PT symmetry in nonlinear twisted multicore fibers.

We address propagation of light in nonlinear twisted multicore fibers with alternating amplifying and absorbing cores that are arranged into the parity-time (PT)-symmetric structure. In this structure, the coupling strength between neighboring cores and global energy transport can be controlled not only by the nonlinearity, but also by gain and losses and by the fiber twisting rate. The threshold level of gain/losses, at which PT-symmetry breaking occurs, is a non-monotonic function of the fiber twisting rate, and it can be reduced nearly to zero or, instead, notably increased just by changing this rate. Nonlinearity usually leads to the monotonic reduction of the symmetry-breaking threshold in such fibers.

Self-structuring of stable dissipative breathing vortex solitons in a colloidal nanosuspension.

The self-structuring of laser light in an artificial optical medium composed of a colloidal suspension of nanoparticles is demonstrated using variational and numerical methods extended to dissipative systems. In such engineered materials, competing nonlinear susceptibilities are enhanced by the light induced migration of nanoparticles. The compensation of diffraction by competing focusing and defocusing nonlinearities, together with a balance between loss and gain, allow for self-organization of light and the formation of stable dissipative breathing vortex solitons. Due to their robustness, the breathers may be used for selective dynamic photonic tweezing of nanoparticles in colloidal nanosuspensions.

Lipid mobilisation and oxidative stress as metabolic adaptation processes in dairy heifers during transition period.

The objective of this study was to evaluate metabolic disorders and oxidative stress in dairy heifers during the transition period. Possible relationships between lipid mobilisation indicators and oxidative stress markers were investigated as well. Nineteen dairy heifers were included in the study. Blood samples were collected at the time of estrus synchronisation in heifers, at insemination, three weeks after insemination, one week before calving, at calving and 1, 2, 4 and 8 weeks postpartum. Common metabolic parameters, beta-hydroxybutyrate (BHB), free fatty acids (FFA), paraoxonase-1 (PON1) activity and total antioxidative status (TAS) were analysed. Around insemination, no significant difference was observed in the majority of tested parameters (P>0.05). However, the transition period markedly affected the concentration of triglycerides, total cholesterol, HDL-C, BHB, FFA, TAS and PON1activity. Positive correlations between PON1 activity and total cholesterol, HDL-C and triglycerides were noted but inverse correlations with FFA, BHB and bilirubin were found indicating that PON1 activity changed with lipid metabolism and was influenced by negative energy balance. These findings suggest that lipid mobilisation and oxidative stress are part of a complex metabolic adaptation to low energy balance which reaches equilibrium later in advanced lactation.

Kinetics of deposition of oriented superdisks.

We use numerical Monte Carlo simulation to study the kinetics of the deposition of oriented superdisks, bounded by the Lame curves of the form |x|(2p)+|y|(2p)=1 on a regular planar substrate. Recently, it was shown that the maximum packing density as well as jamming density ρ(J) exhibit a discontinuous derivative at p=0.5 when the shape changes from convex to concave form. By careful examination of the late-stage approach to the jamming limit, we find that the leading term in the temporal development is also nonanalytic at p=0.5 and offer heuristic excluded-area arguments for this behavior.

Breathing solitons in nematic liquid crystals.

Dynamical and steady-state behavior of beams propagating in nematic liquid crystals (NLCs) is analyzed. A well-known model for the beam propagation and the director reorientation angle in a NLC cell is treated numerically in space and time. The formation of steady-state soliton breathers in a threshold region of beam intensities is displayed. Below the region the beams diffract, above the region spatiotemporal instabilities develop, as the input intensity and the material parameters are varied. Curiously, the only kind of solitons we could demonstrate in our numerical studies was the breathers. Despite repeated efforts, we could not find the solitons with a steady profile propagating in the NLC model at hand.

Ulcus vulvae acutum.

A 29-year-old female presented with a history of sudden onset of genital ulcers with fever and malaise. The clinical picture and course of the disease corresponded to the diagnosis of acute vulvar ulcer. No relevant etiologic factors could be ascertained. Treatment with doxycycline and systemic corticosteroids led to the resolution of the ulcerations after three weeks. Although the disease is rare, it is important to include it in the differential diagnosis of genital ulcers.

