The spin Hall effect — a change in a particle's trajectory associated with its spin — is now seen with photons, by using a metamaterial to overcome the weak interaction of light with matter.
To an observer in a relativistic rest frame, an electrical current looks like an electron moving in an electric field. From its own perspective however, the electron is sitting stationary in a magnetic field. The spin-carrying electron can precess under the influence of this field. This can lead to spin-dependent scattering from impurities, which diverts electrons of opposing spin in opposite directions. An analogous spin Hall effect has been predicted in optics: a beam of light splits according to circular polarization. But the necessary spin–orbit interaction between a photon and matter is small.
Xiaobo Yin and colleagues used a thin metamaterial to induce a much stronger interaction, creating an array of V-shaped gold nanoantennae on a silicon substrate. The shape and orientation of the antennae varied across the surface, inducing a space-dependent phase discontinuity for a normal-incidence beam of infrared light. This separated the refracted beam into right and left circularly polarized light.
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Gevaux, D. Spiralling under control. Nature Phys 9, 262 (2013). https://doi.org/10.1038/nphys2625
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DOI: https://doi.org/10.1038/nphys2625