Photorefractivity in polymers

Transcription

1 Photorefractivity in polymers George G. Malliaras

2 Photorefractivity in Polymers George G. Malliaras Ph.D. Thesis University of Groningen, The Netherlands November 1995 ISBN x

3 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

4 Promotor: Prof. Dr. G. Hadziioannou

5 Contents Preface 1 1. Introduction 3 Abstract The photorefractive effect Standard model for photorefractivity Photorefractive nonlinear optics Photorefractivity in polymers Aim and outline of this thesis References Experimental 21 Abstract Introduction Sample preparation Photoconductivity measurements Electrooptic measurements Second harmonic generation measurements Diffraction efficiency and response time measurements Two beam coupling measurements Transient holographic and photoconductivity measurements References Photorefractivity in poly(n-vinylcarbazole) based composites 43 Abstract Introduction Results and discussion Optical absorption Photoconductivity Orientational mobility of the nonlinear optical molecules and electrooptic response of poly(n-vinylcarbazole) based photorefractive composites Proof for the photorefractive nature of the observed gratings. Properties and comparison with the standard model Asymmetric energy exchange in poly(n-vinylcarbazole) based photorefractive composites The mechanism of the refractive index change Conclusions and outlook References...68

6 4. Charge trapping in photorefractive polymers 71 Abstract Introduction (Quasi-) steady state holographic experiments The response time of the photorefractive grating The phase shift of the photorefractive grating The amplitude of the photorefractive grating Transient holographic experiments Space charge field formation Influence of the trap density on the space charge field formation Conclusions and outlook References The transient behaviour of the space charge field 91 Abstract Introduction Results and discussion Electric field, temperature and drift length dependence of the hole drift mobility Comparison with the standard model The influence of disorder Conclusions and outlook References List of abbreviations 106 List of symbols 107 Summary 109 Samenvating 111 List of publications 113

7 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.

8 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