Unravelling Ultrafast Dynamics of Self‐Hybridized Exciton‐Polaritons in WSe 2 for External Cavity Free Polaritonics

ABSTRACT Exciton‐polaritons (EPs) show intriguing optoelectronic properties. Self‐hybridized EPs without an external cavity mitigate the complexity of cavity fabrication and provide an open and more natural platform for exploring polariton physics and chemistry. However, ultrafast dynamics and many‐body interactions in self‐hybridized EPs remain unclear. In this work, we use transient reflection spectroscopy to investigate ultrafast dynamics of self‐hybridized EPs in WSe 2 flakes. Above‐resonance and lower‐polariton‐resonant excitations reveal two‐step inter‐branch scattering between the exciton reservoir and polaritons in coherent (ultrafast, < 100 fs) and incoherent, phonon‐assisted (slow, 2–30 ps) regimes. The optical nonlinearity of self‐hybridized EPs originates from the many‐body interactions, including Pauli blocking and bandgap renormalization of excitons. A significantly smaller blueshift is seen in the first few picoseconds under lower‐polariton‐resonant excitation compared to above‐resonance excitation, demonstrating that polaritons experience much weaker interparticle interactions than excitons because of the photonic component. These results are valuable for developing external‐cavity‐free polaritonic devices.