Does a Surface Polariton Have Spin?

TL;DR

This paper investigates the spin properties of surface polaritons, revealing that evanescent waves inherently possess a transverse spin energy flow and angular momentum due to their elliptical polarization, linking spin to mode chirality.

Contribution

It demonstrates that surface polaritons have an intrinsic transverse spin energy flow and angular momentum arising from their elliptical polarization, a novel insight into their electromagnetic properties.

Findings

Evanescent waves have a backward spin energy flow.

Surface polaritons possess a well-defined transverse spin angular momentum.

The spin arises from the imaginary component of the electric field.

Abstract

We consider a p-polarized surface electromagnetic wave (a classical surface polariton) at the interface between the vacuum and a metal or left-handed medium. We show that the evanescent electromagnetic waves forming the surface polariton inevitably possess a backward spin energy flow, which, together with a superluminal orbital energy flow, form the total Poynting vector. This spin energy flow generates a well-defined (but not quantized) spin angular momentum of surface polaritons which is orthogonal to the propagation direction. The spin of evanescent waves arises from the imaginary longitudinal component of the electric field which makes the polarization effectively elliptical in the propagation plane. We also examine the connection between the spin and chirality of evanescent modes.