopy is classified as an indirect method for optical absorption

analysis where the effect of optical absorption on the sample is measured rather than directly

measuring the transmission of light used to excite the sample. Optical absorption leads to

sample heating, hence the photothermal spectroscopy signals are light absorption dependent.

Photothermal signals are not produced by scattering and reflection losses; rather

2

photothermal spectroscopy is used to measure optical absorption in scattering solutions, in

solids, and at interfaces, making this technique particularly attractive for application to

surface and solid absorption studies and studies in scattering media (9).

Photothermal spectroscopy usually utilizes laser light sources due to high spectral

purity, power, and spatial coherence. In the linear regime, the temperature change is

proportional to the optical power for an excitation of a sample with a given absorption

coefficient, and the photothermal spectroscopy signal is in turn proportional to the

temperature change. The magnitude of the signal depends on the power or energy of the

incident beam. High powers or pulse energies over very narrow optical bandwidths can be

delivered by lasers, thus enhancing the photothermal signals. The temperature change is also

inversely proportional to the volume over which the light is absorbed since heat capacity

scales with the amount of substance. The laser source can be focused to small diffraction

limited volumes, which enhance signal magnitudes. Thus photothermal spectroscopy can be

used in small volume sample analysis and allows for microscopic analysis of heterogeneous

materials (9).

Fig. 1.1 depicts the basic processes responsible for photothermal spectroscopy signal

generation. Excitation of a sample is accomplished by using optical radiation, usually from a

laser. The internal energy of the sample increases due to absorption of some of this radiation.

Two different modes of hydrodynamic relaxation lead to dispersion of the internal energy.

The increased internal energy results in a temperature change in the sample, which results in

a change in sample density and pressure. The density change results in a change of the

refractive index, which can be probed by a variety of methods (10).

3

Pressure change occurs if the photothermal induced temperature change occurs faster

than the time required for the material to expand or in a few cases contract, and this pressure

perturbation will disperse in an acoustic wave. A density change proportional to the

temperature will remain once the pressure has relaxed to its equilibrium value. The

absorption of optical energy induces a change in temperature in either case, which will in

turn result in a density change in the sample. Other properties of the sample are affected by

the combined temperature and density changes, and photothermal spectroscopy is based on a

measurement of these properties. In particular, the basis of sensitive photothermal methods is

the measurement of the refractive index change that occurs with changes in temperature and

density of the sample (10).

Time-dependent temperature fields in a sample and its surrounding media due to heat

conduction are obtained when a time-dependent heat source is active within a solid state

material. The changing temperature fields are referred to as thermal waves. Thermal

Optical Excitation

Absorption

Excited State Relaxation

Increased Thermal Energy

Pressure Change(Acoustic wave)

Temperature Change(Thermal Diffusion)

Temperature Change Photothermal SignalOptical probe

Fig. 1.1 Basic processes responsible for photothermal spectroscopy signal generation

4

diffusivity can be measured by using the thermal waves. The thermal diffusion equation

shown below describes the heat conduction process in a homogenous and uniform solid state

material (10).

2T = (1/ α) x (δT / δt) (1)

where α = k/ ρ c is the thermal diffusivity and

k = thermal conductivity

ρ = density

c = specific heat of the material

T = temperature

t = time

A special class of solutions to the above equation are one dimensional thermal waves

which occur when the heat source is periodic in time.

Fig. 1.2 Thermal waves for characterizing thin films

Air

Film

Substrate

Laser Beam Laser Beam

Low ω High ω

5

In Figure 1.2, a periodic heat source having an angular frequency, ω is focused at the sample

surface, and the thermal wave is expressed as:

T(x, t) = T0 e-x/μ cos ((2πx/ λ) – ωt) (2)

where μ = (2α/ ω) 1/2 is the thermal diffusion length and

λ = 2π μ is the thermal wavelength

x = thickness of film

T = temperature

T0 = initial temperature

t = time

From Equation 2 it is observed that the thermal wave is heavily damped and heavily

dispersed. The thermal diffusion length μ determines the decay rate of the thermal wave as it

penetrates into the sample, and according to Equation 2 it is seen that the amplitude of the

thermal wave decays exponentially as it propagates. The diffusion length is inversely

proportional to the square root of the frequency, and the penetration depth of the thermal

waves decreases as frequency increases. Short diffusion length is obtained at high

modulation frequency, thus enabling the thermal wave method to be used as a powerful tool

for study of localized properties of thin films at microscopic levels.

