fabrication at the microscale harder, which does not comply with the development tendency of planarization and integration. The development of metasurfaces brings a promise to solve the above dilemma nicely by creating a planar mirror whose reflective property can be engineered at will. Usually, a metasurface consists of a layer of subwavelength particles, with/without a reflecting substrate, which imparts local and space-variant phase delay by their localized resonance. [2][3][4] Metasurfaces have verified their strong power in a wide range of applications like achromatic lensing, [5,6] hologram, [7,8] polarization-flipping, [9,10] and even photonic topology. [11] However, there have been just a few works studying metasurfaces in the cavity application. [12,13] In these works, a metasurface is just used as a frequency filter [12] or a spatial filter [13] to replace the corresponding traditional optical element outside a cavity to manipulate a supported resonant mode. [14] Here, a planar metasurface as a reflecting mirror will be introduced to replace the curved mirror in the concave cavity by imparting a designed phase profile. Utilizing the flexible phase modulation of the metasurface, a plano-planar meta-cavity behaving like the concave cavity can be realized, and the problems brought by the curved shape can be got rid of thoroughly. In constructing the meta-cavity, the most important problem is to find a low-loss metasurface which can nearly totally reflect an incident wave. The required metasurface can be obtained relatively easily by putting dielectric resonant particles of high permittivity on a metal substrate in the microwave range. A meta-cavity will be demonstrated based on such a metasurface in this paper. If one can find a dielectric metasurface with high reflection, an optical meta-cavity can also be realized in a similar method.
Designing an Ideal Plano-Planar Cavity Mimicking a Concave CavityFirst, the mode pattern difference between an FP cavity and a concave cavity is demonstrated simply. Both cavity structures are schematically illustrated in Figure 1a, which consist simply of two planar or curved perfect electrical conductors (PECs) as mirrors positioned in the free space. The cavity structures are assumed to be infinite along the y axis, and then their simulation is simplified into a 2D problem. In this paper, the simulation is all conducted with the COMSOL software unless 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 cons...