Vortex clusters in a stirred polariton condensate

Recently, we realized the formation of a single quantized vortex in nonresonantly optically stirred exciton-polariton condensates [I. Gnusov et al., Sci. Adv. 9, eadd1299 (2023)]. In this work, we demonstrate that the number of emerging vortices depends on the characteristic size of the rotating potential induced by the nonresonant laser excitation. For smaller sizes, we observe only a single vortex with a topological charge of $\pm 1$ defined by the stirring direction. For larger trap sizes, clusters of up to four corotating vortices are observed, also following the stirring direction. We find that the interplay of stirring speed and confining potential size dictates the number of vortices in a cluster. This is confirmed by observed energy distribution of condensed polaritons as a function of rotation frequency. Our findings offer an insight into the behavior of stirred condensates, complementing previous works on optically trapped polariton condensates in static traps.