Transverse spin and momentum in two-wave interference

TL;DR

This paper investigates the complex local momentum and spin structures in the interference pattern of two electromagnetic waves, revealing novel transverse spin and momentum densities with potential for experimental observation.

Contribution

It uncovers previously unknown transverse spin and momentum densities in two-wave interference fields, including the Belinfante spin momentum, with analytical and numerical validation.

Findings

Discovery of transverse, helicity-independent spin density.

Identification of transverse polarization-dependent momentum density.

Numerical simulations suggest experimental detectability.

Abstract

We analyze the interference field formed by two electromagnetic plane waves (with the same frequency but different wave vectors), and find that such field reveals a rich and highly non-trivial structure of the local momentum and spin densities. Despite the seemingly-planar and extensively-studied character of the two-wave system, we find that it possesses a transverse (out-of-plane) helicity-independent spin density, and also a transverse polarization-dependent momentum density with unusual physical properties. The polarization-dependent transverse momentum represents the so-called Belinfante spin momentum, which does not exert the usual optical pressure and it is considered as `virtual' in field theory. We perform analytical estimations and exact numerical simulations of the interaction of the two-wave field with probe Mie particles. The results of these calculations clearly indicate the straightforward detectability of the unusual spin and momentum properties in the two-wave field and strongly motivate their future experimental verifications.