Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum

Non-reciprocal 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 non-reciprocity has attracted significant attention due to the absence of external bias and the ease of integrability within conventional material platforms. So far, nonlinearity-induced non-reciprocity has been demonstrated only in guided platforms using high-quality-factor resonators. Here we demonstrate ultrathin optical metasurfaces with a large non-reciprocal response for free-space radiation based on silicon thermo-optic nonlinearities. Our metasurfaces combine an out-of-plane asymmetry-necessary to obtain non-reciprocity-with in-plane broken symmetry, which finely tunes the radiative linewidth of quasi-bound states in the continuum. Third-order thermo-optic nonlinearities, engaged by the quasi-bound state in the continuum, are shown to enable over 10 dB of non-reciprocal transmission and less than 3 dB of insertion loss, for impinging average intensities smaller than 3 kW cm-2. Numerical calculations suggest that the build-up and relaxation times of the non-reciprocal response can approach sub-microsecond scales, only limited by thermal dissipation. The demonstrated devices merge the field of non-reciprocity with ultrathin metasurface technologies, offering an exciting functionality for signal processing and routing, communications and protection of high-power laser cavities. Bias-free optical metasurfaces with a large non-reciprocal response for free-space radiation are discussed, based on thermo-optic nonlinearities. These ultrathin devices may lead to new approaches for areas ranging from signal processing to protection of high-power laser cavities.