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Nat Commun. 2014 Mar 6;5:3300. doi: 10.1038/ncomms4300.

Extraordinary momentum and spin in evanescent waves.

Author information

1] iTHES Research Group, RIKEN, Wako-shi, Saitama 351-0198, Japan [2] A. Usikov Institute of Radiophysics and Electronics, NASU, Kharkov 61085, Ukraine.
1] Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama 351-0198, Japan [2] I. I. Mechnikov National University, Dvorianska 2, Odessa 65082, Ukraine.
1] Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama 351-0198, Japan [2] Physics Department, University of Michigan, Ann Arbor, Michigan 48109-1040, USA [3] Department of Physics, Korea University, Seoul 136-713, Korea.


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'.

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