Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VII. Localized plasma formation on NaCl single crystal surfaces

Wide band gap insulators containing defects exposed to nanosecond pulses of UV laser radiation at fluences close to the damage threshold often display highly localized flashes of light. In this work, we show that flashes obse rved during irradiation of cleaved, single crystal NaCl at relatively low f luences are due to localized plume fluorescence. By comparing time-resolved optical images of this fluorescence with subsequent scanning electron micr oscope images of surface topography, we show that these flashes are often a ssociated with micron-dimension surface and near-surface damage, typically associated with cleavage steps. With continued laser irradiation, plume flu orescence at previously damaged regions usually grows stronger from pulse t o pulse. In some cases, weak plume fluorescence disappears after one laser pulse, and may or may not reappear with continued irradiation. We interpret these results in terms of localized laser absorption by deformation-induce d defects generated during cleavage. Deliberately deformed material, produc ed by indentation, is damaged at considerably lower laser fluences, consist ent with this interpretation. We suggest that mobile excitations produced b y laser absorption preferentially decay along dislocation cores, which stro ngly localizes laser-induced thermal stresses and damage. (C) 2001 American Institute of Physics.