Dark photovoltaic spatial solitons:: Experiment and numerical solution

We generated dark photovoltaic spatial solitons in the iron doped lithium niobate, and we studied the generation process with a numerical model. The Schrodinger nonlinear equation was simulated with BPM (beam propagation method). This numerical method is also called symmetrical split-step Fourier method. For the generation of dark solitons, we used both the amplitude mask and the phase mask. The amplitude mask generated the even number of solitons, and the phase mask created the odd number of solitons. Every result from our experiment can be verified with BPM. The numerical program was programmed in Matlab. We created dark photovoltaic solitons in a bulk crystal with the optical intensity 1 - 10 mW/cm(2), and the soliton's FWHM about 5-18,mu m. We observed the temporal evolution of the one-dimensional dark photovoltaic solitons under open-circuit condition and the self-defocusing effect of the laser beam. The steady-state measurement (stable soliton) was obtained after a 6-15 min exposure. For the generation the argon ion laser beam at the wavelength of 514nm was used. It was polarized along the optical axis and collimated to a diameter of about 2mm on the input face. The resulting index perturbation forms a planar waveguide.