Transport and Spatial Separation of Valley Coherence via Few Layer WS2 Exciton-Polaritons.

The optical response in two-dimensional transition-metal dichalcogenides (2D TMDCs) is dominated by excitons. The lack of spatial inversion symmetry in the hexagonal lattice within each TMDC layer leads to valley-dependent excitonic emission of photoluminescence. Here, we demonstrate experimentally the spatial separation of valley coherent emission into orthogonal directions through self-resonant exciton polaritons of a free-standing three-layer (3L) WS2 waveguide. This was achieved by patterning a photonic crystal consisting of a square array of holes allowing for the far field probing of valley coherence of engendered exciton-polaritons. Furthermore, we report detailed experimental modal characterization of this coupled system in good agreement with theory. Momentum space measurements reveal a degree of valley coherence in the range 30-60%. This work provides a platform for manipulation of valley excitons in coherent light-matter states for potential implementations of valley-coherent optoelectronics.