  Unconstrained by Lorentz reciprocity, nonreciprocal metasurfaces are uniquely
capable of encoding distinctive optical functions on forward- and
backward-propagating waves. The nonreciprocal metasurfaces reported to date
require external electric or magnetic field biasing or rely on nonlinear
effects, both of which are challenging to practically implement. Here, we
propose and experimentally realize a magnet-free, linear, and passive
nonreciprocal metasurface based on self-biased magnetic meta-atoms. Record
transmittance up to 77% and operation angle reaching 64 degree are
experimentally demonstrated. Moreover, on-demand bidirectional phase modulation
in a "LEGO-like" manner is theoretically proposed and experimentally
demonstrated, enabling a cohort of nonreciprocal functionalities such as
microwave isolation, nonreciprocal beam steering, nonreciprocal focusing, and
nonreciprocal holography. The design can also be extended to MHz and optical
frequencies, taking advantage of the wide variety of self-biased gyrotropic
materials available. We foresee that the nonreciprocal metasurfaces
demonstrated in this work will have a significant practical impact for
applications ranging from nonreciprocal antennas and radomes to full-duplex
wireless communication and radar systems.
