Crystalline/amorphous silicon and silica Bragg reflectors for gravitational wave detectors

Abstract

Coating thermal noise is a major source of noise in gravitational wave detection. In order to reduce this noise, we investigate the use of silicon-on-insulator (SOI) wafers as Bragg reflectors. First, we show different architectures, such as multi-SOI as top and bottom layers, in combination with amorphous silicon or Ti-doped tantala or germania, and silica, which potentially reduce the coating thermal noise by factors between 4.2 and 1.5 (i.e., down by about 24% to 68% of its current value) respectively. We discuss the advantages and disadvantages of each architecture and challenges associated with their fabrication. Then, we present the optical properties of a stack featuring a single SOI and a double SOI. Ion beam analysis combined with ellipsometry allowed to check that the layer composition, density and thickness were as designed. We conclude that such architectures are promising solutions for future detectors provided that they can be fabricated with minimum defects.