Nonlinear and Multidimensional Terahertz Spectroscopy of Liquids and Crystalline Solids

Citation

Lin, Haw-Wei (2024) Nonlinear and Multidimensional Terahertz Spectroscopy of Liquids and Crystalline Solids. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2qt0-mg05. https://resolver.caltech.edu/CaltechTHESIS:01082024-171459002

Abstract

The delocalized and correlated nuclear degrees of freedom in the terahertz (THz) regime strongly influences the room temperature chemical and physical properties of condensed matter systems, yet detailed understanding of the photo-induced dynamics and anharmonicities of the vibrational modes have remained elusive. In hydrogen-bonded liquids, these information facilitates the development of accurate force field models to aid simulations of biological processes of proteins and DNAs. In the field of nonlinear phononics, anharmonic lattice vibrations form the foundation for ultrafast coherent control of material properties, which has become an indispensable technique in the engineering toolbox for quantum materials. In this thesis, we demonstrate the development and application of nonlinear 1D THz Kerr effect (TKE) and 2D THz-THz-Raman (2D-TTR) ultrafast THz spectroscopies, which are specially designed to induce resonant coherent excitations of the correlative nuclear degrees of freedom in liquids and crystalline solids. By analyzing the temporal evolution of the nuclear THz-driven dynamics, insights into the excitation mechanisms, nonlinear coupling interactions, and the dominant source(s) of anharmonicity may be determined. Specifically, we developed a nonlinear imaging method based on the third-order response of electro-optic crystal GaP, which significantly improved the alignment consistency and the signal strength of 2D-TTR spectroscopy. Further, we extended an echelon-based single-shot detection scheme, originally developed in 1D TKE spectroscopy, to 2D-TTR spectroscopy, which led to up to two orders-of-magnitude reduction in acquisition time. Armed with these instrument advancements, we measured 2D-TTR spectra of liquid halogenated methanes with significantly improved signal-to-noise and a larger temporal window, which led to the identification of a novel competing sum-frequency THz excitation pathway. On the other hand, we investigated resonant driven-dynamics of the phonon-polariton modes in semiconductor LiNbO$_3$, which revealed nonlinear coupling interactions between two phonon branches that are attributed to mechanical anharmonicity. In addition, we directly observed photo-induced coherent phonon wavepackets for the layered semiconductors transition metal dichalcogenide using 1D TKE spectroscopy, which are attributed to the sum-frequency excitation pathway. These works highlight the rigorous experimental considerations and careful spectral analysis required to extract essential insight into excitation mechanisms and anharmonic contributions, while avoiding spectral artifacts due to the instrument response function. In order to provide clarity to these often misunderstood spectroscopies in the THz regime, this thesis further summaries the theories behind 1D TKE and 2D-TTR spectroscopies and the lessons we have learned from experimental realization of these exotic instruments and the analysis of complex spectral features.

Thesis Files