Showing papers on "Filamentation published in 2000"
TL;DR: This Letter proposes a novel nonlinear filtering effect that suppresses premature backscattering of the pump in a noisy plasma layer, while the desired amplification of a sufficiently intense seed persists with a high efficiency.
Abstract: The recently proposed scheme of so-called ``fast compression'' of laser pulses in plasma can increase peak laser intensities by ${10}^{5}$ [Phys. Rev. Lett. 82, 4448 (1999)]. The compression mechanism is the transient stimulated Raman backscattering, which outruns the fastest filamentation instabilities of the pumped pulse even at highly overcritical powers. This Letter proposes a novel nonlinear filtering effect that suppresses premature backscattering of the pump in a noisy plasma layer, while the desired amplification of a sufficiently intense seed persists with a high efficiency. The effect is of basic interest and also makes it robust to noise the simplest technologically fast compression scheme.
139 citations
TL;DR: In this paper, the transient Raman backscattering of a laser pump in plasma is used to reach relativistic pumped pulse intensities, like 1017 W/cm2 for λ=1/μm wavelength radiation.
Abstract: Laser energies and powers, significantly much higher than available now through the most advanced chirped pulse amplifiers, might be achieved in much smaller devices. The working medium in such devices is plasma, capable of tolerating ultrahigh laser intensities within times shorter than it takes for filamentation instabilities to develop. The ultrafast amplification mechanism that outruns filamentation instabilities is the transient Raman backscattering of a laser pump in plasma. In principle, this mechanism is fast enough to reach nearly relativistic pumped pulse intensities, like 1017 W/cm2 for λ=1 μm wavelength radiation. Such a nonfocused intensity would be 105 times higher than currently available. This mechanism also produces complete pump depletion. Many amplifiers with expensive and fragile meter-size gratings might then be replaced by a single amplifier comprised of a 1 cm size plasma layer. Raman instabilities of the pump to noise, as the pump traverses plasma layer towards the seed pulse, can ...
122 citations
TL;DR: RIM101 is suggested to be responsible for the pH dependence of hyphal development of Candida albicans.
Abstract: Morphological development of the fungal pathogen Candida albicans is profoundly affected by ambient pH. Acidic pH restricts growth to the yeast form, whereas neutral pH permits development of the filamentous form. Superimposed on the pH restriction is a temperature requirement of approximately 37 degrees C for filamentation. The role of pH in development was investigated by selecting revertants of phr2Delta mutants that had gained the ability to grow at acid pH. The extragenic suppressors in two independent revertants were identified as nonsense mutations in the pH response regulator RIM101 (PRR2) that resulted in a carboxy-terminal truncation of the open reading frame. These dominant active alleles conferred the ability to filament at acidic pH, to express PHR1, an alkaline-expressed gene, at acidic pH, and to repress the acid-expressed gene PHR2. It was also observed that both the wild-type and mutant alleles could act as multicopy suppressors of the temperature restriction on filamentation, allowing extensive filamentation at 29 degrees C. The ability of the activated alleles to promote filamentation was dependent upon the developmental regulator EFG1. The results suggest that RIM101 is responsible for the pH dependence of hyphal development.
99 citations
TL;DR: In this article, the balance of the processes that produce the filament are different in the UV compared to the IR, and various measurements (spectrum, size, conductivity) indicate that the balance process is different in both environments.
97 citations
TL;DR: In this paper, a physical model is derived which explains current filamentation to be due to a possible negative differential resistance during dynamic avalanche breakdown, and simulations of reverse recovery of a diode structure at similar operating conditions were carried out.
Abstract: High voltage, high power bipolar devices like diodes or GTOs exhibit a limited safe operating area (SOA). During turn-off or reverse recovery the maximum permissible losses remain well below values which could trigger thermal runaway. Up to now the limiting destruction mechanism is not understood. 2D-simulations of GTO turn-off at turn-off gain 1 revealed that during dynamic avalanche multiplication current filamentation may occur. Then current and power loss density are locally increased by a factor of about 20 which could explain a destruction. To exclude filamentation caused by specific points of the GTO cell-structure or the underlying grid also simulations of reverse recovery of a diode structure at similar operating conditions were carried out. In this case too, current filamentation can be observed which occurs simultaneously with sudden voltage drops during the general voltage rise. Measurements on diodes confirm the simulated characteristic notches in the voltage curve. A physical model is derived which explains current filamentation to be due to a possible negative differential resistance during dynamic avalanche breakdown.
72 citations
TL;DR: Filamentation of subpicosecond UV laser pulses with only millijoule energy in atmosphere with good agreement with a numerical simulation using a quasi-three-dimensional propagation code.
