Apparatus for the measurement of birefringence maps of optical materials: the case of crystalline silicon for Einstein Telescope

TL;DR

This paper introduces a highly sensitive optical polarimeter for two-dimensional birefringence mapping, applied to crystalline silicon, crucial for optimizing next-generation gravitational-wave detectors like the Einstein Telescope.

Contribution

The paper presents a novel optical polarimeter system capable of spatially mapping birefringence in optical materials, specifically applied to crystalline silicon for gravitational-wave detector substrates.

Findings

Birefringence in silicon samples is around 10^{-7}.

Birefringence varies spatially across silicon samples.

Intrinsic birefringence measured on (110) surfaces.

Abstract

Einstein Telescope (ET) is expected to achieve sensitivity improvements exceeding an order of magnitude compared to current gravitational-wave detectors. The rigorous characterization in optical birefringence of materials and coatings has become a critical task for next-generation detectors, especially since this birefringence is generally spatially non-uniform. A highly sensitive optical polarimeter has been developed at the Department of Physics and Earth Sciences of the University of Ferrara and INFN - Ferrara Section, Italy, aimed at performing two-dimensional birefringence mapping of substrates. In this paper we describe the design and working principle of the system and present results for crystalline silicon, a candidate material for substrates in the low-frequency (LF) interferometers of ET. We find that the birefringence is of order $10^{-7}$ for commercially available samples and is position dependent in the silicon (100)-oriented samples, with variations in both magnitude and axis orientation. We also measure the intrinsic birefringence of the (110) surface. Implications for the performance of gravitational-wave interferometers are discussed.