Photorefractivity in polymers
George G. Malliaras
Photorefractivity in Polymers George G. Malliaras Ph.D. Thesis Univ...
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Photorefractivity in polymers
George G. Malliaras
Photorefractivity in Polymers George G. Malliaras Ph.D. Thesis University of Groningen, The Netherlands November 1995 ISBN 90-367-0569-x
RIJKSUNIVERSITEIT GRONINGEN
Photorefractivity in polymers Proefschrift
ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus Dr. F. van der Woude in het openbaar te verdedigen op maandag 18 december 1995 des namiddags te 2.45 uur precies
door George Malliaras geboren op 5 juni 1969 te Serres, Griekenland
Promotor: Prof. Dr. G. Hadziioannou
Contents Preface
1
1. Introduction 3 Abstract ..............................................................................................................3 1.1. The photorefractive effect......................................................................4 1.2. Standard model for photorefractivity .....................................................6 1.3. Photorefractive nonlinear optics...........................................................10 1.4. Photorefractivity in polymers...............................................................13 1.5. Aim and outline of this thesis...............................................................17 1.6. References ..........................................................................................18 2. Experimental 21 Abstract ............................................................................................................21 2.1. Introduction ........................................................................................22 2.2. Sample preparation .............................................................................22 2.3. Photoconductivity measurements.........................................................26 2.4. Electrooptic measurements ..................................................................27 2.5. Second harmonic generation measurements..........................................29 2.6. Diffraction efficiency and response time measurements........................31 2.7. Two beam coupling measurements ......................................................36 2.8. Transient holographic and photoconductivity measurements.................39 2.9. References ..........................................................................................42 3. Photorefractivity in poly(N-vinylcarbazole) based composites 43 Abstract ............................................................................................................43 3.1. Introduction ........................................................................................44 3.2. Results and discussion.........................................................................44 3.2.1. Optical absorption ....................................................................45 3.2.2. Photoconductivity .....................................................................47 3.2.3. Orientational mobility of the nonlinear optical molecules and electrooptic response of poly(N-vinylcarbazole) based photorefractive composites........................................................48 3.2.4. Proof for the photorefractive nature of the observed gratings. Properties and comparison with the standard model...................53 3.2.5. Asymmetric energy exchange in poly(N-vinylcarbazole) based photorefractive composites..............................................62 3.2.6. The mechanism of the refractive index change...........................65 3.3. Conclusions and outlook......................................................................67 3.4. References ..........................................................................................68
4. Charge trapping in photorefractive polymers 71 Abstract ........................................................................................................... 71 4.1. Introduction........................................................................................ 72 4.2. (Quasi-) steady state holographic experiments..................................... 73 4.2.1. The response time of the photorefractive grating ....................... 75 4.2.2. The phase shift of the photorefractive grating............................ 77 4.2.3. The amplitude of the photorefractive grating............................. 78 4.3. Transient holographic experiments...................................................... 80 4.3.1. Space charge field formation .................................................... 81 4.3.2. Influence of the trap density on the space charge field formation ......................................................................... 86 4.4. Conclusions and outlook..................................................................... 88 4.5. References.......................................................................................... 88 5. The transient behaviour of the space charge field 91 Abstract ........................................................................................................... 91 5.1. Introduction........................................................................................ 92 5.2. Results and discussion........................................................................ 95 5.2.1. Electric field, temperature and drift length dependence of the hole drift mobility......................................... 95 5.2.2. Comparison with the standard model ........................................ 99 5.2.3. The influence of disorder........................................................ 100 5.3. Conclusions and outlook................................................................... 104 5.4. References........................................................................................ 105 List of abbreviations
106
List of symbols
107
Summary
109
Samenvating
111
List of publications
113
Preface Polymers are not considered as traditional materials for optoelectronics. Although there is by far more plastic than silicon in every computer, isn’t it in the wrong place: around the microchip instead of inside it? In recent years however, more and more devices made almost entirely out of polymers, such as nonlinear optical elements, transistors, light emitting diodes, solar cells, optical fibers, etc. become available. The observation of the photorefractive effect in a polymer in 1991 was yet another manifestation of this “plastic revolution”. The field of polymer photorefractivity is fairly young and most of the research effort has been concentrated on exploring the limits of performance of these materials. A great variety of polymer architectures that show this effect has been synthesized as the race for the best one (in terms of gain coefficient, diffraction efficiency, response time, etc.) still goes on. In this thesis, however, I followed a different line: by focusing on one model polymer composite and systematically studying its properties, I hoped to gain insight into the mechanism of photorefractivity in this class of materials. Although this composite exhibited the highest gain ever in the beginning, it is by now rather moderate in terms of performance compared to newly developed photorefractive polymers. Several more efficient compounds have been synthesized in our laboratory and even the same composite has been improved to show a diffraction efficiency which is more than ten times as high. However, instead of outlining how to optimize a material, I have tried to put together experimental results that show the underlying physics governing the photorefractive effect in polymers. Although it is still too early to tell whether photorefractive polymers will find any practical application, my personal view is optimistic. The tremendous pressure to succeed in efficiently processing and storing the billions of data bytes of information produced every day, combined with the great potential of these materials, may lead to the computer of the next generation.