Abstract: We report filamentation of subpicosecond UV laser pulses with only millijoule energy in atmosphere. The results are in good agreement with a numerical simulation using a quasi-three-dimensional propagation code.
68 citations
TL;DR: Reported are interactions of high-intensity laser pulses with plasmas in a new parameter regime, in which the pulse duration corresponds to 0.6-2.6 plasma periods, and relativistic filamentation is observed to cause laser-beam breakup and scattering of the beam out of the vacuum propagation angle.
Abstract: Reported are interactions of high-intensity laser pulses ( lambda = 810 nm and I
62 citations
TL;DR: In this article, the growth rate of the filamentary modulational instability is computed as a function of the transverse wavenumber and frequency of periodic perturbations, and the stability domains are expressed in terms of the electron density and pulse peak power, when the inertial plasma response together with the group-velocity dispersion of the wave are taken into account.
Abstract: The filamentation of ultra-short pulses is investigated for plane waves propagating in gases ionized by multiphoton sources. The spatial growth rate of the filamentary modulational instability is computed as a function of the transverse wavenumber and frequency of periodic perturbations. Classical results for optical filamentation in Kerr media are recovered when the density of the electron plasma produced by ionization is close to zero. However, when the electron density is high enough, a beam with an input power above the Kerr self-focusing threshold is shown to stop forming filaments. Stability domains are expressed in terms of the electron density and pulse peak power, both for stationary perturbations and when the inertial plasma response together with the group-velocity dispersion of the wave are taken into account. These theoretical results are supported by numerical simulations and the influence of a delayed Kerr response on filament formation is finally discussed.
61 citations
TL;DR: In this paper, the phase shift of the interference fringes produced in water by the femtosecond optical breakdown was investigated using two-color interferometric imaging technique together with a conventional Mach-Zehnder interferometer.
56 citations
TL;DR: It is shown that the threshold external electric field driving the current is relatively large in dusty plasmas because of the large dissipation rates induced by the dusts.
Abstract: Current-driven dust-acoustic wave instabilities in a collisional plasma with variable-charge dusts are studied. The effects of electron and ion capture by the dust grains, the ion drag force, as well as dissipative mechanisms leading to changes in the particle numbers and momenta, are taken into account. Conditions for the instability are obtained and discussed for both weak and strong ion drag. It is shown that the threshold external electric field driving the current is relatively large in dusty plasmas because of the large dissipation rates induced by the dusts. The current-driven instability may be associated with dust cloud filamentation at the initial stages of void formation in dusty RF discharge experiments.
54 citations
TL;DR: Intensity-dependent beam filamentation is shown to be closely connected to the periodic evolution of quadratic solitons, and implications for the generation of three-dimensional spatiotemporalsolitons are discussed.
Abstract: We present experimental and numerical observations of transverse instability in quadratic media under conditions that emphasize the inherently spatiotemporal and multidimensional nature of the wave propagation. Intensity-dependent beam filamentation is shown to be closely connected to the periodic evolution of quadratic solitons, and implications for the generation of three-dimensional spatiotemporal solitons are discussed.
TL;DR: A unified treatment of electron parametric instabilities driven by ultraintense laser light in plasma is described in this article, which is valid for laser light of any polarization and intensity, for any plasma density both rarefied and classically overdense, and for any scattering geometry.
Abstract: A unified treatment of electron parametric instabilities driven by ultraintense laser light in plasma is described. The method is valid for laser light of any polarization and intensity, for any plasma density both rarefied and classically overdense, and for any scattering geometry. A fully relativistic self-consistent three-dimensional (3D) analysis, based on the cold electron fluid and Maxwell’s equations without further assumption, is carried out in the laser pulse proper frame allowing a concise description in terms of a system of six ordinary differential equations. The approach is applied to linearly polarized laser light in underdense plasma, and overdense plasma accessible by self-induced transparency. Electron charge displacement and relativistic mass fluctuations give rise to hybrids of stimulated Raman scattering, the two plasmon decay, the relativistic modulational and filamentation instabilities, and to stimulated harmonic generation. There is vigorous growth over a wide range of wave numbers...
TL;DR: In this paper, the authors show that the antiguiding factor in a thick 4 nm quantum well is considerably smaller than that for a narrow 2 nm one, and that the thicker quantum well maintains fundamental-mode operation with wider stripe widths and at significantly higher excitation levels.
Abstract: Filamentation and, consequently, output beam quality in InGaN quantum-well lasers are found to be strong functions of quantum-well width because of the interplay of quantum-confined Stark effect and many-body interactions. For an In0.2Ga0.8N/GaN gain medium, the antiguiding factor in a thick 4 nm quantum well is considerably smaller than that for a narrow 2 nm one. As a result, lasers with the thicker quantum well maintain fundamental-mode operation with wider stripe widths and at significantly higher excitation levels.
