Ultrahigh-finesse, low-mode-volume Fabry–Perot microcavity

Ultralow-loss concave micromirrors with radius of curvature below 60μm were fabricated by laser ablation and reflective coatings. A 10-μm-long microcavity with a mode volume of 40μm3 was set up with two such mirrors, and the cavity linewidth was measured both spectrally and temporally. The smallest linewidth obtained was 96MHz, corresponding to a quality factor of 3.3×106 and a finesse in excess of 1.5×105. With these parameters, we estimate that a variety of solid-state quantum emitters coupled to the cavity may enter the strong coupling regime.