Extraordinary momentum and spin in evanescent waves

Abstract

Momentum and spin represent fundamental dynamic 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'.