Biomedical subjects

D J Frantzeskakis

Publications and source records attributed to D J Frantzeskakis.

26 records · Page 2Linked to original sources

PubMed2003

Domain walls in two-component dynamical lattices.

We introduce domain-wall (DW) states in the bimodal discrete nonlinear Schrödinger equation, in which the modes are coupled by cross-phase modulation (XPM). The results apply to an array of nonlinear optical waveguides carrying two different polarizations of light, or two different wavelengths, with anomalous intrinsic diffraction controlled by direction of the light beam, and to a string of drops of a binary Bose-Einstein condensate, trapped in an optical lattice. By means of continuation from various initial patterns taken in the anticontinuum (AC) limit, we find a number of different solutions of the DW type, for which different stability scenarios are identified. In the case of strong-XPM coupling, DW configurations contain a single mode at each end of the chain. The most fundamental solution of this type is found to be always stable. Another solution, which is generated by a different AC pattern, demonstrates behavior which is unusual for nonlinear dynamical lattices: it is unstable for small values of the coupling constant C (which measures the ratio of the nonlinearity and coupling lengths), and becomes stable at larger C. Stable bound states of DWs are also found. DW configurations generated by more sophisticated AC patterns are identified as well, but they are either completely unstable, or are stable only at small values of C. In the case of weak XPM, a natural DW solution is the one which contains a combination of both polarizations, with the phase difference between them 0 and pi at the opposite ends of the lattice. This solution is unstable at all values of C, but the instability is very weak for large C, indicating stabilization as the continuum limit is approached. The stability of DWs is also verified by direct simulations, and the evolution of unstable DWs is simulated too; in particular, it is found that, in the weak-XPM system, the instability may give rise to a moving DW. The DW states can be observed experimentally in the same parameter range where discrete solitons have been found in the lattice setting.

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PubMed2002

Targeted transfer of solitons in continua and lattices.

We propose a robust mechanism of targeted energy transfer along a line, as well as on a surface, in the form of transport of coherent solitary-wave structures, driven by a moving, spatially localized external ac field ("arm") in a lossy medium. The efficiency and robustness of the mechanism are demonstrated analytically and numerically in terms of the nonlinear Schrödinger (NLS) equation, and broad regions of stable operation are identified in the model's parameter space. Direct simulations show that the driving arm can manipulate solitons equally well in a lattice NLS model. A salient feature, which is revealed by simulations and explained analytically, is a resonant character of the operation of the driving arm in the lattice medium, both integer and fractional resonances being identified. Numerical experiments also demonstrate that the same solitary-wave-transport mechanism works well in two-dimensional lattice media.

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PubMed2002

One- and two-dimensional solitons in second-harmonic-generating lattices.

In a model of a dynamical lattice with the on-site second-harmonic-generating nonlinearity and harmonic intersite couplings (that may be equal or different for the fundamental and second harmonics), various solitary-wave solutions are considered in one and two dimensions (1D and 2D). Fundamental (single-hump) solitons are identified in either dimension and their stability is examined and compared to previous results as well as to what is known for the model's continuum counterpart. Stability limits in terms of the coupling constants, which depend on the value of the phase-mismatch parameter, are found for solitons of the twisted-mode type in the 1D lattice, and for their counterparts of two different types (one being a discrete vortex) in the 2D lattice. When the twisted-mode soliton is unstable, the instability, which may be either oscillatory or due to imaginary eigenfrequency pairs, transforms the unstable soliton into a stable fundamental one, in both 1D and 2D cases.

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PubMed2002

Multichannel pulse dynamics in a stabilized Ginzburg-Landau system.

We study the stability and interactions of chirped solitary pulses in a system of nonlinearly coupled cubic Ginzburg-Landau (CGL) equations with a group-velocity mismatch between them, where each CGL equation is stabilized by linearly coupling it to an additional linear dissipative equation. In the context of nonlinear fiber optics, the model describes transmission and collisions of pulses at different wavelengths in a dual-core fiber, in which the active core is furnished with bandwidth-limited gain, while the other, passive (lossy) one is necessary for stabilization of the solitary pulses. Complete and incomplete collisions of pulses in two channels in the cases of anomalous and normal dispersion in the active core are analyzed by means of perturbation theory and direct numerical simulations. It is demonstrated that the model may readily support fully stable pulses whose collisions are quasielastic, provided that the group-velocity difference between the two channels exceeds a critical value. In the case of quasielastic collisions, the temporal shift of pulses, predicted by the analytical approach, is in semiquantitative agreement with direct numerical results in the case of anomalous dispersion (in the opposite case, the perturbation theory does not apply). We also consider a simultaneous collision between pulses in three channels, concluding that this collision remains quasielastic, and the pulses remain completely stable. Thus, the model may be a starting point for the design of a stabilized wavelength-division-multiplexed transmission system.

