Anti-crossing of modes and singularity points in dielectric metasurface-on-mirror microcavities

Over the past decade, single layer metasurfaces have been used to realize a variety of passive and active flat optical devices. In the pursuit of broadening the range of potential applications and exploring newer physical phenomena, metasurface-on-mirror configuration has been widely explored; but mostly in the plasmonic regime. Dielectric metasurfaces have emerged as a better alternative to their plasmonic counterparts owing to their low losses, capability to support electric and magnetic multipole resonance with associated scattering interference effects, and better compatibility with semiconductor fabrication technologies. In this work, we numerically study the coupling of modes of a resonant dielectric metasurface with the cavity modes obtained by placing the metasurface above a metallic mirror. We consider two case studies-(1) Huygens' (HM) metasurface, which has overlapped electric dipole (ED) and magnetic dipole (MD) resonances, and (2) non-Huygens' metasurface (n-HM), which has well-separated ED and MD modes. The cavity formed by the Huygens' metasurface-on-mirror leads to a strong coupling of modes and the signature of anti-crossing of modes in reflection. The avoided crossing gives rise to large phase modulation of 4π, with a great potential for tunable metadevices with continuous wavefront modulation beyond standard 2π. On the other hand, for a n-HM metasurface-on-mirror, we observe a pair of phase singularities around each metasurface resonance. These singularities are associated with reflectance zeros resulting in perfect absorption. We explain the properties of these photonic systems and support the obtained results with a semi-analytic approach employing the (scattering) S-matrix formalism. We believe this study will help in better understanding the far-field coupling of dielectric metasurface resonances and cavity modes while leveraging the best of both worlds in realizing multifunctional, high-Q, robust, and potentially tunable nano and quantum photonic devices.