Abstract
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. In addition to spin, optical waves may have complex structure of vector fields associated with orbital angular momentum or nonuniform intensity variations. 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. The predicted photonic spin dynamics is experimentally verified with four kinds of nondiffracting surface structured 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 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.
Highlights
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
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 [6,7,8]
For an arbitrary electromagnetic wave propagating in a homogeneous medium, the curl of the energy flow density can be presented as (SI Appendix, section I)
Summary
Peng Shia, , Luping Dua,, Congcong Lia, Anatoly V. 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 Z4 invariant and can transport spin unidirectionally [9] This intrinsic optical spin– momentum locking is a basis for many intriguing phenomena such as spin-controlled unidirectional excitation of surface and waveguided modes and offers potential applications in photonic integrated circuits, polarization manipulation, metrology, and quantum technologies for generating polarization entangled states [14,15,16,17,18,19,20]. PNAS 2021 Vol 118 No 6 e2018816118 transverse spin from electromagnetic waves to fluid, acoustic, and gravitational waves [30,31,32]
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