Non-reciprocity and Doppler Effect Control by Using Time-varying Metamaterials and Metasurfaces

In this contribution we present the opportunities offered by artificial non-reciprocity and frequency shifting induced by time-varying metamaterials and metasurfaces. Such artificial metastructures are characterized by constitutive parameters (permittivity, permeability and/or surface impedance) that are modulated in both space and time through an external control. At first, the physical insights of the unusual interaction arising between the electromagnetic field and time-modulated metamaterials are presented, with emphasis on the resulting non-reciprocal behavior and Doppler cloaking capabilities offered by the control of the frequency content of reflected and transmitted fields. We present some applications of Doppler cloak, such as motion invisibility and restoration of antenna matching in moving systems. Finally, we introduce the time-varying metasurfaces, which are able to (red-)blue-shift the frequency content of an illuminating electromagnetic wave by changing continuously the reflection phase, emulating their motion in a certain direction and velocity.