Topological valley-locked silicon photonic crystal waveguides

By introducing photonic crystals with Dirac point based on valley edge states, we design heterostructure wave guides on the silicon-on-insulator platform, promising waveguides with different widths to operate in the single mode state. Benefiting from the unidirectional transmission and backscattering-immunity characteristics enabled by the topological property, there is no scattering loss induced by the mode-mismatch at the transition junction between the waveguides with different widths. Therefore, the valley-locked heterostructure waveguide possesses unique width degrees of freedom. We demonstrate it by designing and fabricating waveguides with expanding, shrinking, and Z-type configurations. Thanks to the free transition between waveguides with different widths, an interesting energy convergency is observed, which is represented from the imaging of the enhanced third harmonic generation of the silicon slab. Consequently, these heterostructure waveguides can be more flexibly integrated with existing on-chip devices and have the potential for high-capacity energy transmission, energy concentration, and field enhancement.