Showing papers on "Filamentation published in 1998"
TL;DR: Two regulatory pathways govern filamentation in the pathogenic fungus Candida albicans, and recent virulence studies of filamentation regulatory mutants argue that both yeast and filamentous forms have roles in infection.
255 citations
TL;DR: Deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltame p1 Deltamesp2/Delta p2 and Deltagpa2/deltagma2 strains, suggesting that stress conditions other than nitrogen deprivation may also promote filamentous growth.
Abstract: Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Galpha subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Deltamep2/Deltamep2 and Deltagpa2/Deltagpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltamep1 Deltamep2/Deltamep2 and Deltamep1/Deltamep1 Deltagpa2/Deltagpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Deltamep1/Deltamep1 Deltamep2/Deltamep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth.
171 citations
TL;DR: In this paper, a detailed stability analysis of one-and two-ring solitary waves with central phase dislocation in self-focusing saturable and quadratic nonlinear media is presented.
Abstract: We present a detailed stability analysis of one- and two-ring solitary waves with central phase dislocation in self-focusing saturable and quadratic nonlinear media. Varying parameters, we demonstrate transitions between different filamentation scenarios. An analytical approach is developed for the study of filament dynamics after ring breakup. Approximate expressions for the angular separation rate of filaments based on the conservation of the angular momentum and on the conservation of the Hamiltonian are derived and compared. The stability analysis and analytical results are tested by an extensive series of numerical simulations of the original models, and good agreement is found. @S1063-651X~98!14609-3#
119 citations
TL;DR: In this article, the authors studied the linear and nonlinear evolution of the electromagnetic beam-plasma instability in a collisionless inhomogeneous plasma by using a set of two-fluid electron equations in the nonrelativistic and relativistic regimes.
Abstract: Beams of fast electrons in a cold electron background play a key role in the generation of a magnetic field in the wake of an ultrashort ultraintense laser pulse propagating in an underdense plasma. Here we study the linear and nonlinear evolution of the electromagnetic beam-plasma instability in a collisionless inhomogeneous plasma by using a set of two-fluid electron equations in the nonrelativistic and relativistic regimes. We show the characteristic spatial structures in the current and magnetic field distributions that are generated by this instability. These structures can be used as a signature of the physical mechanism at play in the analysis of the numerical and experimental results of the laser-plasma interaction.
103 citations
TL;DR: In this paper, a simulation of the temporal evolution of laser light filamentation and stimulated Brillouin forward scattering in plasmas, under conditions that are relevant to laser fusion, is presented and analyzed.
Abstract: Numerical simulations of the temporal evolution of laser light filamentation and stimulated Brillouin forward scattering (SBFS) in plasmas, under conditions that are relevant to laser fusion, are presented and analyzed. Long term unsteady behavior of filaments is observed to be the norm. Temporal and spatial incoherence due to filamentation and SBFS are impressed upon time-independent incident laser beams. The bandwidth and angular divergence imposed upon the beam increase with the strength of the interaction. In addition, the spectrum of the transmitted light is redshifted by an amount that increases with the interaction strength. Spectral analysis of the transmitted light reveals that SBFS plays a role in the generation of the observed temporal incoherence. Incident beams with some spatial incoherence but no temporal smoothing are compared to those with ab initio temporal beam smoothing (TBS). Under typical conditions, TBS beams will undergo far less angular and spectral spreading and far less SBFS than...
94 citations
TL;DR: In this paper, a detailed theoretical analysis of the properties and formation of single solitons and higher-order bound dipole pairs in media with anisotropic nonlocal photorefractive material response is presented.
Abstract: We present a detailed theoretical analysis of the properties and formation of single solitons and higher-order bound dipole pairs in media with anisotropic nonlocal photorefractive material response. The single solitons are elliptical beams, whereas the dipole pairs are formed by a pair of displaced elliptical beams with a p phase shift between their fields. The theory predicts convergence of Gaussian beams to the solitary states within a certain basin of attraction. Experimental observation of these solitons has been presented elsewhere. The experimental portion of the present paper concentrates on the region further away in parameter space, where complex spatial oscillations, including asymmetric filamentation into several beamlets, occurs. @S1050-2947~97!03011-4# The possibility of creating two-transverse-dimensional @(211)D# soliton-type structures of light in nonlinear media is of considerable interest due to potential applications in optical information processing systems @1#. The dynamics of nonlinear propagation equations resulting in the formation of such structures can be very complex and may result in the generation of higher-order and multisoliton solutions. These solutions have been investigated extensively in the ~111!D ~one-transverse-dimensional! case @2#, where a number of exact analytical methods have been developed including the inverse scattering method, which provides an exact solution
88 citations
TL;DR: A stable regime looking like a waveguide appears in certain conditions, in which the beam to propagate in a narrow tube, is reported here: a stable regime is simply modeled in terms of saturated reorientation of the nematic.
