<title>Role of defects in the ablation of wide-bandgap materials</title>

Abstract

In previous work we examined the particle emissions and changes in the surface topography of sodium trisilicate glass (Na2O(DOT)3SiO2) accompanying exposure to pulsed 248 nm excimer laser light. This material ablates readily over a wide range of fluences (> 2.6 J/cm2) after a fixed number of preliminary laser pulses (an incubation effect). The effectiveness of laser bombardment in removing material is strongly dependent on defects produced by high fluence 248 nm radiation. We show a dramatic synergism in the ablation process at sub-threshold and near-threshold fluences by simultaneous bombardment of the glass surface with 0.5 - 2 keV electrons and laser pulses. Similar results are obtained on single crystal NaCl, LiF, and UV grade fused silica. We attribute this effect to surface defects produced by the electron beam. A model is discussed involving surface and near-surface defects created by the electron beam which allow for single photon excitations of electrons to the conduction band and subsequent free-electron/laser heating. Preliminary results with MgO suggest that certain impurities can have the opposite effect by trapping electrons at defects from which single photon absorption does not yield conduction band electrons. This reduction in laser-material coupling may occur despite enhanced absorption at the laser wavelength. Thus the effectiveness of defects in enhancing laser-material interactions depends strongly on their ability to provide high cross section excitations to the conduction band. The potential of these effects in the control of laser-material interactions in optical materials and in laser processing of materials is discussed.