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U Peschel

Publications and source records attributed to U Peschel.

13 recordsLinked to original sources
PubMed2003

Dynamics and stability of solitary waves in optical-microwave interaction.

We study the dynamics and the stability of localized bound states of optical and microwave fields, which are linked together by a quadratic nonlinearity. The system is an example of an intense interaction between low and high frequency waves, as appears in many areas of physics. Perturbed solitary waves show a number of regular but damped oscillations with strong radiation from the microwave. It is demonstrated that these oscillations are caused by the excitation of several quasibound asymmetric linear modes of the solitary wave. The associated eigenvalues are found to be complex leading to a decay of the oscillations as observed numerically. Additional quasibound linear modes with a complex eigenvalue corresponding to exponential growth also exist, but due to physical constraints cannot be excited. Therefore, in contrast to systems solely with high frequency waves, the stability of the solutions is retained.

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PubMed2003

Discrete solitons in inhomogeneous waveguide arrays.

The existence and dynamical properties of discrete solitons in inhomogeneous waveguide arrays with a Kerr nonlinearity are studied in two different configurations. First we investigate the effect of a longitudinal periodic modulation of the coupling strength on the dynamics of discrete solitons. It is shown that resonances of internal modes of the soliton with the longitudinal structure may lead to soliton oscillations and decay. Second we study the existence and stability of discrete solitons in arrays exhibiting a linear variation of the waveguide effective index in the transverse direction. We find that resonant coupling between conventional discrete solitons and linear Wannier-Stark states leads to the formation of so-called hybrid discrete solitons.

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PubMed2002

Hybrid discrete solitons.

The existence and stability of discrete solitons in waveguide arrays exhibiting a linear variation of the effective index and a Kerr nonlinearity is studied. We find that the resonant coupling of the conventional discrete soliton to a linear Wannier-Stark state does not entail soliton decay. We rather observe the formation of a bound state where the Wannier-Stark state gets nonlinearly modified. This results in an infinite number of isolated branches of hybrid discrete solitons.

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PubMed2002

Modulational instability in optical-microwave interaction.

The stability of continuous optical and microwave fields is studied in the presence of dispersion and second order nonlinearity. The cascade combination of optical rectification and the electro-optic effect induces modulational instability (MI) in a wide range of system parameters. It is demonstrated that MI can lead potentially to filamentation of high power optical pulses as well as the generation of terahertz radiation.

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PubMed2002

Anomalous refraction and diffraction in discrete optical systems.

We experimentally prove that light propagation in a discrete system, i.e., an array of coupled waveguides, exhibits striking anomalies. We show that refraction is restricted to a cone, irrespective of the initial tilt of the beam. Diffraction can be controlled in size and sign by the input conditions. Diffractive beam spreading can even be arrested and diverging light can be focused. The results can be thoroughly theoretically explained.

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PubMed2001

Perturbation theory for domain walls in the parametric Ginzburg-Landau equation.

We demonstrate that in the parametrically driven Ginzburg-Landau equation arbitrarily small nongradient corrections lead to qualitative differences in the dynamical properties of domain walls in the vicinity of the transition from rest to motion. These differences originate from singular rotation of the eigenvector governing the transition. We present analytical results on the stability of Ising walls, deriving explicit expressions for the critical eigenvalue responsible for the transition from rest to motion. We then develop a weakly nonlinear theory to characterize the singular character of the transition and analyze the dynamical effects of spatial inhomogeneities.

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PubMed2001

Effects of spatial inhomogeneities on the dynamics of cavity solitons in quadratically nonlinear media.

We study the dynamics of cavity solitons under the influence of spatial inhomogeneities and derive generalized equations of motions. For perturbations large compared to the soliton size we find the modulus of the soliton velocity to be proportional to the gradient of the respective perturbation and that the proportionality coefficient changes sign when the soliton peak power drives the cavity beyond the resonance. For short scale perturbations solitons may be trapped at the extrema of the inhomogeneities. Shape and stability of these trapped solitons can be quasianalytically described by means of a perturbation theory. If both types of perturbations act solitons are either trapped or move depending on the strength of the respective perturbation. In the framework of a quasiparticle approach this dynamics is governed by a differential equation that holds for particle motion in a strongly viscous fluid under the action of a constant and harmonically varying force. We also show that in addition to acquiring a velocity the very existence conditions of the solitons (hysteresis curve) are affected by both kinds of perturbations. We find good quantitative agreement between our analytical results and numerical findings, which were obtained for a two wave interaction in a cavity filled with a quadratically nonlinear material.

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PubMed2001

Universal criterion and amplitude equation for a nonequilibrium Ising-Bloch transition.

We identify a universal criterion for the onset of a nonequilibrium Ising-Bloch (NIB) transition, and describe the behavior near the bifurcation by a generic amplitude equation. We found that a NIB transition is caused by an antisymmetric eigenvector passing the translational mode of the system at a critical point. In this context we discuss Hamiltonian and dissipative systems. We report on a NIB in nonlinear optics, manifesting itself in a transition from static to moving polarization fronts.

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PubMed1981

[Leukocyte-adherence-inhibition reactivity in patients with colorectal and pancreatic cancer (author's transl)].

The immunoreactivity of patients with colorectal or pancreatic cancer was investigated by a modified leukocyte adherence inhibition test (LAI). The microcapillary-LAI-test was easy to practise and measurements had a low intra-assay coefficient of variation. The number of false positive results was low, only 1 of 39 patients with benign disease and not one of 13 healthy volunteers reacted in the presence of colorectal carcinoma extracts. In 43 patients with colorectal cancer 17 of 25 (68%) with Dukes' B & C and 15 of 18 (83%) with Dukes' D cancer were LAI positive. Of the 19 patients with colon polyps 6 positive results were observed, exclusively in case of large villous adenomas, polyposis coli and one polyp with a focal carcinoma. 10 of 11 patients with pancreatic cancer reacted in the test. Leukocytes from patients with limited cancer of other source, when tested with colorectal or pancreatic carcinoma extracts, showed a negative LAI response. However, in case of metastatic diseases some crossreactions were observed, which resulted in false positive tests. Following our results, the LAI test could be of value in the diagnosis of gastrointestinal carcinoma.

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PubMedDate not supplied

["Interface problems" in medical rehabilitation: development of a brief questionnaire for assessing the need for information and communication among general practitioners].

Against the background of current and future demographic changes the relevance of medical rehabilitation in the German health care system is generally acknowledged. It is a central element of treatment programmes especially for elderly and chronically ill patients. In recent years, problems due to lack of co-operation and communication as well as insufficient links between acute care and rehabilitation treatment have found increasing attention. Previous attempts to increase involvement especially of general practitioners in the rehabilitation process were only slightly successful. In a pilot study conducted by the Rehabilitationswissenschaftlicher Arbeitskreis Schleswig-Holstein a new procedure was tested to increase involvement of general practitioners. A brief questionnaire was attached to the report of discharge of rehabilitation patients and was sent to 130 practitioners. With this questionnaire the general attitude and need for information of the practitioners should be analysed. Knowledge about these variables should help to improve communication and co-operation. The response rate was quite high: about 75% of the contacted practitioners answered the questionnaire. All practitioners described medical rehabilitation as important and also had a quite positive attitude toward the same. Two-thirds of the physicians were interested in more intensive contacts and requested further information.

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