Abstract: We report on experiments dealing with the propagation of a collimated laser beam in a dye doped nematic confined in a capillary of optical fiber size. The nematic is aligned in a such a way that the source beam is self focused. The behavior of the beam - focusing, multifocus regime, filamentation and undulation - already observed in larger cylindrical geometry and pure nematic is shown here to be reproduced in the dye doped medium, at much lower powers. Another feature is reported here: a stable regime looking like a waveguide appears in certain conditions, in which the beam to propagate in a narrow tube. This regime is simply modeled in terms of saturated reorientation of the nematic.
73 citations
TL;DR: In this paper, a simulation of the filamentation of an intense speckled laser beam in a hot, high-density plasma is performed using polarization smoothing, which consists of irradiating the plasma with two superimposed, orthogonally polarized and uncorrelated speckle patterns.
Abstract: Dramatic reductions in the filamentation of an intense speckled laser beam in a hot, high-density plasma are reported in simulations of polarization smoothing (PS), which consists of irradiating the plasma with two superimposed, orthogonally polarized and uncorrelated speckle patterns. PS instantaneously smooths the spatial beam structure, and thus can be more effective than temporal smoothing techniques. Even greater reduction in filamentation, offering improved prospects for inertial fusion, results from combining these two methods.
63 citations
TL;DR: In this article, the master oscillator power amplifier using a semiconductor laser model that is fully time and space (laterally and longitudinally) resolved is investigated, and numerically examined the stability of the device and identify the nature of the different instabilities.
Abstract: We investigate theoretically the master oscillator power amplifier using a semiconductor laser model that is fully time and space (laterally and longitudinally) resolved. We numerically examine the stability of the device and identify the nature of the different instabilities. These can arise from undamped relaxation oscillations, beating between the longitudinal modes of any of the cavities that comprise the device, or lateral filamentation.
52 citations
TL;DR: Mora and Antonsen as mentioned in this paper interpreted the relativistic self-focusing by means of the particle code WAKE using the ponderomotive approximation to describe the laser-plasma interaction.
Abstract: Existing experimental results on relativistic self-focusing are interpreted by means of the particle code WAKE using the ponderomotive approximation to describe the laser–plasma interaction [P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997)]. Novel features of the code, such as gas medium ionization and an enhanced paraxial approximation, allow more confidence in data interpretation. Simulations where the pulse power is less or close to the critical value match the experimental data. The transmitted pulse spectrum is shown in this case to shift towards longer wavelengths. The pulse is shown to focus over the vacuum diffraction limit, while the energy is slowly depleted. Simulations of pulses above the critical power match experiment with reduced precision. This can be ascribed to beam filamentation. High energy depletion is expected in this case due to Raman instability.
49 citations
TL;DR: In this article, the interaction of copropagating electromagnetic and matter waves is described with a set of coupled higher-order nonlinear Schrodinger equations, where self-focusing modulates an initially planar wave leading to the generation of dipole forces on the atoms.
Abstract: The interaction of copropagating electromagnetic and matter waves is described with a set of coupled higher-order nonlinear Schr\"odinger equations Optical self-focusing modulates an initially planar wave leading to the generation of dipole forces on the atoms Atomic channeling due to the dipole forces leads, in the nonlinear regime, to filamentation of the atomic beam Instability growth rates are calculated for atomic beams with both low and high phase space densities In one transverse dimension an exact solution is found that describes a coupled optical and atomic soliton
TL;DR: Campbell et al. as discussed by the authors showed that scattering losses and effects of filamentation are reduced when the beam is spatially smoothed with a random phase plate or kino-form phase plate.
Abstract: Gas-filled hohlraums are presently the base line ignition target design for the National Ignition Facility. Initial Nova [E. M. Campbell et al. Rev. Sci. Instrum. 57, 2101 (1986).] experiments on gas-filled hohlraums showed that radiation temperature was reduced due to stimulated Brillouin and stimulated Raman scattering losses and that implosion symmetry had shifted compared with vacuum hohlraums and calculations. Subsequent single beam experiments imaging thermal x-ray emission showed the shift is due to laser–plasma heating dynamics and filamentation in a flowing plasma. Experiments using a single beam have shown that scattering losses and effects of filamentation are reduced when the beam is spatially smoothed with a random phase plate or kinoform phase plate. Scattering is further reduced to less than 5% of the incident laser energy when temporal smoothing is added.
