High finesse microcavities in the optical telecom O-band

Abstract: 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.