Zosteriform lichen planus-like eruption.

Lichen planus (LP) is a relatively common papulosquamous skin disease of unknown etiology, it is believed to be a T-cell mediated disorder. In addition to the cutaneous eruptions it may also affect mucous membranes, nails or cause scarring alopecia. Lichen planus appears in various clinical variants which are categorized according to the morphology and configuration of lesions. We present a 34-year-old man who developed a papular eruption localized unilaterally on the right side of the body in a linear-zosteriform pattern within the L5-S1 nerve segments. The skin lesions clinically and histologically mimicked LP. Topical treatment with betamethason ointment for one month led to remarkable improvement, but a zosteriform hiperpigmentation persisted. According to the clinical findings in our patient and a review of the literature, we believe that lichenoidzosteriform eruption is a variant of lichen planus or a herpes zoster infection.

Counterpropagating self-trapped beams in optical photonic lattices.

Dynamical properties of counterpropagating (CP) mutually incoherent self-trapped beams in optically induced photonic lattices are investigated numerically. A local model with saturable Kerr-like nonlinearity is adopted for the photorefractive media, and an optically generated two-dimensional fixed photonic lattice introduced in the crystal. Different incident beam structures are considered, such as Gaussians and vortices of different topological charge. We observe spontaneous symmetry breaking of the head-on propagating Gaussian beams as the coupling strength is increased, resulting in the splitup transition of CP components. We see discrete diffraction, leading to the formation of discrete CP vector solitons. In the case of vortices, we find beam filamentation, as well as increased stability of the central vortex ring. A strong pinning of filaments to the lattice sites is noted. The angular momentum of vortices is not conserved, either along the propagation direction or in time, and, unlike the case without lattice, the rotation of filaments is not as readily observed.

Counterpropagating beams in nematic liquid crystals.

The behavior of counterpropagating self-trapped optical beam structures in nematic liquid crystals is investigated. A time-dependent model for the beam propagation and the director reorientation in a nematic liquid crystal is numerically treated in three spatial dimensions and time. We find that the stable vector solitons can only exist in a narrow threshold region of control parameters. Below this region the beams diffract, above they self-focus into a series of focal spots. Spatiotemporal instabilities are observed as the input intensity, the propagation distance, and the birefringence are increased. We demonstrate undulation, filamentation, and convective dynamical instabilities of counterpropagating beams. Qualitatively similar behavior as of the copropagating beams is observed, except that it happens at lower values of control parameters.

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.

Counterpropagating beams in biased photorefractive crystals: anisotropic theory.

We formulate an anisotropic nonlocal theory of the space charge field induced by the coherent counterpropagating beams in biased photorefractive crystals. We establish that the competition between the drift and diffusion terms has to be taken into account when the crystal c axis is tilted with respect to the propagation direction of the beams. We demonstrate that this configuration combines the features of both spatial soliton formation without energy exchange and two-wave mixing with energy exchange leading to pattern formation.

Spatiotemporal optical instabilities in nematic solitons.

We investigate numerically the propagation of self-trapped optical beams in nematic liquid crystals. Our analysis includes both spatial and temporal behavior. We display the formation of stable solitons in a narrow threshold region of beam intensities for fixed birefringence, and depict their spatiotemporal instabilities as the input intensity and the birefringence are increased. We demonstrate the breathing and filamentation of solitons above the threshold with increasing input intensity, and discover a convective instability with increasing birefringence. We consider the propagation of complex beam structures in nematic liquid crystals, such as dipoles, beam arrays, and vortices.

Counterpropagating optical vortices in photorefractive crystals.

We present a comprehensive numerical study of (2+1)D counterpropagating incoherent vortices in photorefractive crystals, in both space and time. We consider a local isotropic dynamical model with Kerr-type saturable nonlinearity, and identify the corresponding conserved quantities. We show, both analytically and numerically, that stable beam structures conserve angular momentum, as long as their stability is preserved. As soon as the beams loose stability, owing to radiation or non-elastic collisions, their angular momentum becomes non-conserved. We discover novel types of rotating beam structures that have no counterparts in the copropagating geometry. We consider the counterpropagation of more complex beam arrangements, such as regular arrays of vortices. We follow the transition from a few beam propagation behavior to the transverse pattern formation dynamics.

