Jennifer Dionne1

1, Stanford University, Stanford, California, United States

The propagation of free-space electromagnetic signals is generally governed by time-reversal symmetry, meaning that forward- and backward-travelling waves will trace identical paths when being reflected, refracted or diffracted at an interface. Breaking time-reversal symmetry promises significantly improved photo-voltaic efficiencies and optical diodes, but is challenging to achieve in compact optical devices. Here, we introduce two nanophotonic designs that enable nonreciprocal transmission of visible and near infrared light within subwavelength optical paths. First, we design an all-dielectric, 100-nm-thick Si metasurface for non-reciprocal signal propagation. 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 beam steering with appropriate patterning to form a phase gradient metasurface. Secondly, we design a plasmonic metamaterial that exhibits broadband and wide angle nonreciprocity. A parity-time symmetric distribution of saturable loss and gain leads to nonreciprocal transmission over a 50 nm wavelength range and 60 degree angular range at visible frequencies. Compared to existing schemes, these platforms enable time-reversal-symmetry breaking for arbitrary free-space and modal optical inputs in a simple, robust materials platform.