A mechanically stable and tunable cryogenic Fabry-Pérot microcavity.

High-finesse, open-geometry microcavities have recently emerged as a versatile tool for enhancing interactions between photons and material systems with a range of applications in quantum optics and quantum information science. However, mechanical vibrations pose a considerable challenge to their operation within a closed-cycle cryostat, particularly when spatial tunability and free-space optical access are required. Here, we present the design and characterization of a system that can achieve ∼16 pm-rms passive mechanical stability between two high-finesse mirrors with 34% duty cycle while permitting both three-dimensional positioning of the cavity mode and free-space confocal imaging. The design relies on two cascaded vibration isolation stages connected by leaf springs that decouple axial and lateral motion and incorporates tuned-mass and magnetic damping. Furthermore, we present a technique for quantifying cavity length displacements similar to or larger than the cavity linewidth, allowing for the in situ measurement of vibrations with and without active feedback. Our results facilitate operation of a tunable, high-finesse cavity within a closed-cycle cryostat, representing an enabling technology for cavity coupling to a variety of solid-state systems.