Fig. 1: Spin and momentum properties of a plane wave in free space, a surface plasmon plane wave at a metal/dielectric interface and a single evanescent wave at an interface.
From: Dynamical and topological properties of the spin angular momenta in general electromagnetic fields
For (a) an elliptically polarized plane wave propagating in the x-direction, the kinetic momentum density Πis equal to the canonical momentum density P, and the whole SAM (S) is the L-spin (Sl), which is homogeneous through the space; for (b) a linearly polarized surface plasmon plane wave17 propagating in the x-direction, the kinetic momentum density Πand the canonical momentum density P are paralleled with the spin momentum along the x-direction, and the whole SAM is the T-spin (St), which decays exponentially along the +z-direction; for (c) a single evanescent wave, the canonical momentum density P is along the x-direction, whereas the spin momentum density Ps = ∇⨯S/2 has an x-component and y-component because the SAM density contains an L-spin and T-spin simultaneously and these spins decay exponentially along the +z-direction. Therefore, the kinetic momentum density Πalso has an x-component and y-component simultaneously.
