One-dimensional steady-state photorefractive spatial solitons in centrosymmetric paraelectric potassium lithium tantalate niobate

Abstract

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...

Exact bright and dark spatial soliton solutions in saturable nonlinear media

“…Moreover, the dependence of the full width at half maximum (FWHM), ∆ξ, as a function of the peak amplitude u 0 can be easily obtained from Eq. dependence is often measured in experiments [32]. In our present case there are several interesting features.…”

“…The case with α = 1 (which corresponds to non-centrosymmetric photorefractive media) has been throughly studied, and solitary wave solutions in the form of bright, black and grey solitons have been found numerically [20,21,22,30]. It was also shown theoretically (by means of numerical calculations) [31], and experimentally [32,33], that the nonlinearity with α = 2 also appears in the, socalled, centrosymmetric photorefractive media and exhibits bright and black spatial solitons. Motivated by these studies, here we go beyond the previous numerical studies and obtain three classes of exact solutions to Eq.…”

Soliton electro-optic effects in paraelectrics

“…(1a) Fig. (1b), where a 11µm soliton is formed for u Experiments are carried out with an apparatus that is well documented in literature [13] [14]. An enlarged TEM 00 Gaussian beam from a CW Argon-ion laser operating at λ =514nm, is focused be means of an f=150mm cylindrical lens onto the input facet of an 3.7 (x) × 4.6 (y) × 2.4 (z) mm sample of zero-cut paraelectric KLTN, at T=20…”

Self-focusing and self-trapping in unbiased centrosymmetric photorefractive media

“…As far as paraelectric KLTN is concerned, recent results for spatial screening solitons 4 were obtained for crystals characterized by a room-temperature phase transition that, however, possesses low peak values of r ͑´r ഠ 3 3 10 4 ͒. This implies that we can explore values for at most m ഠ 1 1.2 and that, therefore, we can expect only slight self-focusing effects described by Eq.…”

“…4 In this case, because the nonlinear refractive-index contribution for unbiased centrosymmetric crystals is proportional to the square of E sc , its asymmetry is no longer relevant, and it is natural to look for the existence of self-confined propagating beams with no self-bending. In this Letter we study the general propagation equation in unbiased centrosymmetric PR crystals and demonstrate the existence of such solutions in the form of both selfconfined and self-focused beams.…”