TL;DR: In this article, the spatial and temporal characteristics of filamentation in broad-area semiconductor lasers were investigated and it was shown that the sparing between filaments decreases as either the pumping level or linewidth enhancement factor is increased.
Abstract: We experimentally study the spatial and temporal characteristics of filamentation in broad-area semiconductor lasers. Near-field measurements show that the sparing between filaments decreases as either the pumping level or the linewidth enhancement factor is increased. Spectral measurements reveal that spatial filaments are accompanied by periodic or chaotic temporal variations. Our experimental results show excellent qualitative agreement with a theory based on a linear stability analysis of the rate equations for an infinitely wide stripe laser.
TL;DR: In this paper, an inhomogeneously pumped broad area diode laser was fabricated using a digitated electrical contact pad to smooth the edges of the transverse distribution of injected carriers.
Abstract: The authors have fabricated inhomogeneously pumped broad area diode lasers using a novel digitated electrical contact pad to smooth the edges of the transverse distribution of injected carriers. Experiments demonstrate the effectiveness of this scheme in increasing the filamentation threshold and reducing near field modulation up to 35 times threshold. The inhomogeneously pumped design has reduced threshold current and increased slope efficiency relative to a comparable standard broad area device.
TL;DR: In this article, the authors show that epitaxial layer structures with low optical confinement are much more insensitive to beam filamentation because of their reduced differential gain, and they find that the beam quality of tapered laser oscillators can be improved by an order of magnitude, when epitaxia with reduced modal gain are used for the device fabrication.
Abstract: In high-power, high-brightness laser diodes, beam filamentation is one of the main physical effects that limit the device performance. Due to the interaction between the optical power and the carrier density in the active region of broad area devices, spatial hole-burning leads to an inhomogeneous optical index that causes the degradation of the optical beam profile. We show, that epitaxial layer structures with low optical confinement are much more insensitive to beam filamentation because of their reduced differential gain. Experimentally we find, that the beam quality of tapered laser oscillators can be improved by an order of magnitude, when epitaxial layer structures with reduced modal gain are used for the device fabrication. Two mm long tapered devices with a 200 micrometer wide output facet show near diffraction limited farfield profiles up to output powers of more than 2 W cw.
TL;DR: In this article, the stability and dynamics of higher-order solitary waves in quadratic media were investigated, and the existence of two qualitatively different behaviors were found for positive and negative phase mismatches.
Abstract: We investigate stability and dynamics of higher-order solitary waves in quadratic media, which have a central peak and one or more surrounding rings. We show the existence of two qualitatively different behaviors. For positive phase mismatch the rings break up into filaments that move radially to the initial ring. For sufficient negative mismatches rings are found to coalesce with the central peak, forming a single oscillating filament.
TL;DR: C. glutamicum has a unique regulation mechanism, that is, the inhibition of cell division leads to cessation of cell elongation, which is similar to those observed with the temperature-sensitive ts mutants of C.glutamicum.
Abstract: In order to characterize the cell-division mechanism of coryneform bacteria, we tried to isolate cell-division mutants from Corynebacterium glutamicum after N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis, such as Escherichia coli fts mutants, which form long filaments at the restrictive temperatures. At the non-permissive temperature, most of the mutants formed club-shaped or dumbbell-shaped, elongated rod cells, but no filament formers were isolated. Then we examined the effects of cell division inhibitors on this organism. Cephalexin and sparfloxacin, which are the inhibitors of septation and DNA synthesis respectively, and are known to cause cell filamentation in E. coli, did not cause filamentation in C. glutamicum but induced morphological changes that were similar to those observed with the temperature-sensitive ts mutants of C.␣glutamicum. These results suggest that C. glutamicum has a unique regulation mechanism, that is, the inhibition of cell division leads to cessation of cell elongation.
TL;DR: In this paper, two-dimensional drift-diffusion simulations were performed to study the transient response of silicon avalanche shaper (SAS) devices that are used in high-power switching and pulse sharpening applications.
Abstract: Two-dimensional (2-D) drift-diffusion simulations were performed to study the transient response of silicon avalanche shaper (SAS) devices that are used in high-power switching and pulse sharpening applications. The role of transverse doping variations on the transient device response has been studied. Our results clearly reveal a potential for filamentary current conduction. The filamentation, however, is shown to be strongly dependent on the transverse doping characteristics, and hence in principle, could he tailored.
TL;DR: Aerobic or 100% CO 2 cultures of Aeromonas hydrophila g rowing in Tryptone Soya Broth at 5, 10 or 28°C, were examined for filamentation and chain formation and filamentation was noted in aerobically grown cells.