Thermal waves being highly dispersive, the propagation velocity υ depends strongly

on frequency as shown by the Equation 3.

6

υ = ω (λ /2π) = (2ω) ½ (3)

Accordingly, increasing the frequency will result in an increase in thermal wave velocity.

Thus the two features of thermal waves (heavily damped and heavily dispersed) make

them suitable for characterizing thin film coatings. Fig. 1.2 shows a sample with a single

layer and the thermal waves generated. The waves generated are a combination of

contributions from the films, the substrate, and the film substrate interface (10).

Different diffusion length and thermal wave velocities are obtained at different

frequencies. Depth profiling of the thin film system and separation of the contributions of

the different parts can be done as each part plays a significantly different role in the detected

signal.

There are several methods used to monitor the thermal state of the analytical sample

(12-17). Older methods include photoacoustic spectroscopy, photothermal calorimetry and

photothermal radiometry, which did not make use of lasers (9, 18, 19). Table 1 lists major

techniques with their applications.

7

Table 1

Detection techniques based on thermodynamic properties

The following presents a brief review of some of the above techniques based on refractive

index changes.

Photothermal Interferometry

Sample heating occurs due to optical absorption, which subsequently changes the

refractive index and thus induces a phase shift in light passing through the heated region.

This optical phase shift can be detected with an interferometer. A change in power at a

photoelectric detector is observed when the phase of monochromatic light passes through the

heated sample, relative to the phase passing through the reference arm. There are various

interferometry methods used to detect changes in the optical path length induced by the

photothermal effect, and these methods are classified as photothermal interferometry (9, 20).

Thermodynamic property Measured Property Detection Technique

Temperature Infrared emission Photothermal radiometry

Pressure Acoustic wave velocity Photoacoustic spectroscopy

Refractive index Photothermal Lens

Photothermal Interferometry

Photothermal Deflection

Photothermal Reflectance

Density

Surface Deformation Photothermal Diffraction

Photothermal Deformation

8

Measuring refractive index changes using optical interferometry while using an

excitation laser to heat the sample is a fairly new concept. Most photothermal interferometry

apparatuses are based on laser excitation sources, and both coherent and wide-band

incoherent sources could be used. Photothermal interferometry is a highly sensitive

technique.

Fig. 1.3 Experimental set-up of photothermal interferometry

Photothermal Reflectance

This technique is based upon generating a photothermal signal due to induced

modulation of the reflectivity of the illuminated material (10, 21). The modulated reflectance

of the probe beam is based on the temperature dependence of the sample’s optical

reflectivity. A fractional change in the reflectivity (∆R) of the probe beam occurs due to a

small local temperature excursion (∆T), as defined by Equation 4:

∆ R / R0 = (1/R0) x (δR/ δT) x (∆T) (4)

where R0 = the reflectivity at a reference temperature T0

Optical Excitation

Detector

Sample

Mirror

Probe Laser

Mirror

9

(1/R0) x (δR/ δT) = the temperature coefficient of reflectivity, which is a function of the

material properties of the sample, sample temperature, and wavelength of the probe beam.

As shown in Fig. 1.4, a reflected signal without any angular deflection is obtained

when a probe beam is focused to a spot that is directly in the center of the pump beam area.

Due to variation in reflectance with temperature, the intensity of the reflected probe beam

shows a periodic variation in phase with the periodic variation in the pump beam intensity.

Fig. 1.4 Schematic of photothermal reflectance

Photothermal Diffraction

Photothermal diffraction is based upon measuring signals due to volume phase

diffraction gratings formed by the photothermal effects. A periodic spatial refractive index

modulation results in a volume phase diffraction grating, which will diffract light at an angle

that meets the requirements from Bragg's Law. The amount of light diffracted is proportional

to the refractive index change, and the diffracted light is measured using a photoelectric

detector (9).

Pump Laser

Photodetector

Sample

Probe Laser

10

Fig. 1.5 Experimental set-up of photothermal diffraction

Photothermal Deformation

Photothermal deformation is a sensitive method in which the thermal waves are

generated by a modulated pump beam. As the slope of the transient thermal lens curvature

changes, the angle of the reflected beam also changes as the probe beam is reflected from the

surface of the lens at a point on the lens profile away from the pump beam. A quad cell

detector monitors the change in the deflection of the beam and records both the surface

normal and parallel components of the reflected beam. Lock in amplifiers collect these

signals and selectively amplify the ac component to a measurable strength (10, 22).

