Polaritons are Not Weakly Interacting: Direct Measurement of the Polariton-Polariton Interaction Strength

摘要

Exciton-polaritons in a microcavity are composite two-dimensional bosonicquasiparticles, arising from the strong coupling between confined light modesin a resonant planar optical cavity and excitonic transitions, typically usingexcitons in semiconductor quantum wells (QWs) placed at the antinodes of thesame cavity. Quantum phenomena such as Bose-Einstein condensation (BEC),quantized vortices, and macroscopic quantum states have been reported attemperatures from tens of Kelvin up to room temperatures, and polaritonicdevices such as spin switches \cite{Amo2010} and optical transistors have alsobeen reported. Many of these effects of exciton-polaritons depend crucially onthe polariton-polariton interaction strength. Despite the importance of thisparameter, it has been difficult to make an accurate experimental measurement,mostly because of the difficulty of determining the absolute densities ofpolaritons and bare excitons. Here we report the direct measurement of thepolariton-polariton interaction strength in a very high-Q microcavitystructure. By allowing polaritons to propagate over 40 $\mu$m to the center ofa laser-generated annular trap, we are able to separate the polariton-polaritoninteractions from polariton-exciton interactions. The interaction strength isdeduced from the energy renormalization of the polariton dispersion as thepolariton density is increased, using the polariton condensation as a benchmarkfor the density. We find that the interaction strength is about two orders ofmagnitude larger than previous theoretical estimates, putting polaritonssquarely into the strongly-interacting regime. When there is a condensate, wesee a sharp transition to a different dependence of the renormalization on thedensity, which is evidence of many-body effects.