Fig. 3: Measured normalized differential reflectivity.

The probe injects polaritons at an extended range of densities covering condensation threshold. Results in (a–d) are shown for an average probe intensity of 45 mW cm⁻², 92 mW cm⁻², 117 mW cm⁻² and 149 mW cm⁻², respectively at cavity-exciton detuning δ = +5.1 meV and for a fixed pump peak intensity of 10.2 GW cm⁻². The LP and the UP are represented by thick purple arrows and green narrow arrows, respectively. The Stark shift at Δt ≈ 0 is associated with the coherent oscillations at ∆t < 0. The onset of condensation, observed in (c), is marked by a significant prolongation of the oscillations. Below condensation threshold (a, b), the oscillatory features are associated with the LP and UP. Above threshold (c, d), the energy of the major part of the LP polaritons is blueshifted, giving rise to the dominant, higher-energy oscillations, while the UP-associated features fade. The bright arrows in (c, d) represent the uncondensed portion of the LP. The dashed lines mark pump-probe temporal overlap (Δt = 0). τ represents the decay time constant of the oscillations.