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Search Results: 1 - 10 of 9862 matches for " Stefan Skupin "
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Modelling arbitrarily shaped and tightly focused laser pulses in electromagnetic codes
Illia Thiele,Stefan Skupin,Rachel Nuter
Physics , 2015,
Abstract: Investigation of laser matter interaction with electromagnetic codes requires to implement sources for the electromagnetic fields. A way to do so is to prescribe the fields at the numerical box boundaries in order to achieve the desired fields inside the numerical box. Here we show that the often used paraxial approximation can lead to unexpected field profiles with strong impact on the laser matter interaction results. We propose an efficient numerical algorithm to compute the required laser boundary conditions consistent with the Maxwell's equations for arbitrarily shaped, tightly focused laser pulses.
Filamentation of ultrashort laser pulses in silica glass and KDP crystal: A comparative study
Jérémie Rolle,Luc Bergé,Guillaume Duchateau,Stefan Skupin
Physics , 2014, DOI: 10.1103/PhysRevA.90.023834
Abstract: Ionizing 800-nm femtosecond laser pulses propagating in silica glass and in potassium dihydrogen phosphate (KDP) crystal are investigated by means of a unidirectional pulse propagation code. Filamentation in fused silica is compared with the self-channeling of light in KDP accounting for the presence of defect states and electron-hole dynamics. In KDP, laser pulses produce intense filaments with higher clamping intensities up to 200 TW/cm$^2$ and longer plasma channels with electron densities above $10^{16}$ cm$^{-3}$. Despite these differences, the propagation dynamics in silica and KDP are almost identical at equivalent ratios of input power over the critical power for self-focusing.
Stable rotating dipole solitons in nonlocal optical media
Servando Lopez-Aguayo,Anton S. Desyatnikov,Yuri S. Kivshar,Stefan Skupin,Wieslaw Krolikowski,Ole Bang
Physics , 2005, DOI: 10.1364/OL.31.001100
Abstract: We reveal that nonlocality can provide a simple physical mechanism for stabilization of multi-hump optical solitons, and present the first example of stable rotating dipole solitons and soliton spiraling, known to be unstable in all types of realistic nonlinear media with local response.
Spectral self-action of THz emission from ionizing two-color laser pulses in gases
Eduardo Cabrera-Granado,Yxing Chen,Ihar Babushkin,Luc Bergé,Stefan Skupin
Physics , 2014, DOI: 10.1088/1367-2630/17/2/023060
Abstract: The spectrum of terahertz (THz) emission in gases via ionizing two-color femtosecond pulses is analyzed by means of a semi-analytic model and finite-difference-time-domain simulations in 1D and 2D geometries. We show that produced THz signals interact with free electron trajectories and thus influence significantly further THz generation upon propagation, i.e., make the process inherently nonlocal. This self-action plays a key role in the observed strong spectral broadening of the generated THz field. Diffraction limits the achievable THz bandwidth by efficiently depleting the low frequency amplitudes in the propagating field.
Directionality of THz emission from photoinduced gas plasmas
Christian K?hler,Eduardo Cabrera-Granado,Ihar Babushkin,Luc Bergé,Joachim Herrmann,Stefan Skupin
Physics , 2011, DOI: 10.1364/OL.36.003166
Abstract: Forward and backward THz emission by ionizing two-color laser pulses in gas is investigated by means of a simple semi-analytical model based on Jefimenko's equation and rigorous Maxwell simulations in one and two dimensions. We find the emission in backward direction having a much smaller spectral bandwidth than in forward direction and explain this by interference effects. Forward THz radiation is generated predominantly at the ionization front and thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model.
Azimuthons in weakly nonlinear waveguides of different symmetries
Yiqi Zhang,Stefan Skupin,Fabian Maucher,Arpa Galestian Pour,Keqing Lu,Wieslaw Królikowski
Physics , 2010, DOI: 10.1364/OE.18.027846
Abstract: We show that weakly guiding nonlinear waveguides support stable propagation of rotating spatial solitons (azimuthons). We investigate the role of waveguide symmetry on the soliton rotation. We find that azimuthons in circular waveguides always rotate rigidly during propagation and the analytically predicted rotation frequency is in excellent agreement with numerical simulations. On the other hand, azimuthons in square waveguides may experience spatial deformation during propagation. Moreover, we show that there is a critical value for the modulation depth of azimuthons above which solitons just wobble back and forth, and below which they rotate continuously. We explain these dynamics using the concept of energy difference between different orientations of the azimuthon.
