Dark (b) planar spatial solitons are predicted for photovoltaic photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing (focusing) due to the photovoltaic field and electro-optic effect. These solitons may have steady-state irradiances of microwatts to milliwatts per square centimeter and widths as small as 10 mum in lithium niobate. Optical control is provided by incoherent illumination, and the nonlinear index of a dark soliton may be used to trap a bright soliton by rotating the plane of polarization of the soliton field.
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"Only a year later Duree et al.  were able to observe them experimentally. Till date, theoretical as well as experimental investigations have been made on different types of PR spatial solitons of which screening solitons (SS)     , photovoltaic (PV) solitons   and screening photovoltaic (SP) solitons   , being the most widely and thoroughly investigated categories. These solitons have been found to exist in bright, dark and grey, scalar as well as vector configurations . "
[Show abstract][Hide abstract] ABSTRACT: This paper presents an investigation on the propagation characteristics of optical spatial solitons in a biased centro-symmetric photorefractive medium. Unlike earlier attempts on photorefractive solitons, in the present investigation, we have given equal significance to the effects of charge drift and their diffusion. We have obtained dynamical equations for solitons employing paraxial ray approximation and examined criteria for stationary propagation. Trajectories of stationary solitons have been examined.
"During the last decade, the optical spatial solitons based on photorefractive effect have attracted much interest, for these photorefractive spatial solitons can be formed at low light intensity and are potentially useful for all-optical switching, beam steering, and optical interconnects. At present, three types of steady-state scalar solitons (screening solitons   , photovoltaic solitons     and screening-photovoltaic solitons   ) have been predicted theoretically and found experimentally. The diffusion process introduces an asymmetric tilt in the light-induced photorefractive waveguide, which results in the self-deflection process of solitons . "
[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of higher-order space charge field on the self-deflection of bright screening spatial solitons due to two-photon photorefractive effects by a numerical method under steady-state conditions. The expression for an induced space charge electric field including higher-order space charge field terms is obtained. Numerical results indicate that bright screening solitons undergo self-deflection process during propagation, and the solitons always bend in the opposite direction of the c axis of the crystal. The self-deflection of bright screening solitons can experience considerable increase especially in the regime of high bias field strengths. Relevant examples are provided.
"The first is the screening soliton. Both bright and dark screening solitons (SS) in the steady state are possible when an external bias voltage is applied to a nonphotovoltaic PR crystal   . The second kind is the photovoltaic soliton [3,5–15], the formation of which, however, requires an unbiased PR crystal that exhibits the photovoltaic effect, i.e., generation of dc current in a medium illuminated by a light beam. "
[Show abstract][Hide abstract] ABSTRACT: The existence and nonlinear dynamics of two-component incoherently coupled composite solitons in two-photon photorefractive materials under open circuit conditions have been investigated. In the steady-state regime, these incoherently coupled solitons can propagate in bright–dark, bright–bright and dark–dark configurations. These photovoltaic soliton families can be established provided that the carrier beams share the same polarization and wavelength. The influence of the gating beam on the dynamics of stable solitons is discussed. Numerical simulations show that these solitons are stable for small perturbation on amplitude.