Showing papers on "Filamentation published in 2003"
TL;DR: In this article, it was shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of light field on the whole transverse scale of the beam, including its edges.
Abstract: We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp.
246 citations
06 Jun 2003
TL;DR: In this article, the backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied and it shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE).
Abstract: The backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied It shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE)
230 citations
TL;DR: In this paper, the competition between femtosecond laser pulse induced optical breakdown and filamentation in condensed matter is studied both experimentally and numerically using water as an example, and the coexistence of filamentation and breakdown is observed under tight focusing conditions.
Abstract: The competition between femtosecond laser pulse induced optical breakdown and femtosecond laser pulse filamentation in condensed matter is studied both experimentally and numerically using water as an example. The coexistence of filamentation and breakdown is observed under tight focusing conditions. The development of the filamentation process from the creation of a single filament to the formation of many filaments at higher pulse energy is characterized systematically. In addition, strong deflection and modulation of the supercontinuum is observed. They manifest themselves at the beginning of the filamentation process, near the highly disordered plasma created by optical breakdown at the geometrical focus.
178 citations
TL;DR: The fabrication of 2-mm directional couplers to split the coupled beam into 1:1 at a wavelength of 632.8 nm is demonstrated and the realization of three-dimensional directional coupler and wavelength division in the output from the coupler is also demonstrated.
Abstract: Refractive-index changes can be induced by filamentation of 800-nm, 1-kHz femtosecond laser pulses in silica glass. Two-dimensional translation of a 40-microm-long filament leads to the formation of a curved waveguide because of bending by the previously induced refractive-index change. The fabrication of 2-mm directional couplers to split the coupled beam into 1:1 at a wavelength of 632.8 nm is demonstrated. The realization of three-dimensional directional couplers and wavelength division in the output from the couplers is also demonstrated.
156 citations
TL;DR: The experiments show that input beam ellipticity can dominate the effect of noise (transverse modulational instability), giving rise to predictable and highly reproducible MF patterns.
Abstract: The standard explanation for multiple filamentation (MF) of intense laser beams has been that it is initiated by input beam noise (modulational instability). In this study we provide the first experimental evidence that MF can also be induced by input beam ellipticity. Unlike noise-induced beam breakup, the MF pattern induced by ellipticity is reproducible shot to shot. Moreover, our experiments show that ellipticity can dominate the effect of noise, thus providing the first experimental methodology for controlling the MF pattern of noisy beams. The results are explained using a theoretical model and simulations.
130 citations
TL;DR: It is shown that energetic electron beams generated from ultrahigh intensity laser interactions with dense plasmas are unstable to filamentation and "hosing" instabilities in the regime where the beam density approaches the density of the background plasma.
Abstract: Measurements of energetic electron beams generated from ultrahigh intensity laser interactions (I>1019 W/cm2) with dense plasmas are discussed. These interactions have been shown to produce very directional beams, although with a broad energy spectrum. In the regime where the beam density approaches the density of the background plasma, we show that these beams are unstable to filamentation and 'hosing' instabilities. Particle-in-cell simulations also indicate the development of such instabilities. This is a regime of particular interest for inertial confinement fusion applications of these beams (i.e., "fast ignition").
129 citations
TL;DR: The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.
Abstract: The competition between optical breakdown (OB) and laser-pulse filamentation (FL) in bulk fused silica is investigated by using a 1-kHz femtosecond infrared laser. We measure input powers corresponding to the threshold of OB and FL in terms of external focusing conditions. The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.
120 citations
TL;DR: In this paper, the authors compared near field and beam quality measurements on narrow stripe quantum-dot and quantum-well (QW) laser with identical structure, both emitting at 1100 nm.
Abstract: Comparative near-field and beam-quality (M2) measurements on narrow stripe quantum-dot (QD) and quantum-well (QW) lasers of identical structure, both emitting at 1100 nm, are presented. Intrinsic suppression of filamentation in the QD lasers is observed. QD lasers emitting at 1300 nm again show no filamentation. For a 6-μm-stripe, QW laser, M2 increases from 2.6 to 6.1 with output power increasing from 5 to 60 mW and with increasing stripe width (20 mW, 3→10 μm, M2=2.6→4.7). In the QD lasers, filamentation is suppressed up to 8 μm (1100 nm) and 9 μm (1300 nm) stripe width and no dependence on output power is observed.
108 citations
TL;DR: Self-compression of weakly relativistically intense laser pulses in subcritical plasmas using one- and three-dimensional direct particle-in-cell (PIC) simulations to develop an analytical model and results agree well with the model.
