Figure shows cuts at the energy of the lower polariton branch (in blue) alongside the emission attributed to the neat film. As fluence increases, the intensity of the lower polariton branch grows at a lower rate than the excitonic emission; however, both emissions plateau after a pumping fluence of 3 mJ/cm² is reached. The difference in the rate of increase between the two PL intensities (lower polariton at �𝑘𝑘 ∥ ���⃗ � = 0 and PL2) suggests that the exciton reservoir predominantly scatters through PL2 as the fluence increases. Therefore, in cavities where the lower polariton mode is radiatively pumped by PL1, the PL2 emission acts as a depletion mechanism for the exciton reservoir, radiating at an energy region that does not populate polaritons. This fluence-dependent behavior of the exciton reservoir PL can be observed in the bare film PL collected at different pumping fluences, depicted in Figure . As fluence rises from 0.18 to 1.77 mJ/cm², the normalized intensity of the lower energy peak grows at a higher rate than that of the higher energy peak. The fluence-dependent emission of this lower-energy feature in the PL had been reported previously and was interpreted as biexciton emission. Furthermore, by increasing the perovskite gain medium, amplified stimulated emission from this lower-energy peak was used by Polimeno et al. to feed the lower polariton mode through radiative pumping and drive the system into condensation. The sublinear photoluminescence intensity as a function of fluence of the lower polariton (LP) and excitonic emission in thin-top-mirror microcavities (Figure ), is indicative of nonlinear quenching mechanisms manifesting at higher excitation densities . To further understand such nonlinearities in the emission, and, more importantly to rationalize the different intensity trends of the LP and PL2 peaks, we perform excitation correlation photoluminescence (ECPL) spectroscopy. In this experiment, the sample is photo-excited with two identical pump pulses with tunable time delay (𝜏𝜏) between them. Then, the photoluminescence measured from the sample will be composed of PL due to the excitation of each of the pulses and an additional cross-component (∆PL), which arises only in the presence of nonlinear interactions between the photo-excited states.