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Correlation of UV damage threshold with post-annealing in CVD-grown SiO2 overlayers on etched fused silica substrates

Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present a study relating structural and electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition annealing temperature (THT). Plasma-enhanced CVD, based on SiH4/N2O under oxygen-rich conditions, was used to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO2 substrates. Rapid annealing was performed using a scanned CO2 laser beam up to THT~2100 K. The films were then characterized using X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual transition in the damage threshold of annealed films was observed at THT up to 1600 K, correlating with a decrease in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850 K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is proposed, and the potential of using high-THT CVD SiO2 to mitigate optical damage is also discussed.