A metasurface is a thin array of subwavelength elements with designable scattering responses, and metasurface holography is a powerful tool for imaging and field control. The existing metasurface holograms are classified into two types: one is based on phase-only metasurfaces (including the recently presented vectorial metasurface holography), which has high power efficiency but cannot control the phases of generated fields; while the other is based on phase-amplitude-modulated metasurfaces, which can control both field amplitudes and phases in the region of interest (ROI) but has very low efficiency. Here, for the first time, it is proposed to synthesize the field amplitudes and phases in ROI simultaneously and independently by using high-efficiency phase-only metasurfaces. All points in ROI may have independent values of field amplitudes and phases, and the requirements for X and Y components may be different in achieving spatially varied polarization states. To this end, an efficient design method based on equivalent electromagnetic model and gradient-based nonlinear optimization is proposed. Full-wave simulations and experimental results demonstrate that the phase-only metasurface designed by the method has 10 times higher efficiency than the phase-amplitude-modulated metasurface. This work opens a way to realize more complicated and high-efficiency metasurface holography.

Direction-of-arrival (DOA) estimation is one of the most critical technologies of radar, remote sensing, and wireless communications. The traditional DOA estimation is closely related to phased array antennas, which require complicated and expensive hardware and high-power consumption. Metasurface can manipulate electromagnetic waves without using massive transceivers, which makes it possible to realize antenna arrays in a cost-effective way. Here, we propose a strategy of DOA estimations by using a time-domain-coding digital metasurface and a single receiver. Specifically, the incident wave on the metasurface is modulated by the time-domain orthogonal codes impressed on the meta-atoms, and their amplitude and phase distributions are precisely retrieved from the signals detected by the receiver antenna. The effectiveness and accuracy of the proposed strategy are verified by experiments on a two-dimensional metasurface with individually addressable meta-atoms. The strategy features low cost and high flexibility and will facilitate various wireless applications.