Abstract: We study self-compression of weakly relativistically intense laser pulses in subcritical plasmas using one- (1D) and three-dimensional (3D) direct particle-in-cell (PIC) simulations. The self-compression works in the density window from $1/4$ critical to slightly below critical density, where the Raman instability is prohibited. An analytical model is developed to describe the self-compression. The model admits pulsing Gaussian solutions and a long-lived running soliton solution. The 1D PIC results agree well with the analytical model, and compressions by an order of magnitude are observed. In the 3D geometry, the longitudinal self-compression competes with the transverse self-focusing/filamentation. To damp the filamentation we use a periodic plasma-vacuum structure. The 3D PIC simulations suggest that a 30 fs long laser pulse is efficiently compressed to 5 fs.
96 citations
TL;DR: In this article, a combination of simulation and experiment is used to study the nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers using the wide-angle finite-difference beam propagation method for the analysis of optical propagation within the cavity.
Abstract: The nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers are studied using a combination of simulation and experiment. Optical pumping, electrical overpumping, filamentation, and spatial hole burning are shown to be the key nonlinear phenomena influencing the operation of tapered lasers at high output powers. In the particular tapered laser studied, the optical pumping effect is found to have the largest impact on the output beam quality. The simulation model used in this study employs the wide-angle finite-difference beam propagation method for the analysis of the optical propagation within the cavity. Quasi-three-dimensional (3-D) thermal and electrical models are used for the calculation of the 3-D distributions of the temperature, electrons, holes, and electrical potential. The simulation results reproduce key features and the experimental trends.
73 citations
TL;DR: Candida glabrata STE12 is the first virulence factor encoding gene to be described in this increasingly important fungal pathogen and can functionally complement the nitrogen starvation induced filamentation and mating defects of Saccharomyces cerevisiae ste12 mutants.
Abstract: The highly conserved fungal Ste12 transcription factor family of proteins play critical roles in the regulation of many cellular processes including mating, cell wall biosynthesis, filamentation and invasive growth. They are also important mediators of fungal virulence. The Candida glabrata STE12 homologue was cloned. The encoded protein has a single DNA binding homeodomain but lacks both a C2H2 zinc finger DNA binding domain and an apparent Dig1/Dig2 regulatory motif. Candida glabrata STE12 can functionally complement the nitrogen starvation induced filamentation and mating defects of Saccharomyces cerevisiae ste12 mutants. We also show that C. glabrata STE12 is required for nitrogen starvation-induced filamentation as ste12 mutants rarely produce pseudohyphae on nitrogen depleted media. Finally we describe a novel murine model of C. glabrata systemic disease and use this to demonstrate that C. glabrata ste12 mutants, although still able to cause disease, are attenuated for virulence compared with STE12 reconstituted strains. Candida glabrata STE12 is therefore the first virulence factor encoding gene to be described in this increasingly important fungal pathogen.
TL;DR: In this article, the authors investigated the filamentation of optical beams in focusing Kerr media and proposed analytical criteria for the mutual coalescence of filaments in the context of collapsing and saturating nonlinearities.
TL;DR: In this article, a change in the spectrum and the energy distribution of the light field of a high-power femtosecond laser pulse is studied numerically in a broad range of its spatial scales.
Abstract: A change in the spectrum and the energy distribution of the light field of a high-power femtosecond laser pulse is studied numerically in a broad range of its spatial scales. It is shown that the effect of filamentation, the generation of a supercontinuum and conical emission, the formation of a ring structure in the distribution of the pulse energy and other effects observed during the propagation of the laser pulse in air are caused by the nonlinear-optical transformation of the light field in the region with dimensions exceeding substantially the transverse size of the filament. The pulse filamentation is accompanied by the redistribution of power in its cross section. The spatio — temporal characteristics of emission calculated for femtosecond laser systems are in quantitative agreement with the experimental data.
TL;DR: In this article, it was shown that the initial phase modulation of the pulse results in a shift of the beginning of the filament from the output aperture of the laser system and in a significant increase in the filament length.
Abstract: The possibility of controlling the processes of filamentation and generation of a supercontinuum during propagation of a high-power femtosecond laser pulse in air is investigated. Using a numerical simulation, it is shown that the initial phase modulation of the pulse results in a shift of the beginning of the filament from the output aperture of the laser system and in a significant increase in the filament length. The efficiency of the generation of the short-wavelength part of the supercontinuum in a pulse with a negative phase modulation is more than two orders of magnitude higher than for a pulse with a limited spectrum.
TL;DR: In this article, the authors discuss the possibility of Weibel-type color-current filamentation in high-energy nuclear collisions and show that filamentation helps to speed up the equilibration of the parton plasma and it may lead to nonstatistical azimuthal patterns in the hadron final state.
