We present the fabrication of exceptionally small-radius concave microoptics on fused silica substrates using CO2 laser ablation and subsequent reactive ion etching. The protocol yields on-axis near-Gaussian depressions with a radius of curvature ≲5 μm at shallow depth and low surface roughness of 2 Å. This geometry is appealing for cavity quantum electrodynamics where small mode volumes and low scattering losses are desired. We study the optical performance of the structures within a tunable Fabry-Pérot type microcavity and demonstrate near-coating-limited loss rates () and small focal lengths consistent with their geometrical dimensions.
The tested top-down craters #E and #G belong to an earlier fabrication batch than #A, where the photoresist was manually applied as a droplet. The resulting geometry, i.e., radius and depth, remains the same.