Persistent, controllable circulation of 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.

 

Our experiments focus on polariton ring condensate. An optical trap was used to create the ring condensate. The continuous wave pump laser was tuned by SLM to a target-shape, and it generates polaritons and also creates a Mexican hat potential due to repulsive exciton-polariton interaction. The ring condensate was observed at the valley of the potential. We chose the “flat” region of the sample and made the optical trap uniform so that the condensate would not circulate.

 

When a second, pulsed laser (probe) was introduced, the condensate circulated with winding number 1. By simply moving the probe position, we could flip the circulation direction. The streak images show that the circulation persists for 13.2ns, which is much longer than the polariton lifetime. A theoretical model is built to explain the experimental results. It shows that the asymmetry of the probe is essential to start the circulation.