Michael Tripepi

As in all physical models, the response of a system to external perturbations is only approximately linear and with greater driving force, becomes nonlinear. The study of light interactions in solids is no different and with the development of laser technology, this can be explored with great detail. The use of femtosecond, and even few-cycle pulse lasers, allows for the study of very intense, localized interactions, which can range from frequency generation and spectral broadening of the incident pulse to photoionization of electrons and optical damage of the material itself. This work presents a trio of experiments that investigate the interaction of strong laser pulses with crystalline solids using femtosecond pulses. The first experiment characterizes the supercontinuum generation (SCG) in undoped, single-crystal yttrium aluminum garnet (YAG) optical fibers. Pump wavelengths near the zero-dispersion wavelength of YAG (1600 nm) are used to study SCG in both the normal and anomalous group-velocity dispersion (GVD) regimes. The output spectra of the fiber over many input pulse energies are collected for each pump wavelength and presented in 2D "energy scans." Three different criteria are developed to help characterize the SCG in each energy scan. The second experiment looks at laser damage of gallium nitride (GaN) and gallium oxide (Ga2O3) using few-cycle pulses (FCPs) with a central wavelength near 760 nm. These experiments report on the damage thresholds as well as examine the crater morphology of single and multi-shot damage. Simulations using the Keldysh photoionization model and finite-difference time-domain method coupled to ionization and plasma effects are also employed to understand the carrier dynamics for single-shot exposure and possible mechanisms that differentiate FCP from longer femtosecond mechanisms. The final experiment returns to YAG in order to study damage under FCPs using a pump-probe technique known as time-resolved surface microscopy. The dynamics of the damage process are imaged from < 85 fs to 2 ns and analyzed using a Python code to extract the relative change in transmission along a vertical trace of the image. Using the Drude-Lorentz model, the change in transmission can be converted to the free electron concentration in the conduction band, which is used to estimate the relaxation time(s) for the carriers in YAG. An algorithm is also developed to overlay images of the damage crater and focal spot to quickly and robustly determine the damage threshold. Atomic force microscopy images of the damage craters revealing unusual characteristics for short pulse damage are also discussed.