The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics. When embedded in optical microcavities the large oscillator strength of excitonic transitions in TMDs allows the formation of hybrid quasiparticles which are a superposition of the exciton and cavity photon states called polaritons. Here, we report the valley addressability of MoSe2 cavity polaritons under non-resonant helical excitation with a polarisation degree observed in photoluminescence that can be controlled by the exciton-cavity frequency detuning. Moreover, we observe both strong trion-cavity and exciton-cavity coupling demonstrating the formation of valley-polarised polaritons which are a linear superposition of excitons, trions and photons. In contrast to the very low circular polarisation degrees seen in MoSe2 exciton and trion resonances, we observe a significant enhancement of up to 20% when in the polaritonic regime. We further extend this work to the control of valley coherence in magnetic field in the polariton regime in WSe2 observed as rotation of linear polarisation imprinted by the pump laser. An unexpected feature here is the very different rotation angles of the linear polarisation in the lower and upper polariton branches, that is in addition controlled by the exciton-cavity-mode detuning. The effect originates from the modified exciton dynamics of the high-momentum excitons in the presence of the low-momentum polariton states.
Alexander I. Tartakovskii, "Valley phenomena in the microcavity polariton regime in transition metal dichalcogenides (Conference Presentation)," Proc. SPIE 10534, 2D Photonic Materials and Devices, 105340L (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2293066.5751542090001.
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