We report what is believed to be the first experimental observation of spontaneous nucleation of arrays of optical vortices in the process of second-harmonic generation in a crystal with moderate Poynting vector walk-off, pumped with focused beams containing screw phase dislocations. Our experiments were conducted in lithium triborate under conditions of small-efficiency second-harmonic generation pumped by nanosecond pulses at 1064 nm.
Molina, G., Dmitri, V., Recolons, J., Torner, L. Observation of optical vortex streets in walking-second harmonic generation. "Optics letters", 15 Abril 2002, vol. 27, núm. 8, p. 625-627.