Birefringence in a Silicon Beamsplitter at 2um for Future Gravitational Wave Detectors

TL;DR

This paper measures the birefringence of silicon beamsplitters at 2 micrometers, highlighting its implications for future cryogenic gravitational wave detectors and supporting the use of silicon in this context.

Contribution

It provides the first detailed measurement of birefringence in <100> float zone silicon beamsplitters at 2 micrometers, emphasizing its significance for detector sensitivity.

Findings

Birefringence varies between 3.44e-7 and 1.63e-7

Estimated birefringence along <110> axis is 1.64e-6

Results support the use of <100> silicon at 2 micrometers

Abstract

The next generation of gravitational wave detectors will move to cryogenic operation in order to reduce thermal noise and thermal distortion. This necessitates a change in mirror substrate with silicon being a good candidate. Birefringence is an effect that will degrade the sensitivity of a detector and is of greater concern in silicon due to its crystalline nature. We measure the birefringence in a <100> float zone silicon beamsplitter since we expect there to be a large inherent birefringence due to the spatial dispersion effect. We observe that the birefringence varied between $3.44 \pm 0.12 \times 10^{-7}$ and $1.63 \pm 0.05 \times 10^{-7}$ and estimate the birefringence along the <110> axis to be $1.64 \pm 0.5 \times 10^{-6}$ at 2um. We demonstrate this effect and argue that it strengthens the case for 2um and <100> silicon.