Abstract: Employing a previously derived formulation, and extending the treatment from purely transverse modes to wave vectors having a longitudinal component, we discuss the prospects for the occurrence of Weibel-type color-current filamentation in high-energy nuclear collisions. Numerical solutions of the dispersion equation for a number of scenarios relevant to RHIC and LHC suggest that modes with (predominantly transverse) wave numbers of several hundred MeV may become moderately agitated during the early collision stage. The emergence of filamentation helps to speed up the equilibration of the parton plasma and it may lead to nonstatistical azimuthal patterns in the hadron final state.
TL;DR: A significant reduction of the spatial filamentation and suppression of the spatiotemporal instabilities are shown, which are both intrinsic emission properties of standard high-power broad-area lasers.
Abstract: We demonstrate the control of the spatiotemporal emission dynamics of a broad-area semiconductor laser in an external optical feedback configuration formed by a spatially filtering mirror. The emission properties are studied with a single-shot streak camera with temporal resolution of ~7 ps and spatial resolution of ~5 μm . Our results show a significant reduction of the spatial filamentation and, furthermore, suppression of the spatiotemporal instabilities, which are both intrinsic emission properties of standard high-power broad-area lasers. Associated with the control of the emission dynamics, strong improvement of the beam quality, which is essential for numerous high-power applications, is possible.
TL;DR: In this paper, the authors investigated the effect of third-harmonic generation on the spectrum of femtosecond IR laser pulses focused in air and showed that a two-color filamentation effect occurs, in which the thirdharmonic conversion efficiency remains almost constant over the whole filament length.
Abstract: The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV.
TL;DR: In this article, an interpretation for the dynamics of light filaments formed when a femtosecond laser pulse propagates in air is proposed, where the pulse evolves as a set of coupled nonlinear oscillators forming a coherent structure over several Rayleigh lengths.
Abstract: An interpretation is proposed for the dynamics of light filaments formed when a femtosecond laser pulse propagates in air. The pulse evolves as a set of coupled nonlinear oscillators forming a coherent structure over several Rayleigh lengths. The plasma generated by photoionization is numerically shown to be the single saturation mechanism of the beam self-focusing. Other physical processes such as group velocity dispersion and the quintic susceptibility ${\ensuremath{\chi}}^{(5)}$ for the polarization promote different propagation regimes. From theoretical expressions of the oscillation period for the spatial profile of the pulse, we show that the electron density in a femtosecond filament may be estimated.
TL;DR: In this article, a model for the propagation of femtosecond laser pulses in transparent media and the formation of light bullets is proposed, which enables us to generalize Marburger's formula for the position of the nonlinear focus of the beam in the presence of delayed Kerr effect.
Abstract: A model for the propagation of femtosecond laser pulses in transparent media and the formation of light bullets is proposed. This model enable us to generalize Marburger’s formula for the position of the nonlinear focus of the beam in the presence of delayed Kerr effect. It is shown that an instantaneous higher-order saturation tuned to mimic the defocusing effect due to the plasma generated by multiphoton ionization does not properly represent the dynamics of femtosecond filamentation as it violates causality and artificially promotes long distance propagation in the form of periodic oscillations around a spatial soliton. A causal description of multiphoton ionization leads to an extended moving focus model from which the formation of a synchronized structure exhibiting several focusing-defocusing cycles before eventual diffraction can be inferred.
TL;DR: In this paper, the filamentation of a focused femtosecond pulse in fused silica was systemically investigated both experimentally and numerically, and it was found that the filamentations are connected with different temporal reshaping behaviors, and the temporal compression of the reshaped laser pulse in propagation is responsible for the two-foci structure of an electron plasma filament.
Abstract: The filamentation of a focused femtosecond pulse in fused silica was systemically investigated both experimentally and numerically. We observed the different filamentation of a laser pulse under different external focusing conditions and input energy experimentally. For instance, a focused femtosecond laser pulse with an energy of 3.3 \ensuremath{\mu}J induced a long electron plasma filament for an effective numerical aperture (NA) of 0.01 of focusing objective, an electron plasma filament with two-foci structure for 0.04 effective NA, and a short filament for effective NA of 0.09. In theory, the numerical results of the extended nonlinear Schr\"odinger (NLS) equation agreed with experimental results under different focusing conditions and energy. Moreover, under different focusing conditions we found the filamentation is connected with different temporal reshaping behaviors, and the temporal compression of the reshaped laser pulse in propagation is responsible for the two-foci structure of an electron plasma filament.
TL;DR: In this paper, the authors derived analytical estimates for the length of an infrared light filament as a function of the pulse duration and beam energy for a fixed energy per pulse, and obtained a maximum filamentation length for a specific pulse duration.
Abstract: From a theoretical model for the propagation of high-power laser pulses in air over long distances, we derive analytical estimates for the length of an infrared light filament as a function of the pulse duration and beam energy. For a fixed energy per pulse, a maximum filamentation length is shown to be obtained for a specific pulse duration. These estimates are in agreement with the results of numerical simulations and measurements available in the literature.
