We investigate, experimentally and theoretically, light propagation in one-dimensional waveguide arrays exhibiting a saturable self-defocusing nonlinearity. We demonstrate low-intensity “discrete diffraction”, and the high-intensity formation of spatial gap solitons arising from the first band of the transmission spectrum. The waveguide arrays are fabricated by titanium in-diffusion in a photorefractive copper-doped lithium niobate crystal, and the optical nonlinearity arises from the bulk photovoltaic effect.

The observation of nonlinear staggered surface states at the interface between a substrate and a one-dimensional self-defocusing nonlinear waveguide array is reported. Launching of staggered input beams of different power in the first channel of the array results in formation of localized structures in different channels. Our experimental results are confirmed numerically.

We report on the formation of dark discrete solitons in a nonlinear periodic system consisting of evanescently coupled channel waveguides in defocusing lithium niobate. Localized nonlinear dark modes displaying a phase jump in the center that is located either on-channel (mode $A$) or in-between channels (mode $B$) are formed, which is to our knowledge the first experimental observation of mode $B$. By numerical simulations we find that the saturable nature of the nonlinearity is responsible for the improved stability of mode $B$. The ability of the induced refractive index structures to guide light of a low-power probe beam is demonstrated.

We report on a new method to form reconfigurable channel waveguides in lithium niobate crystals, based on a combination of low-dose O³⁺ ion implantation and selective white light illumination. The fabricated structures show low loss as well as rather high resistivity against optical erasure with red or infrared light, while at the same time reconfiguration of the structures remains possible using homogeneous white light illumination. The transmission properties of the channel waveguide modes can be well simulated numerically by the beam propagation method, which allows for the fabrication of tailored optical interconnections.

We have obtained stationary bright spatial solitons in a planar photorefractive strontium barium niobate waveguide for visible light ranging from 514.5 to 780 nm. Even for larger wavelengths $\left(\lambda =1047 \mathrm{nm}\right)$ strong self-focusing of the beam was observed; however, input power had to be some orders of magnitude higher than for visible light for self-focusing to occur. Furthermore, we found transient self-trapping of red light $\left(\lambda =632.8 \mathrm{nm}\right)$ that corresponds to the formation of bright quasi-steady-state solitons.

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