Nonreciprocal Wavefront Engineering with Time-Modulated Gradient Metasurfaces

Gradient metasurfaces have enabled the control of electromagnetic waves for arbitrary wavefront shaping in both optical near and far fields. So far the responses of such a structure in transmission and reception have been identical, due to time-reversal symmetry. This work shows that time-modulated gradient metasurfaces can efficiently convert between only two desired frequencies, allowing one to break and manipulate reciprocity by imposing drastically different phase gradients in frequency up- and down-conversion. Thus a wide variety of functionalities may be implemented, including beam steering and focusing, with strong and angle-insensitive nonreciprocal responses.