Direct measurement of polariton-polariton interaction strength

Exciton-polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions. Quantum phenomena such as Bose-Einstein condensation, superfluidity, quantized vortices, and macroscopic quantum states have been realized at temperatures from tens of kelvin up to room temperatures. Crucially, many of these effects of exciton-polaritons depend on the polariton-polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty in determining the absolute densities of polaritons and bare excitons. Here we report a direct measurement of the polariton-polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 20 mu m to the centre of a laser-generated annular trap, we are able to separate the polariton-polariton interactions from polariton-exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons in the strongly interacting regime.