Universal spin-momentum locking of light

We show the existence of an inherent handedness (spin) of evanescent-electromagnetic-waves which is fundamentally locked to the direction of propagation (momentum). It is universal and accompanies evanescent waves in total internal reflection, waveguides/fibers and surface-states. The hallmark of the recently discovered quantum spin hall (QSH) state of matter is the existence of electronic surface states which are robust to disorder (non-magnetic impurities). This property arises since the spin of the electron is intrinsically locked to the direction of propagation (momentum) and the electrons cannot backscatter unless there is a spin-flip. Recent work on topological photonics has shed light on electromagnetic surface states which are immune to disorder and show similar effects as the electrons. However, the presence of spin-orbit coupling has been considered fundamental to the origin of this spin-momentum locking effect in both electronics and photonics.