# A hybrid silicon-sapphire cryogenic Fabry-Perot cavity using hydroxide catalysis bonding

## TL;DR

This paper demonstrates a hybrid sapphire-silicon cryogenic Fabry-Perot cavity with hydroxide catalysis bonding, showing it can withstand thermal cycling from room temperature to 5.5 K while maintaining high finesse, advancing cryogenic gravitational wave detector technology.

## Contribution

It introduces a novel bonding technique for sapphire-silicon interfaces that survives cryogenic thermal cycling, enabling improved test masses for gravitational wave detectors.

## Key findings

- Bonded sapphire-silicon samples survive thermal cycling from room temperature to 5.5 K.
- The measured bond strength is approximately 3.6 MPa after cooling.
- The hybrid cavity maintains high finesse after repeated thermal cycles.

## Abstract

The third-generation gravitational wave detectors are under development by operating the detector in cryogenic temperature to reduce the thermal noise. Silicon and sapphire are promising candidate materials for the test masses and suspension elements due to their remarkable mechanical and thermal properties at cryogenic temperature. Here we present the performances of the cryogenic thermal cycling and strength testing on hydroxide catalysis bonding between sapphire and silicon. Our results suggest that although these two materials have very different coefficients of thermal expansion, but if the flatness and the thermally grown $\mathrm{SiO_2}$ oxidation layer on the silicon surface are controlled well, the bonded samples can still survive thermal cycling from room temperature to 5.5 K. A breaking strength of 3.6$\pm 0.6$ MPa is measured for the bonds between sapphire and silicon with a 190 nm silicon oxidation thickness after cooling cycle. We construct a hybrid sapphire-silicon Fabry-Perot cavity with the developing bonding technique in our lab. The measurement results reveal that the cavity can survive repeated thermal cycling while maintaining a good finesse.