Nat. Phys. (2017)

Polariton-based optoelectronics may turn out to be considerably more efficient than originally expected. Scientists in the US from the Massachusetts Institute of Technology, Princeton University and the University of Pittsburgh have now directly measured the strength of polariton–polariton interactions and determined that it is about two orders of magnitude stronger than previous theoretical estimates. The finding suggests that polariton-based devices would be operating in the strongly interacting regime with an effective nonlinearity that could be larger than anticipated. Exciton-polaritons (commonly referred to as just polaritons) are a type of quasiparticle generated in a semiconductor optical microcavity — in essence, a mixed state of light and matter formed by the strong coupling between an exciton and a photon. To date, it has been difficult to accurately experimentally measure the interaction strength as it relies on being able to distinguish between polariton–polariton interactions and interactions between polaritons and excitons. The US team circumvent this problem by using a very high-Q microcavity (Q = 320,000) so that the polaritons have a long lifetime (of the order of >200 ps) and are able to propagate to an optical trap where they can be separated from free charge carriers and hot excitons. Spectral data from the centre of the trap is then used to determine the polariton interaction strength. The microcavity consists of four GaAs quantum wells sandwiched between two distributed Bragg reflectors.