Frequency conversion and parametric amplification using a virtually rotating metasurface

We analyze the scattering of circularly polarized electromagnetic waves from a time-varying metasurface having a time-dependent surface susceptibility that locally mimics a rotating, anisotropic surface. Such virtually rotating metasurfaces (VRM) can be realized by means of electronically tunable surface elements and reach microwave-range rotation frequencies. It is shown that the scattered field contains the incident tone, as well as a single up-or down converted tone which differs by twice the rotation frequency of the surface. A simple full frequency converter is then proposed by augmenting the VRM with a metal screen separated by a proper distance. It is shown that after reflection from this system, the incident tone is fully converted to a single down- or up-converted tone, and shows amplification in the case of up conversion. The analysis of these time-rotating scenarios is carried out by switching to a rotating frame for the fields, leading to time-invariant equations, and thus using common phasor-representation. All results are also validated against an in-house 1D-FDTD code showing excellent agreement. A lumped element model using a 2D periodic metal mesh grid loaded with time-varying capacitive nodes is also presented that enables the VRM concept. This model is then further used to design a 3D realization, verified with static full-wave simulations for different values of the capacitor arrangement. Furthermore, the effect of piece-wise constant changes of surface susceptibility in a general virtually rotating metasurface is studied and it is shown to operate with acceptable results, which is of practical importance. The results of this paper can open new ways for realization of frequency conversion and amplification, in a magnetless and linear time-varying system. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement