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 - 500 046 India March 2000.
For my Parents ii
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)
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)
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.
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.
vii Table of Contents Declaration Certificate Acknowledgments CHAPTER 1 Introduction 1.1 Introduction to Nonlinear Optics and its applications 1 1.2 Nonlinear Optical Materials 2 1.3 Measurement techniques for excited state dynamics 4 1.4 Incoherent Laser Spectroscopy: Tool for measuring excited state dynamics in the ps / fs domain 5 1.5 Aim and scope of the thesis 6 1.6 References 9 CHAPTER 2 Experimental Techniques and Details: Degenerate Four Wave Mixing and Z-scan 2.1 Introduction 12 2.1.1 Degenerate Four Wave Mixing (DFWM) 13 2.2 Degenerate Four Wave Mixing with Incoherent Light (DFWM-IL) 16 2.2.1 Characterization of the Broadband Dye Laser 16 2.2.2 Phase-Conjugate configuration 20 2.2.3 Boxcar Configuration 23 2.2.4 Self-Diffraction Configuration 25 2.3 Degenerate Four-Wave Mixing with 35 ps pulses (DFWM-PS) 25 2.4 Z-scan 28 2.4.1 Closed aperture scan for sign and refractive nonlinearity 28 2.4.2 Open aperture scan for absorptive nonlinearity 30 2.5 Photo-physics of a typical organic molecule 30 2.6 Results on the samples Rhodamine B and CS 2 34 2.7 Conclusions 37
viii 2.8 References 38 CHAPTER 3 Theoretical model for the DFWM signals observed in materials exhibiting Reverse Saturable Absorption 3.1 Introduction 40 3.2 DFWM-IL for ultra-fast relaxation measurements 41 3.3 Results and Discussion 43 3.4 Measurement of Population Relaxation Time (T 1 ) 50 3.4.1 Rhodamine B: Concentration dependent PC Signal 58 3.4.2 DFWM signals in the boxcar geometry 62 3.4.3 PC signals recorded with cells of different thickness 62 3.5 Theoretical modeling of line shapes observed in the experiment 65 3.5.1 Numerical Results and Discussion 79 3.6 Conclusions 85 3.7 References 86 CHAPTER 4 Tetra Tolyl Porphyrins: Studies of third-order optical nonlinearity and nonlinear absorption. 4.1 Introduction 90 4.2 Experimental Details 93 4.2.1 Degenerate Four Wave Mixing 93 4.2.2 Z-scan 94 4.3 Results and Discussion 94 4.3.1 Origin of large nonlinearity in the ns regime 104 4.3.2 Comparison with other class of porphyrins 109 4.4 Conclusions 112 4.5 References 115 CHAPTER 5 Excited State Dynamics in Tetratolyl Porphyrins studied using Incoherent Light and 35 psec pulses. 5.1 Introduction 119 5.2 Experimental Details 120
ix 5.3 Results and Discussion 122 5.3.1 DFWM IL Results 122 5.3.2 DFWM PS Results 127 5.4 Conclusions 136 5.5 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 142 6.2 Degenerate Four Wave Mixing 143 6.2.1 χ (3) Measurements 143 6.2.2 Temporal response of the DFWM signal 147 6.3 Z-scan studies 153 6.3.1 Reverse Saturable Absorption in C 60 153 6.3.2 Dispersion studies of nonlinear absorption 156 6.3.3 Z-scans at 532 nm and 600 nm using Nd: YAG laser 167 6.4 Conclusions 170 6.5 References 172 CHAPTER 7 Phthalocyanines: DFWM-IL Results 7.1 Introduction 179 7.2 Experimental Details 180 7.3 DFWM-IL results on Phthalocyanines 180 7.4 References 184 Conclusions 186 Future Scope 188 Curriculum Vitae