Valley Exciton Polaritons*

The valley degree of freedom in exciton-polaritons (half-light half-matter quasiparticles) provide a largely unexplored control parameter for all-optical manipulation of polaritons. Owing to the large exciton binding energy and oscillator strength in two-dimensional (2D) transition metal dichalcogenides (TMDs), exciton-polariton formation at room temperature has been reported in variety of cavity architectures. One of the important distinctions of polaritons formed in 2D TMDs is their valley polarization property arising from the quantum mechanically distinct valley origin of their excitonic component. We will discuss the formation and optical control of the valley exciton-polaritons in 2D WS₂ embedded in a microcavity. These valley polaritons show in-plane momentum dependent helicity arising from their exciton-photon composite nature. Interestingly, even when the polariton state is optically excited at energies below the exciton reservoir, the polariton emission shows valley polarization. This unambiguously shows that the distinct valley origin of the exciton component is imprinted onto the hybrid polariton state, and that they can be addressed via their photonic component. Following this, we will briefly discuss the possibility to electrically control the exciton polaritons achieved through gating in a field effect transistor geometry. The realization of half-light half-matter valley polaritons at room temperature presents an attractive approach towards optical control and manipulation of valley degree of freedom in 2D semiconductors.