Excitonic oscillator-strength saturation dominates polariton-polariton interactions

Exciton-polaritons in semiconductor microcavities exhibit large two-body interactions that, thanks to ever-refined nanotechnology techniques, are getting closer and closer to the quantum regime where single-photon nonlinearities start being relevant. To foster additional progress in this direction, in this work we experimentally investigate the microscopic mechanism driving polariton-polariton interactions. We measure the dispersion relation of the collective excitations that are thermally generated on top of a coherent fluid of interacting lower polaritons. By comparing the measurements with the Bogoliubov theory over both the lower- and upper-polariton branches simultaneously, we find that polariton-polariton interactions stem dominantly from a mechanism of saturation of the exciton oscillator strength.