Acknowledgements I will always recall with fondness my four-year stay in Groningen. It has truly been a privilege and a rewarding experience to work in the creative atmosphere of the group of Prof. Dr. Georges Hadziioannou. First and foremost I would like to thank him for the trust he has shown in me, his continuous guidance, support and encouragement and for being an inexhaustible source of ideas and inspiration. I am grateful to Dr. Victor Krasnikov, who gave me expert guidance and generous professional support. Apart from being my closest colleague he became my best friend. I am also grateful to the chemist of the “photorefractive team” , Henk Bolink, for synthesizing and purifying the compounds used in this thesis, but also for the fun we had together in the lab. My special thanks go to the members of the reading committee, Prof. Dr. Albert Pennings, Prof. Dr. Douwe Wiersma and Prof. Dr. Campbell Scott (IBM Almaden Research Center, USA) for carefully reading the manuscript and for their insightful comments.
2
Preface
I would also like to thank all those who helped me with my graduate studies: Dr. Paul van Hutten for the very interesting scientific and philosophical discussions and for carefully reading and correcting most of my papers; the theoreticians of the group of Polymer Chemistry, Henk Angerman and Prof. Dr. Gerrit ten Brinke for fruitful discussions; Prof. Dr. Albert Pennings and his group for helping me with their expertise in polymer processing; Prof. Dr. Douwe Wiersma, Dr. Koos Duppen, Ben Hesp and generally the group of Chemical Physics for stimulating discussions and for helping with setting up the optoelectronics laboratory; Prof. Dr. Jan Kommandeur for helpful discussions and valuable suggestions on the subject of charge trapping; Dr. Homer Antoniadis (Hewlett-Packard Laboratories, Palo Alto, USA) for suggestions on the subject of charge transport; Prof. Dr. Teun Klapwijk for fruitful discussions; my former supervisors Prof. Dr. Kostas Kambas (Aristotle University, Thessaloniki, Greece) and Dr. Theo Rasing (University of Nijmegen) for their continuous interest and encouragement; finally, my colleagues Jurjen Wildeman, Hendrik-Jan Brouwer, Gerald Belder, Vagelis Manias, Jan Herrema, Richard Gill, Diny Hissink, Vasilis Koutsos, Dominique Morichère, Yiannis Papantoniou, Alain Hilberer, Amar Mavinkurve, Geert Berentschot, Erik Kroeze and the rest of the group for help, stimulating discussions and lots of fun! While setting up the optoelectronics lab, I enjoyed the assistance from my student Roy Gerritsen and the help of Aldert Meulema and Adams Verweij in technical and administrative matters. I am grateful to Nanno Herder, Anne Appeldoorn and Henk Knol, for helping me with the electronics, mechanical constructions and glassware. Thanks are due to Ulco and Sjouke from the library, for assistance with the literature research. I would like to express my special thanks to Hans Beekhuis and his colleagues for checking up the lab every night and to our secretaries, Marjan, Daphne, Ingrid and Betty, for arranging thousand things for me. Last, but not least, my family, though so far away, has always been close to me, making me feel safe. There is no word in the dictionary to fully express my gratefulness. I am also grateful to Stefania, for her unconditional love and support during all this period. The optoelectronics lab was set up with the financial support from the Faculty of Mathematics and Physical Sciences of the University of Groningen, the Dutch Ministry of Economic Affairs, the Stichting Toegepaste Wetenshappen (STW) and the Stichting Scheikundig Onderzoek Nederland (SON). This thesis was shaped on the basis of feedback that I got from presentations at various conferences. This was made possible with the generous financial support from the University of Groningen, the Department of Polymer Chemistry, the Materials Science Centre, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) and Shell Nederland B.V. All the above are gratefully acknowledged. George G. Malliaras November 1995
Stellingen 1. The existence of photorefractivity in polymers creates the opportunity to study the processes of charge transport and trapping in this class of amorphous materials with optical techniques. 2. Some ideas that are promoted as “new” can be found in older Russian journals. 3. The electrooptic coefficients r33 and r13 do not apply in the waveguide geometry used by Yu at al. Yu et al., Macromolecules 26, 2216 (1993) 4. The zero degrees phase shifted gratings observed by Silence at al. in the polymer PVK:Lophine 1:TNF can also be attributed to a photorefractive response, limited by a large amount of traps. S.M. Silence et al., Appl. Phys. Lett. 64, 712 (1994) 5. The recent development of a holographic lock by Holoplex (briefly described by Psaltis et al.), shows the tremendous potential of optical computing schemes. D. Psaltis and F. Mok, Scientific American, p. 52, November 1995 6. Unnecessary use of complicated terminology and formalism does not imply great science but charlatanism. 