Self-induced passive nonreciprocal transmission by nonlinear bifacial dielectric metasurfaces

The breaking of the Lorentz reciprocity law is a nontrivial task, since it usually requires bulky magnets or complicated time-modulation dynamic techniques to be accomplished. In this work, we present a simple and compact design of a nonlinear bifacial dielectric metasurface to achieve strong self-induced passive nonreciprocal transmission without the use of external biases. The proposed design is ideal for free-space-optics applications, can operate under both incident polarizations, and requires very low input excitation power to reach the nonreciprocal regime. It is composed of two passive silicon-based metasurfaces exhibiting Fano and Lorentzian resonances embedded in an ultrathin glass substrate. Highly asymmetric field enhancement is achieved with the proposed design that leads to strong nonreciprocity at low excitation intensities due to the large Kerr nonlinearity of silicon. Moreover, cascade designs are presented to further reduce the insertion loss, broaden the nonreciprocal intensity range, and increase the isolation ratio by enhancement of the transmission contrast. Finally, it is demonstrated that the proposed nonlinear metasurface is robust with regard to fabrication imperfections and can achieve large isolation for a relatively broad input power range even in the case of two incident waves impinging at the same time from both directions. The current work is expected to lead to several compact nonreciprocal nanophotonic devices, such as all-optical diodes, isolators, circulators, and ultrathin protective layers for sensitive optical components.