Gap solitons supported by optical lattices in biased centrosymmetric photorefractive crystals

We analyze the existence and stability of gap solitons supported by optical lattices with self-focusing nonlinearity in biased centrosymmetric photorefractive crystals. It is shown that, in first finite bandgap, gap solitons are symmetric in transverse dimension, single humped, entirely positive and linearly stable, while these solitons are antisymmetric with similar profiles, the stable and unstable intervals of the gap solitons are intertwined in the second finite bandgap.     

Incoherent spatial optical solitons in photorefractive crystals

In this paper we have presented a theoretical investigation on the propagation properties of incoherent solitons in photorefractive media which is characterized by noninstantaneous saturating nonlinearity. Using mutual coherence function approach, we have obtained the equation of existence curve for such solitons. We have discussed the coherence characteristics of these solitons. We have found that solitons with a particular radius can possess two different critical powers. The dynamical evolution of these solitons has been discussed in detail by both analytical and numerical simulation.     

Surface solitons supported by one-dimensional composite Bessel optical lattices

We address the existence of surface solitons at an interface in a defocusing cubic medium with an imprinted one-dimensional （1D） composite Bessel optical lattice. This setting is composed of two Bessel lattices with different orders and different modulation depths, separated beside both sides of an interface. Stability analysis and numerical propagation simulations prove that solitons supported by the model are dynamically stable in the entire domain of their existence. The order of lattice determines the shape of soliton, and the amplitude of soliton depends on the lattice modulation depth. The experimental realization of the scheme is also proposed. Our results may provide another effective way of controlling the shapes of surface solitons and thus their evolutions by introducing a new freedom degree.     

Defect modes supported by optical lattices in photovoltaic-photorefractive crystals

We study defect modes in optically induced one-dimensional lattices in photovoltaic-photorefractive crystals. These defect modes exist in different bandgaps due to the change of defect intensity. For a positive defect, defect mode branches exist not only in the semi-infinite bandgap, but also in the first and second bandgaps. When the defect mode branch is fixed, the confinement of defect modes increases with the defect strength parameter. For a negative defect, defect mode branches exist only in the first and second bandgaps. For a given defect mode branch, the strongest confinement of the defect modes appears when the lattice intensity at the defect site is not the smallest in its branch. On the other hand, when the defect strength parameter is fixed, the most localized defect modes arise in the semi-infinite bandgap for the positive defect and in the first bandgap for the negative defect.     

非对称光折变全息空间光孤子的存在曲线

证明了在加外电场的光折变晶体中存在非对称全息空间光孤子对，其特征是孤子对中的两个 孤子具有相同的宽度、不同的振幅.给出了这种孤子对的存在曲线，其中存在三个具有不同 特征的区域：对于给定的外加电场来说，当孤子对中两个孤子的入射峰值强度之和与背景光 强度之比足够大或者足够小时，孤子宽度强烈依赖于这个比值，而在此中间的一段区域，孤 子宽度几乎不随这个比值而变化.非对称情况下这个中间区域的范围比对称情况的窄. 关键词： 空间光孤子 光折变非线性光学 全息光栅     

光晶格中超流费米气体的能隙孤子

研究了一维光晶格中超流费米气体的能隙孤子. 应用平均场理论和超流费米气体的流体动力学模型, 利用变分法得到了在整个跨越区超流费米气体在光晶格中存在带隙孤子的条件, 即原子间的非线性相互作用项与系统化学势以及晶格深度的相互关系. 通过对超流费米气体的基态能隙孤子空间分布的分析与对比, 揭示了在一维情况下超流费米气体能隙孤子的存在并发现超流费米气体能隙孤子在整个跨越区当系统从Bose-Einstein凝聚端跨越到BCS端时孤子存在的条件与孤子空间分布存在明显的差别.     

Holographic solitons in centrosymmetric photorefractive crystals

The conditions necessary for obtaining holographic solitons in centrosymmetric photorefractive crystals are discussed. When both the intensity modulation of the two interacting waves and the angle between the vector of each beam and the longitudinal axis are small, centrosymmetric photorefractive crystals with a positive (or negative ) effective quadratic electro-optic coefficient are able to support bright (or dark) holographic solitons. The intensity width at half maximum of bright and dark solitons is inversely proportional to the external bias field. The holographic solitons can also exit in dissipative centrosymmetric photorefractive crystals when the applied external bias field is large enough.     

Spatial soliton steering induced by weak nonlocality in optical lattices

We investigate the soliton steering in weakly nonlocal nonlinear media with harmonic modulation of the refractive index. It is shown that the weak nonlocality originated from the nonlocal nonlinear response of medium induces a transverse modulation of the refractive index, which leads to the occurrence of interplay between the weak nonlocality-induced transverse modulation and the harmonic modulation. Thus, one can control the soliton steering in weakly nonlocal medium, such as nematic liquid crystal, by employing the competition between the weak nonlocality parameter and the lattice depth.     

