Low-amplitude spatial solitons in photovoltaic photorefractive crystals with external bias field

A theory of spatial solitons is developed for closed-circuit photorefractive-photovoltaic crystals. Spatial solitons result from both the photovoltaic effect and the spatially nonuniform screening of the external bias field. The properties of these solitons differ from those observed previously for steady-state photorefractive spatial solitons. The nonlinear wave equation is derived; it describes the stationary (soliton) propagation of wave in photovoltaic crystals with external applied field. When the external bias field is much stronger such that the photovoltaic effect can be neglected, the nonlinear wave equation is similar to that for screening solitons. If the external bias field is absent, the nonlinear wave equation is similar to that for screening solitons. If the external bias field is absent, the nonlinear wave equation is similar to that for photovoltaic solitons in a closed or open circuit. In the low-amplitude regime, an approximate analytic solution is obtained for the nonlinear wave equation. Under the appropriate conditions, these solitons change into screening and photovoltaic solitons. When external electric field is of a certain value, these solitons may be switched from bright to dark by changing the polarity of the external electric field and by rotating the polarization of the light.