Metasurface for Constructing a Stable High‐Q Plano‐Planar Open Cavity

Traditional open cavities are constructed by planar or concave mirrors. A Fabry–Pérot (FP) cavity is convenient to integrate and fabricate, but highly sensitive to the parallelism of the end mirrors. In contrast, a concave cavity is more stable and possesses a lower leaky rate, but the curved geometry brings inconvenience. Here, it is suggested that a planar meta‐cavity can simultaneously possess the advantages of both of the above open cavities by using a metasurface to create a planar meta‐mirror that mimics the behavior of a concave mirror. For demonstration, a microwave meta‐cavity is constructed, whose meta‐mirror consists of subwavelength ceramic blocks on a metal substrate. Compared to an FP cavity, the meta‐cavity is more robust against the alignment error of the end mirrors, and 15‐fold enhancement of the quality factor is observed numerically. Experimental results are in good agreement with the simulated results. The suggested method may be extended to work in high frequency ranges. This work would support the planarization and miniaturization of an open cavity and may also increase the functionality. A plano‐planar open cavity sustaining a stable Gaussian‐like cavity mode is constructed based on a reflecting metasurface. Compared to a concave cavity, it is more convenient to be integrated and fabricated, and compared to a Fabry–Pérot cavity, it supports more stable resonant modes of higher quality factor. This work supports the planarization and miniaturization of open cavities.