Coupled exciton–photon systems are investigated using the pump–probe technique in order to gain a better understanding of the important mechanisms governing cavity–polariton nonlinear behaviour and dynamics. A semi-classical model is used to describe the nonlinear behaviour and identify the dominant nonlinear mechanisms. Our phenomenological model produces very good agreement with experiment. The influence of free carriers on nonlinearities in the strong coupling regime was studied directly using non-resonant pump–probe.