Konstantin Y. Bliokh, Aleksandr Y. Bekshaev, Franco Nori
Momentum and spin represent fundamental dynamical properties of quantum particles. It is known that the photon's momentum is determined by the wave vector and is independent of polarization. The spin of the photon is associated with circular polarization and is also collinear with the wave vector. We show that exactly the opposite can be the case for evanescent optical waves. First, a single evanescent wave possesses a spin angular momentum, which is largely independent of the polarization and is orthogonal to the wave vector. Second, such a wave carries a momentum component, which depends on the circular polarization and is also orthogonal to the wave vector. Although these extraordinary properties seem to be in contradiction with what is known about photons, we show that they reveal a fundamental spin momentum, introduced by Belinfante in field theory more than 70 years ago, which is unobservable in propagating fields. We demonstrate, both theoretically and numerically, that the unusual transverse momentum and spin push and twist an absorbing Mie particle in an evanescent field. Thus, evanescent waves offer a unique possibility to observe 'impossible' properties of light and a fundamental field-theory quantity, which was previously considered as 'virtual'.
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http://arxiv.org/abs/1308.0547
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