Speaker
Description
Mitigating the intrinsic and stress-induced birefringence of silicon test masses is paramount to meeting the stringent sensitivity requirements of the Einstein Telescope (ET). In this work, the Ferrara ET Research Unit, in collaboration with the ET-Pathfinder group at Maastricht University, presents recent advancements in the optical characterization of ET candidate substrates. We investigated various IKZ-grown silicon samples by measuring their birefringence at 1550 nm using a custom, highly sensitive optical polarimeter. Spatial mapping of thick samples ($L \ge 20$ mm) revealed pronounced high-birefringence poles, initially raising concerns regarding bulk crystal uniformity. However, our investigation demonstrated that these features are localized artifacts caused by lateral surface stress induced during mechanical cutting processes. To counter this, we implemented a controlled lateral surface treatment. The resulting birefringence maps show the suppression of the high-stress poles, revealing the true, highly uniform bulk birefringence of the silicon. These results demonstrate that specific surface treatments are an effective mitigation strategy for surface-induced birefringence in thick silicon optics, representing a significant milestone in the preparation and qualification of large-scale silicon substrates for the ET core optics.