Dispersion engineering in unidirectional excitation of the surface wave of photonic crystal.

The discovery of transverse spin angular momentum (SAM) of evanescent and guided modes presents a novel spin-orbit interaction (SOI), i.e., transverse SOI, to affect and control the intensity distribution and propagation path of light. In this Letter, we first theoretically verify the transverse SAM property of the surface wave of a photonic crystal (PhC) slab. Then we realize the polarization-controllable unidirectional excitation of such (forward) surface wave by means of transverse SOI. Furthermore, taking advantage of dispersion engineering of PhC, we design another PhC slab capable of sustaining a backward surface wave and find that, compared to a forward surface wave, the backward surface wave is related to inverse unidirectional excitation with incident of a circularly polarized beam. In other words, dispersion engineering of PhC provides another route to control the excitation direction of surface modes. The combination of dispersion engineering and transverse SOI will facilitate the design of functional devices based on PhC in the field of nanophotonics and nanoplasmonics.