design and fabrication of an organic dbr laser with meh ... · meh-ppv as active layer design and...
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Boxuan Gao
MEH-PPV as active layerDesign and fabrication of an organic DBR laser with
Academic year 2015-2016Faculty of Engineering and ArchitectureChair: Prof. dr. ir. Rik Van de WalleDepartment of Electronics and Information Systems
Master of Science in Photonics EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Ir. Michiel CallensSupervisor: Prof. dr. ir. Kristiaan Neyts
Boxuan Gao
MEH-PPV as active layerDesign and fabrication of an organic DBR laser with
Academic year 2015-2016Faculty of Engineering and ArchitectureChair: Prof. dr. ir. Rik Van de WalleDepartment of Electronics and Information Systems
Master of Science in Photonics EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Ir. Michiel CallensSupervisor: Prof. dr. ir. Kristiaan Neyts
Acknowledgements
On the completion of my thesis, there comes the realization that so much I have experienced
and learned, the way I view on my topic and the studying has also been refreshed. All of these
are attributed to the help and guidance I received during my thesis, I would like to express my
deepest gratitude to all those whose kindness and advice that have made this happen.
First, I am very thankful to my promoter Professor Kristiaan Neyts for giving me the oppor-
tunity to be able to complete my thesis at the Liquid Crystal group, it is really a valuable
experience for me. Your advice have always provided me with new ways of thinking and help
me make improvement in my thesis studying. I am also greatly indebted to Michiel Callens, a
responsible supervisor who has spent a lot of time on guiding me during my thesis and giving
me multitudes of valuable instructions on the process of my thesis as well as the thesis writing.
I am very appreciate for his help and patience in every stage of my thesis, I have benefited a
lot from all those discussion with him not only for the thesis but also for my future studying.
Without his tutoring and impressive kindness, this thesis could not be kept proceeding in the
right direction and completed in the end.
Moreover, I shall also convey my sincere thanks to all the other people who has offered me
advice during my studying and all the other aspects, which help me smooth away difficulties as
well as giving me confidence.
Last my thanks would go to my beloved family and friends. During my two years studying in
Gent, they have always been so supportive for me no matter where they are. I can only get
over all the difficulties and finally complete my study with their encouragement. Thank you for
being there for me all the time.
Boxuan Gao
June, 2016
Permission for user on loan
The author gives permission to make this master dissertation available for consultation and to
copy parts of this master dissertation for personal use. In all cases of other use, the copyright
terms have to be respected, in particular with regard to the obligation to state explicitly the
source when quoting results from this master dissertation.
June, 2016
Design and fabrication of an organic DBR laser
with MEH-PPV as active layer
Boxuan Gao
Supervisor: Prof. dr. ir. Kristiaan Neyts
Counsellor: Ir. Michiel Callens
Master’s dissertation submitted in order to obtain the academic degree of
Master of Science in Photonics Engineering
Department of Electronics and Information Systems
Chair: Prof. dr. ir. Rik Van de Walle
Faculty of Engineering and Architecture
Academic year 2015-1016
Abstract
This thesis describes the design and fabrication of an organic DBR laser with conjugated
polymer MEH-PPV as amplifying medium. As organic semiconducting materials com-
bine the advantages of both semiconductors and organic materials, they become promising
candidates for laser development. Conjugated polymers such as MEH-PPV, has a four-
level system and large cross section, make it a proper amplifying medium for lasers. In
this thesis, a net thin MEH-PPV film was made to be the amplifying material by spin
coating, because it is soluble in common organic solvents. Furthermore, the light propa-
gation is also controlled by an organic component–an organic DBR consists of alternative
BDAVBi and TAPC layers, together with a silver mirror to provide high reflectivity, a
laser resonator is achieved, leading to a vertical surface emitting modality. A green laser
functioned as the pump for this organic laser.
Keywords
Organic semicondutors, MEH-PPV, DBR laser
Abstract—this thesis is concerned with the design
and fabrication of an organic DBR laser. A net thin
film of conjugated polymer MEH-PPV is spin coated
as amplifying medium of the laser and an organic
DBR comprising alternative BDAVBi and TAPC
layers serves as the resonator mirror, while the other
mirror is silver. A green laser functions as the pump
device.
Keywords—organic semiconductors, MEH-PPV,
spin coating, organic DBR
I. INTRODUCTION
N recent years, organic semiconductors have
emerged as a promising new material for its
combination of the advantages of both
semiconductors and organic material, providing
possibilities to make easy fabricated tunable
devices, thus being used in various electronics
and optoelectronics applications, for example
light emitting diodes[1] and thin film
transistors[2]. The conjugated polymer
2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-
vinylene (MEH-PPV) with its large absorption
cross-section and high photoluminescence
quantum yield has already been studied as
amplifying medium for organic lasers [3][4] and
proven to be a versatile semi-conducting polymer
for use in optoelectronic devices[5]. However
mostly, the MEH-PPV was used in a dilute
solution. Moreover, light propagation in the
organic semiconductor devices has always been
controlled by inorganic materials such as metals
and dielectric insulators, the refractive indices
difference in organic semiconductors are barely
considered to affect the propagation of light. In
this thesis, a net MEH-PPV thin film is spin
coated on a silver coated substrate as the
amplifying medium and an organic distributed
Bragg reflector (DBR) is deposited on top of the
film to form the laser cavity, controlling the light
reflection. Finally, a green laser is used to provide
population inversion for lasing.
II. BASIC PRINCIPLE OF LASER
LASER is the acronym for Light Amplification by
Stimulated Emission of Radiation. To obtain the
stimulated emission, three basic components are
needed, namely an amplifying medium, a
resonator and a pump device, as illustrated in
Figure 1.
The resonator is formed by two mirror to provide
feedback, light in the resonator will bounce back
and forth. If constructive interference is achieved,
standing waves will be built, which is called
lasing modes.
Figure 1 Schematic of a basic laser set up.
Reprinted from "Lasers", Chapter 1[6]
The condition for the wavelengths of these modes
are expressed below:
][2 LN
Where [L] is the optical path length and N is the
longitudinal mode numbers. Only light with
certain wavelengths can lase.
The amplifying medium is the place where
population inversion occurs, which is necessary
Supervisor: Prof. dr. ir. Kristiaan Neyts
Counsellor: Ir. Michiel Callens
Boxuan Gao
Design and fabrication of an organic laser
with MEH-PPV as active layer
I
to achieve stimulated emission. To have a laser in
stable state, the net round trip gain should be
equal to 1, as shown below:
1)1)(1( ][221 LgeTT
Where 1T and 2T represent the losses at two
mirrors, respectively, and g the gain factor. It
could be realized that with a certain gain factor,
the optical path length should be long enough to
satisfy this condition. In this thesis, the
amplifying medium is a thin film of MEH-PPV.
The pump is provided by a green laser in this
thesis.
III. AMPLIFYING MEDIUM—MEH-PPV
A. Spin coating procedure
Because MEH-PPV can dissolve in common
organic solvents, the thin film is obtained by
casting from solution. In order to produce
uniform film, different solvents have been tried.
According to the formula to obtain stable state
of lasing, the thickness for the film should be
several hundreds of nanometers. First the
MEH-PPV was dissolved in chloroform with
two different concentration 1mg/ml and 3mg/ml.
After completely dissolved, the solution was
spin coated with spin speed varied from
2000rpm to 3000rpm. However, the resulting
films were too thin to meet the requirement.
Then MEH-PPV in Tetrohydrofuran (THF) with
concentration 7.65mg/ml was tested. The
difference is that the spin speed was set to be
600rpm. This time the film seemed much thicker.
As indicated in Figure 2. All the work above
were done in cleanroom.
Figure 2 MEH-PPV flim cast from MEH-PPV in THF
mixture with concentration 7.65mg/ml
B. Optical characteristics
The optical characteristics correspond to lasing
were measured in the lab. For the emission
property, the sample was excited by a green laser
with wavelength 532nm and a broad band
emission spectrum was shown. By adjusting the
parameters of the incident laser beam (repetition
rate, incident intensity) and the position of the
sample, amplified spontaneous emission (ASE)
was obtained with peak at wavelength of around
620nm, suggesting the realization of population
inversion and the possibility of getting lasing
once the resonator is ready.
Figure 3 Up: transmission spectrum of MEH-PPV.
Down: emission spectrum shows the peak of
amplified spontaneous emission
One drawback is that under high incident laser
intensity, there is bleaching effect. To solve this,
the sample was put in a vacuum chamber.
The transmission spectrum is also shown above.
It can be observed that the absorption at
wavelength 532nm is high and for the emitted
light the absorption is low.
IV. ORGANIC DBR
DBR is a layered structure with alternative high
refractive index and low refractive index layer.
The thickness for each layer is calculated using
the following formula:
4
nd
Where n is the refractive index of that layer and d
is the corresponding thickness. For emission
wavelength 620nm, the thicknesses for
4,40-Bis[4-(diphenylamino)styryl]biphenyl
(BDAVBi) and 1,1-bis[4-[N,N-di(p-tolyl)amino]
-phenyl]cyclohexane (TAPC) are 81.7nm and
93.1nm, respectively. The layers were deposited
on the MEH-PPV film in a vacuum chamber by
physical vapor deposition in cleanroom. These
two materials were chosen because they have
relatively high refractive indices contrast at
620nm. As the reflectivity curve of DBR can shift
by changing the thicknesses of the layers and low
reflection of incident laser at 532nm is preferred,
the BDAVBi layer was made 6nm thicker. The
final reflectivity curve simulated in Matlab is
shown in Figure 4.
Figure 4 Reflectivity of organic DBR
V. THE COMPLETE ORGANIC LASER
When spin coating the MEH-PPV film on the
silver coated substrate and then depositing the
organic DBR on top of the film, the organic laser is
completed. As in Figure 5.
Figure 5 Complete organic DBR laser
The small dots are the silver mirrors, the red film
is MEH-PPV film and the green strip indicates the
positon of the DBR. This sample was excited by a
green laser in the effort to obtain lasing. However,
the lasing did not appear in the end and the DBR
was damaged to some extent. Possible reasons
maybe the thickness of the film cannot support
the lasing mode, the DBR has been burned by the
incident laser energy and the aggregation of the
MEH-PPV polymers. Future works are required
for the acquirement of lasing.
VI. CONCLUSION
In this thesis, a DBR laser was designed and
fabricated using organic materials. The
fabrication procedure has proved to be simple
and the broad band emission spectrum promises
the possibility for tunable laser. Although it is a
pity that the lasing was not achieved in the end,
the appearance of ASE has shown that lasing is
available if all the conditions are met. Therefore
future efforts are needed for the achievements of
a working organic DBR laser.
