Direct observation of unidirectional exciton polaritons in layered van der Waals semiconductors

Abstract 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 has been investigated for unidirectional routing of surface plasmons and valley exciton polaritons, without exploiting the complicate magneto-optical effects and parity symmetry breaking. So far, the direct nanoimaging of such exotic polaritonic modes in near fields has remained elusive. Here, we report the real-space nanoimaging of unidirectional exciton-polariton in van der Waals semiconductors. We couple photonic spins into the tip of a scattering-type scanning near-field optical microscopy for circular dipolar excitations of spin-orbit interactions, thus enabling the unidirectional exciton propagation (with remarkable ratio of unidirectionality R=3.44 for TM mode). Via switching to the opposite helicities, we observe the reversed opposite directions. Our work offers a promising avenue for detecting and processing spin information for future communication technology at the nanoscale.