Effect of vacuum on the occurrence of UV-induced surface photoluminescence, transmission loss, and catastrophic surface damage

Vacuum degrades the transmittance and catastrophic damage performance of fused-silica surfaces, both bare and silica-sol anti-reflective coated. These effects may be important in certain space application of photonics devices. When exposed to hundreds of 355-nm, 10-ns laser pulses with fluences in the 2-15 J/cm(2) range, transmittance loss is due to both increased reflectance and absorption at the surface. Spectroscopic measurements show that the absorbed light induces broadband fluorescence from the visible to infrared and that the peak photoluminescence wavelength depends cumulative fluence. The effect appears to be consistent with the formation of surface SiOx (x<2) with progressively lower x as cumulative fluence increases. Conversely, low fluence CW UV irradiation of fluorescent sites in air reduces the fluorescence signal, which suggests a photochemical oxidation reaction back to SiO2. The occurrence of catastrophic damage (craters that grow on each subsequent pulse) also increases in a vacuum relative to air for both coated and uncoated samples. In both cases, the 50% damage probability for 100 one-mm sites decreases from about 45 to 35 J/cm(2) for superpolished fused silica at pressures in the 10(-6) Torr range. The damage probability distribution in 10 Torr of air is close to that at one atmosphere of air. The damage morphology of the crater formed in vacuum differs substantially from that in air and has a more melted appearance and does not show cracking and flaking. These differences are possibly due to more coupling of the plasma shock wave into the surface with air present but slower heat dissipation in a vacuum. While it is attractive to propose that formation of sub-stoichiometric silica on the surface in a vacuum environment enhances the probability of catastrophic damage, initial experiments have not yet been able to establish a mechanistic link between the two phenomena.