Correlating optical damage threshold with intrinsic defect populations in fused silica as a function of heat treatment temperature

Chemical vapour deposition (CVD) is used for the production of fused silica optics in high-power laser applications. However, relatively little is known about the ultraviolet laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present here a study relating structural and electronic defects in CVD films to 355 nm pulsed-laser damage threshold as a function of post-deposition annealing temperature ( T HT ). Plasma-enhanced CVD based on SiH 4 /N 2 O under oxygen-rich conditions was used to deposit 1.5, 3.1 and 6.4 µ m thick films on etched SiO 2 substrates. Rapid annealing was performed using a scanned CO 2 laser beam up to T HT ∼ 2100 K. The films were then characterized using x-ray photoemission spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. A gradual transition in the damage threshold of annealed films was observed for T HT values up to 1600 K, correlating with a decrease in non-bridging silanol and oxygen deficient centres. 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- T HT CVD SiO 2 to mitigate optical damage is also discussed.