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06 Feb 1998TL;DR: In this article, the shape of the trumpet-flare is optimized to delay the onset of filamentation and the degradation of the output laser beam due to reflections to higher output powers for the MFA-MOPA device.
Abstract: An MFA-MOPA that includes a semiconductor laser with separate master oscillator and trumpet-flared power amplifier regions. Within the trumpet-flared active gain region of the uniformly-pumped power amplifier of the MFA-MOPA device, the density distribution of carriers and reflections of the laser beam are analyzed to determine the output powers at which filamentation and beam degradation due to reflections occur. The shape of the trumpet-flare is optimized to delay the onset of filamentation and the degradation of the output laser beam due to reflections to higher output powers for the MFA-MOPA device.
TL;DR: In this article, the second harmonic (SH) and X-ray emissions from Al plasmas produced by 3-ns, 1.064-μm laser pulses at 10 14 W/cm 2 were investigated.
Abstract: An investigation of second harmonic (SH) and X-ray emissions from Al plasmas produced by 3-ns, 1.064-μm laser pulses at 10 14 W/cm 2 is reported. The SH and X-ray yields are strongly correlated as a function of the target position with respect to the laser beam focus. The SH originates from the underdense coronal plasma and has a filamentary source, while the X-ray source is uniform. The results suggest that, although the X-ray emission is significantly enhanced by the filamentation of the laser light in the corona, there is a smoothing effect in the energy transport process toward the overdense region.
25 Jun 1998
TL;DR: In this article, a model for the scaling of strongly-driven stimulated Brillouin and Raman scattering was proposed, which postulates an intensity dependent correlation length associated with spatial incoherence due to filamentation and stimulated forward scattering.
Abstract: An improved understanding of strongly-driven laser plasma coupling is important for optimal use of the National Ignition Facility (NIF) for both inertial fusion and for a variety of advanced applications. Such applications range from high energy x- ray sources and high temperature hohlraums to fast ignition and laser radiography. We discuss a novel model for the scaling of strongly-driven stimulated Brillouin and Raman scattering. This model postulates an intensity dependent correlation length associated with spatial incoherence due to filamentation and stimulated forward scattering. We first motivate the model and then relate it to a variety of experiments. Particular attention is paid to high temperature hohlraum experiments, which exhibited low to modest stimulated Brillouin scattering even though this instability was strongly driven. We also briefly discuss the strongly nonlinear interaction physics for efficient generation of high energy electrons either _ by irradiating a large plasma with near quarter-critical density or by irradiating overdense targets with ultra intense laser
TL;DR: In this article, a family of (2+1) D spatial solitary waves which can stably propagate in bulk media in the presence of coexisting diffraction, self-focusing Kerr and quadratic nonlinearities is identified.
TL;DR: In this paper, numerical magnetohydrodynamical studies are used to show that ideal boundary perturbations (like, e.g., Kelvin-Helmholtz modes) can result in filamentary current structures.
Abstract: Astrophysical plasmas typically have to be considered as highly collision-free from a kinetic point of view or highly ideal in a fluid description. This refers to near-Earth space plasmas, like the magnetosphere, or extragalactic objects like radio jets, as well. On the other hand, magnetic reconnection is discussed to play an important role as an effective mechanism for conversion of magnetic energy into kinetic energy by particle acceleration. As magnetic reconnection demands for local deviations from adiabatic plasma conditions, in the highly collision-free plasma regime inertia driven reconnection should be expected. In the framework of a magnetohydrodynamic description, anomalous resistivity due to current driven microinstabilities is discussed to provide sufficient nonidealness. Both classes of processes demand for sufficiently thin current sheets. This constraint can be fulfilled by current filamentation. In the present paper numerical magnetohydrodynamical studies are used to show that ideal boundary perturbations (like, e.g., Kelvin–Helmholtz modes) can result in filamentary current structures. This fundamental process is discussed with respect to extragalactic jets.
08 May 1998
TL;DR: In this article, a three-dimensional numerical simulation of the propagation of an intense femtosecond laser pulse through atmosphere is performed to obtain precise information on the physical origin of temporal self-compression.
Abstract: We have observed that a pulse temporal self-compression takes place during the filamentation process. Pulse durations have been measured inside the filament by time-resolved self-diffraction between two identical filaments. We show this effect by comparing the pulse duration recorded before and well beyond the onset of filamentation. To obtain precise information on the physical origin of temporal self-compression, we have performed a three-dimensional numerical simulation of the propagation of an intense femtosecond laser pulse through atmosphere.
