Revealing Ultrafast Optical Nonlinearity of Trapped Exciton Polaritons in Atomically Thin Semiconductors

Nonlinearities are fundamental to modern optical technologies. Exciton polaritons in semiconductor microcavities provide a promising route to strong nonlinearities. Monolayer TMDs, with tightly bound excitons and strong oscillator strength, enable polaritonic phenomena under ambient conditions but face challenges from weak polariton interactions due to small exciton Bohr radius. Although spatial confinement can boost polariton nonlinearity, the dynamics of trapped polaritons remain underexplored. Here we study the transient nonlinearities of confined polaritons in monolayer WS₂ mesa cavities. We observe increasingly pronounced blueshifts within the first few picoseconds as trapping sizes decrease or excitonic fractions increase. Furthermore, our findings reveal that exciton-photon detuning, not trapping size, predominantly influences the time to reach the peak of transient nonlinearity. This insight aligns with the experimentally observed and theoretically simulated relaxation dynamics of trapped polaritons. Our findings pave the way for developing ultrafast all-optical polaritonic devices in TMD systems.