Nonreciprocal and time-modulation metasurface for direct imaging

Image reconstruction has become more attractive in electromagnetic (EM) areas, especially when metasurface emerges to introduce various methods for manipulating spatial EM waves. Here, we propose a new method for achieving direct EM imaging in the near field by applying a co-aperture active metasurface combined with nonreciprocal and time-modulation features at a single frequency. In our metasurface design, the transmitting and receiving EM signals can be manipulated independently in each meta-atom by integrating the dual-direction power amplifiers (PAs), which can be regarded as isolators for one-way propagation. In the transmitting aperture, a uniform wavefront is realized to illustrate the imaging plane; in the receiving aperture, the phase shift of 360-degree coverage in each meta-atom is dynamically controlled by time-modulation of different frequencies to establish direct relations between the spatial pixels of the imaging plane and spectra of Doppler shift. A metasurface prototype is fabricated, and several targets composed of metal stripes are successfully reconstructed directly with one co-aperture metasurface at a single frequency in experiments. The good measurement results verify the validity of the proposed method for direct imaging and sensing.