Towards a sub-kelvin cryogenic Fabry-Perot silicon cavity

TL;DR

This paper presents the development of a sub-kelvin silicon Fabry-Perot cavity aimed at reducing thermal noise for ultrastable lasers, involving cryogenic operation, optimized mechanical design, and analysis of birefringence-related noise.

Contribution

The work introduces a cryogenic silicon cavity with high finesse and optimized heat transfer, addressing thermal noise limits and birefringence effects at sub-kelvin temperatures.

Findings

Achieved room-temperature finesse of 220,000.

Designed a cryogenic mount with efficient heat transfer.

Measured birefringent mode splitting of about 250 kHz.

Abstract

We report on the development of a sub-kelvin, single-crystal silicon Fabry-Perot cavity. Operating such a cavity below 1~K should reduce the thermal noise limit of the cavity, and by this way address the current limitations of ultrastable lasers. To further decrease mechanical losses, mirrors with silicon substrates and crystalline coatings are optically contacted to the spacer, resulting in a room-temperature finesse of 220,000. To operate our cavity at sub-kelvin temperatures, we use a dilution refrigerator able to reach temperatures down to 10 mK. We have designed a mechanical mount to house our cavity in such a cryostat, with optimized heat transfer that will decrease the cooldown time for temperatures below 1~K. The estimated thermal noise is projected to be $\sim 7{\times}10^{-19}$ at 100~mK. However, silicon cavities with crystalline mirror coatings at cryogenic temperatures have shown birefringence correlated frequency fluctuations as well as unknown additional noise mechanisms \cite{yu2023, kedar2023}. We have measured a room-temperature TEM$_{00}$ birefringent mode splitting of about 250 kHz. Understanding and measuring these noise mechanisms will be a key to attaining fractional frequency stabilities beyond state-of-the-art.