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While conventional semiconductor technology relies on the manipulation ofelectrical charge for the implementation of computational logic, additionaldegrees of freedom such as spin and valley offer alternative avenues for theencoding of information. In transition metal dichalcogenide (TMD) monolayers,where spin-valley locking is present, strong retention of valley chirality hasbeen reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalouslylow valley polarisation retention. In this work, chiral selectivity of MoSe$_2$cavity polaritons under helical excitation is reported with a polarisationdegree that can be controlled by the exciton-cavity detuning. In contrast tothe very low circular polarisation degrees seen in MoSe$_2$ exciton and trionresonances, we observe a significant enhancement of up to 7 times when in thepolaritonic regime. Here, polaritons introduce a fast decay mechanism whichinhibits full valley pseudospin relaxation and thus allows for increasedretention of injected polarisation in the emitted light. A dynamical modelapplicable to cavity-polaritons in any TMD semiconductor, reproduces thedetuning dependence through the incorporation of the cavity-modified excitonrelaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ whichis an order of magnitude faster than those reported in other TMDs. The valleyaddressable exciton-polaritons reported here offer robust valley polarisedstates demonstrating the prospect of valleytronic devices based upon TMDsembedded in photonic structures, with significant potential forvalley-dependent nonlinear polariton-polariton interactions.

Valley addressable exciton-polaritons in atomically thin semiconductors