High average power properties of porous silica thin films and all-silica high-reflectivity mirrors

Abstract

In this project, deposition techniques were developed to put down porous silicafilms, with the goal of integrating them into optical multilayer structures to maximize their optical resistances. Since laser induced damage thresholds (LIDT) increase with increasing bandgap, and silica films have the highest bandgap of commonly used optical materials, multilayers made of a single material, silica, should have high optical resistance. In the first part of this project, porous nano-columnar silica films (n~1.27) were fabricated by oblique angle deposition (OAD) in an e-beam evaporator and characterized for their optical properties. All-silica mirrors were then fabricated by putting down OAD porous silica as low-index layers and evaporated dense silica as high-index layers. Their contamination- induced LIDTs are higher than those of corresponding high-reflectivity (HR) mirrors made of two materials. In the second part, a new process was developed to deposit an ultra-low index silica film to replace OAD silica since using lower index silica would significantly reduce the number of layers required in HR mirrors to maximize reflectivity. Silica aerogel thin films were fabricated by spin-coating a silica sol, synthesized by a two-step acid/base catalyzed technique at ambient pressure. Silica aerogel films showed refractive index as low as 1.06. They also have high contamination-induced LIDTs, comparable to those of conventional PECVD silica films. Impact of high-power laser fluence on silica aerogel films were also investigated. XPS analysis showed that aerogel films started to lose their surface hydrophobic layers when the laser heated them beyond 450℃, which made them vulnerable to attracting vapor and particles from the environment and lowered their optical transparency and laser-induced damage thresholds.