  Nonreciprocity is important for optical information processing, full-duplex
communications, and protection of sensitive laser equipment from unwanted
reflections. However, it is very challenging to obtain strong nonreciprocal
response in optical frequencies, especially when nanoscale configurations are
considered. In this work, we solve this problem by demonstrating a compact
bifacial plasmonic metasurface that acts as a new ultrathin nonreciprocal
transmission filter at near-infrared frequencies. The proposed nanostructure is
simple to be practically implemented, since it is composed of two silver
nanostripes with different dimensions placed on both sides of an ultrathin
active dielectric spacing layer. The introduced gain leads to an exceptional
point formation in the linear operation regime, where unidirectional perfect
transmission due to loss compensation is achieved from the presented
non-Hermitian parity-time symmetric nanoscale system. The demonstrated
plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when
its nonlinear response is considered, mainly due to its spatially asymmetric
geometry combined with enhanced nonlinearity. Strong and broadband
nonreciprocity with unity transmission contrast from opposite directions is
realized under relatively low input laser intensity values. Furthermore,
asymmetric nonlinear third harmonic generation with a pronounced contrast is
also illustrated by the same nonreciprocal metasurface design. The findings of
this work can lead to the realization of various new self-induced nonreciprocal
nanophotonic configurations, such as integrated ultrafast switches, ultrathin
protective coatings, and asymmetric directional imaging devices.
