Supercavity modes in stacked identical Mie-resonant metasurfaces

Modes with a high-$Q$ factor are crucial for photonic metadevices with advanced functionalities. In sharp contrast to recent techniques which generate a supercavity mode by bound states in the continuum via symmetry breaking, we reveal a general and different route, by stacking two parallel and identical Mie-resonant metasurfaces with an air separation. The supercavity mode can be designed by the established theoretical model by overlapping Mie resonant modes with tailor-made Fabry-P\'erot modes. The simplified system, with free-space field concentration which can exist in plane as well as out of plane of the metasurfaces, provides for ease of integration with added matter, creating exciting opportunities for the study of fundamental light-matter coupling. This work deepens our understanding of light manipulation using metasurfaces. It paves a different and general route for generating supercavity modes which can be easily engineered and controlled for different applications of all dielectric metaoptics.