REFERENCES
[1] Christopher J Tonzola, Maksudul M Alam,
and Samson A Jenekhe. “New soluble n-type
conjugated copolymer for light-emitting
diodes.” Advanced Materials, 14(15):
1086-1090, 2002.
[2] Henning Sirringhaus, Nir Tessler, and
Richard H Friend. “Integrated optoelectronic
devices based on conjugated polymers”.
Science, 280(5370):1741-1744, 1998.
[3] Daniel Moses. “High quantum efficiency
luminescence from a conducting polymer in
solution: A novel polymer laser dye.” Applied
Physics Letters, 60(26):3215-3216, 1992.
[4] Ifor David Williams Samuel and Graham
Alexander Turnbull. “Organic semiconductor
lasers.” Chemical Reviews, 107(4):1272-1295,
2007.
[5] F. Hide, B. J. Schwartz, M. A. Díaz-García, and
A. J. Heeger. “Laser emission from solutions
and films containing semiconducting polymer
and titanium dioxide nanocrystals.” Chemical
Physics Letters, vol. 256, no.4, pp.424–430, 1996.
[6] Prof.Geert Morthier(UGent), Prof.Nathalie
Vermeulen(VUB). “Laser.” 2014.
Table of Contents
List of Figures iii
List of Tables vi
Abbreviations vii
1 Introduction 1
2 Organic Semiconducting Materials 3
3 Laser 7
3.1 Basic Discipline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Amplifying Medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Resonator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.1 DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.2 DFB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Amplifying Medium–MEH-PPV 17
4.1 Introduction of Amplifying Material . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2 Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.1 Spin-coating Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.2 Literature Study and Realization . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.2.1 Chloroform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.2.2 Tetrohydrofuran . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2.3 Solution Chosen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3 Optical Characterization of Amplifying Medium . . . . . . . . . . . . . . . . . . 21
4.3.1 Emission Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.3.2 Transmission Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5 Resonator Mirror–DBR 28
5.1 Material Chosen and Composition . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.2 Properties Verification by Simulation . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2.1 Simulation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.2 An Ideal Referential DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.2.1 Contrast in Refractive indices . . . . . . . . . . . . . . . . . . . 30
5.2.2.2 Number of Layer pairs . . . . . . . . . . . . . . . . . . . . . . . . 30
i
TABLE OF CONTENTS ii
5.2.2.3 Thickness Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 32
5.2.2.4 Variation of Incident angle . . . . . . . . . . . . . . . . . . . . . 33
5.2.3 Organic DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2.3.1 Number of Pairs of Layers . . . . . . . . . . . . . . . . . . . . . 35
5.2.3.2 Thickness Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 36
5.2.3.3 Variation of Incident Angle . . . . . . . . . . . . . . . . . . . . . 37
5.2.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3 Fabrication of Organic DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.4 Transmission Measurement and Comparison . . . . . . . . . . . . . . . . . . . . . 40
5.4.1 Angle Variation Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.4.2 Thickness Uncertainty Measurement . . . . . . . . . . . . . . . . . . . . . 42
5.5 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6 Organic DBR Laser 44
6.1 Fabricated Structure of Organic DBR laser . . . . . . . . . . . . . . . . . . . . . 44
6.1.1 Silver Coated Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.1.2 MEH-PPV and DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.1.3 Fabricated Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.2 Property Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6.2.1 Experimental Setup and Devices . . . . . . . . . . . . . . . . . . . . . . . 48
6.2.2 Measurement Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.3 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
7 Conclusion and Perspective 54
Bibliography 56
List of Figures
2.1 The structure of energy bands in semiconductor materials. Source:http://www.optique-ingenieur.org/[1]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Overview of diverse III-V semiconductors in lattice constant vs.band-gap dia-gram. Reprinted from [2]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Schematic of line defection. Reprinted from [3]. . . . . . . . . . . . . . . . . . . 4
2.4 Chemical structures of typical organic semiconductors used for lasers: (a) an-thracene; (b) aluminum tris(quinolate); (c) generic poly(para-phenylene vinylene)derivative; (d) generic polyfluorene derivative; (e) bisfluorene cored dendrimer;(f) spirolinked oligomer. Reprinted from I.D.W.Samuel, G.A.Turnbull, “OrganicSemiconductor Lasers”, Chem.Rev.2007, 107,1272-1295[4]. . . . . . . . . . . . . 5
2.5 Energy level diagram for organic semiconductor gain medium with vibrationallevels. Reprinted from I.D.W.Samuel, G.A.Turnbull, “Organic SemiconductorLasers”, Chem.Rev.2007, 107,1272-1295[4]. . . . . . . . . . . . . . . . . . . . . 6
3.1 Schematic of stimulated emission by a two-level system. Reprinted from Prof.NathalieVermeulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . 8
3.2 Schematic of a basic laser set up. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . . . . . . . . . . . . 9
3.3 Schematic of the first laser. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . . . . . . . . . . . . 9
3.4 Thin slice of an amplifying medium. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . . . . . . . . . . . . 10
3.5 Cross section as a function of frequency. Reprinted from Prof.Nathalie Ver-meulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . . . 11
3.6 Small-signal saturated gain as functions of frequency. Reprinted from Prof.NathalieVermeulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5]. . . . . 12
3.7 Lasing modes of the laser. Reprinted from Prof.Nathalie Vermeulen(VUB), Prof.GeertMorthier(UGent), “Lasers”, Chapter1[5]. . . . . . . . . . . . . . . . . . . . . . 12
3.8 Schematic of Bragg reflector. Reprinted from Prof.Dries Van Thourhout, Prof.RoalBaets (UGent), Prof.Heidi Ottevaere (VUB), “Microphotonics”[6]. . . . . . . . 13
3.9 Reflection mechanism of Bragg reflector. . . . . . . . . . . . . . . . . . . . . . . 13
3.10 Reflection curve for an ideal DBR. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.11 Schematic of a VCSEL. Reprinted from Prof.Nathalie Vermeulen(VUB), Prof.GeertMorthier(UGent), “Lasers”, Chapter7[5]. . . . . . . . . . . . . . . . . . . . . . 15
3.12 Schematic of a DFB laser. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter7[5]. . . . . . . . . . . . . . . . 15
iii
List of Figures iv
3.13 Waveguide structure of DFB laser. Reprinted from G.A.Turnbull, P.Andrew,W.L.Barnes, I.D.W.Samuel,“Operating characteristics of a semiconducting poly-mer laser pumped by a microchip laser”,2003[7]. . . . . . . . . . . . . . . . . . 16
4.1 Chemical structure of MEH-PPV. Reprinted from Operating characteristics of asemiconducting polymer laser pumped by a microchip laser”,2003[[7]. . . . . . . 18
4.2 Process of spin coating. First the solution applying, then the spin makes anuniform, thin film. Reprinted from [8]. . . . . . . . . . . . . . . . . . . . . . . . 18
4.3 Spin coating film of MEH-PPV dissolved in chloroform with concentration 1mg/ml,spin speed:3000rpm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4 Spin coating film of MEH-PPV dissolved in chloroform with concentration 3mg/ml,spin speed:2300rpm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5 Spin coating film of MEH-PPV dissolved in THF with concentration 7.65mg/ml,spin speed:600rpm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.6 Experimental set up for measuring emission spectrum. A: lens with focal length50mm. B: sample position. C: lens system to converge the emission light. D:fiber of spectrometer. E: filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.7 Emission spectrum of MEH-PPV film. . . . . . . . . . . . . . . . . . . . . . . . 23
4.8 Emission spectrum of MEH-PPV film with ASE peak. . . . . . . . . . . . . . . 23
4.9 Bleached sample. The round spots indicate the places that have been bleached. 24
4.10 Bleaching effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.11 Comparison of bleaching effect with/without vacuum. . . . . . . . . . . . . . . . 25
4.12 Transmission curve for MEH-PPV, extracted from article (with chloroform assolvent) and measured from experimental samples. . . . . . . . . . . . . . . . . . 25
4.13 Transmission curve of a glass substrate. . . . . . . . . . . . . . . . . . . . . . . . 26
4.14 Comparison of transmission and emission spectra of MEH-PPV film . . . . . . . 27
5.1 Molecule structures of BDAVBi and TAPC, adapted from[9]. . . . . . . . . . . . 29
5.2 Difference in refractive indices for BDAVBi and TAPC. The large difference inrefractive indices of these two materials at wavelength 600nm could make a goodorganic DBR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.3 Reflectivity of DBR versus wavelength with different refractive indices contrast.Higher contrast gives higher reflectivity and broader bandwidth. . . . . . . . . . 31
5.4 Reflectivity of DBR versus wavelength with different number of pairs. The morethe number of pairs, the better the reflectivity is. . . . . . . . . . . . . . . . . . . 31
5.5 Schematic description of thickness uncertainty of layers. . . . . . . . . . . . . . . 32
5.6 Reflectivity of DBR versus wavelength with thickness uncertainty, with thickerlayers than the design, reflectivity curve shifts to longer wavelength. . . . . . . . 33
5.7 Schematic description of different incident angles. . . . . . . . . . . . . . . . . . . 33
5.8 Reflectivity of DBR versus wavelength with incident angle variations, with largerincident angle, the curve shifts to shorter wavelength. . . . . . . . . . . . . . . . 34
5.9 Change in optical path length because of incident angle, which becomes shorterthan before. Reprinted from[6]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.10 Reflectivity of organic DBR versus wavelength with different number of pairs.Same explanation as for Fig.5.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.11 Reflectivity of organic DBR versus wavelength with thickness uncertainty, in-creased thickness of the high refractive index layer makes the reflectivity at 532nmreach the bottom of the curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
List of Figures v
5.12 Reflectivity of organic DBR versus wavelength with different incident angles,same explanation as for Fig.5.8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.13 Reflectivity of organic DBR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.14 Physical vapor deposition chamber. Reprinted from[10]. . . . . . . . . . . . . . . 38
5.15 DBR sample with uniform thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . 39
5.16 DBR sample with continuously changing thicknesses. . . . . . . . . . . . . . . . . 39
5.17 Simulation reflectivity of the organic DBR with different angles. . . . . . . . . . . 40
5.18 Comparison between experiment and simulation: angle 0. . . . . . . . . . . . . . 41
5.19 Comparison between experiment and simulation: angle 18.9. . . . . . . . . . . . . 41
5.20 Comparison between experiment and simulation: angle 27.5. . . . . . . . . . . . . 42
5.21 Comparison between experiment and simulation for all three angles. . . . . . . . 42
5.22 Measurements for sample 2, different thickness were taken at different position. . 43
6.1 Silver coating present as small round dots. . . . . . . . . . . . . . . . . . . . . . 44
6.2 Reflectivity curve of silver coated on glass substrate. . . . . . . . . . . . . . . . 45
6.3 Comparison of transmission curves among MEH-PPV film, organic DBR andtheir combination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.4 Sample with small silver dots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.5 Sample with non-uniform MEH-PPV film thickness. . . . . . . . . . . . . . . . . 47
6.6 Sample with thick MEH-PPV film, the device holding it is a vacuum chamber. . 47
6.7 Lasing measurement setup. A: green laser. B: half wave plate. C: polarized beamsplitter. D: bi-convex lens, f=75mm. E: sample to be measured. . . . . . . . . . 48
6.8 Detection device of the power meter. . . . . . . . . . . . . . . . . . . . . . . . . 49
6.9 Damage of the DBR on sample. A: caused by focused 1000Hz beam. B:causedby focused 50Hz beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.10 Damage of the DBR on another sample. . . . . . . . . . . . . . . . . . . . . . . 50
6.11 Damage of the DBR on small mirror dots sample. . . . . . . . . . . . . . . . . . 51
6.12 ko and ke value of BDAVBi, adjusted from [9]. . . . . . . . . . . . . . . . . . . . 52
6.13 k value of TAPC, adjusted from [9]. . . . . . . . . . . . . . . . . . . . . . . . . . 52
List of Tables
4.1 Solvents used for spin coating with corresponding results . . . . . . . . . . . . . . 21
6.1 Incident versus reflected laser intensity . . . . . . . . . . . . . . . . . . . . . . . . 49
vi
Abbreviations
ASE Amplified Spontaneous Emission
BDAVBi 4,4′-Bis[4-(diphenylamino)styryl]biphenyl
CP Conjugated Polymer
DBR Distributed Bragg Reflector
DFB Distributed Feed Back
MEH-PPV 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene
TAPC 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane
THF Tetrohydrofuran
VCSEL Vertical Cavity Surface Emitting Laser
vii
Chapter 1
Introduction
Ever since the first ruby laser was introduced in the 1960s, leading to a revolution in science,
multitudes of applications have been developed in different aspects. Plenty of them are found
in our daily life such as printers, anti-theft gates, smoke detections and others, making them an
indispensable part of our lives.
