Influence of polarization and material on Brownian thermal noise of binary grating reflectors

Grating reflectors are a potential low-noise replacement for amorphous multilayer mirrors. We investigate the influence of polarization and refractive index on Brownian thermal noise of binary grating reflectors using Maxwell's stress tensor. Our results demonstrate that the refractive index of the grating material is a critical parameter for thermal noise in these structures. In contrast to multilayer mirrors, a low coating thickness does not necessarily lead to a low thermal noise amplitude for structures with low refractive index. We find that an improved noise performance of grating reflectors requires materials of refractive index ≳2.5. We present a factorized expression for the thermal noise of grating reflectors made of arbitrary materials by simply scaling the noise amplitude with the related material parameters.