Photonic spin-Hall effect in efficient and desired mode excitation of a plasmonic nanosandwich on a metallic film to scatter high-power directive surface waves

One branch of the spin-orbit interaction of light is the photonic spin-Hall effect (PSHE), which leads to spin-momentum locking of an incident optical beam into a scattered evanescent surface wave at a scatterer. Two longitudinal spin states of an incident beam cause the transverse spin photons of the surface wave to propagate perpendicular to the direction of incidence and in two opposite directions. As photons with two opposite spins illuminate a nanoparticle (NP) near a metal–dielectric interface, the electric fields of incident light-beam photons are coupled into the evanescent electric fields of scattered spin-dependent directive surface plasmon polaritons (SPPs). The NP may be replaced with a nanosandwich with two nanodisks to provide a desired unidirectional scattering directivity pattern (SDP). Despite traditionally maximizing the localized surface plasmon (LSP) power of a nanosandwich’s mode, an analytical method is developed based on minimizing the delay angle ($ δ $) between two internal electric polarizations of the nanodisks, which are tunably excited by a circularly polarized wave. This method and simulation clarify power coupling from LSPs to launched SPPs by the far-field Green’s tensor of the scattered SPPs, thereby forming a desired unidirectional PSHE-affected SDP. $ δ $ can be minimized by incidence wavelength variation and nanodisks’ spacing variation. Accordingly, the desired mode of the excited nanosandwich can be easily found and tuned, by which PSHE affects the unidirectional scattering of the SPP. Our findings pave the way for tunable unidirectional SPP-based engineering and adaptive photonic communications.