7. It is very important for Ph.D. students to present their work at international conferences (not forgetting, it's important for post-docs too!). 8. It takes courage to give up hope, but more courage to keep it. 9. The fact that one can "buy" in advance his own funeral is yet another sign of the degradation of family values. 10. There are two types of optimists: Those who claim things could not be better and those who realize that things could have been much worse. 11. A car is not just a means of transportation from point A to point B: Quality makes the difference. 12. Multitasking in OS/2 can greatly improve someone’s productivity. However, it requires at least 16 MB to run decently. 13. All those who claim to be 100% politically correct should not say "walkman" but "walkperson". George G. Malliaras December 1995
Chapter One Introduction
Abstract In this chapter, a concise introduction to photorefractivity is made. Using the simplest case of the band transport model, the steady state and the dynamics of the space charge field are surveyed. The theory of two beam coupling in photorefractive media is briefly outlined, as it will be useful for the understanding of the experimental results. The main lines behind the engineering of polymers which show the effect follow and the introduction closes with the outline of this thesis. References are given not only to the original work but to several recent reviews, which can serve as a guide to the literature.
4
Chapter One
1.1. The photorefractive effect The photorefractive effect was accidentally discovered in 1966 in LiNbO3 and LiTaO3 as detrimental optically induced refractive index inhomogeneities [1]. It was referred to as "optical damage" because it caused a degradation of the performance of nonlinear optical devices based on these materials. Two years later, holographic optical storage has been demonstrated in LiNbO3 using this newly discovered effect [2]. In 1969, Chen proposed a model based on the migration of photoexcited electrons which explained the main experimental observations and set the basis for future experimental and theoretical work [3]. The term photorefractive, which literally means light induced change of the refractive index, was introduced later on and since then has been reserved for this particular mechanism. In 1976, Kukhtarev et al. derived the dependence of the refractive index change on light intensity and material parameters and described the coupling of beams in thick photorefractive gratings [4]. Today, almost 30 years after its first discovery, photorefractivity is a blooming field of interdisciplinary research. Over the years several materials like BaTiO3, KNbO3, Bi4Ti3O12, Sr1-xBaxNb2O6 (SNB), Ba2-xSrxNayNb5O12 (KNSBN), Bi12SiO20 (BSO), Bi12GeO20 (BGO), GaAs, InP, CdTe, (Pb,La)(Zr,Ti)O3 and many other have been shown to exhibit the photorefractive effect [5,6], which makes it a quite general property of electrooptic crystals. Numerous applications in optical data storage, image processing and amplification, self and mutually pumped phase conjugation, photorefractive resonators, programmable optical interconnects, simulation of neural networks etc. have been proposed and demonstrated on a laboratory scale [6,7]. Apart from potential applications, intensive research has been triggered for the understanding of the microscopic origin of the photorefractive effect, resulting in the discovery of new phenomena, such as the bulk photovoltaic effect [8] and the excited state polarization [9]. Today, the mechanism of photorefractivity, although not fully, is quite well understood. This led to the recent observation of photorefractivity in new classes of materials such as organic crystals [10], polymers [11] and liquid crystals [12]. In figure 1.1, the basic mechanism is explained. The photorefractive effect is observed in materials which are both electrooptic and photoconducting. If such a sample is illuminated with a nonuniform light intensity pattern resulting from the interference of two mutually coherent beams, charge generation will take place at the bright areas of the fringes. These photogenerated charges will migrate and eventually get trapped at the dark areas, a process which can take place through several circles of photogeneration, diffusion and trapping. The resulting charge redistribution creates an internal electric field, the space charge field ESC, which changes the refractive index via the electrooptic effect. The space charge field forces the charges to drift in the opposite direction than diffusion and a dynamic equilibrium is reached when it has grown strong enough to
Introduction
5
cause a drift current which totally compensates the diffusion current. Application of an external electric field assists charge separation through drift and generally a higher space charge field can be produced in this way.