Fig. 1.6 Schematic of photothermal deformation

Probe Laser Sample Detector

Undiffracted probeIndex grating

Excitation Lasers

Pump LaserQuad-Cell Detector

Sample Surface

Probe Laser

11

Transverse Photothermal Deflection

Transverse photothermal deflection spectroscopy is based on the mirage effect. This

method is used to examine optical absorption at or near the surface of solid samples. When a

sample absorbs optical radiation, it results in the heating of the gas or liquid above the

surface. The heated fluid behaves like a prism and deflects the probe laser incident tangent

to the surface. Spatial gradients in refractive index change the direction in the propagation of

a ray of light, and the light exits a medium with a refractive index gradient at an angle

relative to the incident ray. This bending of light path is commonly called photothermal

deflection. A position-sensing detector monitors the deflection of the probe beam. This

technique is very easy to set up and produces highly sensitive measurements of surface

absorption. It is very versatile and can be used for solid, surface, liquid, and gas phase

analysis. Excitation sources can be either pulsed or chopped continuous (10, 23, 24).

Fig. 1.7 Schematic of the transverse photothermal technique

Probe Laser

Pump Laser Modulator

Detector

Lens

Lens

Mirror

Sample

12

Collinear Thermal Lensing

Fig. 1.8 shows the schematic of a collinear photothermal technique, which is an

alternative to the transverse photothermal deflection method. This method is employed to

detect the change of refractive index inside the sample due to the pump beam by its focusing

or defocusing of the collinear probe beam. Earlier work done using collinear thermal lensing

technique reported maximum sensitivity values as several parts per billion (ppb), and

standard photothermal deformation techniques have reported sensitivities of 0.1 ppm. The

advantages of this technique are that it is a straightforward method to model the signal and

offers high spatial and time resolution (10).

Fig. 1.8 Schematic of the collinear thermal lensing (3)

A beam deflection signal is not obtained if the pump and probe beams are aligned

collinearly in Fig. 1.8. The pump beam causes localized heating, and the refractive index

profile generated behaves as a lens. Depending on whether the derivative of the refractive

index with respect to temperature is positive or negative, the pump and the probe beams will

Probe Laser

Pump Laser Modulator

Detector

Lens

Lens

Sample

Filter

13

be focused or defocused. Small variations in the refractive index are sufficient to generate a

measurable thermal lens effect. Though the sensitivity of the thermal lens method is less

than the photothermal deflection method, this is a widely used technique for absorption

measurement in various materials.

14

CHAPTER 2

Surface Thermal Lensing (STL)

Highly localized heating of transparent materials is observed due to absorption of

even a very small fraction of an incident laser beam. This causes numerous physical changes

in the sample and surrounding media, which is subjected to secondary heating. Thermal

waves, which are modulated temperature fields, are created when the laser beam is

modulated and the heating effect is time dependent. Thermal conductivity, density, and

specific heat capacity of the sample control the magnitude, velocity of propagation, and

damping of the thermal waves (25).

Photothermal techniques have been useful for characterization of weak absorption,

measurement of thermal properties of solid samples, and analysis of local defects. One of the

primary disadvantages of using photothermal techniques for characterizing weak absorptions

is signal calibration requirements.

Characterization of the thermal properties of solids is important in studying their

structure over a wide range of scales. Measurement of thermal conductivity gives significant

information about defects and impurities, structural anisotropies, finite size effects and

effects of interfaces and grain boundaries. This is particularly important for optical materials

since local defects in solids may lead to laser-induced damage.

Surface thermal lensing is a technique that combines the sensitivity of photothermal

deformation methods and the simplicity of the thermal lensing technique. It is a powerful

tool for photothermal characterization of weakly absorbing optical thin films and has been

15

extensively used to study thin films only and bulk polymers have not been investigated until

recently. This method uses a probe beam that has a larger beam size than the pump beam.

Fig. 2.1 demonstrates the principle of STL in which the modulated pump beam is the heat

source (26-33).

