Abstract
We investigate higher-order space charge field effects on the evolution of bright spatial solitons in biased photorefractive crystals under steady-state conditions. Numerical simulations demonstrate that these optical solitons can experience considerable increase in their self-deflection especially in the regime of very high bias field strengths. This process is further studied using perturbation techniques. Our analysis indicates, that for very high bias fields, the self-bending process is further enhanced by a factor that varies cubically with the applied field. Relevant examples are provided.

Abstract

Abstract
We show that the vector beam evolution equations in properly oriented biased photorefractive media can exhibit bright-dark soliton pair solutions under steady-state conditions. These wave pairs are obtained perturbatively provided that the intensities of the two optical beams are approximately equal. Our analysis indicates that these bright-dark vector solitons exist irrespective of the polarity of the external bias field. The stability of these vector pairs has been investigated numerically and it has been found that they are stable only in the regime of positive bias polarity.

Abstract
The modulational instability of quasi-plane-wave optical beams in biased photorefractive media is investigated under steady-state conditions. The spatial. growth rate of the sideband perturbations is obtained by globally treating the space-charge field. Our analysis indicates that the growth rates depend on the strength of the externally applied electric field and, moreover, on the ratio of the optical beam's intensity to that of the dark irradiance. Our results are then compared to previous local treatments of the space-charge field equation. The two approaches are found to be in good agreement in the low spatial-frequency regime provided that the external bias field is sufficiently high. Conversely, in the high spatial-frequency region notable differences may exist. Relevant examples are provided.

Abstract
The so-called screening solitons that occur in steady state when an external bias voltage is appropriately applied to a PR crystal are known to occur when the drift term dominates in the expression of the space charge field. In a recent study, the effects arising from first-order diffusion terms have also been investigated. It was found that the PR soliton can experience adiabatically self-deflection that varies linearly with the applied electric field. However, recent experiments suggest that this self-deflection can exceed that predicted by theory, especially in the regime of quite high bias fields. To account for this discrepancy, this paper investigates the effects of other higher-order electric field terms on the evolution of bright steady-state solitons in PR media.

Abstract
It is shown that a new type of incoherently coupled soliton pairs is possible under steady-state conditions provided that their carrier beams share the same polarization and wavelength and are mutually incoherent. They can be readily realized in a simple experimental setup where the optical beams co-propagate collinearly in a biased photorefractive crystal, in which case they experience equal electro-optic coefficients. The two optical beams can be also slightly misaligned at the input so as to differentiate them at the output plane.