To further expand the investigation, we spectrally resolve the ECPL dynamics and Figure shows the ECPL map as a function of the photon-energy and time delay. We note that if the ECPL response entirely arises due to the nonlinear interactions in the reservoir and given that a common reservoir feeds both the lower polariton and the PL2 state, we expect stationary ECPL intensity (Δ𝑃𝑃𝑃𝑃/𝑃𝑃𝑃𝑃) over the entire spectral range of the PL. Contrary to this, we observe that the negative nonlinear ECPL response is relatively higher for the lower polariton state in comparison to the PL2 state (see SI). The larger nonlinearity within the LP can be interpreted as enhanced nonlinear interactions of the polariton states, promoting the population loss of the lower polariton. This is consistent with the intensity dependence of the integrated PL, where the PL2 peak rises more rapidly than the LP peak with increasing excitation intensity (Figure ).