  Optical microcavities allow to strongly confine light in small mode volumes
and with long photon lifetimes. This confinement significantly enhances the
interaction between light and matter inside the cavity, with applications such
as optical trapping and cooling of nanoparticles, single-photon emission
enhancement, quantum information processing, and sensing. For many
applications, open resonators with direct access to the mode volume are
necessary. Here we report on a scalable, open-access optical microcavity
platform with mode volumes < 30 $\lambda^3$ and finesse approaching $5x10^5$.
This result significantly exceeds the highest optical enhancement factors
achieved to date for Fabry-P\'erot cavities. The platform provides a building
block for high-performance quantum devices relying on strong light-matter
interaction.