Materials developments have been playing a significant role in the new lasers development.
Conventional lasers are made of inorganic materials, for example, neutral-gas laser He-Ne laser,
molecular gas laser CO2 laser, dye lasers, Nd:YAG laser and other kinds [11]. Recently, the
fact that organic semiconductors combine different attractive optoelectronic properties makes
them competitive candidates in this field. Dating back to 1992, the first semiconducting laser
was reported by Moses [12], of which the amplifying medium is a dilute solution of the poly-
mer poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene), MEH-PPV, making use of the
interesting properties and ease of manufacturing of this semiconducting polymer. However,
light propagation in the organic semiconductor devices has always been controlled by inorganic
materials such as metals and dielectric insulators, the refractive indices difference in organic
semiconductors are barely considered to affect the propagation of light.
In this thesis, as indicated by the title, an organic laser is built, meaning that besides the
gain medium, organic semiconductors also occupy a place in the light propagation control in
this device. Conjugated polymer (CP) MEH-PPV is chosen as the gain medium as it has
been used in many organic lasers [4, 12]. Distributed Bragg reflectors (DBRs) consisting of
vacuum-deposited amorphous organic semiconductor films are selected for light control. Be-
cause of the linear molecular shape of the 4,4′-Bis[4-(diphenylamino)styryl]biphenyl (BDAVBi)
1
CHAPTER 1. INTRODUCTION 2
molecules, large optical anisotropy is achieved due to the different molecular orientation, thus
it has a much higher ordinary refractive index than the extraordinary refractive index. Tak-
ing advantage of BDAVBi’s ordinary refractive index in combination with a low-index isotropic
organic material 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC), a DBR with high
reflectance demonstrates that light control is obtained. Also, as these vacuum-deposited amor-
phous organic films have smooth surfaces, it will not cause troubles in stacking the films and
allows flexible thicknesses design.
Chapter 2 gives more insight into the organic materials, their structures and advantages as
components of lasers. In Chapter 3, general laser structures are introduced to understand the
principles of a laser and how it works, brief introduction of the amplifying medium, resonator
and pump is given. The two mirror like structures DBR and Distributed Feedback (DFB) Laser
are paid special attention to as they have been used in the development of organic lasers [7, 9].
Chapter 4 focuses on the amplifying medium. The detailed description of the material MEH-
PPV, the deposition technology to get uniform and thick film for required lasing threshold, and
the related properties as amplifying medium are present. In Chapter 5, the Bragg reflector,
built by stacking two organic materials, is treated. First, different aspects of properties are
tested by simulation, then the deposited reflector is measured to obtain the realistic parameters
of these properties, showing its ability to function well in the laser. In Chapter 6, the complete
organic laser is described, including the deposition, the metallic mirror, the pump method and
the attempt to get it lasing. Finally, Chapter 7 gives the general conclusion and perspective.
Chapter 2
Organic Semiconducting Materials
Among the daily life materials, there are semiconductors and organic materials. Semiconductors
such as Si and GaAs, which are inorganic materials, are common materials used in electronics
and optoelectronics, especially in integrated circuits. Some examples of integrated circuits
containing devices are laptops and cellphones. They have also been used in lasers[11]. As shown
in Fig.2.1, the band-gap between the valence band and the conduction band of the semiconductor
is relatively low, makes it possible for the realization of stimulated emission, the basic principle
of laser emission. Fig.2.2 gives examples of different semiconductors used for fabricating laser
Figure 2.1: The structure of energy bands in semiconductor materials.Source:http://www.optique-ingenieur.org/[1].
diodes with their band-gap and lattice constant. Semiconductors have crystalline structures, in
order to grow these materials on top of each other, their lattice constants have to be extremely
close, otherwise the mismatch of the lattice could cause defect in the growth and thick film is
beyond reach.
3
CHAPTER 2. ORGANIC SEMICONDUCTING MATERIALS 4
Figure 2.2: Overview of diverse III-V semiconductors in lattice constant vs.band-gap diagram.Reprinted from [2].
Figure 2.3: Schematic of line defection. Reprinted from [3].
Another widespread type of material is organic materials. For example polymers, which are
involved in applications as packaging and tapes. These materials have a huge range of structures,
resulting in various and tunable properties. Also, the amorphous structure of organic materials
makes it much easier to fabricate. Compared with the demanding epitaxial growth of inorganic
semiconductors, vacuum evaporation and solution spin coating for amorphous organic materials
are more accessible.
The organic semiconducting materials acquire the advantages of both semiconductors and or-
ganic materials, and as such are promising materials for laser development. The recent use
of organic semiconductors based on the organic light emitting diodes is already commercially
available. The semiconducting behavior of the organic materials originates from the alternating
CHAPTER 2. ORGANIC SEMICONDUCTING MATERIALS 5
π− and σ−bonding, converting the two p orbitals, causing the delocalization of the π− elec-
trons with respect to the conjugated bond, forming the freely moved carrier charges, for which
the energy needed is decreasing with the increasing number of conjugations in the material[13].
There are several types of organic semiconductors, as shown in Fig.2.4, defined according to how
the structural features are combined and the procession. Common ones are small molecules,
conjugated polymers (used as gain medium in this thesis), conjugated dendrimers[14] and spiro-
compounds[15–18].
Figure 2.4: Chemical structures of typical organic semiconductors used for lasers: (a)anthracene; (b) aluminum tris(quinolate); (c) generic poly(para-phenylene vinylene) deriva-tive; (d) generic polyfluorene derivative; (e) bisfluorene cored dendrimer; (f) spirolinkedoligomer. Reprinted from I.D.W.Samuel, G.A.Turnbull, “Organic Semiconductor Lasers”,
Chem.Rev.2007, 107,1272-1295[4].
Roughly speaking there are 5 advantages in organic semiconducting materials for the use in
lasers.
1. Due to the various structures, light can be emitted with different wavelengths, covering
from the visible spectrum to the near ultraviolet and infrared. By adjusting the structure
of the materials, tunable laser can be realized.
2. The absorption coefficients of these materials are strong and light can be absorbed in very
short distance. Since the gain factor is related to the absorption of light, large gain is
possible and high amplification is obtained.
CHAPTER 2. ORGANIC SEMICONDUCTING MATERIALS 6
3. The electronic energy levels of these materials can be subdivided into vibronic sublevels,
see Fig.2.5. The excited molecule will first experience a rapid transition to the lowest
vibrational level before the radiative decay. As a result, the absorption and fluorescence
spectra are separated, avoiding the absorption of emitting light.
Figure 2.5: Energy level diagram for organic semiconductor gain medium with vibra-tional levels. Reprinted from I.D.W.Samuel, G.A.Turnbull, “Organic Semiconductor Lasers”,
Chem.Rev.2007, 107,1272-1295[4].
4. The organic semiconductors are also able to transport charges, providing the possibilities
for electronic pump.
5. The simple fabrication methods as vacuum evaporation and spin coating (see more details
in Chapter 4 and Chapter 5) compared with inorganic semiconductors.
For more information, refer to references [2, 13–18].
Chapter 3
Laser
Before we start making a laser, it is necessary to review the general principles. In this chapter,
a brief introduction will be given to LASERs. Especially, two kinds of “mirror” DFB and DBR
will be discussed.
3.1 Basic Discipline
LASER is the acronym for Light Amplification by Stimulated Emission of Radiation, plainly
speaking, laser is an amplifying device for light, functioning based on one special mechanism,
stimulated emission, which leads to the special properties of a laser beam. In order to describe
the principle of lasing, quantum mechanics will be needed and atoms are considered as systems
with discrete energy levels, introduced by Albert Einstein in 1916. Fig.3.1 shows a model of
two-level system where light is amplified by stimulated emission. When a photon is incident on
an excited atom with energy identical to the energy difference between the two levels, the atom
will be stimulated to return to its ground state, at the same time, a new photon will be created.
Similarly, if a light beam is incident on this matter, a new beam will be created.
The new beam has the following properties.
• Monochromatic, the new light beam has the same frequency as the incident one.
• Collimated, the new light beam is emitted in the same direction as the incident one.
• Coherent, the new light beam has the same phase as the incident one.
7
CHAPTER 3. LASER 8
Figure 3.1: Schematic of stimulated emission by a two-level system. Reprinted fromProf.Nathalie Vermeulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
Due to these properties, it is obvious that the light beam is amplified, which is the basis of laser
action.
