High average power optical properties of silica aerogel thin film

Abstract

Silica aerogels synthesized by different techniques have been studied for their electrical and thermal insulating properties, mostly in their bulk structures; however, the optical properties of a silica aerogel thin film have remained largely unexplored. Due to their high porosities, silica aerogel thin films may be useful as very low-index optical materials, especially in multilayer coatings for high intensity and high-power lasers, where the large bandgap of silica is particularly valuable. In this paper, silica aerogel thin films were fabricated by spin-coating a silica sol, derived from a two-step acid/base catalyzed technique at an ambient pressure, on fused silica substrates. The films showed very low refractive indices (n) around 1.1 (approximately 72% porosity) and low absorptions between about 6 and 16ppm, lower than plasma-enhanced chemical vapor deposition (PECVD) comparison films. Optical scatterings of the silica aerogel films were measured and found to be comparable to a PECVD silica film and a bare fused silica substrate, with most films showing slightly higher scattering but one film showing lower. The laser-induced damage thresholds (LIDT) of all films were measured using carbon particle contamination, which allows testing over statistically large areas using continuous wave (CW) laser illumination. The LIDT of silica aerogels was similar to that of pure high-density silica and much higher than that of other common high-LIDT films such as hafnia and alumina. Most damage spots on silica aerogel samples only showed slight discoloration at irradiance levels of 150 kW/cm2 (1.5 × 109 W/m2), while similar tantala high reflectivity coatings failed catastrophically at 75 or 86 kW/cm2 (7.5 × 108 or 8.6 × 108 W/m2). Moreover, aerogels performed somewhat better than PECVD silica, where most damage occurred at a lower irradiance level of 100 kW/cm2 (1 × 109 W/m2) with a larger discolored spot.