Fig. 2.1 Schematic of STL signal

A Gaussian distribution of temperature is generated in the interaction volume, due to

absorption of the pump beam. A thermal lens is formed due to the competition between the

heat injected by absorption and thermal diffusion in the medium. The thermal lens alters the

propagation of the probe beam along the sample, and the resulting distortions in the spatial

profile of the transmitted beam are observed in the far field. Due to the periodic formation of

the surface lens, the reflected probe beam has an embedded diffraction pattern, which is

modulated in phase with the chopped beam.

A change in light intensity can be measured by placing a detector behind a pinhole so

that some portion of the circular beam profile is sampled. In regions where the modulated

Probe beam

Pump beam

Detector

16

diffraction pattern forms, the modulated change in light intensity will produce an AC signal.

Areas of the beam profile outside the diffraction pattern will simply give a steady DC signal

with no AC component. The profile of the diffraction pattern can be plotted by scanning a

detector in one or two dimensions, or an entire diffraction pattern in principle might be

obtained by a CCD camera or an array detector.

Generation of the STL signal is demonstrated as shown in Fig. 2.2.

Fig. 2.2 Schematic of STL signal generation

17

The reflected beam sampling is performed using a photodiode detector equipped with

a pinhole. With the probe beam OFF, there is no current in the photodiode detector, but with

it being ON and the pump beam OFF, steady DC current flows in the photodiode detector.

With only the pump beam being chopped and both the pump and probe beams ON, a

corresponding modulation occurs in the reflected probe beam, which has the form of a

diffraction pattern embedded in the Gaussian profile of the probe beam (34).

The modulation frequency of the pump beam chopper is fed into a lock-in amplifier

as a reference signal. Thus, only the modulated AC component of the photodiode signal that

matches the reference signal is amplified by the detector, ignoring the constant background

of the Gaussian profile. An STL signal is generated in the form of peaks by a line scan

across the reflected probe beam, and this signal corresponds to the areas of the beam that are

being modulated by the diffraction pattern.

Glass Transition Temperature

Glass transition temperature (Tg) is that temperature when a changeover in behavior from

that typical of a disordered solid to a liquid occurs with increase in temperature (and vice

versa with decrease in temperature). This change occurs in a narrow temperature range, and

the plexus of changes observed at Tg can be divided into two families; one set involves

measurements made in equilibrium, which include changes in macroscopic properties such as

thermal expansion and heat capacity, and in microscopic quantities such as temporal

variation of the mean squared displacement of individual atoms. The other set involves

response to externally imposed stimuli that causes a shift from a system’s equilibrium state,

which also include changes in macroscopic category such as viscosity and acoustic

18

attenuation, and microscopic quantities such as drag forces on an atom constrained to move

with a specified velocity (35).

As mentioned in the previous section, highly localized heating results in formation of

a thermal lens, which alters propagation of the probe beam (basis of the STL technique). The

utilization of this technique to determine Tg of solid polymers has been studied and reported

in this thesis. With gradual increase in temperature of the solid polymer, a sudden change in

STL signal intensity is expected at Tg.

Fig. 2.3: Expected STL signal behavior at Tg with gradual increase in temperature

0

5

10

0 25 50 75 100 125 150

Temperature (˚C)

ST

L a

mp

litu

de

(nA

)

Tg

19

CHAPTER 3

Equipment Set-up

This section provides an in-depth view of the experimental setup and the various

configurations of the instrumentation used. The schematic is as shown in Fig. 3.1.

Fig. 3.1 Equipment set up for STL

The pump laser is a Spectra Physics Millennia II Diode pumped solid state CW Nd-

YAG laser that provides greater than 2W of green 532 nm from a standard 110 or 220V

single phase outlet. It is focused at right angles to the sample surface and behaves as the

excitation laser. The probe laser is a low power 20mW CW He-Ne laser operating at 632 nm

in the visible range and is used to monitor the changes caused by the pump beam. The probe

He-Ne Laser

Sample holder

532 nm DPSS Laser

Shutter

CRT Monitor

Video Camera

ChopperLock in amplifier

Detector

STL signal

Thermocouple reader

+Z

+Y

+X

Motion Controller

Lens

Heater Control Attenuator

20

beam is incident at 16º to the sample. The attenuator is used to control the power of the

probe beam. It is placed in the path of the probe beam to control its power using a motorized

actuator attached to it. A linear actuator (not shown in the schematic) is connected to the

attenuator and controls the power of the beam.

