  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.
