Showing papers on "Filamentation published in 2002"
TL;DR: Results suggest that hyphal formation is pivotal for biofilm development in C. albicans, however, the sessile lifestyle associated with adherent cells confers antifungal resistance, regardless of coherent biofilm formation.
Abstract: Candida albicans biofilms are structured microbial communities composed of a mixture of yeast cells and hyphal elements, suggesting a pivotal role for the dimorphic switch in the development of biofilms. We have used C. albicans mutants defective in genes involved in filamentation (Δcph1, Δefg1, Δhst7, and Δcst20) and compared these mutants to wild-type strains to determine whether filamentation is an integral factor for biofilm formation. Scanning electron microscopy revealed that Δcph1, Δhst7 and Δcst20 mutants were able to filament and form structured biofilms displaying three-dimensional architecture similar to those formed by wild-type strains. However, Δefg1 and Δcph1/Δefg1 mutants were unable to filament and did not form biofilms, but rather sparse monolayers of loosely attached elongated, rod-like, cells. Antimicrobial susceptibility testing showed intrinsic resistance of all mutant strains to fluconazole and amphotericin B when attached to the surface of biomaterials. These results suggest that hyphal formation is pivotal for biofilm development in C. albicans. However, the sessile lifestyle associated with adherent cells confers antifungal resistance, regardless of coherent biofilm formation.
315 citations
TL;DR: Als1p expression is controlled by the transcription factor Efg1p, which is known to be a key regulator of filamentation in C. albicans and functions as a downstream effector of the EFG1 filamentation pathway, and it is found that Als1 p mediates both flocculation and adherence of C.Albicans to endothelial cells in vitro.
Abstract: Summary
Filamentation and adherence to host cells are critical virulence factors of Candida albicans. Multiple filamentation regulatory pathways have been discovered in C. albicans using Saccharomyces cerevisiae as a model. In S. cerevisiae, these pathways converge on Flo11p, which functions as a downstream effector of filamentation and also mediates cell–cell adherence (flocculation). In C. albicans, such effector(s) have not yet been identified. Here, we demonstrate that the cell surface protein Als1p is an effector of filamentation in C. albicans. We show that Als1p expression is controlled by the transcription factor Efg1p, which is known to be a key regulator of filamentation in C. albicans. Further, disruption of ALS1 inhibited filamentation, and autonomous expression of Als1p restored filamentation in an efg1 homozygous null mutant. Thus, Als1p functions as a downstream effector of the EFG1 filamentation pathway. In addition, we found that Als1p mediates both flocculation and adherence of C. albicans to endothelial cells in vitro. As a cell surface glycoprotein that mediates filamentation and adherence, Als1p has both structural and functional similarity to S. cerevisiae Flo11p. Consistent with our in vitro results, Als1p was required for both normal filamentation and virulence in the mouse model of haematogenously disseminated candidiasis.
234 citations
TL;DR: In this paper, the supercontinuum spectrum of an intense femtosecond Ti:sapphire laser pulse propagating in condensed optical media (water, chloroform and glass), at various input laser energies, below and above the threshold for filamentation of the laser pulse was measured.
228 citations
TL;DR: In this article, a model of femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses.
Abstract: Formation and wandering of filaments in air are studied both experimentally and numerically. Filament-center deflections are collected from 1100 shots of 190-fs and 800-nm pulses in the plane perpendicular to the propagation direction. To calculate the filament wandering in air we have developed a model of powerful femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses. Statistical processing of experimental and numerical data shows that filament-center displacements in the transverse plane obey the Rayleigh-distribution law. Parameters of the Rayleigh distribution obtained for numerical and experimental data are close to each other.
113 citations
TL;DR: It is proposed that Czf1p interacts with Efg1p, altering its function, and the complex regulation of filamentation may reflect the versatility of C. albicans as a pathogen.
Abstract: Filamentation of Candida albicans occurs in response to many environmental cues. During growth within matrix, Efg1p represses filamentation and Czf1p relieves this repression. We propose that Czf1p interacts with Efg1p, altering its function. The complex regulation of filamentation may reflect the versatility of C. albicans as a pathogen.
105 citations
TL;DR: Spectral data exhibit a large broadening in the IR filament and a limited one for UV, which justifies the white-light generation associated with IR filamentation only.
Abstract: The propagation of femtosecond UV laser pulses in air is numerically shown to form intense light filaments over several tenths of Rayleigh lengths. We compare UV filamentation with IR filamentation and show that the balance of the physical processes supporting the filaments is identical in both cases. For IR and UV wavelengths, it is shown that the intensity in the filament and the density of the electron plasma created by ionization of air molecules reach similar values as high as 10(14) W/cm(2) and 10(17) cm(-3). Spectral data exhibit a large broadening in the IR filament and a limited one for UV, which justifies the white-light generation associated with IR filamentation only.
