Extraordinary momentum and spin in evanescent waves

TL;DR

This paper reveals that evanescent optical waves uniquely possess transverse spin and momentum components orthogonal to the wave vector, uncovering fundamental properties linked to Belinfante's spin momentum and enabling novel optical manipulations.

Contribution

It demonstrates that evanescent waves exhibit extraordinary transverse spin and momentum properties, distinct from propagating waves, linked to fundamental field-theory concepts.

Findings

Evanescent waves have orthogonal spin and momentum components independent of polarization.

These properties reveal Belinfante's spin momentum in optical fields.

Transverse momentum and spin can manipulate particles in evanescent fields.

Abstract

Momentum and spin represent fundamental dynamical properties of quantum particles and fields. In particular, propagating optical waves (photons) carry momentum and longitudinal spin determined by the wave vector and circular polarization, respectively. Here we show that exactly the opposite can be the case for evanescent optical waves. A single evanescent wave possesses a spin component, which is independent of the polarization and is orthogonal to the wave vector. Furthermore, such a wave carries a momentum component, which is determined by the circular polarization and is also orthogonal to the wave vector. We show that these extraordinary properties reveal a fundamental Belinfante's spin momentum, known in field theory and unobservable in propagating fields. We demonstrate that the transverse momentum and spin push and twist a probe Mie particle in an evanescent field. This allows the observation of `impossible' properties of light and of a fundamental field-theory quantity, which was previously considered as `virtual'.