TL;DR: In this paper, a mechanism of filamentation instability due to ponderomotive forces in laser produced plasmas is discussed, where a small fluctuation in the intensity profile of a plane electromagnetic beam grows as it propagates in the plasma where the self-generated axial magnetic field cannot be neglected.
Abstract: Self generated axial magnetic field due to ponderomotive force in a laser produced plasmas is discussed. When an intense laser beam propagates in an expanding plasma, spatial inhomogeneities present in the intensity profile of the incident laser beam can grow rapidly due to self-focusing effect, which result in localized filamentation of higher intensity. This filamentation can directly cause localized heating of the target, thus destroying the uniformity in the ablative acceleration necessary for high compression implosions. In view of experimental observations a small fluctuation in the intensity profile of a plane electromagnetic beam grows as it propagates in the plasma where the self-generated axial magnetic field cannot be neglected. Based on these observations we discuss a mechanism of filamentation instability due to ponderomotive forces in laser produced plasmas.
TL;DR: In this paper, the nonlinear behavior of a single laser beam with power above critical is investigated in three dimensions without the paraxial approximation, and it is shown numerically that the hot spot contracts spatially in all directions, leading to transverse dimensions smaller than the laser wavelength, even when the ion response is nonlinearly saturated.
Abstract: The nonlinear behavior of a single laser beam with power above critical is investigated in three spatial dimensions without the paraxial approximation. In the case where stimulated Brillouin scattering (SBS) is suppressed by strong damping, it is found numerically that the hot spot contracts spatially in all directions, leading to transverse dimensions smaller than the laser wavelength, even when the ion response is nonlinearly saturated. This contraction leads to the three dimensional collapse of the transverse wave in the absence of nonlinear saturation of the ion response. When the saturation of the ion response is properly retained, a narrow filament forms initially, This filament then appears to be unstable and exhibits violent modulations of the laser wave intensity along the direction of its propagation, leading to a complicated dynamical behavior. In the case of moderate SBS damping, one still observes the initial spatial contraction in all directions and the formation of a narrow channel. SBS then occurs in violent bursts, and does not stop the complex dynamical behavior of the filament.
31 Dec 1998
TL;DR: In this article, the authors identified and confirmed that ultrafast intense Ti-sapphire laser pulses after propagating through an optical medium self-transform into chirped white light laser pulses as manifested by the generation of supercontinuum.
Abstract: : We have identified and confirmed that ultrafast intense Ti-sapphire laser pulses after propagating through an optical medium self-transform into chirped white light laser pulses as manifested by the generation of supercontinuum. During the propagation of such pulses in air, self-focusing and filamentation take place together with strong field interaction with molecules in its path. Some important physical processes were studied. In particular, we observed a new type of fluorescence from the air molecules characteristic of intense field ionization and fragmentation unexpected from normal wisdom of plasma emission. An analytical tunnel ionization model of air molecules that agree with our experimental results was developed. It could be used in any theoretical modelling of such propagation. New techniques were developed to study the dynamics of filamentation. They allowed us to observed directly for the first time the signature of re-focusing The dependence of filamentation on various laser parameters such as the chirp, the divergence, etc. was also measured. Preliminary results agree with what we expect based on current knowledge of ultrafast nonlinear optics.
TL;DR: In this paper, the effect of recombination and the filamentation instability of Alfven waves propagating along homogeneous magnetic field in low-beta plasmas were investigated using an MHD simulation code.
Abstract: We investigate plasma heating associated with the effect of recombination and the filamentation instability of Alfven waves propagating along homogeneous magnetic field in low-beta plasmas, by using an MHD simulation code. The linear instability of Alfven waves leading to the filamentation is investigated by imposing small density perturbations across a magnetic field. We show results of the nonlinear stage of the above filamentation instability and the plasma heating through a two-dimensional simulation. It is shown that the plasma heating is caused by localized heating and whole heating, which are associated with the filamentation instability and the effect of recombination, respectively. We discuss the implication of these results for plasma heating processes observed in the chromosphere of the Sun.
TL;DR: In this paper, the filamentation tendencies of a wide input-aperture tapered amplifier are examined and compared to theory, and a numerical simulation of the device operation is presented to address non-uniform current injection.
Abstract: Filamentation currently limits the amount of diffraction- limited power that can be obtained from broad-area semiconductor amplifiers. This paper examines the filamentation tendencies of a wide input-aperture tapered amplifier. Experimental measurements of filament gain under varying duty-cycle are offered and compared to theory. A numerical simulation of the device operation, which addresses non-uniform current injection, is also presented.