Birefringence Measurements on Crystalline Silicon

TL;DR

This study measures and models birefringence in crystalline silicon, a candidate material for gravitational wave detectors, finding low birefringence levels and suggesting minimal impact from elastic strains.

Contribution

It introduces a polarization-dependent resonance frequency analysis method for measuring silicon birefringence and presents a model explaining the observed birefringence sources.

Findings

Silicon birefringence is approximately 10^{-7} at 1550nm

Elastic strains cause superimposed birefringence effects

Elastic strains are unlikely to critically affect ET test mass performance

Abstract

Crystalline silicon has been proposed as a new test mass material in third generation gravitational wave detectors such as the Einstein Telescope (ET). Birefringence can reduce the interferometric contrast and can produce dynamical disturbances in interferometers. In this work we use the method of polarisation-dependent resonance frequency analysis of Fabry-Perot-cavities containing silicon as a birefringent medium. Our measurements show a birefringence of silicon along the (111) axis of the order of $\Delta\, n \approx 10^{-7}$ at a laser wavelength of 1550nm and room temperature. A model is presented that explains the results of different settings of our measurements as a superposition of elastic strains caused by external stresses in the sample and plastic strains possibly generated during the production process. An application of our theory on the proposed ET test mass geometry suggests no critical effect on birefringence due to elastic strains.