Crossover from self-defocusing to discrete trapping in nonlinear   waveguide arrays
  We predict a sharp crossover from nonlinear self-defocusing to discrete self-trapping of a narrow Gaussian beam with the increase of the refractive index contrast in a periodic photonic lattice. We demonstrate experimentally nonlinear discrete localization of light with defocusing nonlinearity by single site excitation in LiNbO$_3$ waveguide arrays. 
Michal Matuszewski, Christian R. Rosberg, Dragomir N. Neshev, Andrey   A. Sukhorukov, Arnan Mitchell, Marek Trippenbach, Mic...