Dynamics of counterpropagating multipole vector solitons.

Dynamical behavior of counterpropagating (CP) mutually incoherent vector solitons in a 5 x 5 x 23 mm SBN:60Ce photorefractive crystal is investigated. Experimental study is carried out, displaying rich dynamics of three-dimensional CP solitons and higher-order multipole structures, and a theory formulated that is capable of capturing such dynamics. We find that our numerical simulations agree well with the experimental findings for various CP beam structures. Linear stability analysis is also performed, predicting a threshold for the modulational instability of CP beams, and an appropriate control parameter is identified. We attempt at utilizing these results to CP solitons, but find only qualitative agreement with the numerical simulations and experimental findings. However, when broader hyper-Gaussian CP beams are used in simulations, an improved agreement with the theory is obtained.

Transverse modulational instabilities of counterpropagating solitons in photorefractive crystals.

We study numerically the counterpropagating vector solitons in SBN:60 photorefractive crystals. A simple theory is provided for explaining the symmetry-breaking transverse instability of these solitons. Phase diagram is produced that depicts the transition from stable counterpropagating solitons to bidirectional waveguides to unstable optical structures. Numerical simulations are performed that predict novel dynamical beam structures, such as the standing-wave and rotating multipole vector solitonic clusters. For larger coupling strengths and/or thicker crystals the beams form unstable self-trapped optical structures that have no counterparts in the copropagating geometry.

Solitonic lattices in photorefractive crystals.

Two-dimensional spatial solitonic lattices are generated and investigated experimentally and numerically in a Sr(x)Ba(1-x)Nb(2)O(6):Ce crystal. An enhanced stability of these lattices is achieved by exploiting the anisotropy of coherent soliton interaction, in particular the relative phase between soliton rows. The manipulation of individual soliton channels is achieved by the use of supplementary control beams.

Self-trapped bidirectional waveguides in a saturable photorefractive medium.

A time-dependent model for the generation of joint waveguides by counterpropagating light beams in photorefractive crystals is introduced. Depending on initial conditions and parameter values, the beams form stable structures or display periodic and irregular dynamics. Steady-state solutions nonuniform in the direction of propagation are found, representing a general class of self-trapped waveguides that include counterpropagating spatial vector solitons as a particular case.

Dynamic counterpropagating vector solitons in saturable self-focusing media.

We display rich spatial and temporal dynamics of light fields counterpropagating in a saturable self-focusing medium numerically, and analyze instabilities that counterpropagating solitons experience. An expression for the maximum length that the medium must not exceed for the solitons to be stable is derived and connected to the coupling strength of beam interaction. The instability can lead to periodic or irregular temporal dynamics of the light beams. By considering mutually incoherent counterpropagating beams, we show that differences to the copropagating case are due to the different boundary conditions.

Isotropic versus anisotropic modeling of photorefractive solitons.

The question of the isotropic versus anisotropic modeling of incoherent spatial screening solitons in photorefractive crystals is addressed by a careful theoretical and numerical analysis. Isotropic, or local, models allow for an extended spiraling of two interacting scalar solitons, and for a prolonged propagation of vortex vector solitons, whereas anisotropic, nonlocal, models prevent such phenomena. In the context of Kukhtarev's material equations, the difference in behavior is traced to the continuity equation for the current density. We further show that neither an indefinite spiraling of two solitons nor stable propagation of vortex vector solitons is generally possible in both isotropic and anisotropic models. Such systems do not conserve angular momentum, even in the case of an isotropic change in the index of refraction.

Dynamics of formation and interaction of photorefractive screening solitons.

An experimental and numerical investigation of the dynamical, time-dependent effects accompanying the formation and interaction of two-dimensional spatial screening solitons in a photorefractive strontium barium niobate crystal is performed. These effects include initial diffraction, collapse to the soliton shape, the oscillation of beam diameters, beam bending, and the rotation, twisting, and turning of soliton pairs. The dynamics of complex spiraling of two incoherent solitons is considered in more detail.