We report an experimental observation of the transverse spin and associated spin-momentum locking of surface plasmon polaritons (SPPs) excited in a plasmonic single crystalline silver nanowire (AgNW). In contrast to the SPPs excited in metal films, the electromagnetic field components of the evanescent SPP mode propagating along the long axis (x axis) of the NW can decay along two longitudinal planes (xy and xz planes), resulting in two orthogonal transverse spin components (sz and sy). Analysis of the opposite circular polarization components of the decaying SPP mode signal in the longitudinal plane (xy) reveals spin dependent biasing of the signal and, hence, the existence of transverse spin component (sz). The corresponding transverse spin density (s3) in the Fourier plane reveals spin-momentum locking, where the helicity of the spin is dictated by the wave-vector components of the SPP evanescent wave. Furthermore, the results are corroborated with three-dimensional numerical calculations. The presented results showcase that how a chemically prepared plasmonic AgNW can be harnessed to study optical spins in evanescent waves and can be extrapolated to explore sub-wavelength effects, including directional spin coupling and optical nano-manipulation.

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