  Nonreciprocal devices - in which light is transmitted with different
efficiencies along opposite directions - are key technologies for modern
photonic applications, yet their compact and miniaturized implementation
remains an open challenge. Among different avenues, nonlinearity-induced
nonreciprocity has attracted significant attention due to the absence of
external bias and integrability within conventional material platforms. So far,
nonlinearity-induced nonreciprocity has been demonstrated only in guided
platforms using high-Q resonators. Here, we demonstrate ultrathin optical
metasurfaces with large nonreciprocal response for free-space radiation based
on silicon third-order nonlinearities. Our metasurfaces combine an out-of-plane
asymmetry - necessary to obtain nonreciprocity - with in-plane broken symmetry,
which finely tunes the radiative linewidth of quasi-bound states in the
continuum (q-BICs). Third-order nonlinearities naturally occurring in silicon,
engaged by q-BICs, are shown to enable over 10 dB of nonreciprocal transmission
while maintaining less than 3 dB in insertion loss. The demonstrated devices
merge the field of nonreciprocity with ultrathin metasurface technologies,
offering an exciting functionality for signal processing and routing,
communications, and protection of high-power laser cavities.
