Electromagnetic Isolation Induced by Time-Varying Metasurfaces: Nonreciprocal Bragg Grating

In this letter, we propose a magnet-less nonreciprocal isolating system based on time-varying metasurfaces. Two parallel time-varying metasurfaces, one for frequency up-conversion and one for down-conversion by the same amount, are used for realizing a region of space where incident waves from opposite directions experience an opposite Doppler frequency shift. As a result, any device within this region becomes sensitive to the illumination direction, exhibiting a different scattering response from the opposite directions, and thus breaking reciprocity. Very importantly, thanks to the opposite frequency shift of the metasurfaces, the frequency of the transmitted electromagnetic field is the same as for the incident one. Here, we demonstrate this general approach by using a Bragg grating as the device between the time-varying metasurfaces. The combined structure of the metasurfaces and the grating exhibits different transmission and reflection properties for opposite illumination direction, thereby realizing an isolator. More broadly, this letter presents a strategy for converting any conventional electromagnetic device to a nonreciprocal one by placing it between two time-varying metasurfaces. This approach opens the door to several new nonreciprocal components based on thin and lightweight metasurfaces, which are simpler to realize compared to their volumetric counterparts.