Gap solitons in a model of a superfluid fermion gas in optical lattices

We consider a dynamical model for a Fermi gas in the Bardeen-Cooper-Schrieffer (BCS) superfluid state, trapped in a combination of a 1D or 2D optical lattice (OL) and a tight parabolic potential, acting in the transverse direction(s). The model is based on an equation for the order parameter (wave function), which is derived from the energy density for the weakly coupled BCS superfluid. The equation includes a nonlinear self-repulsive term of power 7/3, which accounts for the Fermi pressure. Reducing the equation to the 1D or 2D form, we construct families of stable 1D and 2D gap solitons (GSs) by means of numerical simulations, which are guided by the variational approximation (VA). The GSs are, chiefly, compact objects trapped in a single cell of the OL potential. In the linear limit, the VA predicts almost exact positions of narrow Bloch bands that separate the semi-infinite and first finite gaps, as well as the first and second finite ones. Families of stable even and odd bound states of 1D GSs are constructed, too. We also demonstrate that the GS can be dragged without much distortion by an OL moving at a moderate velocity (, in physical units). The predicted GSs contain ∼10³-10⁴ and ∼10³ atoms per 1D and 2D settings, respectively.     

Extraordinary solitons in non-centrally symmetric photorefractive crystals

The directions of the phase normals for which extraordinary bright screened solitons can be formed are described analytically. Formulas are derived for the directions in which extraordinary solitons can exist in lithium niobate for arbitrary orientations of the external electric field. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 403–410, May–June, 2009.     

Anisotropic photonic lattices and discrete solitons in photorefractive media

We study experimentally two-dimensional periodic photonic lattices optically imprinted in photorefractive nonlinear media, and explore the effect of anisotropy on the induced refractive-index patterns. The orientation anisotropy is demonstrated by comparing square and diamond lattices, while the polarization anisotropy is shown to distinguish ordinarily and extraordinarily polarized light. In particular, the extraordinarily polarized lattice induces much stronger refractive-index modulation for the same conditions. Finally, we exploit the photorefractive anisotropy to generate a quasi-one-dimensional refractive-index pattern for the observation of two-dimensional solitons and corroborate these experiments by numerical simulations. PACS 42.65.Tg; 42.65.Wi     

Anisotropic photonic lattices and discrete solitons in photorefractive media

Table grapes were irradiated with UV nanosecond laser pulses in searching for resonant photo-elicitation of trans-resveratrol, a known antioxidant compound naturally produced by grapevines and other plants. To this end, the irradiation time as well as the wavelength dependence of the induced trans-resveratrol content was investigated by comparing the elicitation level of this compound at two laser wavelengths. One wavelength was selected right at the maximum of the absorption band (302.1 nm, the resonant wavelength for this compound) while the second was selected (300 nm, a non-resonant wavelength) such that trans-resveratrol absorption is negligible. It was found that the resonant irradiation enhances the resveratrol content in grapes by up to six times more than that of non-resonant irradiation, the rest of the conditions being the same. This work demonstrates how selective laser excitation of fruits can open new possibilities for the development of functional foods with enhanced nutritional and beneficial properties. PACS 42.62.-b; 82.50.Hp     

Optical solitons supported by finite waveguide lattices with diffusive nonlocal nonlinearity

We investigate the properties of fundamental, multi-peak, and multi-peaked twisted solitons in three types of finite waveguide lattices imprinted in photorefractive media with asymmetrical diffusion nonlinearity. Two opposite soliton self-bending signals are considered for different families of solitons. Power thresholdless fundamental and multi-peaked solitons are stable in the low power region. The existence domain of two-peaked twisted solitons can be changed by the soliton self-bending signals. When solitons tend to self-bend toward the waveguide lattice, stable two-peaked twisted solitons can be found in a larger region in the middle of their existence region. Three-peaked twisted solitons are stable in the lower (upper) cutoff region for a shallow (deep) lattice depth. Our results provide an effective guidance for revealing the soliton characteristics supported by a finite waveguide lattice with diffusive nonlocal nonlinearity.     

Rotary solitons in bessel optical lattices

We introduce solitons supported by Bessel photonic lattices in cubic nonlinear media. We show that the cylindrical geometry of the lattice, with several concentric rings, affords unique soliton properties and dynamics. In particular, in addition to the lowest-order solitons trapped in the center of the lattice, we find soliton families trapped at different lattice rings. Such solitons can be set into controlled rotation inside each ring, thus featuring novel types of in-ring and inter-ring soliton interactions.     

