Ballistic transport and persistent circulation in a polariton ring condensate

Exciton-polaritons are quasiparticles that are a superpositions of excitons and photons. In a microcavity, exciton-polaritons have an effective mass and can form a Bose-Einstein condensate (BEC). Experimentally, this condensate is generated by pumping light into a microcavity structure with quantum wells at the antinodes of the light field. The features of the polaritons are carried by the light they emit, so we can detect those by using conventional optical methods.

My experiments focus on polariton ring condensate. One way to create a ring condensate is to etch the top mirror of the cavity to a ring shape, and inject the polariton condensate at one point in the etched ring. The condensate transports ballistically, and forms orbital-like patterns. Although the condensate is in a single energy state, its polarization precesses around the ring, because of a term in the Hamiltonian analogous to the spin-orbit coupling.

Another way to create ring condensate is to use an optical trap. The target-shape pump laser not only generates polaritons but also creates a Mexican hat potential due to exciton-polariton interaction. The ring condensate was observed at the valley of the potential and was not circulating. When a second, pulsed laser (probe) was introduced, the condensate circulated with winding number 1. We can flip the circulation direction by changing the probe position. We also observed that the circulation persists for 13.2ns, which is much longer than the polariton lifetime.