Steady state multiple dark spatial solitons in the biased photorefractive-photovoltaic crystals

We theoretically study the evolution of dark solitons in the biased photorefractive-photovoltaic crystal by using beam propagation method(BPM). We find that when the absolute value of the extra bias field is less than the photovoltaic field, the dark screening-photovoltaic(SP) solitons can be observed. The initial width of the dark notch at the entrance face of the crystal is a key parameter for generating an sequence of dark coherent solitons. If the initial width of the dark notch is small, only a fundamental soliton or Y-junction soliton pair is generated. When the initial width of the dark notch is increased, the dark notch tends to split into an odd(or even) number of multiple dark solitons, which r ealizes a progressive transition from the low-order solitons to a sequence of higher-order solitons.