![]() ![]() Although photons are bosons with integer spin and surface and waveguided electromagnetic modes suffer from backscattering ( 13), in contrast to the helical fermion behavior of surface Dirac modes, they possess the topological ℤ 4 invariant and hence can transport spin unidirectionally ( 9). For example, surface plasmon polaritons (SPPs) as surface modes propagating at an insulator–metal interface ( 12) exhibit features of spin–momentum locking that are analogous to the behavior of surface state of a topological insulator ( 6– 8). On the other hand, due to an “intrinsic” spin–orbit coupling governed by the Maxwell’s field theory, the spin–momentum locking of light was reported and linked to the modes with the evanescent field components, such as surface waves or waveguided modes ( 9– 11). The photonic analogy of unidirectional surface spin states was demonstrated with the pseudospin by engineering an “extrinsic” spin–orbit interaction and breaking the time-reversal symmetry in artificial photonic structures ( 6– 8). ![]() ![]() Spin–momentum locking, characterized by unidirectional surface spin states, has been extensively studied in topological insulators ( 1), superconductors ( 2), magnons ( 3), and cold-atom ( 4) and Bose–Einstein condensates ( 5). The proposed framework opens up opportunities for designing the spin structure and topological properties of electromagnetic waves with practical importance in spin optics, topological photonics, metrology and quantum technologies and may be used to extend the spin-dynamics concepts to fluid, acoustic, and gravitational waves. In contrast to the one-dimensional uniform spin of a guided plane wave, a two-dimensional chiral spin swirl is observed for structured guided modes. The predicted photonic spin dynamics is experimentally verified with four kinds of nondiffracting surface structured waves. Here, we derive a set of spin–momentum equations which describes the relationship between the spin and orbital properties of arbitrary complex electromagnetic guided modes. In addition to spin, optical waves may have complex structure of vector fields associated with orbital angular momentum or nonuniform intensity variations. Spin–momentum locking, a manifestation of topological properties that governs the behavior of surface states, was studied intensively in condensed-matter physics and optics, resulting in the discovery of topological insulators and related effects and their photonic counterparts. ![]()
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