Theoretical treatments of ultrashort pulse laser processing of transparent materials: towards explanations of extraordinary phenomena

Abstract

The dynamics of ultrashort-laser-induced generation of free electron plasma inside bulk glass (fused silica as an example) is analyzed. The results of modeling are presented for typical glass modification regimes, obtained on the basis of the Maxwell equations supplemented with the equations describing electron plasma formation and the laser-induced electric current. We demonstrate that the model allows revealing important features of laser beam propagation in the regimes of dense electron plasma generation such as strong scattering up to complete displacing of light from the plasma region followed by beam refocusing. The results are compared with those obtained in the frames of the non-linear Schrödinger equation (NLSE). It is demonstrated that a unidirectional approximation for laser beam propagation under the laser breakdown conditions does not provide an adequate description. Moreover, the NSLE may lead to overestimating the absorbed laser energy up to several times as compared to a more rigorous approach based on the Maxwell equations. A controversial issue of the density level of free electron plasma generated inside bulk glass by ultrashort laser pulses is discussed. The energy balance of excited matter is considered with introducing an "Ee - ne" diagram which matches the level of transient excitation with the maximum temperature of the glass matrix.
From the simulation data on the geometry of the laser energy absorption zone, the glass temperature is mapped which may be foreseen at the end of electron - glass matrix relaxation. This, in turn, allows calculations of the laser-induced stress levels and making conclusions on the routes of glass modification. Based on the modeling results, we propose the mechanisms responsible for two unusual effects, formation of volume nanogratings in a number of transparent solids under the action ultrashort laser pulses and laser direct writing anisotropy observed for laser pulses with a tilted front. Finally we address the question on the material properties which are responsible for nanograting imprinting to the glass matrix.

More information

Identifiers

Catalogue record

Export record