Ultrahigh-contrast-ratio silicon Fano diode

We report a distinguished mechanism of using two interaction-free (uncoupled) cavities to realize an all-optical silicon diode with an extremely high contrast ratio. In contrast to existing examples of all-optical diodes, our unidirectional transmission results from mimicking Fano resonance in a compact and finite silicon waveguide. A Fano spectrum with an arbitrary asymmetric factor, resonant center, and linewidth is explicitly designed with two symmetric resonances, which can be separately realized and tuned in an insightful manner by two cascaded uncoupled cavities. The field localizations are inherently asymmetric. We show that if these double asymmetries (Fano spectrum and field localization) are integrated and controlled, an ultrahigh-contrast-ratio silicon Fano diode is accomplished. This finding provides a promising avenue for achieving an extremely compact optical switch and incorporating Fano resonance in silicon photonics.