Thesis submitted for the degree of Doctor of Philosophy. Soma Venugopal Rao

Transcription

1 Studies of Excited State Dynamics, Third Order Optical Nonlinearity and Nonlinear Absorption in C 60, Porphyrins, and Phthalocyanines Using Incoherent Laser Spectroscopy Thesis submitted for the degree of Doctor of Philosophy by Soma Venugopal Rao School of Physics University of Hyderabad Hyderabad India March 2000.

2 For my Parents ii

3 iii DECLARATION I hereby declare that the matter embodied in this thesis entitled Studies of Excited State Dynamics, Third Order Optical Nonlinearity, and Nonlinear Absorption in C 60, Porphyrins and Phthalocyanines Using Incoherent Laser Spectroscopy is the result of investigations carried out by me in the School of Physics, University of Hyderabad, India, under the supervision of Dr. D. Narayana Rao. Place: Hyderabad Date: (Soma Venugopal Rao)

4 iv CERTIFICATE This is to certify that the work described in this dissertation entitled Studies of Excited State Dynamics, Third Order Optical Nonlinearity and Nonlinear Absorption in C 60, Porphyrins and Phthalocyanines Using Incoherent Laser Spectroscopy has been carried out by Mr. Soma Venugopal Rao under my direct supervision for the full period prescribed under PhD. ordinances of the University and the same has not been submitted for any other degree or diploma at this or any other University. Place: Date: (Dean, School of Physics) (Dr. D. Narayana Rao)

5 v ACKNOWLEDGEMENTS It is, indeed, a great pleasure to thank all those who have, directly or indirectly, helped me in successful completion of this thesis. At the outset I wish to thank my guide Dr. D. Narayana Rao who was by my side always, patiently and constantly inspiring, encouraging and guiding me throughout the course. I had learnt a lot from his meticulous planning and implementation, dedication and hard work. My association with him for over five years was a rewarding experience, which I never wish to forget. I would like to thank the Dean, School of Physics, for making available all the facilities required for the experiments. I also thank all the non-teaching staff for their co-operation. I would like to thank Dr. Bhaskar G. Maiya and Mr. L. Giribabu, School of Chemistry for providing me with large amounts of samples of Tetra Tolyl Porphyrins when required, for their useful discussions and co-operation extended. I would like to thank Prof. D. Mathur for allowing me to use the picosecond laser facility at TIFR, Dr. s G. Ravindra Kumar and Reji Philip for their kind help in the experiments carried out at TIFR. I would like to thank Prof. s G.S. Agarwal, S.P. Tewari, Dr. s S. Dutta Gupta, T.P. Radhakrishnan, and P. Ananta Lakshmi for their constant support and inspiring association. I would like to thank Dr. Suneel Singh for his excellent theoretical support through out the course of my work. I extend my thanks to Prof. D.V.G.L.N. Rao, U. Mass. Boston, USA, for providing us with some of the Tetrabenzporphyrins. The dispersion studies reported in Chapter VI are carried out in the lab of Prof. Sukant Tripathy, U. Mass., Lowell, USA. Financial assistance, in the form of Junior Research Fellowship and Senior Research Fellowship, from University Grants Commission is gratefully acknowledged.

6 vi I wish to thank Mr and Mrs. T.V.K. Murthy, Mr. Suresh for their continuous and excellent moral support, kind concern and well wishes. I wish to thank all my senior lab-mates Dr. s V. Nirmal Kumar, S. Sivaprakasam, V.S. Ashoka, M. Ravi, and G. Vijaya Prakash. Thanks are also due to my colleagues Harsha, Jolly, Ramana, G.V. Rao, Rammohan, Kalyan, Hari, Chandu, Raji, Srinivas, Prem, Rajani, Manoj, Ranita, Palas, Sonika, Subbalakshmi, Sastri, Srinath, Anita, Senthil, Satheesh, Loganathan, Azher, and Sudhakar for their support throughout. Thanks are also due to all those whose names are missing in this list and have helped me in various stages of my work. Last, but not the least, I wish to thank all my family members without whose co-operation, completion of this task would have been highly impossible.

7 vii Table of Contents Declaration Certificate Acknowledgments CHAPTER 1 Introduction 1.1 Introduction to Nonlinear Optics and its applications Nonlinear Optical Materials Measurement techniques for excited state dynamics Incoherent Laser Spectroscopy: Tool for measuring excited state dynamics in the ps / fs domain Aim and scope of the thesis References 9 CHAPTER 2 Experimental Techniques and Details: Degenerate Four Wave Mixing and Z-scan 2.1 Introduction Degenerate Four Wave Mixing (DFWM) Degenerate Four Wave Mixing with Incoherent Light (DFWM-IL) Characterization of the Broadband Dye Laser Phase-Conjugate configuration Boxcar Configuration Self-Diffraction Configuration Degenerate Four-Wave Mixing with 35 ps pulses (DFWM-PS) Z-scan Closed aperture scan for sign and refractive nonlinearity Open aperture scan for absorptive nonlinearity Photo-physics of a typical organic molecule Results on the samples Rhodamine B and CS Conclusions 37

8 viii 2.8 References 38 CHAPTER 3 Theoretical model for the DFWM signals observed in materials exhibiting Reverse Saturable Absorption 3.1 Introduction DFWM-IL for ultra-fast relaxation measurements Results and Discussion Measurement of Population Relaxation Time (T 1 ) Rhodamine B: Concentration dependent PC Signal DFWM signals in the boxcar geometry PC signals recorded with cells of different thickness Theoretical modeling of line shapes observed in the experiment Numerical Results and Discussion Conclusions References 86 CHAPTER 4 Tetra Tolyl Porphyrins: Studies of third-order optical nonlinearity and nonlinear absorption. 4.1 Introduction Experimental Details Degenerate Four Wave Mixing Z-scan Results and Discussion Origin of large nonlinearity in the ns regime Comparison with other class of porphyrins Conclusions References 115 CHAPTER 5 Excited State Dynamics in Tetratolyl Porphyrins studied using Incoherent Light and 35 psec pulses. 5.1 Introduction Experimental Details 120

9 ix 5.3 Results and Discussion DFWM IL Results DFWM PS Results Conclusions References 138 CHAPTER 6 C 60 : Studies of Third-order Nonlinearity, Excited State Dynamics using DFWM-IL and Dispersion Studies of Nonlinear Absorption using Z-scan techniques 6.1 Introduction Degenerate Four Wave Mixing χ (3) Measurements Temporal response of the DFWM signal Z-scan studies Reverse Saturable Absorption in C Dispersion studies of nonlinear absorption Z-scans at 532 nm and 600 nm using Nd: YAG laser Conclusions References 172 CHAPTER 7 Phthalocyanines: DFWM-IL Results 7.1 Introduction Experimental Details DFWM-IL results on Phthalocyanines References 184 Conclusions 186 Future Scope 188 Curriculum Vitae