104 citations
TL;DR: The interaction of relativistic laser light with overdense plasmas is studied by three-dimensional particle-in-cell simulations and it is found that fast electron energy flow is not only inside the magnetic channels but also it is widely distributed outside the channels.
Abstract: The interaction of relativistic laser light with overdense plasmas is studied by three-dimensional particle-in-cell simulations. Generation of layered current sheets and quasistatic magnetic fields is observed near the target surface owing to anisotropic laser filamentation and Weibel instabilities. Later these current sheets tear into filaments that partially merge with each other to form isolated magnetic channels penetrating into the dense plasmas. It is found that fast electron energy flow is not only inside the magnetic channels but also it is widely distributed outside the channels. This is possible because of electron anomalous diffusion across self-generated magnetic fields. Consequently, the total hot electron current exceeds a few hundred kiloamperes and is much larger than the Alfven current. Hence a considerable amount of energy flows towards the plasma core. Significant heating of the bulk plasma electrons is also observed.
90 citations
TL;DR: In this paper, the three-dimensional dynamics of a monochromatic wave propagating along an ambient magnetic field is simulated by direct numerical integration of the Hall-magnetohydrodynamics equations, in spite of the presence of competing, possibly linearly dominant, instabilities that in some instances distort the above structures.
Abstract: The three-dimensional dynamics of a small-amplitude monochromatic Alfven wave propagating along an ambient magnetic field is simulated by direct numerical integration of the Hall-magnetohydrodynamics equations. As predicted by the two-dimensional nonlinear Schrodinger equation or by more general amplitude equations retaining the coupling to low-frequency magnetosonic waves, the transverse instability of the pump leads to wave collapse and formation of intense magnetic filaments, in spite of the presence of competing, possibly linearly dominant, instabilities that in some instances distort the above structures. In computational boxes, including a large number of pump wavelengths, an early arrest of the collapse is possible under the effect of quasi-transverse instabilities that drive magnetosonic waves and also prescribe the directions of the filaments.
71 citations
TL;DR: Analysis and simulations of this system show that multiple filamentation is suppressed for circularly polarized beams in a Kerr medium.
Abstract: We derive a new system of equations that describes the propagation of circularly polarized laser beams in a Kerr medium. Analysis and simulations of this system show that multiple filamentation is suppressed for circularly polarized beams.
52 citations
TL;DR: The predictions of a theoretical model of the filamentation process are in good agreement with observations of transverse pattern formation leading to highly regular structures in both the near and far fields when a near-resonant laser beam propagates without feedback through an atomic sodium vapor.
Abstract: We have observed transverse pattern formation leading to highly regular structures in both the near and far fields when a near-resonant laser beam propagates without feedback through an atomic sodium vapor. One example is a regular far-field honeycomb pattern, which results from the transformation of the laser beam within the vapor into a stable three-lobed structure with a uniform phase distribution and highly correlated power fluctuations. The predictions of a theoretical model of the filamentation process are in good agreement with these observations.
51 citations
TL;DR: In this paper, a femtosecond laser pulse filamentation in competition with optical breakdown in condensed matter is studied both experimentally and numerically using water as an example, and strong random deflection and modulation of the supercontinuum under tight focusing conditions are observed.
Abstract: Ti:sapphire femtosecond laser pulse filamentation in competition with optical breakdown in condensed matter is studied both experimentally and numerically using water as an example. Strong random deflection and modulation of the supercontinuum under tight focusing conditions were observed. They manifest the beginning of the filamentation process near the highly disordered plasma created by optical breakdown at the geometrical focus.
TL;DR: From this observation, one can directly analyze and recognize the propagation process, which is significantly affected by pulse-energy fluctuation, and (or) atmospheric turbulence from shot to shot.
Abstract: We have demonstrated femtosecond time-resolved and picosecond time-interval successive observations of a single femtosecond optical pulse propagating in air with ultrafast self-modulation such as filamentation. A quadruple femtosecond probe pulse crossing an intense propagating pulse at picosecond intervals was able to capture directly four successive images of the propagating pulse as in a movie but with femtosecond time resolution. From this observation, we can directly analyze and recognize the propagation process, which is significantly affected by pulse-energy fluctuation, and (or) atmospheric turbulence from shot to shot.
TL;DR: In this paper, the authors investigated the use of adaptive mesh refinement in the simulation of laser plasma filamentation and constructed a numerical algorithm to solve model equations consisting of a fluid approximation of a quasineutral plasma combined with a paraxial light propagation model.