The sample holder is capable of moving in the X, Y, and Z directions. It has heating

coils wrapped around the exterior with a thermocouple attached to the sample stage, which is

connected to the temperature controller for feedback control. Another thermocouple is

attached to the surface of the sample to measure the temperature at the irradiated spot. A

mechanical chopper (Stanford Research Systems SR540) modulates the pump beam intensity

with a chopping frequency ranging from 1 Hz to 4000 Hz and a modulation ratio of 50:50.

The pump beam is focused on 100 µm of the sample surface, and the probe beam on 200 µm

of the sample surface. The position of the probe beam spot can be adjusted by movable

mirrors and by using lenses to collimate the probe laser. The motion controller helps in

making a 1-D or 2-D scan of the sample surface. It is also used to scan the detector across

the reflected probe beam. A CCD camera (Model # 250677 [COHU] 6400 Series) is used

mainly to check if the probe and pump beam are centered on the same spot. An

epifluorescence microscope (Leica DMRB) coupled with a DEI-470 Video Camera System

was used to capture images of the sample. The reflected probe beam that diverges from the

sample passes through a lens to focus into a small spot at a pinhole photodetector that can be

scanned across the reflected probe beam. The detector acquires the STL signal and sends it

to a Stanford Research System SR830 Dual-Phase DSP lock-in amplifier, which measures

the modulated changes in the probe beam intensity as a function of chopping frequency. The

lock-in amplifier is connected to a computer that uses software developed by Andrew Zhao

21

in Visual Basic to acquire and analyze real time data. The software package has the

following basic functions:

1. It controls the shutter to determine if the pump laser passes through the shutter.

2. It sets and maintains the chopping frequency by controlling the modulator SR540.

3. It controls the motion controller to make one or two dimensional scanning for the

sample stage and detector.

4. It acquires real time data from the lock-in amplifier and analyzes the signal.

22

CHAPTER 4

Experimental Procedure

The experimental set-up is as shown in Fig. 3.1. In order to keep dust out, to reduce

noise levels, and to maintain a controlled environment, the entire set-up was enclosed in

plexiglass. A thick dark cloth was used to cover the enclosure in order to protect the user’s

eyes and to avoid stray radiation from entering.

The pump beam laser source was set in the range 0.2W-1.0W according to the

experimental need, and the beam was projected perpendicular to the surface of the sample at

a specified spot. The probe beam laser source was set to give approximately 4.5mW-16.5

mW by using an attenuator, and this beam was projected on the sample at an off-normal

angle. Using mirrors, the pump and the probe beams were aligned to coincide on the same

sample spot. The alignment was checked using a CCD camera that was connected to a

monitor outside the enclosure to avoid affecting the stability of the output signal due to

inaccurate alignment.

The position of the detector was optimized by chopping the probe beam with the

pump beam blocked. The motion controller was used to adjust the detector position to obtain

an optimized signal. The reference frequency was supplied by the probe beam chopper, and

the signal for the lock-in-amplifier came from the detector. Whenever the lock-in-amplifier

showed overload, it was set for higher input current values until steady value was reached.

Further, if overload was observed, the average of values on either side of the overload for

both x and y positions on the motion controller were taken.

The probe beam chopper was turned off and with the pump beam chopper turned on,

the pump beam chopper serves as the reference frequency for the lock-in-amplifier. The

23

reference frequency was set at 67 Hz for strong and stable signal values. The lock-in-

amplifier’s time constant was set to one second, and the computer acquired one data point per

second. The modulated pump beam was unblocked, and an actual irradiation time of 5

minutes was used. A localized deformation, i.e. a thermal bump, was produced, which flexed

with the same frequency as that of the chopped pump beam, and the incident probe beam was

reflected from the fluctuating thermal bump, which produced an oscillating diffraction

pattern with the same frequency as that of the thermal bump. Its maximum intensity signal

entered the pinhole detector, sending the signal to the lock-in-amplifier, which amplified the

AC part of the signal whose frequency matched with that of the reference frequency, and the

rest was filtered out. The phase shift between the input (chopped pump beam) and the output

(reflected probe beam with diffraction pattern embedded in it) and the intensity of the signal

were measured by the lock-in-amplifier. The lock-in-amplifier then sent the data to the

computer, which used software (PTM YC using Visual Basic 6.0) for data acquisition. The

data were displayed in the form of plots (signal amplitude versus time and phase versus

time).

