Date of Award
2026
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Physics
Department
Physics
First Advisor
Feruz Ganikhanov
Abstract
This dissertation applies time-resolved coherent anti-Stokes Raman scattering (tr-CARS), to measure ultrafast vibrational dynamics in technologically important solid-state and van der Waals molecular systems. The central goal is to use time-domain Raman methods to resolve mode-specific phonon and vibrational dephasing processes with high temporal precision, high spectral selectivity, and sufficient experimental flexibility to probe materials ranging from wide-bandgap semiconductors and oxide crystals to energetic molecular crystals. Across the work, I show that carefully designed tr-CARS measurements can access vibrational lifetimes from hundreds of femtoseconds to several picoseconds, enabling direct observation of decay pathways, relaxation mechanisms, and dispersion effects that shape the fidelity of ultrafast Raman measurements.
A major theme of the dissertation is the measurement of optical phonon decay in wide-bandgap materials using time-domain coherent Raman spectroscopy. In these studies, vibrational modes were selectively excited and monitored with better than 120 fs time resolution and equivalent spectral resolution approaching 0.1 cm-1, allowing intrinsic phonon dynamics to be traced across multiple decay orders. The measured phonon decay times spanned approximately 0.45 to 1.7 ps, demonstrating substantial mode dependence and revealing the importance of third- and fourth-order parametric phonon interactions as dominant decay channels. These results provide quantitative estimates for zero-temperature decay rates and Raman linewidths for key Raman-active vibrations, establishing time-domain coherent Raman spectroscopy as a powerful tool for probing lattice dynamics in materials relevant to next-generation electronics and photonics.
The dissertation also extends tr-CARS to energetic molecular crystals, where ultrafast vibrational relaxation plays a central role in energy redistribution and initiation dynamics. Using pentaerythritol tetranitrate (PETN) as a model system, mode-specific dephasing was measured over a broad range of coherence lifetimes, from sub-picosecond to greater than 4 ps. These measurements show that different vibrational modes relax on markedly different timescales, underscoring the importance of mode selectivity in understanding how energy flows through molecular solids following impulsive excitation. By resolving these lifetimes directly in the time domain, the work provides experimental constraints on microscopic relaxation pathways that are difficult to obtain through conventional steady-state spectroscopy alone.
In addition to materials measurements, the dissertation includes the design and optimization of the tr-CARS instrumentation needed to support high-resolution vibrational spectroscopy. A Ti:sapphire laser system synchronized to periodically poled stoichiometric lithium tantalate (PPSLT) optical parametric oscillators (OPOs) was analyzed in detail to quantify the influence of group-delay dispersion, pulse compression, and power-dependent broadening on the effective temporal resolution at the sample plane. Optical components were found to broaden pulses from 157 fs to more than 200 fs, while prism-pair compression restored near-transform-limited durations of about 135 fs at the OPO crystals and about 165 fs at the sample. The OPOs themselves were shown to exhibit intracavity dispersion, yielding uncompressed pulse widths near 267 fs and compressed pulses near 90 fs but with a significant residual pedestal caused by higher-order phase distortions. These studies establish practical dispersion-management strategies and define the operating limits of a stable ultrafast source for precision tr-CARS experiments.
A further contribution of the dissertation is the use of periodically poled lithium niobate (PPLN) based femtosecond optical parametric oscillators as dual-wavelength excitation sources for time-resolved CARS. The PPLN OPO produced signal output at 1064 nm and 1111 nm, demonstrating a flexible excitation architecture for coherent Raman measurements. This platform was then applied to the A1(LO) phonon mode of lithium niobate, where polarization-dependent measurements yielded a phonon decay time of approximately 380 fs. This result reinforces the broader conclusion of the dissertation: coherent Raman techniques can directly resolve ultrafast lattice dynamics in functional oxide materials.
Overall, this dissertation demonstrates that time-resolved coherent Raman spectroscopy is a versatile and highly resolving approach for studying nonequilibrium vibrational dynamics in materials of practical and fundamental interest. By combining instrument development with application-driven measurements, the work links ultrafast nonlinear optics to phonon physics, materials characterization, and energy-transfer dynamics. The results advance understanding of lattice and vibrational relaxation in wide-bandgap semiconductors, energetic molecular crystals, and nonlinear crystals, while also providing a framework for future tr-CARS studies aimed at resolving even faster and more complex vibrational processes.
Recommended Citation
Senarathna, Dinusha Madushanka, "ULTRAFAST VIBRATIONAL DYNAMICS IN IMPORTANT WIDE-BANDGAP SEMICONDUCTORS, DIELECTRICS AND ENERGETIC MATERIALS" (2026). Open Access Dissertations. Paper 4600.
https://digitalcommons.uri.edu/oa_diss/4600