Spontaneous rotating vortex rings in a parametrically driven polariton fluid J. O. Hamp, A. K. Balin, F. M. Marchetti, D. Sanvitto and M. H. Szymańska EPL, 110 5 (2015) 57006 Published online: 26 June 2015 DOI: 10.1209/0295-5075/110/57006