Direct Observation of Unidirectional Exciton Polaritons in Layered van der Waals Semiconductors.

Unidirectional excitation of highly confined guided modes is essential for nanoscale energy transport, photonic integrated devices, and quantum information processing. Among various feasible approaches, the mechanism based on optical spin-orbit coupling is investigated for unidirectional routing of surface plasmons and valley exciton-polaritons, without needing the use of complicated magneto-optical effects and parity symmetry breaking. So far, the direct near-field nanoimaging of such exotic polaritonic modes based on optical spin-orbit coupling has remained elusive. Here, the real-space nanoimaging of unidirectional polaritons in van der Waals semiconductors are reported. Specifically, photonic spins are coupled into the tip of a scattering-type scanning near-field optical microscopy for circular dipolar excitations of spin-orbit interactions, thus enabling the unidirectional waveguide exciton-polariton propagation with remarkable unidirectionality (ratio of spectrum amplitudes under opposite circularly polarized illumination) R = 0.291 for TM mode. Via switching to the opposite helicities, the reversed opposite directions are observed. The work offers a promising avenue for detecting and processing spin information for future communication technology at the nanoscale.