On the Saturation of the Nonlinear Refractive Index in Atomic Gases
Christian K?hler,Roland Guichard,Emmanuel Lorin,Szczepan Chelkowski,André D. Bandrauk,Luc Bergé,Stefan Skupin
Physics , 2013, DOI: 10.1103/PhysRevA.87.043811
Abstract: Motivated by the ongoing controversy on the origin of the nonlinear index saturation and subsequent intensity clamping in femtosecond filaments, we study the atomic nonlinear polarization induced by a high-intensity and ultrashort laser pulse in hydrogen by numerically solving the time dependent Schr\"odinger equation. Special emphasis is given to the efficient modeling of the nonlinear polarization at central laser frequency corresponding to 800 nm wavelength. Here, the recently proposed model of the Higher-Order Kerr Effect (HOKE) and two versions of the Standard model for femtosecond filamentation, including either a multi-photon or tunnel ionization rate, are compared. We find that around the clamping intensity the instantaneous HOKE model does not reproduce the temporal structure of the nonlinear response obtained from the quantum mechanical results. In contrast, the non-instantaneous charge contributions included in the Standard models ensure a reasonable quantitative agreement. Therefore, the physical origin for the observed saturation of the overall electron response is confirmed to mainly result from contributions of free or nearly free electrons.
Supercontinuum generation of ultrashort laser pulses in air at different central wavelengths
S. Skupin,L. Berge
Physics , 2007, DOI: 10.1016/j.optcom.2007.06.067
Abstract: Supercontinuum generation by femtosecond filaments in air is investigated for different laser wavelengths ranging from ultraviolet to infrared. Particular attention is paid on the role of third-harmonic generation and temporal steepening effects, which enlarge the blue part of the spectrum. A unidirectional pulse propagation model and nonlinear evolution equations are numerically integrated and their results are compared. Apart from the choice of the central wavelength, we emphasize the importance of the saturation intensity reached by self-guided pulses, together with their temporal duration and propagation length as key players acting on both supercontinuum generation of the pump wave and emergence of the third harmonics. Maximal broadening is observed for large wavelengths and long filamentation ranges.
Calcium Signals Driven by Single Channel Noise
Alexander Skupin ,Helmut Kettenmann,Martin Falcke
PLOS Computational Biology , 2010, DOI: 10.1371/journal.pcbi.1000870
Abstract: Usually, the occurrence of random cell behavior is appointed to small copy numbers of molecules involved in the stochastic process. Recently, we demonstrated for a variety of cell types that intracellular Ca2+ oscillations are sequences of random spikes despite the involvement of many molecules in spike generation. This randomness arises from the stochastic state transitions of individual Ca2+ release channels and does not average out due to the existence of steep concentration gradients. The system is hierarchical due to the structural levels channel - channel cluster - cell and a corresponding strength of coupling. Concentration gradients introduce microdomains which couple channels of a cluster strongly. But they couple clusters only weakly; too weak to establish deterministic behavior on cell level. Here, we present a multi-scale modelling concept for stochastic hierarchical systems. It simulates active molecules individually as Markov chains and their coupling by deterministic diffusion. Thus, we are able to follow the consequences of random single molecule state changes up to the signal on cell level. To demonstrate the potential of the method, we simulate a variety of experiments. Comparisons of simulated and experimental data of spontaneous oscillations in astrocytes emphasize the role of spatial concentration gradients in Ca2+ signalling. Analysis of extensive simulations indicates that frequency encoding described by the relation between average and standard deviation of interspike intervals is surprisingly robust. This robustness is a property of the random spiking mechanism and not a result of control.
Rotating soliton solutions in nonlocal nonlinear media
S. Skupin,M. Grech,W. Krolikowski
Physics , 2008, DOI: 10.1364/OE.16.009118
Abstract: We discuss generic properties of rotating nonlinear wave solutions, the so called azimuthons, in nonlocal media. Variational methods allow us to derive approximative values for the rotating frequency, which is shown to depend crucially on the nonlocal response function. Further on, we link families of azimuthons to internal modes of classical non-rotating stationary solutions, namely vortex and multipole solitons. This offers an exhaustive method to identify azimuthons in a given nonlocal medium.
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