Speakers
Description
The Einstein Telescope will require highly optimized optical substrates to achieve its sensitivity targets. Silicon is one of the primary candidates for cryogenic test masses. We present 2D birefringence maps of silicon samples measured at 1550 nm using a highly sensitive optical polarimeter. Initial measurements of thick substrates ($L \ge 20$ mm) revealed four distinct "poles" of high birefringence, significantly degrading optical uniformity.
To isolate bulk properties from surface effects, we applied a dedicated surface treatment to the lateral walls of the samples. This process completely eliminated the anomalous poles, drastically reducing the measured ellipticity $\psi$ and confirming that the excess birefringence originated from manipulation-induced surface stress rather than bulk crystal defects.
Our results demonstrate that specific surface treatments are an effective mitigation strategy for surface-induced birefringence in thick silicon optics, paving the way for further optical design optimizations in next-generation cryogenic interferometers.