Besides stimulated emission, there also exists other phenomena in this system, absorption and
spontaneous emission, compared with stimulated emission, spontaneous emission is the tran-
sition of an excited quantum system to a lower energy state, the emission is not coherent. To
make the stimulated emission the dominant one, the number of atoms in the excited state must
be larger than in the ground state. This condition is called population inversion, maintained by
the pumping process. The material in which population inversion is realized acts as a amplifying
medium.
Another problem is that the amplification through the amplifying medium is rather small. To
solve this problem, the amplifying medium is enclosed between two mirrors, thus light reflected
by the mirrors can bounce back and forth in the medium. With tens of hundreds of times passing
through the amplifying medium, the light will gain enough amplification. The two mirrors form
the laser resonator. One of the mirrors has reflectance less than 100% to couple the light out of
the resonator, giving rise to lasing action.
To sum up, a laser should contain three basic blocks, a pump system, an amplifying medium
and a resonator. The schematic for a basic laser set up is shown in Fig.3.2.
The first laser is a ruby laser built by Ted Maiman. It was realized in 1960, a ruby crystal
constituted the amplifying medium. Fig.3.3 shows the structure of the first laser. The amplifying
medium for the first laser was a ruby crystal with chromium ions dissolved in as active atoms.
The flash tube around the ruby served as a optical pump. The two facet ends were polished
CHAPTER 3. LASER 9
and covered with silver to form the laser resonator. This configuration is still present in some
lasers nowadays.
Figure 3.2: Schematic of a basic laser set up. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
Figure 3.3: Schematic of the first laser. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
CHAPTER 3. LASER 10
3.2 Amplifying Medium
As explained in last section, light bounces back and forth in the amplifying medium, if con-
structive interference is achieved, standing waves will be built, which are called modes. The
condition for the wavelengths of these modes are expressed as:
Nλ = 2[L] (3.1)
Where [L] is the optical path length and N is the longitudinal mode number. From equation3.1,
it can be concluded that there can exist more than one mode. For example, a laser cavity with
[L] ≈ 6µm and λ ≈ 600nm, the number of mode is 10.
Fig.3.4 shows a simple model of gain mechanism in the amplifying medium. Consider a two-
Figure 3.4: Thin slice of an amplifying medium. Reprinted from Prof.Nathalie Ver-meulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
level system with population N1 and N2 for high energy level and low energy level, respectively.
Then the optical power increment is related with the incident power as:
dP = Pσ(N2 −N1)dz (3.2)
CHAPTER 3. LASER 11
where σ is defined to be the cross section, surface of one atom. Then the power growth after
propagation through the medium can then be expressed by an exponential equation:
P (z) = P (0)egz (3.3)
In equation 3.3, g is the gain factor and z is the path length. For one roundtrip, the path length
equals two times the laser cavity length. The expression for g is given by:
g = σ(N2 −N1) (3.4)
The unit of g is often given in cm−1 or mm−1. However, the interaction between the quantum
system and the incident light can also happen at frequencies close to but not exactly at the
resonant frequency, thus the cross section σ depends on the frequency f (Fig.3.5), resulting in
a frequency dependent gain according to equation 3.4, indicated by Fig.3.6. This phenomenon
is called line broadening. The line broadening indicates that as long as the modes are in
the gain region, they can be amplified, meaning that the gain modes are not limited to one
certain frequency, i.e. more than one modes can experience lasing action. Fig.3.6 also indicates
saturation, the decreasing of gain when power increases because the increasing power takes too
much energy from the pump and the population inversion decreases, not strong enough to keep
up the gain. Then the system gradually comes to the steady state, where the roundtrip gain
becomes 1. As a result, only part of the modes get to be amplified, shown in Fig.3.7. These are
the lasing wavelengths.
Figure 3.5: Cross section as a function of frequency. Reprinted from Prof.Nathalie Ver-meulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
CHAPTER 3. LASER 12
Figure 3.6: Small-signal saturated gain as functions of frequency. Reprinted fromProf.Nathalie Vermeulen(VUB), Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
Figure 3.7: Lasing modes of the laser. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter1[5].
3.3 Resonator
In this section, two special light reflection mechanisms-DFB and DBR-are present, as they are
used in organic lasers.
3.3.1 DBR
DBR is the acronym for Distributed Bragg Reflector, which is a layered structure consisting
of alternating quarter-wave layers of two different materials, a schematic view of this kind of
structure is shown in Fig.3.8. One high refractive index layer and one low refractive index layer
is called one pair of layers.
For the incident light with certain wavelength λ0, the thicknesses d1 and d2 of the two materials
with refractive indices n1 and n2 are calculated by the following formula:
n1d1 = n2d2 =λ04
(3.5)
CHAPTER 3. LASER 13
Figure 3.8: Schematic of Bragg reflector. Reprinted from Prof.Dries Van Thourhout,Prof.Roal Baets (UGent), Prof.Heidi Ottevaere (VUB), “Microphotonics”[6].
Then the reflected light will interfere constructively, a high reflectance is obtained at this par-
ticular wavelength. A graphic explanation is given below. As shown in Fig.3.9, light incident on
Figure 3.9: Reflection mechanism of Bragg reflector.
the surface will be reflected at the interfaces of different layers. The first reflection happens at
the interface between air and the first layer. Because the refractive index of air is smaller than
n2, there is a π shift in the reflection compared with the incident light. When the reflection at
the second interface goes back in the air, there is also a π shift because it experienced an extra
λ2n2
path length (no phase shift when light is incident from low refractive index material to high
refractive index material). Similarly, the phase shift for the reflection from the third and forth
layers is 3π and so forth, the differences between these phase shifts are always the multiples
of 2π. Then the reflected light at the air-first layer interface interferes constructively, a high
reflection is formed.
The peak reflection can be given using the matrix method,
RHR,max =
(1− (ns
na)(n1n2
)2N
1 + (nsna
)(n1n2
)2N
)2
(3.6)
CHAPTER 3. LASER 14
Where ns is the refractive index of the substrate, na that of air and N the number of periods.
It is seen that the reflection converges to 1 with increasing periods. Increasing the contrast
between the refractive indices of the two materials can also increase the reflection.
The frequency bandwidth for the stop band is given by equation3.7,
δf0f0
=4
πarcsin
(n2 − n1n2 + n1
)(3.7)
Where refractive indices contrast is again seen to play a role, larger contrast gives rise to wider
bandwidth.
Figure 3.10: Reflection curve for an ideal DBR.
Compared to conventional mirror, DBR serves as a wavelength selective element of laser. A
mirror reflects all the incident light regardless of the wavelengths, resulting in broadband modes,
while a narrow band of wavelengths is achieved by using a DBR. Fig.3.10 shows a reflection
curve for a DBR, the refractive indices for the two layer materials are 2 and 1, respectively and
the number of pairs is set to be 5. The thicknesses of the layers are calculated by equation 3.5
assuming the wavelength is 550nm. The bandwidth is around 300nm wide. More information
shall be given in Chapter 5.
Fig.3.11 presents a Vertical Cavity Surface Emitting Laser (VCSEL) with DBRs, the cavity
length is the thickness of the amplifying medium. This structure is similar to the structure we
will use in this thesis.
CHAPTER 3. LASER 15
Figure 3.11: Schematic of a VCSEL. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter7[5].
3.3.2 DFB
DFB is the acronym for Distributed Feedback. The principle is similar to DBR. For DFB laser,
there is a diffracting grating below or above the active layer, see Fig.3.12. By tuning the period
of the grating, it can act as a Bragg reflector, light wave with desired wavelength then can
bounce back and forth in the cavity. Different from the DBR case, the cavity length is in the
surface parallel with the DFB, direction is dependent on the choosing of facets. When used in
an organic laser, the organic film itself can form the polymer layer with an average thickness of
∼ 100nm in this structure[7], as shown in Fig.3.13.
Figure 3.12: Schematic of a DFB laser. Reprinted from Prof.Nathalie Vermeulen(VUB),Prof.Geert Morthier(UGent), “Lasers”, Chapter7[5].
CHAPTER 3. LASER 16
Figure 3.13: Waveguide structure of DFB laser. Reprinted from G.A.Turnbull, P.Andrew,W.L.Barnes, I.D.W.Samuel,“Operating characteristics of a semiconducting polymer laser
pumped by a microchip laser”,2003[7].
3.4 Conclusion
In this chapter, the lasing mechanism was treated which originates from stimulated emission
and amplified by the lasing mode bouncing back and forth in the laser cavity. In order to gain
stimulated emission, a pump laser is needed to provide population inversion for the amplifying
medium, which is made of organic material in this thesis. The laser resonator is enclosed by two
mirrors for the feedback of light. The optical length of the laser cavity determines the lasing
modes.
Chapter 4
Amplifying Medium–MEH-PPV
In this chapter, the fabrication and characterization of the amplifying medium–MEH-PPV–are
described. The fabrication is realized by the deposition of the material by spin-coating wherein
different solutions were tested. The related optical properties, more specifically, emission and
transmission properties of this material are discussed.
4.1 Introduction of Amplifying Material
As mentioned in Chapter 1, organic semiconductors such as conjugated polymers are of great
interest for its combination of both semiconductor and polymer properties, thus being used
in various electronics and optoelectronics applications, for example light emitting diodes[19]
and thin film transistors[20]. Substituted PPV’s emerge as promising candidates as laser am-
plifying medium because of their large absorption cross-section and high photoluminescence
quantum yield, among which MEH-PPV (shown in Fig.4.1) has been studied as amplifying
medium for organic lasers[4, 12] and has proven to be a versatile semi-conducting polymer for
use in optoelectronic devices[21], research have also been conducted regarding to the deposition
methodology[22, 23]. The solubility of MEH-PPV in some common organic solvents comes from
the asymmetric alkoxy side chains, allowing deposition techniques such as spin coating and drop
casting. In this thesis, MEH-PPV material is chosen as the active film for the organic DBR
laser.
17
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 18
Figure 4.1: Chemical structure of MEH-PPV. Reprinted from Operating characteristics of asemiconducting polymer laser pumped by a microchip laser”,2003[[7].
4.2 Deposition
4.2.1 Spin-coating Process
Spin coating has been used for several decades for applying uniform thin films to flat substrates.
The general procedure is presented in Fig.4.2. An excess amount of a solution is dropped onto
a clean substrate mounted on a spin coater. The substrate is then rotated at a high speed,
causing the solution to spread out on it. Excess solution will eventually spin off the edge of
the substrate until the desired thickness is achieved. The applied solvent is usually volatile and
evaporation happens during the process. There are four basic stages of the process:
1. Application of the solution on the substrate.
2. Acceleration of the substrate to the desired rotation speed.
3. Constant spinning, spread of fluid.
4. Constant spinning, evaporation of solvent
Figure 4.2: Process of spin coating. First the solution applying, then the spin makes anuniform, thin film. Reprinted from [8].
