Theoretical treatments of ultrashort pulse laser processing of transparent materials: toward understanding the volume nanograting formation and quill writing effect

The dynamics of ultrashort-laser-induced generation of free electron plasma inside bulk glass is analyzed. The results of modeling are presented for typical glass modification regimes, obtained on the basis of Maxwell's equations supplemented with the equations describing electron plasma formation and the laser-induced electric current. The model has been demonstrated to allow 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 geometry of the laser energy absorption zone is determined, and the glass temperature is mapped which may be foreseen at the end of electron-glass matrix relaxation. This, in turn, allows estimating the laser-induced stress levels and making conclusions on the routes of glass modification. Finally, based on the modeling results, the plausible mechanisms are discussed which may be responsible for the 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.