TL;DR: In this paper, the effect of thermal motion of charged particles in the filamentation of a nonrelativistic current-driven plasma in the ion-acoustic frequency region is investigated.
Abstract: The effect of thermal motion of charged particles in the filamentation of a nonrelativistic current-driven plasma in the ion-acoustic frequency region is investigated. The period and the establishment time of the filamentation structure and the threshold for instability development are obtained.
TL;DR: A nonlocal model of electron heat flow in laser-heated plasmas taking into account the super-Gaussian deformation of the electron velocity distribution function by the laser heating was developed and performs better than previous models.
Abstract: A nonlocal model of electron heat flow in laser-heated plasmas taking into account the super-Gaussian deformation of the electron velocity distribution function by the laser heating was developed. Based on comparisons to Fokker-Planck simulations of hot spot heating, it performs better than previous models. The growth rate of thermal filamentation is considerably changed by this thermal conductivity modification. First results from a formula describing the isotropic component, f 0 (r,υ), of the electron velocity distribution function as a spatial convolution over local Maxwellians are also presented.
TL;DR: In this article, the properties of high power broad area quantum dots lasers are analyzed numerically and it is shown that transverse modes instability is associated with the excess of carriers at the edge of the pumped area.
TL;DR: It is found that OSH3 overexpression promoted filamentation growth of the Sigma1278b wild-type strain and suppressed the filamentations growth defect of the ste12 mutation, which leads to a new understanding of cellular functions of the yeast OSBPs.
TL;DR: This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.
Abstract: An ultrafast imaging technique has been developed to visualize directly a light pulse that is propagating in a medium. The method, called femtosecond time-resolved optical polarigraphy (FTOP), senses instantaneous changes in the birefringence within the medium that are induced by the propagation of an intense light. A snapshot sequence composed of each femtosecond probing the pulse delay enables ultrafast propagation dynamics of the intense femtosecond laser pulse in the medium, such as gases and liquids, to be visualized directly. Other examples include the filamentation dynamics in CS2 liquid and the propagation dynamics in air related to the interaction with laser breakdown plasma. FTOP can also be used to extract information on the optical Kerr constant and its decay time in media. This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.
12 Jun 2002
TL;DR: In this article, a quasi-3D model was developed with the aim of studying the different factors limiting the performance of high-brightness high-power tapered lasers with a gain-guided taper section.
Abstract: A quasi-3D model has been developed with the aim of studying the different factors limiting the performance of high-brightness high-power tapered lasers. The model solves the complete semiconductor and thermal equations, neglecting the flow of carriers and heat along the cavity ax is, together with a 2D Wide-Angle Beam Propagation method solving the optical propagation. The coupling between electrical, thermal and optical equations yields a stable solution which incorporates carrier and temperature induced perturbations of the refractive index. Although tapered lasers have already demonstrated superior beam quality performance in comparison with broad area devices, they still suffer of beam filamentation at high power levels. We analyze the influence of the different competing factors in the self-focusing process for 980 nm lasers with a gain guided taper section. The simulation results indicate that the lasers with the longest taper section provide the highest output power before the filamentation process is triggered, and that the backward propagating field plays a crucial role in the stability of the output beam.
TL;DR: In this paper, Kukushkin and Rantsev-Kartinov pointed out the existence of specific plasma structures of the squirrel-cage type and interpreted them as “wild cables of solid-state nanotubes.
Abstract: Possible configurations of current filaments in Z-pinch and tokamak plasmas are analyzed. A thin current-carrying beam injected in a plasma should be surrounded by a halo of countercurrents, in which case the resulting configuration may resemble a tubular structure. A.B. Kukushkin and V.A. Rantsev-Kartinov pointed out the existence of specific plasma structures of the squirrel-cage type and interpreted them as “wild cables of solid-state nanotubes.” It is shown that these structures can also be attributed to the fundamental mode of the conventional magnetic filamentation in the form of a “hexagonal parquet.” Also, a study is made of the phenomena governing the pattern of plasma structures, namely, tearing filamentation, two types of longitudinal beam bunching, and self-organization of the filaments.
TL;DR: In this article, the steady-state transverse light fields emitted from ring-shaped VCSELs were explored theoretically and experimentally, and the switching on of patterns of increasing spatial frequencies as a function of pump parameter was identified.
