Observation of a gradient catastrophe generating solitons
  We investigate the propagation of a dark beam in a defocusing medium in the strong nonlinear regime. We observe for the first time a shock fan filled with non-interacting one-dimensional grey solitons that emanates from a gradient catastrophe developing around a null of the optical intensity. Remarkably this scenario turns out to be very robust, persisting also when the material nonlocal response averages the nonlinearity over dimensions much larger than the emerging soliton filaments. 
Claudio Conti, Andrea Fratalocchi, Marco Peccianti, Giancarlo Ruocco,   Stefano Trillo