Final film thickness is related to the spin speed, solution concentration and also the viscosity,
in general, for polymer materials, slower speeds and higher concentrations give rise to thicker
films[24].
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 19
4.2.2 Literature Study and Realization
As introduced in Chapter 3, light bouncing back and forth in the amplifying medium will
experience amplification. In order to maintain the steady sate regime of the laser, the total loss
should equal the total gain, which yields:
(1− T1)(1− T2)e2gl = 1 (4.1)
Where T1 and T2 represent the losses at the two mirrors, respectively, g represents the gain factor
and l is the laser cavity length. To achieve this condition, as calculated from equation4.1, the
film thickness of MEH-PPV should be 500nm, with the gain factor used from[25], and assuming
the pump energy is around 10µJ/cm2. For the realization of the desired thickness, different
spin coating settings and several solvents have been tested. The spin coating was conducted on
a glass substrate which was cleaned before use. The cleaning was also done in the cleanroom.
Procedures for cleaning are expressed below.
• Put glass substrates in the substrate holder, immerse the holder in a beaker filled with
soap water. Put the beaker in ultrasonic for 15 minutes.
• Rinse with DI water and dry. Put the holder in another beaker and immersed with
Acetone, put in ultrasonic for 15 minutes.
• Rinse with DI water and dry. Repeat the previous procedure with IPA.
• Rinse with DI water and dry it thoroughly.
The MEH-PPV powder was purchased from SIGMA−ALDRICH.
4.2.2.1 Chloroform
First, chloroform as a solvent was chosen, for it has been used in dissolving MEH-PPV for
spin coating solution[22]. The chloroform was purchased from SIGMA−ALDRICH. To gain
a general idea of the thickness of the film, a piece of tape was attached to cover part of the
substrate, resulting in a no-film region. Starting concentration was 1mg/ml, the spin speed
ranged from 1400rpm to 3000rpm with spin time 15 seconds. The higher speed provides more
uniform and thinner films. The sample with spin speed 3000rpm is shown in Fig.4.3. Part of
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 20
Figure 4.3: Spin coating film of MEH-PPV dissolved in chloroform with concentration1mg/ml, spin speed:3000rpm.
the substrate is covered with the film, the boundary is marked with red dash line. It is obvious
that the boundary is barely seen, meaning the film is too thin.
The second concentration tested was 3mg/ml. The spin speed varied from 1900rpm to 2800rpm
and the spin time stayed the same. One obtained sample with spin speed 2300rpm is shown
in Fig.4.4. Compared with Fig.4.3, the film is visibly thicker and quite uniform. However, the
Figure 4.4: Spin coating film of MEH-PPV dissolved in chloroform with concentration3mg/ml, spin speed:2300rpm.
thickness is still not satisfactory thus can not fulfill the requirement. More research in literature
is needed.
4.2.2.2 Tetrohydrofuran
After a more in depth literature study, tetrahydrofuran (THF) emerged as another solvent
material[26]. MEH-PPV dissolved in THF with concentration 7.65mg/ml was made. The THF
was purchased from SIGMA−ALDRICH. Two spin speeds–600rpm and 800rpm–were tried
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 21
according to the article[26] and the films seem to be better than chloroform. Fig.4.5 shows the
sample with spin speed 600rpm and spin time 15 seconds. The red color is darker than before
Figure 4.5: Spin coating film of MEH-PPV dissolved in THF with concentration 7.65mg/ml,spin speed:600rpm.
and the measured thickness is ∼ 200nm, which fulfils the requirement on the film thickness.
4.2.3 Solution Chosen
The total results are shown in Table 4.1, after comparison, THF was chosen as the solvent for
the amplifying medium deposition.
Table 4.1: Solvents used for spin coating with corresponding results
Solvent Chloroform THF
Concentration 1mg/ml 3mg/ml 7.65mg/mlSpin speed 2000rpm∼3000rpm 600rpm, 800rpmThickness too thin thin acceptable, ∼200nmUniformity uniform uniform uniform
4.3 Optical Characterization of Amplifying Medium
An amplifying medium is the place where laser light is produced and amplified, the emission
and transmission properties of the amplifying medium are of great importance and related to
the characteristic of the laser. In the following, these two properties of the MEH-PPV film are
discussed.
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 22
4.3.1 Emission Properties
The emission wavelength of MEH-PPV is relatively long according to previous research[26], in
this experiment, a green laser PNG −M02010 − 130 which is a pulsed laser with maximum
repetition rate 1000Hz, pulse width smaller than 400ps and working wavelength 532nm served
as the pump, a spectrometer Ocean Optics QE Pro was used for the emission spectrum. The
setup is shown in Fig4.6. As shown in Fig.4.6, the laser beam going out of the green laser
Figure 4.6: Experimental set up for measuring emission spectrum. A: lens with focal length50mm. B: sample position. C: lens system to converge the emission light. D: fiber of spectrom-
eter. E: filter.
shining on the sample will excite the material to obtain the emission beam. The pump laser
beam first goes through the combination of a half wave plate and a polarized beam splitter.
As the polarized beam splitter only allows light with a certain polarization to go through and
the half wave plate is able to change the polarization of light, this combination allows for the
tuning of the outcome intensity of the laser beam. Position A located a lens with focal length
50mm, sliding of the lens will change the laser spot diameter on position B, the MEH-PPV
sample, which was made by spin coating the MEH-PPV in THF mixture with concentration
7.65mg/ml. At position C is a lens system consists two lenses with focal length 50mm and
75mm respectively, the sample at position B sits at the focus. In this way the system transmits
the emission as much as possible to position D. The sample (position B) is put at an angle to
avoid the strong green laser beam destroying the spectrometer. E is a filter to filter out the
lasing wavelength from the green laser, trying to eliminate the influence of the remained laser
beam, there is also a pair of goggle for the same purpose. The obtained emission spectra are
shown in Fig.4.7 and Fig.4.8.
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 23
Figure 4.7: Emission spectrum of MEH-PPV film.
Figure 4.8: Emission spectrum of MEH-PPV film with ASE peak.
When exited by the green laser beam with repetition rate 1000Hz and put in an appropriate
position, the MEH-PPV emission spectrum can be reconstructed by the spectrometer with
wavelength from around 550nm to a little larger than 730nm, shows a broad emission band. By
carefully tuning the sample position to gain the maximum pump energy, population inversion
may occur and amplified spontaneous emission (ASE) is reached, that is the high peak shown
in Fig.4.8 at around 620nm. The amplified spontaneous emission peak is relative narrow with
bandwidth around 10nm and indicates the possibility for the lasing action.
The reflection band of DBR should also be between 550nm and 730nm, corresponding with the
emission band.
Under the light exposure, there is also bleaching effect which destroys the molecules thus re-
duces the emission intensity (Fig.4.9). To verify this effect, the MEH-PPV sample was kept
under exposure and the emission spectra were stored every 20 seconds for 100 times, thus a
total exposure time 2000 seconds. The average emission intensity values around the peak was
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 24
normalized and drawn as a function of time. By changing the position of the lens at spot A in
Fig.4.6, relation between the pump laser intensity and bleaching effect was discovered, which is
drawn in Fig.4.10. The spectrometer used for this detection was QWave compact USB. The
Figure 4.9: Bleached sample. The round spots indicate the places that have been bleached.
Figure 4.10: Bleaching effect.
bleaching effect is stronger for higher laser intensity as shown above. To solve this problem,
the sample was put in a vacuum holder to weaken the effect. The emission spectra were stored
every 1 second for 600 times (in total 600 seconds under exposure) both with and without vac-
uum. Then the normalized average emission intensity value around the peak region was drawn
as a function of time, as shown in Fig.4.11. Because of the change of positions under the two
cases, the starting emission intensities were different. For the case with vacuum, the starting
intensity was 1500.6 (absolute value, as in Fig.4.7 and Fig.4.8) and for without vacuum was
1104.3, meaning the bleaching effect should be stronger for the vacuum case. On the contrary,
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 25
Figure 4.11: Comparison of bleaching effect with/without vacuum.
as in Fig.4.11, with vacuum the emission decline is 2.74% while for its counterpart is 22.9%.
The bleaching effect is indeed suppressed under the vacuum condition.
4.3.2 Transmission Property
The transmission property of MEH-PPV film is measured in PerkinElmer UV/V IS Spectrometer,
the collected data was plot and compared with curve abstracted from an article[22], of which
the MEH-PPV film is spin coated from chloroform mixture on the glass substrate. Results are
shown in Fig.4.12. Also, a transmission spectrum of a glass substrate is shown in Fig.4.13.
Figure 4.12: Transmission curve for MEH-PPV, extracted from article (with chloroform assolvent) and measured from experimental samples.
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 26
Figure 4.13: Transmission curve of a glass substrate.
The samples being measured are MEH-PPV film spin coated from THF mixture with concentra-
tion 7.65mg/ml and different spin speeds and spin times, as shown in the legend. The tendencies
of the curves are similar with an absorption dip around wavelength 500nm, however for THF
as solvent, the absorption range is noticeably wider. The phenomenon that the absorption
spectrum of MEH-PPV film varies when spin coated from different solvents was also observe in
article[22]. The different concentrations of the solution, different solvate of the polymer chain
and different molecule weights can all take on a role in leading to various spectra. When com-
pared with emission spectrum, it verifies that these two spectra are separated, avoiding the
absorption of emitting light, shown in Fig.4.14, as mentioned in Chapter 2.
4.4 Result and Discussion
In this chapter, a detailed discussion about the amplifying medium–MEH-PPV–was given, from
the properties of this material to its deposition then the optical characteristics regarding to
absorption and gain. MEH-PPV is a promising organic material takes on the role as a laser
amplifying medium as it contains advantages of both semiconductors and polymers. The powder
of MEH-PPV dissolved in organic solvent THF formed a solution with concentration 7.65mg/ml,
from which the solid net uniform MEH-PPV film cast could end up in thickness of around 200nm,
thick enough to provide required gain. Excited by a pump laser, this film has shown a broad
spontaneous emission band ranges from 550nm to 730nm and ASE has also been realized in
certain condition, indicating the realization of population inversion. The absorption band is
CHAPTER 4. AMPLIFYING MEDIUM–MEH-PPV 27
Figure 4.14: Comparison of transmission and emission spectra of MEH-PPV film
also well separated from the emission band, avoiding absorbing of the emitted light. Concluded
above, MEH-PPV has proven to be a qualified amplifying medium material. The next step shall
be the design and fabrication of corresponding DBR structure.
