Data Storage and Retrieval using Photorefractive Crystals (Holographic Memories) A Thesis presented for the degree of Doctor of Philosophy in The Faculty of Science University of Calicut, Calicut by RAVINDER KUMAR BANYAL Indian Institute of Astrophysics Bangalore, 560034, INDIA October, 2005 Dedicated to The Sweet Memories of My Grandfather iii DECLARATION I hereby declare that the matter contained in this thesis is the result of the investi- gations carried out by me at the Indian Institute of Astrophysics, Bangalore, under the supervision of Dr. K. Neelakandan. This thesis has not been submitted for the award of any degree, diploma, associateship, fellowship etc. of any university or institute. Dr. K. Neelakandan Ravinder Kumar Banyal (Thesis Supervisor) (Ph.D. Candidate) Indian Institute of Astrophysics Bangalore 560 034, INDIA October, 2005 iv CERTIFICATE This is to certify that the thesis entitled \Data Storage and Retrieval using Photorefractive Crystals (Holographic Memories)" submitted to the Univer- sity of Calicut by Mr. Ravinder Kumar Banyal for the award of the degree of Doctor of Philosophy in the faculty of Science, is based on the results of the investigations carried out by him under my supervision and guidance, at the Indian Institute of Astrophysics, Bangalore. This thesis has not been submitted for the award of any degree, diploma, associateship, fellowship etc. of any university or institute. Dr. K. Neelakandan (Thesis Supervisor) Professor, Department of Physics, University of Calicut, Kerala October, 2005 v DECLARATION I hereby declare that the matter contained in this thesis is the result of the investi- gations carried out by me at the Indian Institute of Astrophysics, Bangalore, under the supervision of Dr. B. Raghavendra Prasad. This thesis has not been submitted for the award of any degree, diploma, associateship, fellowship etc. of any university or institute. Dr. B. Raghavendra Prasad Ravinder Kumar Banyal (Thesis Supervisor) (Ph.D. Candidate) Indian Institute of Astrophysics Bangalore 560 034, INDIA October, 2005 vi CERTIFICATE This is to certify that the thesis entitled \Data Storage and Retrieval using Photorefractive Crystals (Holographic Memories)" submitted to the Univer- sity of Calicut by Mr. Ravinder Kumar Banyal for the award of the degree of Doctor of Philosophy in the faculty of Science, is based on the results of the investigations carried out by him under my supervision and guidance, at the Indian Institute of Astrophysics, Bangalore. This thesis has not been submitted for the award of any degree, diploma, associateship, fellowship etc. of any university or institute. Dr. B. Raghavendra Prasad (Thesis Supervisor) Indian Institute of Astrophysics Koramangala, II Block Bangalore, Karnataka October, 2005 Acknowledgements It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair ... The above opening passage from `A Tale of Two Cities' by Charles Dickens would aptly sum-up the high and low points of bygone years! And now, I am indeed delighted to acknowledge people who mattered most during this phase. First and foremost, I would like to thank my supervisor Dr. B. Raghavendra Prasad for giving me an opportunity to work with him. Without his constant sup- port and able guidance, this thesis would not have been complete. I am also grateful to him for creating excellent lab facilities and a very congenial work atmosphere that helped me to carry out the research work smoothly and independently. I wish to acknowledge The Director, The Board of Graduate Studies, the library and the adminstration sta® of IIA and CREST for their encouraging support and help rendered during my PhD tenure. I am grateful to Prof. K. Neelakandan for readily agreeing to act my co-guide at Calicut University. I sincerely thank Dr. S. Chaterjee and Prof. K. P. J. Reddy from IISc for their valuable suggestions and advice on many occasions. I am obliged to Dr. B. R. S. Babu, Head of Physics Department at Calicut University, for his kind assistance during the PhD registration and other related formalities. I must thank Dr. Gopal Hegde for inviting me to Ngee Ann Polytechnic, Singapore. Personally, the visit was a great learning experience for me. I must thank all my batch-mates and senior students, especially, Dharam, Suresh, Raji, Dilip, Shivarani, and Preeti for being quite considerate and accommodating. vii viii Many people at CREST, including, D.K. Sahu, Parihar, Kemekar, and Ginu were of great help. Not to forget the generosity of the canteen people, security guards and technical sta® who made the stay at CREST quite rememberable. Manoj has been one of the most influential persons around. His indomitable conviction in secular and scienti¯c values have left an indelible impression upon me and in turn compelled me to re-evaluate many of my own beliefs. I thank Pandey Ji, Methi, Nagaraj, Blesson, and