Abstract: We have explored theoretically and experimentally the steady-state transverse light fields emitted from ring-shaped lasers, specifically, those from vertical-cavity surface-emitting lasers (VCSELs). We saw the switching on of patterns of increasing spatial frequencies as a function of pump parameter. Furthermore, we were able to identify the mechanism for such an evolution as geometrical modulational instability within the nonlinear cavity. Other mechanisms such as the conventional gain–loss balance had no effect on the ring configuration that was modeled. The experiments with annular VCSELs gave results that matched the theoretical predictions well, although other mechanisms not considered in our model, such as carrier diffusion, took place in the experimental devices. We conclude that the nonlinear mechanisms presented here can be regarded as limiting cases in the interpretation of more-complex functions, such as patterns, modes, and filamentation switching, in VCSELs.
TL;DR: In this article, a collection of point-like intense vortices arranged symmetrically outside of a uniform circular vortex patch, both enclosed in a free-slip circular boundary, are numerically time evolved for up to 10-15 patch turnover times.
Abstract: Collection of point-like intense vortices arranged symmetrically outside of a uniform circular vortex patch, both enclosed in a free-slip circular boundary, are numerically time evolved for up to 10–15 patch turnover times. These patterns are found to merge with the patch by successively inducing nonlinear dispersive modes (V-states) on the surface of the patch, draw off fingers of vorticity (filamentation), trap the irrotational regions as the fingers symmetrize under the shear flow of the patch and point-like vortices (wave breaking) followed by the vortex–hole capture. While the hole patterns are observed to break up over several turnover periods the vortex patterns appear to evolve into quasistationary patterns for some cases of an initial number of point-like vortices Npv. The bounded V-states, filamentation, and vortex (hole) pattern formation are discussed in some detail and their possible connection to recently observed vortex “crystals” is pointed out.
TL;DR: In this article, the initial stage of the contraction of surface-wave-sustained discharges in a filamentary structure is analyzed, and it is concluded that the discharge contraction is related to a mechanism of ionization-field instability, which is developed under the condition of an increased gas temperature.
TL;DR: In this article, a filamentation instability is shown to be possible in recombination-controlled high-frequency discharges at elevated gas pressures, and a necessary condition for the instability is the increase of the electron-neutral collision frequency with the electron temperature.
Abstract: A filamentation instability is proven to be possible in recombination-controlled high-frequency discharges at elevated gas pressures. A necessary condition for the instability is the increase of the electron-neutral collision frequency with the electron temperature.
10 Dec 2002
TL;DR: In this paper, a tensile strained GaAsP QW with AlGaAs confinement and cladding regions was used to reduce the fast axis divergence and decrease the tendency to filamentation.
Abstract: The 732 nm laser structure consists of a tensile strained GaAsP QW with AlGaAs confinement and cladding regions. The 975 nm structure comprises a strained InGaAs QW embedded in an Al-free optical cavity Both designs employ a large optical cavity to reduce the fast axis divergence and to decrease the tendency to filamentation, with similar values for the vertical confinement factor.
TL;DR: In this article, self-focusing of the intense laser pulses as well as production of the well-collimated electron beams in the moderately underdense plasma of ne/nc 10% were investigated at the maximum laser intensity of 7 × 10 17 W/cm 2.
Abstract: Self-focusing of the intense laser pulses as well as production of the well-collimated electron beams in the moderately underdense plasma of ne/nc 10% were investigated at the maximum laser intensity of 7 × 10 17 W/cm 2 Progresses of a relativistic self-focusing in the plasma of a gaussian density profile were observed by an ultra-fast optical probing Even if the large gas-target of the quasi-uniform density was used, the self-focusing was terminated after traveling approximately 12 mm by the abrupt transition of the propagation mode from the smooth channeling to the complex filamentation, which correlated with the emission of MeV-electron beams High-energy electron beams have the power-law spectrum of E −35 reaching the maximum energy of 2 MeV The maximum number of the electrons was(13±06)×10 11 /sr/shot Model calculations suggest that the most plausible mechanism of the production of high-energy electron beam in the moderately underdense plasma is that the phase of the electron oscillation in filaments of the laser beam randomly changes many times
TL;DR: In this article, the frequency spectra and damping rate of surface waves propagating along a plasma jet are obtained in both of high and low frequency region, and the penetration of the static field in the plasma jet and filamentation of a subsonic plasma jet is studied.
TL;DR: In this paper, a self-organization process was proposed to explain the current filamentation observed in DC gas discharges, which is the result of a nearly spherical double layer known as anode spot.
Abstract: Current filamentation observed in DC gas discharges is explained considering a self-organization process, the result of which is a nearly spherical double layer known as anode spot. Accelerating electrons at energies sufficient to produce ionizations, the double layer acts as an internal source of charged particles. The striated column, working as current filament, appears after the double layer "multiplication." The balance between the repulsive electric forces and the attractive magnetic forces acting between striated columns explains the ordered distribution of the current filaments at the anode surface.