Chapter 5
Resonator Mirror–DBR
As described in Chapter 3, a DBR serves as the resonator mirror that only reflects light at a
certain wavelength range. In this chapter, properties of the DBR and various influencing factors
are treated. Then, special attention is given to the organic DBR used in the organic laser.
5.1 Material Chosen and Composition
Organic semiconductors have different refractive indices and anisotropies depending on the
molecule structures and molecular orientation. By controlling the refractive indices of the
organic semiconductors, an organic DBR with the desired properties, that consisting of these
amorphous vacuum-deposited organic semiconductor films can be used for light control, thus
realizing the mirror function with organic materials.
As mentioned in section3.3.1, a larger refractive index contrast contributes to higher mirror
reflectivity and wider bandwidth. BDAVBi is an organic semiconductor with molecular orien-
tation in the horizontal direction in vacuum-deposited amorphous films because of its linearly
molecular shape. Since it has a certain molecular orientation, birefringence takes place, leading
to a much higher ordinary refractive index no in the horizontal direction than the extraordinary
refractive index ne in the vertical direction. This high no provides a possibility for a high refrac-
tive index layer in our DBR. On the other hand, TAPC molecules have an isotropic orientation
and consequently isotropic optical constants[27, 28]. Because of its saturated hydrocarbons, the
lower molar refractions give rise to a relatively low refractive index of this material[29]. A low
28
CHAPTER 5. RESONATOR MIRROR–DBR 29
refractive index layer for the DBR is obtained. The molecular structures of BDAVBi and TAPC
are shown in Fig.5.1. Fig.5.2 shows the difference in refractive indices of these two materials,
it can be seen that at wavelength 600nm (in MEH-PPV emission range), the difference is ap-
proximately 0.3. Because of a smooth film surface, stacking films of these two materials makes
a functioning organic DBR.
Figure 5.1: Molecule structures of BDAVBi and TAPC, adapted from[9].
Figure 5.2: Difference in refractive indices for BDAVBi and TAPC. The large difference inrefractive indices of these two materials at wavelength 600nm could make a good organic DBR.
5.2 Properties Verification by Simulation
In this section, different factors that influence the behavior of the DBR are discussed, through
which a general idea about the behaviour of the DBR is acquired and the fabrication variation
can be analyzed. First, the simulation was carried out on a simple model of a DBR, isotropic
and without dispersion. Then the parameters for the real materials were added in the code
CHAPTER 5. RESONATOR MIRROR–DBR 30
to simulate the realistic organic DBR. Finally, a comparison between this simulation and the
measurement of the fabricated DBR was performed.
5.2.1 Simulation Method
The simulation method is implemented in Matlab, wherein uniaxial anisotropic films are rep-
resented by their refractive indices, assuming the optical axis is perpendicular to the surface.
The number, the thickness of the films and the incident angle of the light can be modified. As a
result, the refection as well as the transmission can be plot as function of wavelength. Detailed
explanation on the simulation method can be found in[30].
5.2.2 An Ideal Referential DBR
To begin with, a simple model of DBR was simulated. In this case, the high and low refrac-
tive indices were set with certain values, respectively. The films were assumed to be isotropic
without dispersion, that is, the refractive indices stay the same with different incident angle
and are wavelength independent. The surrounding environment was air with refractive index 1.
The factors under simulation are contrast in refractive indices of the layers, number of layers,
thickness uncertainty of layers and variation of incident angle.
5.2.2.1 Contrast in Refractive indices
In this simulation, the incident angle was 0 degree (in the code it was set to be 0.0000001 to
avoid mistake), wavelength range was 300nm–800nm and there were 5 pairs of layers. The
design wavelength for this DBR was 550nm. Three different contrasts, 1.51 , 2
1 , 2.51 were tested
while other parameters remained unchanged during the simulation. As shown in Fig.5.3, with
larger contrast, not only the reflectivity is higher but also the bandwidth is wider, which agrees
with the theory explained in Chapter 3.
5.2.2.2 Number of Layer pairs
Then the refractive indices were fixed to be 2 and 1 for the high refractive index layer and the
low refractive index layer, respectively. The variable for this simulation is the number of pairs
CHAPTER 5. RESONATOR MIRROR–DBR 31
Figure 5.3: Reflectivity of DBR versus wavelength with different refractive indices contrast.Higher contrast gives higher reflectivity and broader bandwidth.
and the other parameters were the same as the simulation in section 5.2.2.1. Fig.5.4 shows
the result. From Fig.5.4, it is clear that with more layers, the reflectivity is higher and the
Figure 5.4: Reflectivity of DBR versus wavelength with different number of pairs. The morethe number of pairs, the better the reflectivity is.
shape of bandwidth is more concentrated, which corresponds with equation 3.6 in Chapter 3,
however the width does not change very much, compared with the contrast difference. With
other optical parameters the same as this model, 5 pairs of layers can already result in a well
performed DBR.
CHAPTER 5. RESONATOR MIRROR–DBR 32
5.2.2.3 Thickness Uncertainty
The third variable is the thickness, which is also the optical path length in this case. During
the fabrication, exactly the same thicknesses as designed may not be guaranteed, thus the
uncertainty of thickness is of importance to be analyzed. A schematic representation of this
phenomenon is shown in Fig.5.5. The number of pairs was fixed to be 5, for the parameters set as
Figure 5.5: Schematic description of thickness uncertainty of layers.
above, from equation 3.5 in Chapter 3 the optimal thickness for the high refractive index layer
and the low refractive index layer were calculated to be 68.75nm and 137.5nm, respectively.
A 10nm variation was introduced to these layers both separately and together. Result is in
Fig.5.6. With increasing thickness, the curve shifts to the longer wavelength. This can be
explained from equation 3.5, the thickness is proportional to the design wavelength, when the
thickness increases, the design wavelength will increase accordingly, resulting in the curve shift.
It is also shown that for the same thickness variation of 10nm, when the variation is on the
high refractive index layer, the curve shift is around 33.5nm while for its counterpart the shift is
around 16.5nm. This is because the high refractive index layer is thinner compared with its low
counterpart, so the relative change in thickness for it is actually higher. If both layers become
10nm thicker, the total influence is simply the sum of the two single influence.
CHAPTER 5. RESONATOR MIRROR–DBR 33
Figure 5.6: Reflectivity of DBR versus wavelength with thickness uncertainty, with thickerlayers than the design, reflectivity curve shifts to longer wavelength.
5.2.2.4 Variation of Incident angle
The last factor that is going to be discussed is the incident angle. Incident light cannot always be
perpendicular to the surface of DBR, as shown in Fig.5.7, different incident angles also influence
the reflectivity. Keeping all the other parameters set to the original values, three incident angles
Figure 5.7: Schematic description of different incident angles.
were chosen, 0 degree, 20 degrees and 40 degrees. The simulation result is shown in Fig.5.8. It
can be seen that the curve shifts to shorter wavelength. This can be explained as follows. The
high reflection comes from the constructively interference of the reflected wave, so the phase
difference between wave reflected at different layers has a certain value, that is, the multiple of
CHAPTER 5. RESONATOR MIRROR–DBR 34
Figure 5.8: Reflectivity of DBR versus wavelength with incident angle variations, with largerincident angle, the curve shifts to shorter wavelength.
2π. The phase difference caused by the propagation of light is expressed as:
φ =2π
λ0ndcosθ (5.1)
Where λ0 is the design wavelength, d is the layer thickness and θ is the incident angle. When
incident angle increases, to maintain the certain value of the phase difference, design wavelength
has to decrease. Fig.5.9 gives a more intuitive explanation. When there is an incident angle, the
Figure 5.9: Change in optical path length because of incident angle, which becomes shorterthan before. Reprinted from[6].
optical path length changes from |AB| to |AB′|, since the phase difference is defined from the
distance between the phase fronts through points A and B. As a result the optical path length
decrease, as explained in section5.2.2.3, the curve should shift to the shorter wavelength.
CHAPTER 5. RESONATOR MIRROR–DBR 35
5.2.3 Organic DBR
After simulation with a simple DBR model, a general idea about the properties of DBR has
been obtained. Now a real organic DBR is considered. As mentioned above, the materials for
this organic DBR are BDAVBi and TAPC, where BDAVBi plays the role of the high refractive
index layer and TAPC the other. Both materials are dispersive and anisotropic (in fact, the
ordinary refractive indices and extraordinary refractive indices of TAPC are so close that this
material can be considered isotropic), meaning that different refractive indices are experienced
by light with different wavelengths and incident angles. According to the emission range of
MEH-PPV mentioned in Chapter 4, design wavelength was set to be 619nm and the thicknesses
were calculated for this wavelength and the corresponding ordinary refractive indices. The
surrounding environment for the DBR is air and the substrate is MEH-PPV, which is also a
dispersive material, so the refractive indices of MEH-PPV should also be considered. After
modifying the code, similar simulations as for the simple DBR case were performed again.
5.2.3.1 Number of Pairs of Layers
For this simulation, the incident angle was 0 degree and the wavelength range was from 500nm
to 700nm as shown in Fig.5.10. Compared with Fig.5.4, the reflectivity for this realistic organic
Figure 5.10: Reflectivity of organic DBR versus wavelength with different number of pairs.Same explanation as for Fig.5.4.
DBR is much smaller and the bandwidth is thinner, around 100nm. Because the contrast of
CHAPTER 5. RESONATOR MIRROR–DBR 36
refractive indices at wavelength 619nm is 1.891.66 ≈
1.141 instead of 2
1 . This again indicates that
contrast of refractive indices plays an important role in the reflectivity.
5.2.3.2 Thickness Uncertainty
As simulated above, the change in thickness can shift the reflectivity curve. By tuning the
thickness, the desired curve can be obtained. For this organic laser, the pump is a green laser
with wavelength 532nm illuminating the amplifying medium, thus the DBR reflectivity should be
as low as possible for better absorption of the pump energy. The original calculated thicknesses
for BDAVBi and TAPC were 81.7nm and 93.1nm, respectively. If 6nm increment is added on
the high refractive index layer, i.e. the thickness for BDAVBi layer is 87.7nm the minimum
reflectivity can be reached at wavelength 532nm. As shown in Fig.5.11. For this simulation, the
incident angle was 0 degree and the number of pairs was 7. From the figure, the reflectivity for
Figure 5.11: Reflectivity of organic DBR versus wavelength with thickness uncertainty, in-creased thickness of the high refractive index layer makes the reflectivity at 532nm reach the
bottom of the curve.
wavelength 532nm is now 3.894%, also the curve has shifted to longer wavelength, the reflectivity
for wavelength 619nm changes from 72.87% to 65.93%, which is still a high value.
