Impact of High Power Laser Fluence on the Chemical Signatures of Silica Aerogel Films

Silica aerogel thin films can be useful as a very low-index optical material in high average power optical coatings due to their combination of high porosity and the intrinsic resistance of silica to laser damage in both clean and particle-rich environments. Water vapor and particles can make an aerogel less optically transparent. To avoid this, hydrophobic functional groups need to be grafted onto the aerogel surface by means of a surface modification process. However, these functional groups can thermally decompose at high temperatures under atmospheric conditions. Since the optics used in high energy laser (HEL) applications, specifically under unwanted particle contamination, can potentially reach temperatures greater than 600 °C, it is important to investigate the chemical stability of hydrophobized silica aerogel thin films under high power laser illumination. Silica aerogel thin films with a very low refractive index near 1.06, were fabricated. The resulting silica thin films were contaminated with carbon particles and tested at different spots over a range of irradiance levels using a high-power continuous wave (CW) laser. The maximum temperature reached at each spot was recorded, and high temperature locations were optically imaged. XPS analysis on these spots showed that silica aerogel thin films showed little change for maximum temperatures lower than 450 °C. Hydrolyzation occurred near 450 °C, and more significant breakdown of hydrophobic functional groups can be seen at 580 °C (or more), at which point visible laser damage is typically seen. Absorption measurements showed no significant changes after the laser shots, except for a slight increase from 15 to 18 ppm near damaged areas. Although silica aerogel films showed reasonable chemical stability below the damage threshold temperature of 580 °C (irradiance level −3 MW/cm2), it is necessary to maintain the temperature below 450 °C to keep their optimal qualities in optical transparency and laser-induced damage thresholds.