Fabry-Perot Resonances in Bilayer Metasurfaces

In this study, we construct Fabry-Perot cavities in which nanostructured, thin resonant metasurfaces act as mirrors. We develop a temporal coupled-mode theory and provide an accurate description of the resonances supported by these cavities, deriving analytically their transmission characteristics. The presence of metasurface mirrors introduces a substantial group delay, causing the field concentration to shift from the bulk of the cavity towards the regions close to the two metasurfaces. This shift is accompanied by a significant increase in the quality factor of the resonances that is associated with the emergence of singular points in frequency space. These singular points exist even when the cavity separations are smaller than the cavity length of the fundamental mode in standard cavities, enabling metamirror Fabry-Perot cavities to outperform traditional cavities by achieving much higher quality factors despite displaying shorter cavity lengths.