THE ROLE OF DEFECTS IN THE REAR SIDE LASER-ABLATION OF MGO AT 308 NM

Over a range of fluences, 308 nm pulsed laser irradiation of cleaved, single crystal MgO can produce a fluorescent plume on the side of the specimen opposite the incident Light (rear side ablation). After one o r two laser pulses, linear patterns of melted material are often obser ved. We present evidence that Fresnel diffraction from heterogeneous f eatures on the front surface control the formation of the localized in teractions on the rear side. At fluences below the threshold for plume formation on the front surface, large areas of the rear surface can b e melted and ablated without fracture and pitting. At higher fluences, defect accumulation on and near the front surface eventually yields a front surface plume and eliminates the rear surface plume. Mechanical treatments strongly affect the evolution of these features due to the production of strongly absorbing point defects. (C) 1996 American Ins titute of Physics.