  Nanostructures that feature nonreciprocal light transmission are highly
desirable building blocks for realizing photonic integrated circuits. Here, a
simple and ultra-compact photonic-crystal structure, where a waveguide is
coupled to a single nanocavity, is proposed and experimentally demonstrated,
showing very efficient optical diode functionality. The key novelty of the
structure is the use of a Fano resonance in combination with spatial symmetry
breaking and cavity enhanced material nonlinearities to realize non-reciprocal
propagation effects at ultra-low power and with a good wavelength tunability.
The nonlinearity of the device relies on ultrafast carrier dynamics, rather
than the thermal effects usually considered, allowing the demonstration of
nonreciprocal operation at a bit-rate of 10 Gbit/s with a low energy
consumption of 4.5 fJ/bit.