TL;DR: In this article, a linear theory of stimulated Brillouin scattering and filamentation instabilities was formulated using nonlocal transport equations for a laser heated plasma, resulting in a model which is fully equivalent to a linearized kinetic description.
Abstract: A linear theory of stimulated Brillouin scattering and filamentation instabilities has been formulated using nonlocal transport equations for a laser heated plasma, resulting in a model which is fully equivalent to a linearized kinetic description. The inverse-Bremsstrahlung heating, nonlocal energy redistribution, and ponderomotive laser–plasma interactions are correctly taken into account contributing to a new generalized driving force for these instabilities. Temporal and spatial growth rates, thresholds and dominant perturbation wavelengths are obtained. This theory predicts substantial modifications of the ponderomotive results for conditions relevant to many laser plasma interaction experiments. A new nonlocal and nonlinear model of laser propagation in weakly collisional plasmas has been derived.
TL;DR: In this paper, near and far field patterns are studied for powerful (above 2-W cw output) quantum-well heterojunction InGaAs/AlGaA/GaAs lasers.
Abstract: Near- and far-field patterns are studied for powerful (above 2-W cw output) quantum-well heterojunction InGaAs/AlGaAs/GaAs lasers. The maximum radiation brightness was 1.7107 W cm-2 sr-1. It is shown that the radiation filamentation is already observed 10 — 20% above the lasing threshold. The filamentation period decreases from 50 to 10 μm with increasing pumping current from almost the lasing threshold to an excess of 20% over the threshold. The filamentation results in the tenfold decrease in the radiation brightness of lasers compared to the theoretical value.
TL;DR: The back-reflected image of a 100 TW laser incident on a long scale length plasma is measured and multiple highly intense spots are observed inside the original focal spot that could be the experimental evidence for the laser beam relativistic filamentation in the plasma.
Abstract: The back-reflected image of a 100 TW laser incident on a long scale length plasma is measured. The plasma is deliberately preformed on a solid planar target in a controlled way. Multiple highly intense spots are observed inside the original focal spot. These spots could be the experimental evidence for the laser beam relativistic filamentation in the plasma. Three-dimensional particle-in-cell (PIC) simulations for parameters close to the experimental values are performed. The experimental observations and the filamentation dynamics obtained in the PIC simulations are in a good agreement.
TL;DR: In this paper, the fabrication and operation characteristics of a grating-based, surface-emitting, tapered unstable resonator laser with a trumpet shaped taper and a focusing outcoupler were reported.
Abstract: We report on the fabrication and operation characteristics of a grating-based, surface-emitting, tapered unstable resonator laser with a trumpet shaped taper and a focusing outcoupler. The laser has a low threshold current density of 225 A/cm/sup 2/. It operates with a spatially coherent field and the near-field intensity indicates stable, filamentation free behavior up to at least 3/spl times/ the threshold current under continuous operation. At low drive currents the laser produces a nearly diffraction limited focused spot. With increasing current, the focused spot size increases and multiple side-lobes develop at high currents. We attribute this to a change in the phase of the emitted wavefront associated with thermal effects in the gain region.
TL;DR: In this article, the authors describe the phenomenon of turbulent photon filamentation occurring in lasers and other active optical media at high Fresnel-numbers and propose solutions to evolution equations in the form of a bunch of filaments chaotically distributed in space and having different radii.
TL;DR: In this paper, the authors investigate current filamentation in n-GaAs in the regime of low-temperature impurity breakdown for different sample and contact geometries and compare with spatially resolved measurements in thin epitaxial layers.
Abstract: We investigate current filamentation in n-GaAs in the regime of low-temperature impurity breakdown for different sample and contact geometries. Computer simulations based on a dynamic microscopic model are compared with spatially resolved measurements in thin epitaxial layers. By varying the applied bias, load resistance and magnetic field, one can effectively control the shape and the size of the filaments in rectangular samples with two point contacts. Multistability and hysteresis due to the successive symmetry-breaking formation of multiple filaments are found in Corbino discs upon sweep-up and sweep-down of the voltage and explained by our model.
TL;DR: The technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas as discussed by the authors, where the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma-forming and interaction beams.
Abstract: The technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long-scale length plasmas where we find that the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma-forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray resulting from these fluctuations is important to understand since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which are being used to benchmark numerical models of beam spray.
TL;DR: In this paper, the process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional magnetohydrodynamic-simulations.
Abstract: The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional magnetohydrodynamic-simulations. This situation is typical for astrophysical systems like jets, and the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus, one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.
