Birefringence of AlGaAs/GaAs Coatings under Above-Band-Gap Illumination, GR Noise and Photo-Optic Transfer Function

TL;DR

This study investigates the birefringence behavior of AlGaAs/GaAs coatings under above-band-gap illumination, developing a theoretical model that explains frequency and intensity dependencies and predicts photon-induced noise relevant for gravitational-wave detectors.

Contribution

The paper introduces a new theoretical model based on a master equation that captures the frequency and intensity dependence of birefringence and predicts photon-induced noise in AlGaAs/GaAs coatings.

Findings

Measured low-frequency birefringence aligns with previous studies.

Identified a frequency-dependent coupling behavior with illumination.

Predicted the presence of generation-recombination noise in the coatings.

Abstract

AlGaAs/GaAs coatings are being considered as coating candidates for gravitational-wave detectors. In this paper we investigate the birefringence properties of this crystalline semiconductor material by modulating the optical illumination on the mirror coating and monitoring the induced birefringence. While the measured low-frequency birefringence values align with previous studies, we observed a frequency-dependent behavior in the illumination-to-birefringence coupling, characterized by a pole increasing with illumination intensity and a DC gain decreasing with illumination intensity. We developed a theoretical mode based on a master equation to characterize the measurement results by considering photon-induced electric fields and electro-optical effects. This model reproduces the frequency and intensity dependencies of the induced birefringence. Additionally, this model predicts a generation-recombination noise (GR noise) will be observable in the coatings birefringence. While the presented measurement cannot predict the exact level of GR noise, for the frequency band and spot sizes relevant for gravitational-wave detectors we expect GR noise to be white below the pole frequency, scale with power the same way laser shot noise does, and for fixed power be independent of spot size.