FEMTOSECOND TIME-RESOLVED COHERENT RAMAN SPECTROSCOPY OF EQUILIBRIUM PHONONS IN CUBIC PEROVSKITES

Essential to green energy technologies are new materials for high-power electronics, especially wide-bandgap semiconductors such as Barium Stannate (BSO) and Strontium Titanate (STO) which promise outstanding performance in high power transistor applications. Design and performance of these materials relies on detailed knowledge of basic physical parameters, including charge-carrier mobility. The mobility values can be as high as 500 cm2/Vs for electron concentrations >1x1019 /cm3 provided that the effect of impurity scattering is reduced using special doping techniques. In this case, the inelastic carrier-phonon scattering is the only mechanism that strongly affects the mobility as the polar optical (LO) phonons can mediate energy exchanges between the hot carriers and the lattice. In turn, carrier-phonon scattering rates directly depend on the phonon population and decay that are governed by phonon-phonon interactions. In addition to the fundamental point of view, lattice vibrations (LO and TO-phonons) and their characteristics are key in quantifying and modeling the oxide material’s complex dielectric function. Precision measurements of the phonon relaxation rates are critical from both the standpoints of practical device applications of the materials and fundamental topics in condensed matter and plasma physics. Precision that can be delivered by time domain coherent Raman techniques cannot be attained by existing traditional measurement techniques, like spontaneous Raman scattering. This study has been focused on direct measurement of the phonon decay time in high-bandgap semiconductors of high interest for high-power applications utilizing a sophisticated time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy setup. These experiments and measurements have been done for the first time and the results provide phonon decay times and linewidth parameters. The precision in linewidth measurement exceeds other techniques. The dissertation is composed of three published papers. The first paper details the experimental setup built to perform time-delayed CARS spectroscopy. It features highpowered output from two synchronously pumped Optical Parametric Oscillators, the second of which is widely and continuously tunable from 1030-1350 nanometers. The versatility of the system allows for probing of Raman resonances from 250-2400 cm-1 with resolution of dynamic processes on the femtosecond timescale. The second paper presents the results of measurements on a sample of barium stannate, BaSnO3 (or BSO). The third paper details the result of measurements on strontium titanate, SrTiO3 (or STO). These materials are wide-bandgap semiconductors with potential uses in high-power systems such as electric vehicles and other components of green technology. ACKNOWLEDGEMENTS The completion of this dissertation has been a multi-year process which would not have been possible without the assistance of several fellow graduate researchers, graduate faculty, and other academic organizations. The majority of this research was completed under grant from the United States Air Force Office of Scientific Research (FA9550-18-10273). It was completed at facilities at the University of Rhode Island, Morril Hall under the tutelage of Dr. Feruz Ganikhanov. I would like to give special thanks to the Center for Quantum Materials Synthesis, Rutgers University, and the Department of Chemical Engineering and materials Science at the University of Minnesota for producing and sharing the samples used in this research. None of this endeavor would have been possible without fellow researchers Chandra Neupane, Helani Singhapurage, and Dinusha Senarathna. Together, our team proved that a large and complex endeavor would not achieve success without a large and diverse skill set. Long hours and hard work across many years was required to produce results for this and other works. Lastly, these experiments would not have occurred without the tutelage and oversight of Dr. Feruz Ganikhanov. His depth and detail of knowledge of solid-state physics, spectroscopy, and optical science has enabled previous and current PhD candidates to begin and complete their efforts towards attaining the highest degree and improve upon the body of knowledge available to humanity. Even as my work terminates, two additional graduate researchers join the team to continue our efforts. I wish them all the best success.