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

AbstractEinstein Telescope (ET) is expected to achieve sensitivity improvements exceeding an order of magnitude comparedto current gravitational-wave detectors. The rigorous characterization in optical birefringence of materials and coatingshas 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 ofthe University of Ferrara and INFN - Ferrara Section, Italy, aimed at performing two-dimensional birefringence mappingof substrates. In this paper we describe the design and working principle of the system and present results for crystal-line silicon, a candidate material for substrates in the low-frequency interferometers of ET. We find that the birefrin-gence is ≲ 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:∆n(110) = −(1.50 ± 0.15) × 10−6 @ λ = 1550 nm. Implications for the performance of gravitational-wave interferom-eters are discussed.