Announcements

Abstract

This thesis studies damage-threshold estimation in ultrashort laser interactions with metallic thin films. The damage threshold is the incident laser fluence at which the film reaches melting onset. Estimating this threshold can be computationally expensive, because each evaluation requires two-temperature-model simulations and repeated threshold searches. This motivates surrogate models for faster threshold prediction.

The physical model is the axisymmetric parabolic two-temperature model (pTTM), which describes nonequilibrium energy exchange between laser-excited electrons and the lattice after femtosecond laser deposition. The work develops optical, thermophysical, and numerical components for threshold calculations, including temperature-dependent material response, ballistic and finite-thickness source corrections, implicit and IMEX time-integration schemes, and bracketing with bisection for threshold search. The numerical schemes are supported by empirical convergence and stability analyses.

The study uses one- and two-dimensional pTTM simulations to generate synthetic data for supervised regression. The surrogate target is the maximum lattice temperature, and threshold fluences are recovered by inverting the predicted temperature-fluence response. The surrogate analysis includes a two-dimensional linear pTTM dataset and a nonlinear two-dimensional extension with temperature-dependent optical properties, electron heat capacity, and electron-phonon coupling. LightGBM and multilayer perceptron models are evaluated on synthetic test sets and material-specific fluence cases.

The results show that the trained surrogates reproduce pTTM temperature targets within the sampled ranges and provide computationally efficient threshold estimates. For Ni and Ti, the nonlinear surrogate reduces the discrepancy between surrogate predictions and pTTM fluence-dependent calculations. Threshold estimates for Ag, Ni, and Ti are compared with reference values from the RIANA/PLAN-hub thermal-response calculator.

Abstract

The discovery of graphene in 2004 marked the emergence of a new class of materials known as two-dimensional (2D) materials. Due to their remarkable physical properties and wide range of applications in optoelectronics and photonics, 2D materials, and in particular the transition metal dichalcogenides (TMDs-with the formula MX₂, M standing for metal and X for chalcogenide) and the 2D monochalcogenides (MX), have become the subject of extensive research. This includes the development of optical characterization techniques, capable of providing 2D crystal structural information, at the atomic level.

In this study, we use the ability of 2D MX₂ and 2D MX to produce optical second harmonic generation (SHG) and we present a formalism based on nonlinear optics in crystals to extract the armchair direction, the twist-angle in MX₂ bilayers and probe the in-plane anisotropy in MX monolayers, by performing polarization-resolved SHG (P-SHG) measurements. Finally, we show that by tuning the twist angle in a MX₂/MX heterobilayer, the in-plane anisotropy of the material can be controlled.