Fig. 5. Strength of nonlinearity vs pump.

The experimental effective interaction constant $g_{\mathrm{X}}^{\mathrm{eff}}$ is shown as a function of the estimated exciton density, n_tot. The $g_{\mathrm{X}}^{\mathrm{eff}}$ is deduced from the energy blueshift of MPB (see Supplementary Note 1.5). The data correspond to four different cavity–exciton detunings (δ):  +8.8 meV (olive),  +2.0 meV (purple),  −2.4 meV (orange), and  −15.4 meV (blue). The error bars (95% CI) are deduced taking into account errors in determining the MPB peak positions at each pump power (exciton density). The red solid curve corresponds to the theoretical values.