Properties of modified silica detected within laser-induced damage sites

Following initiation at absorbing surface flaws, laser-induced damage (λ = 0.35 μm) on polished fused-silica surfaces continues to grow with subsequent laser pulses. The growth process is believed to be associated with a plasma-induced transformation of the silica to a highly absorbing state. A number of optical spectroscopy techniques have been used to characterize the nature of the laser-modified silica. Increases in the concentration of defect states is suggested by the spectral characteristics of the emission. Photoluminescence spectroscopy detects three characteristic emission peaks within damage sites when illuminated at 355 nm. Two of the peaks are likely due to the well-known non-bridging oxygen hole centers (NBOHC; broken Si-O bonds) and oxygen deficiency centers (ODC). The third, and dominant, peak at 565 nm has not been clearly identified, but may be associated with small clusters of Si atoms. Raman spectroscopy suggests densification which is associated with decreases in characteristic ring sizes within the amorphous silica structure. A 3-D model for the nature of laser-damage sites in silica is being developed. This model will provide the basis for the development of strategies to slow or stop the rate of damage growth.