CHAPTER 5. RESONATOR MIRROR–DBR 37
5.2.3.3 Variation of Incident Angle
Last is the variation of the incident angle. Since the lasing direction is perpendicular to the
surface, it will cause some trouble if the incident pump light is also vertical. However, in order
to not change the shape of the reflectivity curve too much, small angles are simulated here.
For this simulation, the number of pairs is 7 and the thicknesses for the layers are 87.7nm and
93.1nm, respectively. The resulting curves are shown in Fig.5.12. From the figure, if the incident
Figure 5.12: Reflectivity of organic DBR versus wavelength with different incident angles,same explanation as for Fig.5.8.
angle is 20 degrees, the curve shifts about 10nm back to shorter wavelength, then the reflectivity
for wavelength at 532nm becomes 9.507% and for 619nm becomes 72.28%, both are higher than
before. However this shift back makes the reflectivity of the pump laser at wavelength 532nm
higher than before, it is still tolerant. Besides, a higher reflectivity for 619nm is beneficial.
5.2.3.4 Conclusion
As a conclusion for the sections above, a organic DBR with 7 pairs of layers, 87.7nm for the
thickness of BDAVBi layer and 93.1nm for the TAPC layer could make a good DBR, the
expected reflectivity curve is shown in Fig.5.13 with reflectivity for wavelength 532nm 3.894%
and for wavelength 619nm 65.93%.
CHAPTER 5. RESONATOR MIRROR–DBR 38
Figure 5.13: Reflectivity of organic DBR.
5.3 Fabrication of Organic DBR
To measure the properties of the deposited organic DBR, it is first deposited on clean glass
substrate instead of on the amplifying medium. The glass substrates are cleaned using the
same procedures described in Chapter 4. The high and low refractive index layer materials
are deposited on the substrate alternatively by thermal evaporation, which is a physical vapor
deposition method. One example is given in Fig.5.14.
Figure 5.14: Physical vapor deposition chamber. Reprinted from[10].
CHAPTER 5. RESONATOR MIRROR–DBR 39
This process takes place in a vacuum chamber, materials in the source boats are heated and
evaporated, and then goes through the chamber to the substrate held on the top of the chamber.
Near the substrate holder there is also a thickness monitor which senses the deposited thickness.
Through this monitor, thicknesses of 83nm and 95nm for BDAVBi and TAPC layer are defined
(these are calculated assuming the design wavelength is 629nm) with 5 pairs of layers. Two
different types of samples are made according to the relative position between the substrate
holder and the source boats. When the substrate holder is exactly on the top of the source
boats, the layers are made uniform. However, if there is horizontal displacement, the thickness
for the layer changes continuously. As shown in Fig.5.15 and Fig.5.16.
Figure 5.15: DBR sample with uniform thicknesses.
Figure 5.16: DBR sample with continuously changing thicknesses.
It is noticed that for Fig.5.16, there is color gradient present, which is a result of thickness
changes.
CHAPTER 5. RESONATOR MIRROR–DBR 40
5.4 Transmission Measurement and Comparison
To make sure that the fabricated DBR functions well, measurements about the reflectivity
properties of it were done and then compared with the simulation results. In this measurement,
PerkinElmer UV/Y IS Spectrometer was used and the transmissions were measured. For
the convenience of comparison, the transmission data was transferred into reflectivity data by
assuming transmission+ reflectivity = 1. Notice that the obtained reflectivity is not exactly
the same as its real value because mechanisms such as absorption was simply ignored in this case.
Also for the two different samples described above, two types of measurements were conducted.
5.4.1 Angle Variation Measurement
For the sample shown in Fig.5.15, measurements were carried out with different incident angles.
To do so, each time the sample was titled a little before the measurements to gain different
Figure 5.17: Simulation reflectivity of the organic DBR with different angles.
incident angles. The simulation results for three angles–0 degree, 18.9 degrees and 27.5 degrees–
are shown in Fig.5.17. These three angles were chosen to be consistent with the experiment
measurements.
As can be predicted, the curve shifts to the shorter wavelength. Then the measured transmission
data was transferred and plot in the same graph with the simulation results for different angles
respectively. These are shown in Fig.5.18, Fig.5.19 and Fig.5.20.
CHAPTER 5. RESONATOR MIRROR–DBR 41
Figure 5.18: Comparison between experiment and simulation: angle 0.
Figure 5.19: Comparison between experiment and simulation: angle 18.9.
From the three comparison graph, the experimental results are observed to shift to the shorter
wavelength compared with the simulation and the reflectivity is lower, for all tested angles. This
shift may result from the thickness variation during the fabrication. Besides, the organic DBR
was deposited on glass substrate, of which the refractive index was not included in the code, so
that for the real DBR the contrast for last layer is smaller, this may explain the descent of the
reflectivity. However, with changing angle, the relative position and shape of the experimental
result to the simulation one stay almost the same, which indicates that the experimental results
agree with the simulation in terms of angle variation. A more intuitive comparison is shown in
CHAPTER 5. RESONATOR MIRROR–DBR 42
Figure 5.20: Comparison between experiment and simulation: angle 27.5.
Fig.5.21.
Figure 5.21: Comparison between experiment and simulation for all three angles.
5.4.2 Thickness Uncertainty Measurement
For the sample shown in Fig.5.16, different measurements were performed at different position
along the direction of thickness changing (or in the horizontal direction from right to left as
shown in the Fig.5.16). Measurements were taken every 2mm and 5 sets of data were saved and
plotted in the same graph, which is shown in Fig.5.22.
CHAPTER 5. RESONATOR MIRROR–DBR 43
Figure 5.22: Measurements for sample 2, different thickness were taken at different position.
Concluded from the figure, the curve shifts to shorter wavelength, meaning along the horizontal
direction from right to left, the thickness is decreasing. The fabricated DBR also shows the
property with the thickness changing as simulated above.
5.5 Result and Discussion
In this chapter, an organic DBR was built and estimated. DBR is a layered structure consisting
of alternative high refractive index and low refractive index quarter-wave layers. Materials were
chosen to be BDAVBi and TAPC to serve as high and low refractive index layer respectively,
because the difference in their refractive indices at emission wavelength are quite high. By sim-
ulation of DBR through Matlab, the influence of the contrast between two refractive indices,
different numbers of layers, thickness uncertainty and varies incident angles were analyzed for
future reference. For the purpose of realizing high reflectivity in emission range and low reflec-
tivity for pump laser at 532nm, the thickness for BDAVBi was set to be 87.7nm and for TAPC
was 93.1nm, 7 pairs of layers were fabricated in a vacuum chamber through physical vapor
deposition. The transmission measurement results of the fabricated DBR suggested compliance
with the simulation, the organic DBR functioned well. On the basis of these experiments, then
the organic DBR can be deposited on the amplifying medium MEH-PPV with silver coating
substrate, a laser resonator is then completed.
Chapter 6
Organic DBR Laser
From previous chapters, the properties of the amplifying medium–MEH-PPV and the resonator
mirror–DBR have been discussed and verified to be able to function as components of a laser.
The next step should be fabricating these components together to form an organic laser. In the
chapter, the fabrication of the laser and the procedure for getting lasing are present.
6.1 Fabricated Structure of Organic DBR laser
6.1.1 Silver Coated Substrate
As introduced in Chapter 3, there should exist two mirror structures for a laser resonator, in
this case, silver coating was chosen for its high reflectivity. Two different kinds of samples were
made. For the first kind, the silver was coated in the form of small round dots, as shown in
Fig.6.1. For each sample, there are 25 dots with equal distance to each other. With all these
Figure 6.1: Silver coating present as small round dots.
44
CHAPTER 6. ORGANIC DBR LASER 45
dots, lasing tests can be conducted at different positions of one sample. For the second kind,
the silver was covered all over the glass substrate.
In order to make the mirror sample cleaner, the samples were once put in a UV ozone system in
90◦C for 15 minutes. This method was discarded after realizing it damaged the silver surface.
The reflectivity curve of silver was drawn in Fig.6.2, generated in Matlab considering the reflec-
tive indices of silver in the range from 500nm to 800nm. As reflected in Fig.6.2, the reflection
in this range is around 100%.
Figure 6.2: Reflectivity curve of silver coated on glass substrate.
6.1.2 MEH-PPV and DBR
Once the silver samples were prepared, the MEH-PPV was applied by spin coating as described
in Chapter 4 and DBR by vacuum deposition on the amplifying medium with the thicknesses
for BDAVBi and TAPC defined in Chapter 5.
The pump laser used was a green laser PNG−M02010−130, the peak power of which is around
30µJ per pulse. Based on this, assuming the intensity of laser on the sample is 25µJ/cm2 (which
should be a relative low value because the laser spot on the sample has a diameter much smaller
than 1cm2), the reflectivity for the silver mirror is 100% and for the DBR is 60% and the gain
factor from article[25], according to equation 4.1, the thickness required for the MEH-PPV film
is around 200nm, which has been achieved in Chapter 4.
CHAPTER 6. ORGANIC DBR LASER 46
For the sample with small silver dots, the MEH-PPV film was spin coated from MEH-PPV/THF
solution with concentration 7.65mg/ml. The spin speed was set 600rpm and two spin time 15
seconds and 45 seconds were used. As a consequence, uniform thin MEH-PPV films were
obtained, the thickness was around 200nm.
For the sample with silver covered all over the substrate, the solution used was also MEH-
PPV/THF with concentration 7.65mg/ml. However, the spin speed was adjusted to 300 −
400rpm to gain non-uniform thicker films for more modes selection (see equation 3.1). One
thick irregular film was obtained by drop-casting.
Then the organic DBR with 7 pairs of layers were deposited on top of the dry MEH-PPV
film in a vacuum chamber. The transmission curve of the combination of the two components
was measured using PerkinElmer UV/V IS Spectrometer, the same as when measuring the
transmission curves of MEH-PPV and DBR separately. Comparison of these transmission curves
is shown in Fig.6.3. It is seen that the combination curve has values near 0 as wavelengths shorter
Figure 6.3: Comparison of transmission curves among MEH-PPV film, organic DBR and theircombination.
than 550nm because of the absorption of MEH-PPV in this region. Then the curve goes up and
transmission of around 40% can be reached at wavelength 620nm, at this point, the absorption
of MEH-PPV film is already quite low, meaning that the reflection of DBR is high enough.
These two materials are still able to function normally when deposited together.
CHAPTER 6. ORGANIC DBR LASER 47
6.1.3 Fabricated Samples
With silver and organic DBR serve as two mirrors and MEH-PPV as amplifying medium, an
organic laser structure can be built. Below are the complete organic lasers.
