Role of defects in the laser ablation of wide bandgap materials

Abstract

In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent), the absorption of laser radiation at 248 nm (5 eV photons) at modest fluences is defect dominated. We describe a technique for imaging the initial defect densities by their luminescence at low laser fluences and show a typical photoluminescence image of cleaved MgO. High defect densities are observed along many cleavage steps, consistent with previous observations of strong point-to-point variations in the ablative response of cleaved MgO surfaces. At fluences below those required for sustained emission, the composition of neutral emissions from the surface can also be strongly influenced by impurity defects, as shown by the intense emission of carbon oxides and the correspondingly weak emissions of atomic and molecular oxygen from arc-fused MgO. We also present evidence for defect-mediated ion emission at these low fluences.