Multi-zone single-shot femtosecond laser ablation of silica glass at variable multi-photon ionization paths

Abstract

Single-shot femtosecond laser ablation of silica glass was studied at moderate peak intensities, providing laser energy deposition in the material via multi-photon absorption prior formation of dense opaque plasma, in order to evaluate the corresponding nonlinear absorption coefficients. For this purpose, tightly focused 515-nm, 220-fs laser pulses produce on a silica glass surface at variable pulse energies single-shot micro-craters, characterized by 3D-scanning confocal laser microscopy. Our analysis of crater diameter dependences on incident laser energy in terms of focusing parameters (Liu analysis) reveals in certain intensity ranges different multi-photon—one-, two-, and four-photon—absorption mechanisms with the increasing integer number of photons. Their depth profiles were considered as sets of distances, representing nonlinear transmission of the incident focused Gaussian beams until the fixed ablation threshold iso-fluence level, and analyzed in terms of the multi-photon absorption mechanisms. These inter-band transitions involve the electronic density-of-states tails in the bandgap (one- and two-photon transitions) and between the main valence and conduction bands (four-photon transitions). In these intensity ranges the depth of the single-shot craters can be fitted, accounting for the derived multi-photon absorption mechanisms, to evaluate the corresponding multi-photon absorption coefficients. The intensity-dependent variation of multi-photon absorption mechanisms provides the three-zone ablation both across the surface and into the bulk of the silica glass.