Exciton-polaritons are quasi-particles with bosonic properties. They can form Bose-Einstein condensate. Due to the short lifetime and strong interaction with non-condensed polaritons and excitons, they are considered to be in a nonequilibrium state. The nonequilibrium character can be reduced by spatial separation of the condensate from the non-condensed particles i.e. excitonic reservoir. This can be performed by a ring-shaped potential that confines polaritons in its centre. Exciton-polaritons created beneath the excitation laser flow outside the spot to the inside of the ring and exhibit properties more similar to the equilibrium state.
Our main motivation was to obtain a thermally-equilibrium condensate, which was possible owing to the use of a ring-shaped optical trap. Polaritons can flow into the interior of the ring, where, due to the lack of additional light, condensate could exist in separation from the cloud of non-condensed particles.