|Abstract or Summary
- In this thesis, I present a new method we developed to study low frequency (< 700 cm⁻¹) vibrational dynamics: Time-resolved third-harmonic generation (TRTHG) spectroscopy. Among a variety of vibrational spectroscopy techniques, TRTHG differentiates itself with robustness, versatility, and simplicity. In TRTHG experiment, that involves only two ultrashort laser pulses, the first pulse is used to impulsively generate coherent Raman vibrations, and potentially electronic coherence which interacts with the time-delayed second pulse and emits a background-free third-harmonic signal in between the two incident pulses. It has been applied to study low-frequency vibrations in various condensed matter, which are intrinsically related to important physical and chemical properties, such as thermal conductivity and mode anharmonicity. We first achieved and characterized third-harmonic generation on BK7 glass-air interface with weak incident laser pulses, and acquired Raman spectrum of the amorphous medium. By increasing the intensity of the pump laser pulse, we characterized in-situ laser-induced crystallization process in glass. We also revealed dynamics of low-frequency optical phonons in CaF₂ and calcite using TRTHG. Finally, by incorporating a flowing liquid sample jet to TRTGH setup, we successfully studied low-frequency vibration dynamics on the sub-picosecond timescale in pure water and electrolyte in an aqueous solution (i.e., 4 M ZnCl₂ in water) and corroborated using femtosecond stimulated Raman spectroscopy.