Tunable Wave Isolators Based on Space-time Modulated Graphene Sheets

In this talk, we propose a tunable metasurface isolator based on space-time (ST) modulated surface impedance. The surface impedance is synthesized by periodic graphene strips with spatiotemporal gating. General and rigorous analytical formulations are developed to calculate all the scattering harmonics for such modulated metasurfaces. We found that with the space modulation of the graphene impedance, the structure can excite evanescent modes that propagate asymmetrically in space. This spatial asymmetry results in strong nonreciprocal effect under time modulation: the device can reflect waves coming from one angle without frequency conversion, while for the oppositely tilted incidence, waves are fully transformed into evanescent fields and absorbed. We also show that the proposed wave isolator can be tuned and robustly operate in a wide frequency spectrum (5.5 12.1 THz) only by modifying the pumping functions.