Various experiments were carried out using solid samples of transparent polymers.

Two types of materials were used, polycarbonate and zeonex discs, which were obtained

from G-S Plastic Optics. Polycarbonates are a group of thermoplastic polyesters that are

easily worked, molded, and thermoformed. They have excellent temperature resistance,

impact resistance, and optical properties that enable their use in modern manufacturing,

between commodity plastics and engineering plastics. Polycarbonate (M.W of repeat unit =

254.3), is synthesized from bisphenol A and phosgene (carbonyl dichloride, COCl2) and has

the following structure:

24

Fig. 4.1: Structure of polycarbonate

Zeonex (Fig. 4.2) is an ultra high purity optical grade Cyclic Olefin Polymer (COP)

with excellent chemical resistance, thermal stability, and optical properties and is used in

optical applications where refractive index stability over high heat and humidity is crucial. It

is also widely used in medical and laser devices.

Fig. 4.2: Structure of Cyclic Olefin Polymer

Time dependence data were obtained for a total of 4 hrs in periods of 1.5, 1.5, and 1

hours to avoid overflow error in the computer. STL signal as a function of pump power,

probe power, and pump frequency were obtained. The pump power dependent runs were

carried out by varying the pump power from 0.2 W – 1.0 W in steps of 0.2 W and the

amplitude noted down immediately.

Finally the effect of temperature over STL signal was observed for zeonex. The solid

zeonex sample was placed in a sample holder, and the temperature was raised gradually from

room temperature to 140˚C. At the end of each experiment, before shutting down, care was

taken to reset the motion controller to 00.00 setting to avoid shifting of its origin to the value

at which it was turned off.

25

CHAPTER 5

Results and Discussion

Experiment 1: Probe beam power calibration

Since the 633 nm He-Ne CW-laser used as the probe beam was a fixed power unit, adjusting

the intensity of the probe beam was accomplished with a Newport Model 935 variable

attenuator. The sensing head of a laser power meter was placed in the path of the probe

beam after it passed through the adjustable beam attenuator in order to calibrate the probe

beam intensity as a function of the actuator position for the attenuator. After warm-up, the

actuator was set at specific values and the probe beam power at the sensor head recorded to

show the expected linear decrease in the power meter reading as attenuation was increased,

as shown in Fig. 5.1.

0

2

4

6

8

10

12

14

0 500 1000 1500 2000

actuator position

pow

er m

eter

rea

ding

(m

W)

Fig. 5.1 Probe beam power calibration

26

Experiment 2: STL signal as a function of time (Sample: Polycarbonate)

A solid polycarbonate sample of 4 mm thickness and 1” diameter was used. The pump

power was 1.0 W and the probe power set at 10 mW. The pump frequency was set at 67 Hz

and the experiment was carried out at a room temperature of 27ºC. The STL signal of the

sample was monitored as a function of time. Time dependence data were obtained for a total

of 4 hours to eliminate initial transient behavior, and the data were collected in periods of

1.5, 1.5, and 1.0 hours to avoid overflow error in the computer. Data were collected from

seven sample positions where the signal was fairly strong as the sample was not uniform

(Fig. 5.2).

0

10

20

30

40

50

0 5000 10000 15000

time (s)

STL

am

plit

ude

(nA

)

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Fig. 5.2 STL signal as a function of time

Similar signal curves were obtained for all of the seven spots as seen. At the end of

the four-hour run, each signal was leveling out to an approximately constant value.

The phase throughout all the runs was relatively constant. The initial and final

27

amplitudes acquired by the lock-in amplifier at each spot were different, and the

differences in the initial and final values for each spot could be due to non-uniformity

in the sample. At other spots, dissimilar curves were obtained as shown in Fig. 5.3.

0

5

10

15

20

25

0 1000 2000 3000 4000 5000 6000

time (s)

STL

am

plit

ude

(nA

)

Position 1

Position 2

Position 3

Fig. 5.3 STL signal as a function of time

During these runs, the phase changed monotonically and these runs were carried out

for just 1.5 hours. Careful inspection of the sample revealed noticeable damage on

the surface of the sample at the three spots where the anomalous curves were

obtained. Epifluorescence microscope studies revealed damage on the surface of the

sample as shown in Fig. 5.4.

