Tunable positive and negative refraction in optically-induced photonic   lattices
  We study tunable refraction of light in one-dimensional periodic lattices induced optically in a photorefractive crystal. We observe experimentally both positive and negative refraction of beams which selectively excite the first or second spectral bands of the periodic lattice, and demonstrate tunability of the output beam position by dynamically adjusting the lattice depth. At higher laser intensities, the beam broadening due to diffraction can be suppressed through nonlinear self-focusing while preserving the general steering properties. 
Christian R. Rosberg, Dragomir N. Neshev, Andrey A. Sukhorukov, Yuri   S. Kivshar, and Wieslaw Krolikowski