We report on the first experimental observation of discrete vortex solitons in two-dimensional optically-induced photonic lattices. We demonstrate strong stabilization of an optical vortex by the lattice in a self-focusing nonlinear medium and study the generation of the discrete vortices from a broad class of singular beams.PACS numbers: 42.65. Tg, 42.65.Jx, 42.70.Qs Periodic photonic structures and photonic crystals recently attracted a lot of interest due to the unique ways they offer for controlling light propagation. Periodic modulation of the refractive index modifies the diffraction properties and strongly affects nonlinear propagation and localization of light [1]. Recently, many nonlinear effects including the formation of lattice solitons have been demonstrated experimentally in one-and twodimensional optically-induced photonic lattices [2,3,4,5]. The concept of optically-induced lattices [6] relies on the modulation of the refractive index of a nonlinear medium with periodic optical patterns, and the use of a weaker probe beam to study scattering of light from the resulting periodic photonic structure.So far, only simple stationary structures have been described theoretically and generated experimentally in optically-induced lattices [2,3,4,5,6]. One of the most important next steps is the study of nonlinear modes with a nontrivial phase such as vortices, the fundamental localized objects appearing in many branches of physics. In optics, vortices are associated with the screw phase dislocations carried by diffracting optical beams [7]. When such vortices propagate in a defocusing nonlinear Kerrlike medium, the vortex core becomes self-trapped, and the resulting structure is known as an optical vortex soliton [1]. Such vortex solitons are usually generated experimentally on a broad background beam [8,9]. They demonstrate many similarities with the vortices observed in superfluids and Bose-Einstein condensates.In contrast, optical vortex solitons do not exist in a self-focusing nonlinear medium; a ring-like optical beam with a phase dislocation carrying a finite orbital angular momentum [10] decays into the fundamental solitons flying off the main ring [11]. This effect was first observed experimentally in saturable Kerr-like nonlinear medium [12], and then in photorefractive [9] and quadratic [13] nonlinear media in the self-focusing regime.Recent theoretical studies of the discrete [14] and continuous models of nonlinear periodic lattices [15,16] suggest that the vortex-like structures can be supported by the lattice even in the self-focusing regime. In this Letter, we report on the first experimental observation of discrete (lattice) vortex solitons and demonstrate, both theoretically and experimentally, that localized optical vortices can be generated in a self-focusing nonlinear medium, being stabilized by the two-dimensional periodic potential of a photonic lattice.To lay a background for our experiment, first we study numerically the generation of discrete vortex solitons in a two-dimensional ...