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PubMed2001

Localized vortices with a semi-integer charge in nonlinear dynamical lattices.

The topological charge S of vortexlike configurations in two-dimensional (2D) dynamical lattices need not necessarily be integer, nor is it a dynamical invariant. Accordingly, we demonstrate that the discrete nonlinear Schrödinger (DNLS) equation in 2D has stationary solutions in the form of a vortex with S=1/2, which does not exist in the model's continuum counterpart. Analysis of the DNLS equation linearized about the vortex shows that it is stable except for, possibly, extremely weak instabilities (at the level of numerical precision). Direct simulations of the full DNLS model in 2D show that the S=1/2 vortex soliton is a stable oscillating solution. This behavior of classical dynamical lattices is in contrast with a recently reported result by Clay et al. [Phys. Rev. Lett. 86, 4085 (2001)], according to which fractional charges in quantum lattices are subject to dynamical rearrangement into integer charges. We also consider S=1 discrete vortices that may be built as a pair of S=1/2 ones. These are different from the cross-shaped S=1 vortices that were recently found in the same 2D model. The S=1 vortices found in this work have larger energy and a slightly smaller stability range. We also find an analog of the S=1/2 vortices in the 1D DNLS model, which also turns out to be a stable oscillating soliton, different from the twisted localized modes recently found in the 1D model.

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PubMed2001

Ring solitons on vortices.

Interaction of a ring dark or antidark soliton (RDS and RADS, respectively) with a vortex is considered in the defocusing nonlinear Schrödinger equation with cubic (for RDS) or saturable (for RADS) nonlinearities. By means of direct simulations, it is found that the interaction gives rise to either an almost isotropic or a spiral-like pattern. A transition between them occurs at a critical value of the RDS or RADS amplitude, the spiral pattern appearing if the amplitude exceeds the critical value. An initial ring soliton created on top of the vortex splits into a pair of rings moving inward and outward. In the subcritical case, the inbound ring reverses its polarity, bouncing from the vortex core, without conspicuous effect on the core. In the transcritical case, the bounced ring soliton suffers a spiral deformation, while the vortex changes its position and structure and also loses its axial symmetry. Through a variational-type approach to the system's Hamiltonian, we additionally find that the vortex-RDS and vortex-RADS interactions are, respectively, attractive and repulsive. Simulations with the vortex placed eccentrically with respect to the RDS or RADS reveal the generation of strongly localized multispot dark and/or antidark coherent structures. The occurrence of spiral-like patterns in many numerical experiments prompted an attempt to generate a spiral dark soliton, but the latter is found to suffer a core instability that converts it into a rotating dipole emitting waves in the outward direction.

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PubMed2001

Collisions between spatiotemporal solitons of different dimensionality in a planar waveguide.

A (2+1)-dimensional nonlinear Schrödinger equation including third-order dispersion is a natural model of a waveguide, in which strong temporal dispersion is induced by a grating in order to make the existence of two-dimensional spatiotemporal solitons possible. By means of analytical and numerical methods, we demonstrate that this model may support, simultaneously, stable dark quasi-one-dimensional (stripe) solitons and two-dimensional elevation solitons ("antidark solitons") in the form of weakly localized "lumps." The spatial position of lumps can be controlled by passing stripe dark solitons through them in an arbitrary direction. To substantiate this mechanism, we analytically calculate a position shift generated by a headon collision between the stripe and lump. The obtained results are in good agreement with direct numerical simulations.

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PubMed1999

Dissipative solitons under the action of the third-order dispersion.

We study the evolution of a solitary pulse in the cubic complex Ginzburg-Landau equation, including the third-order dispersion (TOD) as a small perturbation. We develop analytical approximations, which yield a TOD-induced velocity c of the pulse as a function of the ratio D of the second-order dispersion and filtering coefficients. The analytical predictions show agreement with the direct numerical simulations for two distinct intervals of D. A new feature of the pulse motion, which is a precursor of the transition to blowup, is presented: The pulse suddenly acquires a large acceleration in the reverse direction at D>D(cr) approximately -1.5 and without the reversal at D<D(cr). It is also demonstrated that the laminar-propagation distance L (before the onset of the ultimate turbulent stage) becomes maximum deep inside the normal-dispersion region, while TOD significantly increases L in the anomalous-dispersion region, where, otherwise, it is quite small. The model has a straightforward physical realization in terms of nonlinear optical fibers with losses and bandwidth-limited amplification (gain and filtering).

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