The combined effects of diffraction and Poynting vector walk-off in second-harmonic generation with pump beams that contain screw phase dislocations is addressed for what is believed to be the first time. We predict the spontaneous nucleation of multiple vortex twins whose subsequent explosion can yield quasi-aligned patterns of single-charge vortices.
Molina, G., Torner, L., Petrov, D. Vortex streets in walking parametric wave mixing. "Optics letters", Juliol 1999, vol. 24, núm. 14, p. 899-901.