Experimental observation of transverse spin of plasmon polaritons in a single-crystalline silver nanowire

TL;DR

This paper experimentally demonstrates the transverse spin and spin-momentum locking of surface plasmon polaritons in a single-crystalline silver nanowire, revealing new insights into optical spin phenomena in plasmonic nanostructures.

Contribution

It provides the first experimental observation of transverse spin components and spin-momentum locking in SPPs on a silver nanowire, supported by numerical calculations.

Findings

Observation of transverse spin components ($s_z$ and $s_y$) in SPPs

Spin-dependent biasing of the SPP signal in the $x$-$y$ plane

Confirmation of spin-momentum locking through Fourier plane analysis

Abstract

We report the 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 ($x$-$y$ and $x$-$z$ planes), resulting in two orthogonal transverse spin components ($s_z$ and $s_y$). Analysis of the opposite circular polarization components of the decaying SPP mode signal in the longitudinal plane ($x$-$y$) reveals spin dependent biasing of the signal and hence the existence of transverse spin component ($s_z$). The corresponding transverse spin density ($s_3$) 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. Further, the results are corroborated with three-dimensional numerical calculations. The presented results showcase 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.