We present a numerical model of internal modification in bulk borosilicate glass by high repetition rate picosecond laser pulses. We study free-electron dynamics, nonlinear energy deposition and thermal conduction. The optical absorptivity and modification regions both have good agreements with the experimental results. The smooth outer zone is the molten region and the inner-structure formation is caused by high-density free-electrons generated by thermal ionization. Excitation, relaxation and accumulation of free-electron density in the focal volume are analyzed using different pulse shapes and a double-pulse train. The deposited energy distribution and modification zone are controlled by pulse shaping.

Ultrafast reflection and secondary ablation have been theoretically investigated with a Fresnel-Drude model in laser processing of transparent dielectrics with picosecond pulsed laser. The time-dependent refractive index has a crucial effect on the cascade ionization rate and, thereby, on the plasma generation. The relative roles of the plasma gas and the incident angle in the reflection are discussed in the case of the oblique incidence. The angular dependence of the reflectivity on the laser-excited surface for s- and p-polarization is significantly different from the usual Fresnel reflectivity curve in the low-fluence limit. A road map to the secondary ablation induced by the reflected pulse is obtained on the angles of the first and second incidence. It indicates that the laser-induced plasma plays a major role in the secondary ablation, which could overcome the saturation of the ablation crater depth or generate microcracks underneath the crater wall.

We study the roles of thermal ionization and electronic damage in the internal modification of bulk borosilicate glass by high repetition rate picosecond laser pulses. Laser-induced plasma generation, nonlinear energy deposition and steady temperature distribution are numerically analyzed. The simulated modified regions show good agreement with the experimental results, thereby revealing the roles of thermal damage and electronic damage in the internal modification. While the elliptical outer structure is recognized as the molten region, we found that the teardropshaped inner structure is the damaged zone caused by high-density freeelectrons. In the formation of the inner structure, cascade ionization is seeded by thermal ionization instead of multi-photon ionization and dramatically increases the free-electron density to the damage threshold. The contour of the inner structure is found to be corresponding to a characteristic isotherm of around 3000 ~4000 °C.

This study presents a novel numerical model for laser ablation and laser damage in glass including beam propagation and nonlinear absorption of multiple incident ultrashort laser pulses. The laser ablation and damage in the glass cutting process with a picosecond pulsed laser was studied. The numerical results were in good agreement with our experimental observations, thereby revealing the damage mechanism induced by laser ablation. Beam propagation effects such as interference, diffraction and refraction, play a major role in the evolution of the crater structure and the damage region. There are three different damage regions, a thin layer and two different kinds of spikes. Moreover, the electronic damage mechanism was verified and distinguished from heat modification using the experimental results with different pulse spatial overlaps.