TL;DR: In this paper, the phase shift of the interference fringes produced in water by the femtosecond optical breakdown was investigated using a two-color interferometric imaging technique together with a conventional Mach-Zehnder interferometer.
Abstract: Time-resolved shadow imaging and novel two-color interferometric imaging have been developed for the investigation of optical breakdown induced in water by 100 fs laser pulses with a spatial resolution better than 5 μm. With a femtosecond time resolution, we clearly identified induced-plasma and filamentation formations, as well as the shock wave expansion. Using the two-color interferometric imaging technique together with a conventional Mach-Zehnder interferometer, we investigated the phase shift of the interference fringes produced in water by the femtosecond optical breakdown. For different pump energies, at 40 ns after the breakdown, we estimated the water temperature between 30 and 60 °C and the water pressure between 10 and 40 MPa at the shock wave front.
TL;DR: In this article, the full nonlinear evolution of a four-field plasma model with vanishing resistivity was studied and it was shown that the eventual nonlinear saturation regime is markedly different from extrapolations suggested by investigations of the early nonlinear stage, and that topological transitions akin to filamentation in pressure and current density occur.
Abstract: The full nonlinear evolution of a four-field plasma model with vanishing resistivity reveals that the eventual nonlinear saturation regime is markedly different from extrapolations suggested by investigations of the early nonlinear stage, and that topological transitions akin to filamentation in pressure and current density occur, driven by the nonlinear evolution of the fluid motion.
TL;DR: In this article, the concept of low modal gain is demonstrated to efficiently repel filamentation effects, enabling more than 700 mW to be coupled in a singlemode fiber from a single semiconductor laser diode.
Abstract: Applied to 1.48 /spl mu/m high-power unstable-cavity lasers, the concept of low modal gain is demonstrated to efficiently repel filamentation effects, enabling more than 700 mW to be coupled in a singlemode fibre from a single semiconductor laser diode.
TL;DR: Einaudi et al. as discussed by the authors reviewed the recent research on the formation of the slow solar wind, which they model as a magnetized wake, and found the creation of traveling plasmoids, turbulence production, and streamwise filamentation.
TL;DR: In this paper, an investigation of x-ray emission from Cu plasma produced by 1.054 µm Nd:glass laser pulses of 5 ns duration, at 2 × 1012 −2 × 1013 W cm−2 is reported.
Abstract: An investigation of x-ray emission from Cu plasma produced by 1.054 µm Nd:glass laser pulses of 5 ns duration, at 2 × 1012 − 2 × 1013 W cm−2 is reported. The x-ray emission has been studied as a function of target position with respect to the laser beam focus position. It has been observed that x-ray emissions from ns duration plasma show a volume effect similar to subnanosecond plasmas. Due to this effect the x-ray yield increases when target is moved away relative to the best focal plane of the laser beam. This result supports the theoretical model of Tallents and has also been testified independently using suitably modified theoretical model for our experimental conditions. While above result is in good agreement with similar experimental results obtained for sub-nanosecond laser produced plasmas, it differs from result claiming filamentation rather than pure geometrical effect leading to x-ray enhancement for ns plasmas.
TL;DR: Low energy electron beams generated in a self-modulated laser wakefield accelerator experiment were observed to filament and be deflected away from the laser axis forming radial jets in the electron beam profile.
Abstract: Low energy electron beams (E{approx}300 keV) generated in a self-modulated laser wakefield accelerator experiment were observed to filament and be deflected away from the laser axis forming radial jets in the electron beam profile. At higher energies (E>900 keV), the filamentation and jets were suppressed and smooth electron beams copropagating with the laser were observed. The observed electron beam filamentation likely results from laser beam filamentation in the plasma due to relativistic self-focusing effects. The radial jets of low energy electrons are likely caused by transverse ejection of the electrons due to the radial structure of the wakefield and space charge deflection of electrons as they exit the laser focus. (c) 2000 The American Physical Society.
27 Oct 2000
TL;DR: In this paper, the first rigorous measurements of the characteristics of laser filaments at 400 nm were presented, and the results showed that the laser propagation at 400nm is nonlinear.
Abstract: Summary form only given. Nonlinear propagation of ultrashort laser pulses in air has been a subject of great interest in recent years, with a large amount of work being done at 800nm due to the availability of broadband IR laser materials. Recent studies have begun to examine the nonlinear propagation of laser pulses at wavelengths other than 800nm. In this paper we present what to the best of our knowledge is the first rigorous measurements of the characteristics of laser filaments at 400 nm.
14 Jul 2000
TL;DR: In this article, physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers are presented, where the principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.
Abstract: We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single-mode optical fibers. The principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.