Manakov solitons in centrosymmetric photorefractive crystals due to two-photon photorefractive effect

We present a theoretical study on incoherently coupled soliton pairs in two-photon centrosymmetric photorefractive crystals under steady-state conditions. It is shown that the bright-bright, dark-dark and bright-dark Manakov soliton pairs can be generated in two-photon centrosymmetric photorefractive crystals when the total intensity of two soliton components is much lower than the dark irradiance.     

Coupled matter-wave solitons in optical lattices

We make use of a potential model to study the dynamics of two coupled matter-wave or Bose-Einstein condensate (BEC) solitons loaded in optical lattices. With separate attention to linear and nonlinear lattices we find some remarkable differences for response of the system to effects of these lattices. As opposed to the case of linear optical lattice (LOL), the nonlinear lattice (NOL) can be used to control the mutual interaction between the two solitons. For a given lattice wave number k, the effective potentials in which the two solitons move are such that the well (V_eff(NOL)), resulting from the juxtaposition of soliton interaction and nonlinear lattice potential, is deeper than the corresponding well V_eff(LOL). But these effective potentials have opposite k dependence in the sense that the depth of V_eff(LOL) increases as k increases and that of V_eff(NOL) decreases for higher k values. We verify that the effectiveness of optical lattices to regulate the motion of the coupled solitons depends sensitively on the initial locations of the motionless solitons as well as values of the lattice wave number. For both LOL and NOL the two solitons meet each other due to mutual interaction if their initial locations are taken within the potential wells with the difference that the solitons in the NOL approach each other rather rapidly and take roughly half the time to meet as compared with the time needed for such coalescence in the LOL. In the NOL, the soliton profiles can move freely and respond to the lattice periodicity when the separation between their initial locations are as twice as that needed for a similar free movement in the LOL. We observe that, in both cases, slow tuning of the optical lattices by varying k with respect to a time parameter τ drags the oscillatory solitons apart to take them to different locations. In our potential model the oscillatory solitons appear to propagate undistorted. But a fully numerical calculation indicates that during evolution they exhibit decay and revival.     

Pyroelectric photovoltaic spatial solitons in unbiased photorefractive crystals

A new type of spatial solitons i.e. pyroelectric photovoltaic spatial solitons based on the combination of pyroelectric and photovoltaic effect is predicted theoretically. It shows that bright, dark and grey spatial solitons can exist in unbiased photovoltaic photorefractive crystals with appreciable pyroelectric effect. Especially, the bright soliton can form in self-defocusing photovoltaic crystals if it gives larger self-focusing pyroelectric effect.     

Hybrid nonlinearity supported by nonconventionally biased photorefractive crystals

We theoretically and experimentally demonstrate that a nonconventionally biased photorefractive crystal can support hybrid nonlinearity, i.e., coexistence of self-focusing and self-defocusing nonlinearities under an identical bias condition. It is revealed that the nonlinearity experienced by a one-dimensional (stripe) beam can be switched between self-focusing and self-defocusing solely by changing the beam orientation. For a two-dimensional beam, the hybrid nonlinearity leads to unusual nonlinear beam dynamics with enhanced anisotropy and nonlocality.     

Two dimensional counterpropagating spatial solitons in photorefractive crystals

We demonstrate experimentally the existence of two transverse-dimensional counterpropagating (CP) incoherent spatial solitons in a 5 x 5 x 23 mm SBN:60Ce photorefractive crystal and investigate their dynamical behavior. We carry out numerical simulations that confirm our experimental findings. Substantially different behavior from the copropagating incoherent solitons is found. A symmetry breaking transition from stable overlapping CP solitons to unstable transversely displaced CP solitons is observed. We perform linear stability analysis that predicts the threshold for the split-up transition, in qualitative agreement with numerical simulations and experimental results.     

Gray photorefractive polymeric optical spatial solitons

We show that gray spatial optical solitons are possible in biased photorefractive polymers under steady-state conditions. We find that for a given material parameter the absolute value of a gray photorefractive polymeric soliton's phase decreases with an increase in the beam's grayness, whereas it increases with the material parameter for a given beam's grayness and that the full width half maximum (FWHM) of the gray soliton beam's intensity increases with the beam's grayness when the normalized background intensity and the material parameter are fixed and decreases with an increase in the normalized background intensity when the material parameter is fixed. On the other hand, we also show that N coupled beam evolution equations in biased photorefractive polymers can exhibit multicomponent gray solitons. These multicomponent gray solitons can be obtained provided that the N coupled beams share the same polarization, wavelength, and are incoherent with one another. The characteristics and stability properties of these multicomponent gray solitons are also discussed in detail.