Metasurface-based Doppler cloaks: Time-varying metasurface profile to achieve perfect frequency mixing

Doppler cloaks are spatiotemporally modulated metamaterial covers to be placed in front or wrapped around a moving object to restore locally the source illumination frequency, as if it were not in motion. Although the cloak effectively compensates such a frequency shift, the required thickness of the metamaterial is in the order of dozens of wavelengths of the incident signal for ensuring a complete energy transfer from the Doppler shifted to the original source frequency. In this contribution, we present a bi-dimensional version of the Doppler cloak, realized by a time-varying metasurface with proper time-dependent transmission coefficient. The temporal profile of the transmission coefficient is defined for achieving the same frequency mixing behavior of bulk Doppler cloaks. The operation is verified by a full-wave FDTD numerical simulation in case of normal incidence.