We report the first observation of spatial one-dimensional photorefractive screening solitons in centrosymmetric media and compare the experimental results with recent theoretical predictions. We find good qualitative agreement with theory. © 1998 Optical Society of America OCIS codes: 190.5330, 230.6120. Photorefractive spatial solitons have been a subject of intense study over the past few years. They have been predicted and observed in the quasi-steady-state regime, 1,2 in photovoltaic materials, 3,4 in the screening configuration, 5 -9 and in photorefractive semiconductors. 10More-complicated phenomena have also been reported, giving rise to intriguing observations, such as self-trapping of incoherent light beams. 11All these phenomena have been observed in noncentrosymmetric materials, in which soliton formation is governed by a change in refractive index that is due to the linear electro-optic response to an internal photoinduced space-charge field. Recently, spatial screening solitons of a different nature that should exist in photorefractive centrosymmetric materials were predicted. 12 We report the f irst observation of such solitons and compare our experimental results with the theoretical predictions.Centrosymmetric screening solitons arise from the index change produced by the quadratic electro-optic response to a photoinduced internal f ield. The f ield has, in this case, the double role both of polarizing the crystalline structure and of distorting the electronic polarization. In centrosymmetric crystals the change in refractive index is proportional to the square of the polarization ͑1͞Dn͒ ij g ijkl P k P l and is expressed by Dn 2͑1͞2͒n b 3 g eff e 0 2 ͑e r 2 1͒ 2 E 2 , where E is the internal field, g eff is the effective quadratic electrooptic coeff icient, and n b is the background refractive index and it is assumed that the (dc) polarization is in the linear regime, i.e., P e 0 ͑e r 2 1͒E.Our experiments are performed in potassium lithium tantalate niobate 13 (KLTN), which is treated to have a first-order ferroelectric -paraelectric phase transition slightly below room temperature. Working at room temperature enables one to operate in a centrosymmetric phase close to that transition, thereby enhancing the electro-optic response, 13 making centrosymmetric soliton observation possible with moderate electric fields. In Fig. 1 we show e r as a function of temperature and observe the large increase of e r at the vicinity of the ferroelectric -paraelectric transition (which occurs at ϳ12 ± C). Because Dn scales with ͑e r 2 1͒ 2 , operation at temperatures slightly above the Curie temperature results in an increase of the quadratic electro-optic response. In the specif ic case of KLTN, g eff is positive and thus only bright solitons can be observed; i.e., in the screening regime KLTN is a self-focusing medium. 12Bright centrosymmetric screening solitons in ͑1 1 1͒ D obey the wave equationwhere u͑j͒ is the soliton amplitude normalized to the square root of the sum of background and dark irradianc...

We report the observation of steady-state two-dimensional photorefractive self-trapping and screening spatial soliton formation in a sample of potassium–lithium–tantalate–niobate in the centrosymmetric paraelectric phase.

We investigate experimentally the generation of hexagonal patterns of filaments in a laser beam traversing a thin liquid crystal cell placed in front of a single ‘‘virtual mirror.’’ This mirror is the image of a real mirror and is placed either in front of or behind the cell. The results clearly support the physical idea that the Talbot effect governs the phenomenon. Edge effects due to the limited width of the pump beam are accounted for both theoretically and experimentally.

The combination of charge separation induced by the formation of a single photorefractive screening soliton and an applied external bias field in a paraelectric is shown to lead to a family of useful electro-optic guiding patterns and properties.Apart from their inherent interest as peculiar products of nonlinearity, spatial solitons hold the promise of allowing viable optical steering in bulk environments [1] [2]. Photorefractive screening solitons differ from other known manifestations of spatial self-trapping for their peculiar ease of observation and versatility [3], and recent experiments in photorefractive strontium-bariumniobate (SBN) and potassium-niobate (KNbO 3 ) have demonstrated two conceptual applications of their guiding properties. In the first case, a tunable directional coupler was realized making use of two independent slabsolitons [4]; in the second, self-induced phase-matching was observed to enhance second-harmonic-generation [5]. Although results suggest a means of obtaining all-optical functionality, actual implementation is hampered by the generally slow nonlinear response [6], that can be "accelerated" only at the expense of stringent intensity requirements [7]. In contrast, non-dynamic guiding structures have been observed by fixing a screening soliton [8], or in relation to the observation of spontaneous selftrapping during a structural crystal phase-transition [9]. One possible method of obtaining acceptable dynamics is to make directly use of the electro-optic properties of the ferroelectrics involved, in combination with the internal photorefractive space charge field deposited by the soliton. Since photorefractive charge-activation is wavelength dependent, one can induce charge separation in soliton-like structures at one active wavelength (typically visible), and then read the electrooptic index modulation at a different, nonphotorefractive, wavelength (typically infrared) [10] [11]. For noncentrosymmetric samples (such as the above mentioned crystals) that typically host screening soliton formation, the electro-optic index of refraction modulation is proportional to the static crystal polarization P, and thus to the electric field (linear electro-optic effect). For these, no electro-optic modulation effects are possible: for whatever value of external constant electric field E ext , the original soliton supporting guiding pattern remains unchanged. In centrosymmetrics, such as photorefractive potassium-lithium-tantalate-niobate (KLTN), solitons are supported by the quadratic electro-optic effect [12] [13] [14] [15]. In this case, the "nonlinear" combination of the internal photorefractive field with an external electric field can give rise to new and useful soliton-based electro-optic phenomena, which we here study for the first time.The basic mechanism leading to screening soliton formation is the following: a highly diffracting optical beam ionizes impurities hosted in the lattice of an electro-optic crystal. An externally applied electric field makes these mobile charges drift...