  The emergence of two-dimensional transition metal chalcogenide materials has
sparked an intense activity in valleytronics since their valley information can
be directly encoded and detected by using the spin angular momentum of light.
For their practical applications such as on-chip valley logic gates and
chip-to-chip valley transport, the encoding and processing of valley pseudospin
using light should be extended to an integrated, on-chip nanophotonic system.
Here, we successfully demonstrate, at room temperature, the valley-dependent
directional coupling of light using a plasmonic nanowire-WS2 layers system. Our
calculations show that the local transverse spin angular momentum of the mode
of the plasmonic nanowire provides robust optical spin-path locking of up to 91
%. Experimentally we demonstrate that valley pseudospin in WS2 is coupled with
optical spin of the same handedness and exhibits a high directional coupling
efficiency up to 90 % to the plasmonic guided mode. The result opens up new
avenues of controlling, detecting and processing valley and spin information
with precise optical control at the nanoscale.
