  Spin-orbit coupling is a fundamental mechanism that connects the spin of a
charge carrier with its momentum. Likewise, in the optical domain, a synthetic
spin-orbit coupling is accessible, for instance, by engineering optical
anisotropies in photonic materials. Both, akin, yield the possibility to create
devices directly harnessing spin- and polarization as information carriers.
Atomically thin layers of transition metal dichalcogenides provide a new
material platform which promises intrinsic spin-valley Hall features both for
free carriers, two-particle excitations (excitons), as well as for photons. In
such materials, the spin of an exciton is closely linked to the high-symmetry
point in reciprocal space it emerges from. Here, we demonstrate, that spin, and
hence valley selective propagation is accessible in an atomically thin layer of
MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a
wire-like device, where we can clearly trace the flow, and the helicity of
exciton-polaritons expanding along a channel. By exciting a coherent
superposition of K and K- tagged polaritons, we observe valley selective
expansion of the polariton cloud without neither any applied external magnetic
fields nor coherent Rayleigh scattering. Unlike the valley Hall effect for TMDC
excitons, the observed optical valley Hall effect (OVHE) strikingly occurs on a
macroscopic scale, and clearly reveals the potential for applications in
spin-valley locked photonic devices.
