We have investigated the onset of optical breakdown in silica glass under irradiation of KrF excimer laser by luminescence spectroscopy, luminescence imaging, and SEM observation. When the laser fluence is increased just below the damage threshold, appearance and disappearance of localized luminescence are often observed under cumulative irradiation, and finally a bright luminescence due to optical breakdown occurs. The former is due to micro-plume, and the creation and relaxation of defects should cause the flashing of the luminescence, while the latter is due to bright plume. SEM images of the laser-damaged region shows pits of the order of 1 micron, suggesting that the local heating and vaporization of the particles on the surface layer cause the optical breakdown.
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