Interlayer Topological Transport and Devices Based on Layer Pseudospins in Photonic Valley-Hall Phases

Valley-Hall phases, first proposed in 2D materials, originate from nontrivial topologies around valleys which denote local extrema in momentum space. Since they have been extended into classical systems, their designs draw inspirations from existing quantum counterparts, and their transports show similar topological protections. In contrast, it has been recently established in acoustics that layer pseudospins in valley-Hall phases can give rise to special valley-Hall edge states with fundamentally different transport behaviors at the interfaces compared with various 2D materials. Their realization in other classical systems, such as photonics, would allow to design topological insulators beyond quantum inspirations. Here, it is shown that layer pseudospins exist in photonic valley-Hall phases, using vertically coupled designer surface plasmon crystals, a nonradiative system in open environment supporting tightly confined propagating modes. The negligible thermal and radiative losses in the structure pave the way for the direct observations of the layer pseudospins and associated topological phenomena stem from them in both real and reciprocal spaces. Photonic devices that manipulate the signals based on the layer pseudospins of the topological phases, such as layer convertors and layer-selected delay lines, are experimentally demonstrated, confirming the potential applications of the layer pseudospins as a new degree of freedom carrying information.