Microcavities with suspended subwavelength structured mirrors

We investigate the generic properties of optical cavities with subwavelength structured mirrors whose reflectivity profile exhibit Fano resonances. We focus in particular on short microcavities and mirrors with high quality factor Fano resonances for which the cavity free-spectral range becomes larger than the width of the Fano resonance. In this unusual regime, the transmission spectrum of the microcavity essentially consists in a single mode, whose linewidth can be substantially reduced as compared to both the Fano resonance width and the linewidth of an equally short cavity without structured mirror. The robustness of these narrow transmission features with respect to wavefront curvature and imperfect parallelism effects suggests that such microcavities could be realized in practice using suspended structured thin films in a simple plane-parallel geometry, and could have interesting applications for optomechanics and sensing applications.