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 molecules¹, nonlinear optical waveguides², solid-state systems³ and Bose-Einstein condensates⁴. The underlying dynamics in these systems is dominated by the interplay between tunnelling between adjacent potential wells and nonlinearity^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 onedimensional 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 solitons¹⁸, 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 condensates⁴, which can be observed in optically induced periodic potentials^24,25.