With that brief introduction to ECPL, we proceed to investigate the PL nonlinearities of the thintop-mirror cavity in which PL1 radiatively pumps the lower polariton. As noted earlier, the photoluminescence spectrum of this cavity has two features: emission from the lower polariton and PL2 exciton emission leaking from the cavity (see Figure ). The spectrally-integrated ECPL signal (Figure ) is negative and increases in magnitude with higher excitation fluence, which is consistent with the sub-linear behavior of time-integrated LP and PL2 emission shown in Figure . We also observe that the intensity of the ECPL signal reduces with increasing delay between the pump pulses, following a seemingly mono-exponential decay time of about 30-40 ps. In our previous work , we reported very similar ECPL dynamics in a bare film of the 2D perovskite and a thick-top-mirror cavity with comparable detuning. In that work, given that the dynamics observed were much longer than the polariton lifetime and similar with or without the cavity, we consider them to be representative of the evolution of the population in the reservoir. Interestingly, the rate of decay of the ECPL signal is intensity independent, which suggests that the nonlinear annihilation processes happen within 200 fs, the time-resolution of the experiment. See note SX in the SI for more details.