Nonreciprocal Flat Optics with Silicon Metasurfaces

TLDR

Metasurfaces enable near‑complete control of light through ultrathin subwavelength surfaces, yet all existing designs obey time‑reversal symmetry, causing forward and backward waves to follow identical paths. The study designs a passive metasurface that nonreciprocally transmits both direct and anomalously refracted near‑infrared light over nanoscale optical path lengths. Full‑field calculations were used to design a 100‑nm‑thick, periodically patterned silicon slab that achieves this nonreciprocal behavior. The high‑Q resonances and Kerr nonlinearities of silicon make the metasurface act as an optical diode and enable nonreciprocal anomalous refraction, outperforming existing schemes and providing a straightforward path to experimental realization with potential for one‑way lensing and holography.

Abstract

Metasurfaces enable almost complete control of light through ultrathin, subwavelength surfaces by locally and abruptly altering the scattered phase. To date, however, all metasurfaces obey time-reversal symmetry, meaning that forward and backward traveling waves will trace identical paths when being reflected, refracted, or diffracted. Here, we use full-field calculations to design a passive metasurface for nonreciprocal transmission of both direct and anomalously refracted near-infrared light over nanoscale optical path lengths. The metasurface consists of a 100 nm-thick, periodically patterned Si slab. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal anomalous refraction with appropriate patterning to form a phase gradient metasurface. Compared to existing schemes for breaking time-reversal symmetry, our platform enables subwavelength nonreciprocity for arbitrary free-space optical inputs and provides a straightforward path to experimental realization. The concept is also generalizable to other metasurface functions, providing a foundation for one-way lensing and holography.