_
_ _
_
_
_
illumination charge generation & migration
+
+
+ +
+ +
x
• _
_
+ + +
_
_
_
+ + +
_
_•
charge redistribution
ϕ space charge field and refractive index grating
Fig. 1.1 : Mechanism of the photorefractive effect. A sinusoidal distribution of light intensity causes spatially modulating charge generation. The mobile charges diffuse and get trapped at the dark areas. A space charge field is established which changes the refractive index via the electrooptic effect.
From the above it is clear that the photorefractive effect provides a way to replicate light intensity patterns into refractive index gratings, with obvious potential applications in optical data storage. Several other mechanisms can do the same thing though: photochemical reactions, thermorefraction, formation of excited states, conventional χ(3) etc. can change the refractive index in the illuminated parts of a sample [13]. The photorefractive effect however processes a combination of
6
Chapter One
characteristics which make it unique: Very high nonlinearities can be achieved even with weak laser beams, as a result of the integrating nature of the effect. The resulting refractive index gratings are reversible, as uniform illumination will erase the space charge field. Another very important characteristic, is the existence of a spatial phase shift between the illumination pattern and the refractive index grating. This is the genuine signature of the photorefractive effect: No other mechanism can produce a phase shifted refractive index grating. As will be discussed below, the existence of this phase shift gives rise to steady state asymmetric energy exchange between two laser beams, which is the basis for several specific applications. Apart from applications, the photorefractive effect provides a means to investigate materials properties such as charge transport and trapping, with "clean" optical techniques. Steady state and transient holographic techniques can be employed to measure small photocurrents optically, expelling the need for sensitive electronic equipment. Moreover, the bulk of the sample is directly probed, eliminating electrode problems. Parameters like charge diffusion lengths, mobilities, trap densities and cross sections etc. are measured in this way [14].
1.2. Standard model for photorefractivity The model of Kukhtarev et al. has been very useful in helping to understand the microscopic origin of photorefractivity in inorganic crystals. To some extent, photorefractivity in polymers can also be understood along the same lines. For this reason a basic description of this model is presented here. In figure 1.2, the basic idea for the space charge field formation is illustrated, for the case of electron transport and a single participating impurity level. Depicted are the valence (EV) and the conduction (EC) band, together with donor (D) and acceptor (A) impurity levels. The only role of the acceptors is to deprive some of the donors from their charge, creating an initial concentration of empty traps. Let ND be the total donor density and ND+ the density of the ionized ones which act as traps. In the dark, electrical neutrality demands ND+=NA, where NA is the density of acceptors. Let the crystal be illuminated with a sinusoidal intensity pattern: I=I0(1+mcos(KGx))
(1.1)
where m is the modulation index and KG is the grating wave vector. According to this model, the space charge field is created through the steps of photoionization of a donor in the bright areas of the fringes (step 1 in figure 1.2), transport of the electron in the conduction band (step 2 in figure 1.2) and subsequent trapping at an ionized donor level (step 3 in figure 1.2).
Introduction
7
The rate of formation of ionized donors has a generation term, proportional to the light intensity and the density of donors that can be ionized, plus an annihilation term proportional to the available density of electrons in the conduction band and the trap density: ∂ND+(x)/∂t=sDI(x)(ND-ND+(x))-γDn(x)ND+(x)
(1.2)
where sD is the photogeneration rate, γD is the trapping rate and n is the electron density in the conduction band. 2 EC
hv
3
1
+
+
-
x
D
A EV
Fig. 1.2 : Band transport model for the photorefractive effect. Electrons are photoexcited (1) from donor states (D) to the conduction band (EC), where they migrate (2), until they get trapped at ionized donor sites (3). Acceptors (A) are present to create a few initially empty traps.
Electrons are mobile once in the conduction band and their density changes not only due to photogeneration and trapping, but also due to transport. The continuity equation is written: ∂n(x)/∂t=∂ND+(x)/∂t+(1/e)∂J(x)/∂x
(1.3)
where e is the electron charge and J the current density, which is a result of drift and diffusion: J(x)=µdren(x)E(x)+kBTµdr∂n(x)/∂t
(1.4)
8
Chapter One
where µdr is the electron drift mobility, E is the total electric field (space charge plus externally applied electric field), kB is the Boltzmann constant and T the absolute temperature. The Einstein equation of diffusion has been used in equation (1.4). The total electric field is calculated from Poisson's law: ∂E(x)/∂x=e(ND+(x)-n(x)-NA)/ε
(1.5)
where ε is the dc dielectric constant. In the limit of small modulation (m NA >> n and sDI