Optical micro and nano cavities can trap light in small volumes and enhance the light-matter interaction of intra-cavity emitters. The characterization of these cavities typically uses resonance spectroscopy, which probes the cavity output and the associated transverse mode profiles, as a function of the optical frequency. We study tunable open access Fabry-Perot-type microcavities, which enable easy integration with quantum emitters.

The presented measurements and analyses of resonance spectra and mode profiles is surprisingly rich. We demonstrate how the different mode-shaping effects can be individually distinguished and quantified. Spin-orbit coupling, which is one of the nonparaxial effects, is prominently visible in the intriguing polarization patterns of the resonant modes, while polarization tomography yields the shape-induced birefringence and associated polarization splitting of the fundamental modes.