28

Fig. 5.4 Pictures of the damaged surface

Crater-like pits and sharp-edged holes were observed on the surface with cracks

radiating out from the damaged area, which indicates non-uniformity in the samples. Further

experiments were carried out using a polycarbonate solid sample of thickness 2 mm and 2”

diameter as the above sample was rendered unusable.

29

Experiment 3: STL signal as a function of pump power (Sample: Polycarbonate)

A solid polycarbonate sample of thickness 2 mm and 2” diameter was used. A short test run

at 1.0 W pump power damaged the surface; hence a pump power dependence run was carried

out at a pump frequency of 67 Hz. The STL signal varied linearly with the pump power as

shown in Fig. 5.5. A fairly good signal was obtained at the smallest pump power of 0.2W.

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

pump power (W)

STL

am

plit

ude

(nA

)

Fig. 5.5 STL signal as a function of pump power

30

Experiment 4: STL signal as a function of probe power (Sample: Polycarbonate)

The effect of probe power on the STL signal was studied with the solid polycarbonate sample

of thickness 2 mm and 2” diameter. The pump power was set to 0.2W and the pump

frequency was 70 Hz. Although the correlation coefficient for the data series was somewhat

low, the relationship was approximately linear over the probe power measured. Fig. 5.6

shows the STL signal as a function of probe beam power.

0.60.650.7

0.750.8

0.850.9

0.951

1.051.1

7 8 9 10 11 12 13

probe power (mW)

ST

L a

mp

litu

de

(nA

)

Fig. 5.6 STL signal as a function of probe power

Under ideal conditions, we expect the linear fit to pass through the origin, but extrapolation

to zero probe power gave a Y-intercept at 0.3237 nA. As a fairly good signal was obtained

at the smallest pump power of 0.2W, the next set of time dependence runs were carried out at

pump power of 0.2W and probe power of 12.3 mW, as shown in Fig. 5.7. The pump

frequency was set to 70 Hz, and the experiments were carried out at room temperature.

31

0

1

2

3

4

5

6

7

0 5000 10000 15000

time (s)

STL

am

plit

ude

(nA

)

Position 1

Position 2

Position 3

Fig. 5.7 STL signal as a function of time

An exponential decay with time, as expected, was observed over a period of 4 hours. As

with the previous sample, the initial and final amplitude varied from spot to spot. The initial

amplitude observed was considerably less than that obtained for runs with 1.0 W pump

power. The experiment was repeated at a new spot with a pump power of 0.4 W. Damages

were noticed after 10 minutes of the run, and due to unavailability of sample surface for

further experiments, it was replaced with a Zeonex sample.

32

Experiment 5: STL signal as a function of Pump power (Sample: Zeonex)

The following preliminary experiments were carried out using the Zeonex sample to establish

working conditions with that sample.

The probe power was maintained at 16.5 mW and the pump chopping frequency at

200Hz. The pump power was increased from 0.2 W to 1.0 W in steps of 0.2 W, and the

corresponding maximum STL signal recorded. Then, the pump power was decreased from

1.0 W to 0.2 W in steps of 0.2 W and the amplitude recorded. The STL signal increased

(blue diamond)/ decreased (pink square) linearly with change in pump power as shown in the

Fig. 5.8.

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1 1.2

pump power (W)

STL

am

plit

ude

(nA

)

Fig. 5.8 STL signal as a function of pump power for Zeonex

33

Experiment 6: STL signal as a function of probe power(Sample: Zeonex)

The pump power was set at 1.0 W and the pump chopping frequency at 200Hz. The probe

power was increased from 4.5 mW to 16.5 mW in steps of 2 mW, and the corresponding

maximum STL signal recorded. Then, the probe power was decreased from 16.5 mW to 4.5

mW in steps of 2 mW and the amplitude recorded. The STL signal increased (blue

diamonds)/ decreased (pink square) linearly with change in probe power as shown in

Fig. 5.9.

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20

probe power (mW)

STL

am

plit

ude

(nA

)

Fig. 5.9 STL signal as a function of probe power

34

Experiment 7: STL signal as a function of pump frequency (Sample: Zeonex)

The pump modulation frequency is used as a reference for the lock-in amplifier. The pump

power was set to 1.0 W and the probe power at 16.5 mW. The pump chopping frequency

was varied from 67Hz to 400 Hz and the STL signal recorded (Fig. 5.10). The probe beam

was not chopped.

