Formation of bright discrete gap solitons in onedimensional photonic lattices in lithium niobate

We experimentally investigate some features of nonlinear propagation of light beams within the one-dimensional photonic lattices at light wavelength of 633 nm and optical power of a microwatt range. The one of most exciting features of such lattices is a possibility to reverse a sign of optical nonlinearity when a direction of light beam propagation is close to Bragg angle, e.g. when light propagates in a region of anomalous diffraction. In such a case formation of bright spatial solitons is possible in lithium niobate which is a medium with self-defocusing photorefractive optical nonlinearity. We study the intensity shapes and their time evolution for extraordinarily polarized probe beams of He - Ne laser at the output face of the crystal for the beam waist size at its input face from 8 to 70 micrometers. At excitation of light in the Bragg directions we observe strong localization of light within only one or two waveguide layers at the lattice output during some stage of the light field evolution. We attribute this effect to formation of bright discrete gap solitons which were earlier observed only in periodic structures of coupled channel optical waveguides