Observation of two-dimensional discrete solitons in optically induced nonlinear photonic lattices

Nonlinear periodic lattices occur in a large variety of systems, such as biological molecules(1), nonlinear optical waveguides(2), solid-state systems(3) and Bose-Einstein condensates'. The underlying dynamics in these systems is dominated by the interplay between tunnelling between adjacent potential wells and non-linearity(1-15). A balance between these two effects can result in a self-localized state: a lattice or 'discrete' soliton(1,2). Direct observation of lattice solitons has so far been limited to one-dimensional systems, namely in arrays of nonlinear optical waveguides(2,9-17). However, many fundamental features are expected to occur in higher dimensions, such as vortex lattice solitons(18), bright lattice solitons that carry angular momentum, and three-dimensional collisions between lattice solitons. Here, we report the experimental observation of two-dimensional (2D) lattice solitons. We use optical induction, the interference of two or more plane waves in a photosensitive material, to create a 2D photonic lattice in which the solitons form(11,12). Our results pave the way for the realization of a variety of nonlinear localization phenomena in photonic lattices and crystals(19-23). Finally, our observation directly relates to the proposed lattice solitons in Bose-Einstein condensates', which can be observed in optically induced periodic potentials(24,25).