Studies On Spatial Solitons In Biased Photovoltaic Photorefractive Crystals

A theory of spatial solitons is investigated in biased photovoltaic photorefractive crystals, which predicts a new type of steady-state spatial solitons known as screening-photovoltaic solitons. We show that the photovoltaic nonlinearity that gives rises to spatial solitons can be switched from self-defocusing to self-focusing (or vice versa), and find that under appropriate conditions, bright solitons can change into dark solitons. The self-deflection of screening-photovoltaic solitons is studied using perturbation analysis, which predicts the properties of the soliton beam evolution. Incoherently coupled screening-photovoltaic soliton pairs and incoherently coupled multicomponent spatial solitons are report. A theory on steady-state vector solitons is developed that gives rise to self-coupled and cross-coupled vector screening-photovoltaic solitons and self-coupled and cross-coupled vector photovoltaic solitons. We present a comprehensive study of the modulation instability in biased photorefractive-photovoltaic crystals, and obtain the properties of the modulation instability growth rate. In particular, we show theoretically that under appropriate conditions, the physical system of screening-photovoltaic spatial solitons may change into the physical system of screening and photovoltaic solitons and the theory of screening-photovoltaic solitons may change into the theory of screening solitons and photovoltaic solitons, and acquire the following major results. Steady-state optical spatial solitons, which are known as screening-photovoltaic solitons, are theoretically demonstrated for biased photovoltaic photorefractive crystals, which are due to both the spatially nonuniform screening of the external electric field and the photovoltaic effect. These solitons differ from screening solitons and photovoltaic solitons. Screening solitons result from the nonuniform screening of the external field, while photovoltaic solitons result from the photovoltaic effect. When photovoltaic effect is neglectable, the physical system of screening-photovoltaic solitons becomes the physical system of screening solitons, the space-charge field of screening-photovoltaic solitons becomes the space-charge field of screening solitons, the nonlinear wave equation of screening-photovoltaic solitons becomes the nonlinear wave equation of screening solitons, and screening-photovoltaic solitons becomes screening solitons. When the external field is absent, the physical system of screening-photovoltaic solitons becomes the physical system of photovoltaic solitons, the space-charge field of photovoltaic solitons becomes the space-charge field of photovoltaic solitons, the nonlinear wave equation of screening-photovoltaic solitons becomes the nonlinear wave equation of photovoltaic solitons, and screening-photovoltaic solitons becomes photovoltaic solitons and predicts that gray photovoltaic solitons are possible for closed-circuit condition. Thus, the studies of screening solitons and photovoltaic solitons may change into ones of screening photovoltaic solitons. We show theoretically that screening-photovoltaic solitons can be switched from bright to dark solitons by changing the polarity of the external electric field and by rotating the polarization of the light. Under the appropriate cases, the screening-photovoltaic nonlinearity can be switched from self-defocusing to self-focusing (or self-focusing to self-defocusing) by adding the external electric field. Under a strong bias condition, the screening-photovoltaic nonlinearity can be switched from self-defocusing to self-focusing (or self-focusing to self-defocusing) by changing the polarity of the external electric field or by rotating the polarization of the light. The self-deflection of screening-photovoltaic solitons is investigated by using perturbation analysis, which predicts that the center of the optical beam moves on a parabolic trajectory and, moreover, that the central spatial frequency component shifts linearly with the propagation distance. When photovoltaic effect is neglectable, the self-deflection of screening-photovoltaic solitons is the self-deflection of screening solitos.When the external field is absent, the self-deflection of screening-photovoltaic solitons predicts the self-deflection of photvoltaic solitons. A theory on incoherently coupled soliton pairs is developed that gives rise to incoherently coupled soliton pairs in biased photorefractive-photovoltaic materials, which can propagate in bright-bright, dark-dark, as well as in bright-dark configurations. When the bulk photovoltaic effect is neglectable, the physical system of these soliton pairs is the physical system for screening solitons, the nonlinear wave equations of these soliton pairs are the nonlinear wave equations for screening solitons, and these soliton pairs are soliton pairs for screening solitons. When the external field is absent, the physical system of these soliton pairs is the physical system for photovoltaic solitons, the nonlinear wave equations of these soliton pairs predict the nonlinear wave equations of photovoltaic soliton pairs for open-and closed-circuit conditions, and these soliton pairs predict that photovoltaic soliton pairs are possible for open-and closed-circuit conditions, which propagate in bright-bright, dark-dark, and in bright-dark configurations. Multicomponent spatial solitons are studied in biased photorefractive-photovoltaic materials. These multicomponent spatial solitons can be obtained from N self-trapped mutually incoherent optical beams. We not only obtain bright, dark, and bright-dark multicomponent spatial solitons by use of simple numerical integration procedures, but also determine the analytical solutions of bright, dark, and bright-dark multicomponent spatial solitons in the low amplitude regime. We discuss also the properties of these multicomponent spatial solitons. When N =1, these multicomponent spatial solitons change into steady-state spatial solitons. When N is an even number, these multicomponent spatial solitons change into steady-state soliton pairs. When the bulk photovoltaic effect is neglectable, the nonlinear wave equations of these multicomponent spatial solitons are the nonlinear wave equations for multicomponent screening solitons, and these multicomponent spatial solitons are multicomponent screening solitons. When...