New investigations on the transverse spin of structured optical fields

TL;DR

This paper investigates the transverse spin in structured optical fields, revealing its dependence on propagation, quantization, and effective rest mass, with potential applications in spin-dependent optics and spintronics.

Contribution

It introduces a comprehensive analytical framework for understanding the transverse spin in guided and surface waves, including its quantization and relation to effective rest mass, which is a novel insight.

Findings

Guided waves carry transverse spin dependent on propagation direction

First-time revelation of the quantization form of transverse spin

Transverse spin can be described by photon spin matrix and relates to effective rest mass

Abstract

Guided waves and surface waves can be taken as two typical examples of structured optical fields with the transverse spin. Analytical derivations are developed to demonstrate that (i) guided waves also carry the transverse spin that depends on the mean direction of propagation, which may have important applications in spin-dependent unidirectional optical interfaces; (ii) the quantization form of the transverse spin is for the first time revealed, which is not obvious and related to an ellipticity; (iii) from a unified point of view, the transverse spin can be attributed to the presence of an effective rest mass of structured optical fields; (iv) the transverse spin can also be described by the spin matrix of the photon field; (v) unlike a free optical field whose spin projection on the propagation direction is the only observable, owing to the effective rest mass, the spin projection of structured optical fields on other directions is also an observable, such that one can develop an optical analogy of spintronics. A preliminary idea about the potential applications of the transverse spin is presented, but an in-depth and complete study will be presented in our next work.