Doppler cloak: concept and realistic implementation through space-time modulated metamaterials and time-modulated metasurfaces

A Doppler cloak consists of a non-reciprocal space-time modulated metastructure that allows manipulating the apparent velocity of a moving object by inducing an artificial frequency shift in the reflected signals. In this contribution, we present two Doppler cloaks: one implemented through a spatio-temporal (ST-) metamaterial cover wrapped around or just in front of the object, and one implemented through a time-varying metasurface. We show that, by properly choosing the modulation scheme and frequency of the space-time modulated metastructure, it is possible achieving a full compensation of the Doppler frequency shift, making the composite system appearing to an external observer as stationary, even though it is actually moving. The Doppler cloak may have large impact also in other applications, such as restoration of electromagnetic invisibility and antenna matching in moving systems, just to name a few.