44.14.24.34.44.54.64.74.84.9

0 100 200 300 400 500

frequency (Hz)

STL

am

plit

ude

(nA

)

Fig. 5.10 STL signal as a function of pump frequency

The STL signal was stronger at low chopping frequency and decreases with increases in the

chopping frequency. Experimental working frequency is usually set at around 200-250 Hz

due to the presence of environmental noise at low frequency.

35

Experiment 8: Effect of temperature over STL signal (Sample: Zeonex)

The pump power was set to 1.0 W and the probe power at 16.5mW. The pump frequency

was set to 200Hz and the temperature was increased gradually. The solid Zeonex sample

was placed in a sample holder, and the temperature of the sample was gradually increased

from room temperature to 140 ˚C, and the corresponding STL signal recorded as shown in

Fig. 5.11

0123456789

10

0 20 40 60 80 100 120 140

temperature (˚C)

STL

am

plit

ude

(nA

)

Fig. 5.11 STL signal as a function of temperature

The STL signal was fairly constant with temperature over the range 20 ˚C to 115 ˚C. At

about 115 ˚C, the STL signal rose considerably and leveled out at higher temperatures as

shown in Fig. 5.11. The range over which the sharp change in the STL signal occurred

corresponds to the Tg of Zeonex, 118 ˚C.

36

A similar change in the STL phase was recorded. The phase was almost constant over the

temperature range of 20 ˚C to 115 ˚C as shown in Fig. 5.12. At about 115 ˚C, there was a

sudden decrease in phase, which later stabilized at higher temperatures as shown in Fig. 5.12.

0

20

40

60

80

100

0 50 100 150

temperature (˚C)

ST

L p

has

e (˚

)

Fig. 5.12 Phase as a function of temperature

37

CHAPTER 6

Conclusions

The STL technique, which is based on detecting the modulated interference pattern

resulting from the reflected probe beam off the curved surface thermal lens, has been utilized

to investigate the relationship between the STL signal amplitude and the properties of bulk

polycarbonate and Zeonex discs. The properties of the polymer samples, such as thermal

diffusivity, thermal expansion coefficient, and glass transition temperature, determine the

size of the thermal bump, but as yet there is no definitive mathematical model that can

consistently account for the changes that occur during the process of STL signal generation.

The amplitude decay over time seems to follow a general pattern regardless of the

type of polymer. Constant phase over time was observed for both the types of polymers

used. It was assumed that the amplitude remained constant after the time decay runs were

carried out, which suggests complete relaxation. This was verified through experiments such

as that shown in Fig. 5.8, demonstrating reproducibility after relaxation has occurred. As

expected the STL signal increased with pump and probe power. The maximum amplitude

obtained for each spot (at the same pump and probe power) at the start of any run was

different, which suggests the non-uniformity of the samples (Fig. 5.1, 5.2, and 5.7). Also, the

damage caused at a few spots, which limited the availability of the surface for further studies,

does suggest non-uniformity of the sample in terms of damage resistance.

Following the time decay runs, a number of frequency dependence runs were carried

out by B. Hussain (30) for the polycarbonate samples. It was observed that the STL signal

increased gradually with frequency, reached a peak and then decayed with increasing

frequency. Frequency dependence of the amplitude was studied at a single spot for the

38

Zeonex sample for comparison with the polycarbonate sample. For the Zeonex sample, the

amplitude decayed with increasing frequency from the start of the experiment. There was no

peak in the amplitude observed as in the polycarbonate sample. Although both polymers

were transparent to visible light and thus should have been non-absorbent at either of the

laser frequencies, the polycarbonate samples were distinctly more sensitive to the pump

beam, sustaining significant damage at relatively low powers. Whether this was an inherent

property of the polycarbonate or due to some manufacturing-related problem, such as surface

contamination by absorptive impurities or physical flaws, is not known. Further

investigation is necessary to determine the differential behavior of the two polymer samples.

Finally, a temperature dependent run was carried out with Zeonex, and a sudden

change in the STL signal (as expected in Fig. 2.3) was observed at the glass transition

temperature of the polymer, indicating that thermomechanical properties can be studied using

the STL technique. The number of runs carried out was limited due to the damage caused on

the sample surface and the number of samples available. Further work using different

polymer samples needs to be carried out, and intense experimentation is required to

determine the actual cause of the observed phenomenon.

39

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