Circular polariton currents with integer and fractional orbital angular momenta

Abstract

We study internal dynamics of exciton-polariton condensates created by the nonresonant optical pump in a cylindrical pillar microcavity with an ensemble of embedded quantum wells. The polariton condensates are intrinsically nonequilibrium systems: Their dissipative nature together with a spatial inhomogeneity of a potential landscape and localized pumping leads to formation of steady polariton flows. The gain-loss engineering consisting of a deliberate breaking of the rotational symmetry of the system makes the polariton flows controllable. We demonstrate the switching between the polariton current states characterized by both integer and fractional orbital angular momenta (OAM) by tuning the position and ellipticity of the pump spot. At a weak shift and a small ellipticity, the phase rather than the density of the exciton-polariton condensate is significantly affected. Then the polariton condensate is characterized by an integer OAM per particle coinciding with the topological charge of the polariton vortex state. We demonstrate experimentally the polariton current states with the topological charges from −3 to +2. The further shift of the pump spot perturbs the azimuthal distribution of the polariton density and causes a jump of the phase of the condensate at the density deep. The mean orbital angular momentum characterizing such polariton condensate acquires fractional values. To describe the experimentally observed polariton current states, we propose a model based on the Gross-Pitaevskii equation projected onto the azimuthal states of the ring trap which treats the formation of fractional OAM states as a result of the coherent superposition of integer OAM states sustained due to the gain-loss balance in the system with a broken rotational symmetry.