A small mode volume tunable microcavity: development and characterization

TL;DR

This paper presents a highly tunable, small mode volume microcavity with high finesse, suitable for quantum and optomechanical applications, achieved through innovative fabrication and precise characterization.

Contribution

The development of a miniaturized, tunable air-gap Fabry-Perot microcavity with a concave mirror fabricated by CO2 laser ablation, featuring sub-wavelength beam waists and high mechanical stability.

Findings

Achieved a cubic-wavelength-scale mode volume with high finesse.

Demonstrated effective mode-matching and sub-wavelength beam waists.

Confirmed mechanical stability with sub-5 pm length fluctuations at 4K.

Abstract

We report the realization of a spatial and spectrally tunable air-gap Fabry-Perot type microcavity of high finesse and cubic-wavelength-scale mode volume. These properties are attractive in the fields of opto-mechanics, quantum sensing and foremost cavity quantum electrodymanics. The major design feature is a miniaturized concave mirror with atomically smooth surface and radius of curvature as low as 10 micrometer produced by CO2 laser ablation of fused silica. We demonstrate excellent mode-matching of a focussed laser beam to the microcavity mode and confirm from the frequencies of the resonator modes that the effective optical radius matches the physical radius. With these small radii, we demonstrate sub-wavelength beam waists. We also show that the microcavity is sufficiently rigid for practical applications: in a cryostat at 4K, the root-mean-square microcavity length fluctuations are below 5 pm.