Water as a tunable element for spectral and amplitude modulation of microwave metasurfaces

In this manuscript, using numerical calculations, we demonstrate the spectral tuning and amplitude modulation of reflective microwave metasurfaces based on metal–insulator–metal (MIM) cavities infiltrated with water. Generally, the applications of water as a tunable element for microwave resonators are constrained by huge water losses. In order to solve this issue, the spacer between two metallic layers of the MIM cavities is made of a specially designed water-dielectric bilayer. The additional dielectric layer reduces the imaginary part of the spacer effective permittivity by order of magnitude compared to water itself. At the same time, the real part of the spacer permittivity can be modulated in a wide range by controlling water height level, by metasurface rotation or by temperature control. As a result, the modulated permittivity provides two-fold functionality: 1. the shifting of metasurface resonances in a wide spectral range, and simultaneously 2. the reflectance modulation at single operating frequency. The water-dielectric bilayer is placed in the MIM cavities with deeply subwavelength thickness which provides very efficient modulation since small changes of the water layer thickness or its orientation produce large spectral shifts and reflectance changes. All metasurfaces are analyzed within the framework of temporal coupled mode theory and it is demonstrated that the operation of water-infiltrated metasurfaces is dominantly determined by the ratio between radiative and non-radiative decay rate of resonant modes. By keeping this ratio within a desired working space, we explain the choice of optimal geometrical parameters which provide large relative spectral shifts of more than 100%, and reflectance modulation reaching the theoretical maximum of 1.