Figure 6.4: Sample with small silver dots.
Figure 6.5: Sample with non-uniform MEH-PPV film thickness.
Figure 6.6: Sample with thick MEH-PPV film, the device holding it is a vacuum chamber.
Fig.6.4 is the sample with small silver dots as mirror, the shadow with green color on the edge
represents the organic DBR. Fig.6.5 has silver mirror all over the substrate and the film thickness
is not uniform as a result of the low spin speed. The MEH-PPV film in Fig.6.6 was formed by
CHAPTER 6. ORGANIC DBR LASER 48
drop-casting, hence acquiring extremely thick film. All of these distinct samples shall be under
test.
6.2 Property Measurement
6.2.1 Experimental Setup and Devices
The experimental setup for detecting lasing of the organic laser is similar to the one used for
MEH-PPV emission in Chapter 4. The setup is shown in Fig.6.7. In Fig.6.7, position A locates a
Figure 6.7: Lasing measurement setup. A: green laser. B: half wave plate. C: polarized beamsplitter. D: bi-convex lens, f=75mm. E: sample to be measured.
passively Q-switched Microchip laser PNG−M02010− 130, of which the wavelength is 532nm
and max repetition rate 1000Hz. The constant pulse width range is lower than 400ps and
output energy is around 30µJ per pulse maximum. At position B and C are a half wave plate
and a polarized beam splitter, respectively, which are used to tune the output laser energy.
Position D is a bi-convex lens with focal length 75mm, this lens is able to slide along the rod,
the laser intensity on the sample can be increased further. The sample is place in position D
inside a vacuum device. As demonstrated in Chapter 4, bleaching effect occurs under high laser
exposure, degrading the emission. A vacuum surrounding could help improve the situation. The
sample is put at an angle with the incident laser beam in order to observe the lasing action,
because the lasing direction is vertical to the sample surface according to the vertical laser cavity
structure. A white paper was held manually near the sample for the purpose of detecting.
CHAPTER 6. ORGANIC DBR LASER 49
6.2.2 Measurement Process
Before trying to find the lasing, a measurement determining how much of the incident laser fell
in the MEH-PPV region was conducted. Setting the repetition rate of the pump laser 100Hz
and removing the lens, applying a power meter to measure the laser intensity before the sample
and reflected by the sample. The power meter is Newport with model 2936−C, the wavelength
of which was set as 532nm, in compliance with the pump laser wavelength. Also, the attenuator
on the detection was on, as shown in Fig.6.8. By tuning the angle of the sample (thus change
Figure 6.8: Detection device of the power meter.
the reflectivity of DBR at certain wavelength, as explained in Chapter 5), the following results
were collected. Because the repetition rate of the pump laser was determined 100Hz and the
Table 6.1: Incident versus reflected laser intensity
incident intensity reflected intensity
measured value 341.1µW 44.5µW
laser spot had diameter approximately 2mm, therefore the energy intensity on the sample was
86.15µJ/cm2, much lager than mentioned in section6.1.2. Considering that only 13% of the
incident laser intensity was reflected, it could be assumed that there was enough pump energy.
The experimental steps for getting lasing were similar with those described in Chapter 4, the
difference was that the sample was put at an angle with incident pump laser beam instead of
perpendicular to it, because the stimulated emission to be obtained has a perpendicular direction
to the sample.
CHAPTER 6. ORGANIC DBR LASER 50
First the samples with a complete patch of silver mirror was placed in the vacuum device, which
was located on a stage to control the position change. The repetition rate for the pump laser was
changed back to 1000Hz and the beam was focused on the sample, as obvious ASE phenomenon
has been obtained in this exact situation (Chapter 4). The whole experiment was conducted
under dark situation and a spectrometer was involved at first to find a desirable situation and
position of the sample. There was once a quick increasing in the detected peak, however after
removing the spectrometer and putting a white paper in position, no lasing spot was observed.
Changing the location of pump laser spot also did not help. Then the DBR was realized to
be damaged, as indicated in Fig.6.9. Considering the cause maybe the high repetition rate, it
was decreased to 50Hz, however still ended up in the damage of DBR (also see Fig.6.9). The
same happened to other samples as reflected by Fig.6.10. This damage disappeared only if there
existed no lens, i.e. no focusing of the incident beam, which indicated that it was the focus of
the incident beam that made the DBR damaged. Then the lens was removed from the setup.
Figure 6.9: Damage of the DBR on sample. A: caused by focused 1000Hz beam. B:causedby focused 50Hz beam.
Figure 6.10: Damage of the DBR on another sample.
After removing the lens, the sample was excited by the pump laser again, different repetition
rates of the pump and different positions of the sample were tested. Although the problem of
CHAPTER 6. ORGANIC DBR LASER 51
deteriorating DBR has been overcome, lasing action was still beyond availability.
Then the samples with small silver mirror dots were tested under the same conditions. The same
damage phenomenon was observed with focused incident beam, as shown in Fig.6.11. Removing
the lens and trying exciting different spots on the sample, however lasing still did not appear.
Figure 6.11: Damage of the DBR on small mirror dots sample.
6.3 Result and Discussion
In this chapter, all the laser components tested in the previous chapters were assembled together
to make an organic laser and two kinds of samples were presented. In spite that these components
proved functioning properly separately, the complete organic laser fails in lasing. The interaction
and influence between components were not considered thoroughly. Here are some hypotheses
for the reasons why it fails.
For the sample with small silver mirror dots, the MEH-PPV film was cast on the silver dots
sample from 7.65mg/ml MEH-PPV in THF solution with spin speed 600rpm and spin time 15
seconds. When the exact same film was cast on glass substrate, it has shown ASE with repeti-
tion of pump laser 1000Hz and focused beam, indicating the existence of population inversion
(Chapter 4), consequently this film should be able to lasing. Besides, the silver contact does not
destroy the gain behavior of MEH-PPV, according to article[31]. However, the DBR on the top
layer could be destroyed under high incident intensity as presented above. One cause maybe
the energy absorbed in MEH-PPV results in temperature rise, thus burning the DBR; another
maybe that the DBR itself absorbs some laser energy, as the tail of the 532nm laser goes into
the absorption region of BDAVBi, one material of the organic DBR, as shown in Fig.6.12 and
Fig.6.13.
CHAPTER 6. ORGANIC DBR LASER 52
Figure 6.12: ko and ke value of BDAVBi, adjusted from [9].
Figure 6.13: k value of TAPC, adjusted from [9].
Another reason for this kind of sample may rise from that the film is too thin, only around
200nm. For this thickness the laser mode wavelength is around 720nm, almost out of the range
of the emission band.
For the sample with a whole patch of silver mirror, the MEH-PPV film was formed by either
spin speed of around 300rpm or dropping of the solution on the substrate. ASE has not been
acquired in these films before. Besides the reason for the organic DBR, the MEH-PPV film
could also induce some trouble. For MEH-PPV in solution, the individual polymer chains are
well-separated in space, the fast non-radiative decay and luminescence quenching caused by
the interchain interactions are negligible[32]. When the material is cast into solid film, if the
evaporation is quick enough that no adequate time there for chain-chain and substrate-chain
CHAPTER 6. ORGANIC DBR LASER 53
interactions, the film could also function. But if highly concentrated MEH-PPV solution cast
into a thin film with slow evaporation, polymer aggregation may occur, lowering the cross section
of stimulated emission[33]. To solve this, MEH-PPV film can be cast from MEH-PPV and TiO2
in polystyrene, where TiO2 nanoparticles disperse MEH-PPV polymers.
Chapter 7
Conclusion and Perspective
In this thesis, an organic DBR laser was fabricated with conjugated polymer MEH-PPV as
amplifying medium and BDAVBi and TAPC serve as the quarter-wave layers of the DBR,
leading to vertical surface emission, and silver coated glass substrate functions as the other
mirror, thus constitutes an organic laser.
Chapter 2 and Chapter 3 provide some background information and knowledge. Chapter 2
provides a brief description of the newly developed organic materials used in electronics and
optoelectronics with their novel promising properties. Conjugated polymers as MEH-PPV ini-
tially has a four-level system and large gain cross-section, make it a good candidate for laser
amplifying medium. Chapter 3 mainly gives review on the working mechanism of a laser. A
complete laser consists of a pump device, a resonator and an amplifying medium. In this thesis,
they correspond to a green laser, a resonator formed by an organic DBR and silver mirror, and
a net MEH-PPV thin film. There are certain conditions for a laser to be able to lase, so the
parameters of these components cannot be chosen randomly.
Chapter 4 and Chapter 5 focus more on simulations and experiments. Chapter 4 describes am-
plifying medium MEH-PPV, which is a conjugated polymer soluble in common organic solvent,
therefore MEH-PPV film can be cast from solution. In order to obtain desired uniform and
thick film, distinct solvents with different concentrations and spin parameters were tested and
measured. Finally a uniform film with thickness around 200nm was obtained by spin coating of
7.65mg/ml MEH-PPV in THF mixture and the spin speed was 600rpm. This film has shown
ASE that promises the possibility of lasing. Also the absorption spectrum was well separated
from the emission spectrum. Chapter 5 contains several simulations to gain insight into the
54
CHAPTER 7. CONCLUSION AND PERSPECTIVE 55
DBR properties. Then the organic DBR was fabricated using physical vapor deposition. The
transmission measurement proved that the DBR functioned well.
Chapter 6 assembles all the components described above to make a complete organic DBR,
which is shown in Fig.6.4 and Fig.6.5. Albeit the multiple times of trying, it is a pity that the
laser failed to lasing, possible reasons have been presented in Chapter 6. In spite of the fact that
each part could function well separately, conclusion can not be drawn that they can cooperate
properly, the influence between these components worth consideration.
For future effort to get this kind of organic laser working, MEH-PPV powder can be dissolved
together with TiO2 nanoparticles to make the polymers dispersed, the nanoprticles can also
scatter the emitted photons to achieve gain exceeding the loss. Moreover, a pump laser with
higher wavelength could also be used to avoid the possible energy absorption of the DBR and
gain higher pump energy. However there is trade off between the protection of DBR and
absorption efficiency of MEH-PPV that should be taken into consideration.
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Boxuan Gao
MEH-PPV as active layerDesign and fabrication of an organic DBR laser with
Academic year 2015-2016Faculty of Engineering and ArchitectureChair: Prof. dr. ir. Rik Van de WalleDepartment of Electronics and Information Systems
Master of Science in Photonics EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Ir. Michiel CallensSupervisor: Prof. dr. ir. Kristiaan Neyts