Nonlinear valley phonon scattering under the strong coupling regime.

Research efforts of cavity quantum electrodynamics have focused on the manipulation of matter hybridized with photons under the strong coupling regime1–3. This has led to striking discoveries including polariton condensation2 and single-photon nonlinearity3, where the phonon scattering plays a critical role1–9. However, resolving the phonon scattering remains challenging for its non-radiative complexity. Here we demonstrate nonlinear phonon scattering in monolayer MoS2 that is strongly coupled to a plasmonic cavity mode. By hybridizing excitons and cavity photons, the phonon scattering is equipped with valley degree of freedom and boosted with superlinear enhancement to a stimulated regime, as revealed by Raman spectroscopy and our theoretical model. The valley polarization is drastically enhanced and sustained throughout the stimulated regime, suggesting a coherent scattering process enabled by the strong coupling. Our findings clarify the feasibility of valley–cavity-based systems for lighting, imaging, optical information processing and manipulating quantum correlations in cavity quantum electrodynamics2,3,10–17. Strong exciton–polariton coupling is leveraged as a means to open up phonon scattering channels that are otherwise weak.