Invisibility cloaks, a class of attractive devices that can hide objects from external observers, have become practical reality owing to the advent of metamaterials. In previous cloaking schemes, almost all demonstrated cloaks are time-invariant and are investigated in the system that is motionless, and hence they are limited to hide stationary objects. In addition, the current cloaks are typically static or require manual operation to achieve dynamic cloaking. Here, a smart Doppler cloak operating in broadband and full polarizations is reported, which consists of a time-modulated reflective metasurface and a sensing-feedback time-varying electronic control system. Experimental results show that the smart Doppler cloak is able to respond self-adaptively and rapidly to the ever-changing velocity of moving objects and then cancel different Doppler shifts in real time, without any human intervention. Moreover, the wideband and polarization-insensitive features enable the cloak to be more robust and practical. To illustrate the capabilities of the proposed approach, the smart Doppler cloak is measured in three scenarios with two different groups of linearly-polarized incidences at 3.3 and 4.9 GHz, and one group circularly-polarized incidences at 6.0 GHz, respectively.
Keywords: full polarizations; intelligent manipulation of electromagnetic waves; smart Doppler cloaks; time-modulated broadband metasurfaces; time-varying control systems.
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