Passive Bias-Free Nonreciprocal Metasurfaces Based on Nonlinear Quasi-Bound States in the Continuum

TL;DR

This paper introduces ultrathin silicon-based metasurfaces that leverage nonlinear effects and broken symmetries to achieve significant nonreciprocal light transmission, enabling advanced photonic applications without external bias.

Contribution

It demonstrates a novel approach to nonreciprocal metasurfaces using nonlinear quasi-bound states in the continuum, achieving high nonreciprocity in free-space radiation.

Findings

Over 10 dB nonreciprocal transmission achieved

Less than 3 dB insertion loss

Utilizes silicon third-order nonlinearities with q-BICs

Abstract

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.