Vigeesh for weekend recreations. Many may not have liked the monotony of vegetarian food that I have often cooked at weekends. But, I presume that the disappointment was quickly forgotten amidst the euphoria of drinking sessions that led to many insightful revelations and healthy debates. Ravindra (Bhatti) was a very close friend right from my early days in IIA. Many a times his simplicity and remarkable sense of humour helped me to overcome my own distressful moments. Maheswar has been very supportive and helpful all through out and I really thank him for giving me good company in IIA. My acquaintance with Saigeetha was more of a coincidence. She has been a wonderful and inspiring person. I was immensely bene¯tted from her witty criticism and refreshing wisdom. I'm also grateful to her for introducing me to the fascinating ¯eld of geophysics. Friends back home, namely { Dimmu, Bipan, Ripu, Surender, Sanju, Rajiv, Rockey and Vikas were quite a®ectionate and concerned. It is with great pleasure, I acknowledge their wholehearted support and appreciation. Special thanks to my wife Kanchan who endured my absence without any trace of resentment or complain. Despite my prolonged isolation and total negligence, she displayed a remarkable composure and forbearance. My younger brother Pawan has been a great source of strength for me. His pres- ence at home, spared me from the burden of domestic responsibilities and hardships and allowed me to focus on my studies. Finally, I would like to thank my parents for the overwhelming support and encouragement. Their generosity and liberal attitude has made all the di®erence for me. Preface There are several types of optical materials that exhibit nonlinear properties. The research work reported in this thesis is focused on two of its kind, namely, the inorganic photorefractive systems such as LiNbO3 and BaTiO3 and the bacte- riorhodopsin; which is a biological molecule. The work presented in this thesis is broadly divided into two parts. The ¯rst part of the thesis (Chapter 1 to Chapter 4 ) deals with the photorefractive e®ect in LiNbO3 and BaTiO3 crystals and their use in holographic data storage. The second part (Chapter 5 ) deals with the theoreti- cal and experimental studies of bacteriorhodopsin and its applications in all-optical photonic switching. In its entirety, the most appropriate title for this thesis would have been, \Data Storage and Optical Switching in Photoactive Media." However, in accordance with University's guidelines, we are compelled to retain the topic of research (approved by the University at the time of registration) as our present thesis title even though the content of this thesis is more extensive. In Chapter 1 of the thesis, basics of nonlinear optics and related phenomenon are reviewed. Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media {a media in which the polarization P responds nonlinearly to the electric ¯eld E of the light. Higher order polarization terms are responsi- ble for the di®erent types of nonlinear e®ects. The light-induced refractive index change is a fundamental nonlinear process that occurs in several photorefractive materials. A qualitative discussion on the principle and the physical mechanism of the photorefractive e®ect is outlined. A concise review of holography based data storage in photorefractive crystals is also presented. The importance and require- ments of hight density memories is highlighted subsequently. In the later half of the Chapter 1, a brief introduction to all optical switching in nonlinear media is given. The bacteriorhodopsin molecule and its photophysical and absorption properties are briefly described. All the concepts and material discussed in this chapter provide a necessary background for the work presented in the later chapters. Chapter 2 deals with the characterization of BaTiO3 and LiNbO3 crystals. A short but necessary review of theoretical formulism based on the band transport model and electro-optic e®ect is presented to facilitate physical understanding of the x phenomenon. The work includes detailed experimental studies of the light-induced absorption and two-wave mixing in undoped and Rh-doped BaTiO3 crystals at dif- ferent wavelengths and light intensities. Temporal and the steady-state behaviour of light-induced absorption is studied using a familiar pump-probe technique. A two-center charge transport model is proposed to explain the intensity dependence of the absorption coe±cients in BaTiO3. Important parameters such as build-up and decay rates, thermal excitation rates, and two-wave mixing gain have been ob- tained. Di®raction e±ciency measurements and grating erasure studies in triply doped Fe:Ce:Ti:LiNbO3 crystals are presented in the end. Photorefractive crystals are widely used in the area of holographic data storage and related applications.