Observation of dark spatial photovoltaic solitons in planar waveguides in lithium niobate

We have obtained photovoltaic lenses and dark spatial solitons in planar op tical waveguides in lithium niobate: doped with iron and copper. For TE mod es of lower indices the photovoltaic nonlinearity only partly decreased the width of a dark notch within the outcoupled image of the recording light b eam. The corresponding time to reach a steady state of this light-induced c hange ranged from about 0.1 to 30 s depending on thr waveguide sample. For higher modes we observed a full compensation of the divergence of the dart notch on a time scale of some minutes. In some cases this was followed by a n extinction of the dark solitons because the light was over-defocused in t he highest modes.