Author(s):

We show the existence of an inherent property of evanescent electromagnetic

waves: spin-momentum locking, where the direction of momentum fundamentally

locks the polarization of the wave. We trace the ultimate origin of this

phenomenon to complex dispersion and causality requirements on evanescent

waves. We demonstrate that every case of evanescent waves in total internal

reflection, surface states and optical fibers/waveguides possesses this

intrinsic spin-momentum locking. We derive the Stokes parameters for evanescent

waves which reveal an intriguing result – every fast decaying evanescent wave

is inherently circularly polarized with its handedness tied to the direction of

propagation. We also show the existence of a fundamental angle associated with

total internal reflection (TIR) such that propagating waves locally inherit

perfect circular polarized characteristics from the evanescent wave. This

circular TIR condition occurs if and only if the ratio of permittivities of the

two dielectric media exceeds the golden ratio. Our work leads to a unified

understanding of this spin-momentum locking in various nanophotonic structures

and sheds light on the electromagnetic analogy with the quantum spin hall state

for electrons.

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