TL;DR: Observation of cell morphology revealed that the coexpression of the ftsA and ftsZ genes successfully suppressed filamentation caused by the accumulation of recombinant proteins.
Abstract: During growth of high-cell-density cultures of Escherichia coli, overproduction of recombinant proteins often results in increased stress response, cell filamentation, and growth cessation. Filamentation of cells consequently lowers final achievable cell concentration and productivity of the target protein. Reported here is a methodology that should prove useful for the enhancement of cell growth and protein productivity by the suppression of cell filamentation. By the coexpression of the E. coli ftsA and ftsZ genes, which encode key proteins in cell division, growth of recombinant strains as well as production of human leptin and human insulin-like growth factor I was improved. Observation of cell morphology revealed that the coexpression of the ftsA and ftsZ genes successfully suppressed filamentation caused by the accumulation of recombinant proteins.
TL;DR: Investigation of the filamentation of commensal Escherichia coli, E. coli O157:H7 and Salmonella spp was investigated, following exposure to certain, mildly stressing, levels of temperature, pH or water activity, indicating that cellular filamentation could be demonstrated in all 15 strains of the above organisms.
Abstract: Certain rod-shaped bacteria have been reported to form elongated filamentous cells when exposed to marginal growth conditions, including refrigeration temperatures. To expand upon these observations, the filamentation of commensal Escherichia coli, E. coli O157:H7 and Salmonella spp was investigated, following exposure to certain, mildly stressing, levels of temperature, pH or water activity (aw), with levels of cellular protein being monitored during cell elongation, in some experiments. Our studies indicated that cellular filamentation could be demonstrated in all 15 strains of the above organisms tested, following exposure to marginal conditions achieved by incubation at high or low temperatures, high or low pH values and low aw. The level of environmental stress causing filamentation tended to be specific to the particular organisms. For example, Salmonella spp formed filamentous cells at 44 degrees C, whereas E. coli strains, including O157, grew by binary fission at that temperature, but formed filamentous cells at 46 degrees C. In addition, plate count techniques to enumerate bacteria during filamentation, failed to reflect the increase in cell biomass that was occurring, whereas measurements of protein concentration demonstrated the increase quite strikingly. These findings have important implications for our understanding of the ability of food-borne pathogens to cause disease, since the infectious dose of a microorganism implicated in an outbreak of such disease is typically determined by a viable count method, which could underestimate the number of potential infectious units present in a food that had been stored in such a way as to provide marginal growth conditions.
TL;DR: In this article, the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique is measured using backscattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament.
Abstract: We demonstrate a new method for measuring the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique. The LIDAR involves a detector with a fast response time. The back-scattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament is measured, from which the length of the filament can be determined. We find good agreement between the measured filament length and the length estimated from burn patterns on paper. In addition, good qualitative agreement between the experimental measurement and numerical simulations is obtained for the signal features of the filament. We propose that this new method can be used to quantitatively determine filamentation at longer distances.
TL;DR: In this paper, the evolution of super-broadened spectrum generation by intense 50-fs pulses propagating in bulk fused silica was investigated and a mechanism of supercontinuum generation was proposed based on good agreement between the experimental results and numerical simulations.
Abstract: We report experimental and theoretical investigations of the evolution of super-broadened spectrum generation by intense 50-fs pulses propagating in bulk fused silica. Based on good agreement between the experimental results and numerical simulations, a mechanism of supercontinuum generation (SCG) is proposed. At first, both self-phase modulation (SPM) and stimulated Raman scattering (SRS) contribute substantially but slowly to the broadening before filament formation takes place. After filamentation, a plasma grows rapidly and asymmetric spectral broadening results in a blue-shifted spectrum extending to about 400 nm. A time-resolved experiment of the SCG was also performed using a double-pump technique. The temporal behavior suggests that the vibrational mode excited by the stimulated Raman process by the first pulse contributes to the occurrence of self-focusing.
TL;DR: Fully time-dependent, three-dimensional, nonlinear propagation equations describing the Raman interaction, optical Kerr nonlinearity due to bound electrons, and group velocity dispersion of short laser pulses and pulse trains are presented and discussed.
Abstract: Stimulated rotational Raman scattering (SRRS) is known to be one of the processes limiting the propagation of high-power laser beams in the atmosphere. In this paper, SRRS, Kerr nonlinearity effects, and group velocity dispersion of short laser pulses and pulse trains are analyzed and simulated. Fully time-dependent, three-dimensional, nonlinear propagation equations describing the Raman interaction, optical Kerr nonlinearity due to bound electrons, and group velocity dispersion are presented and discussed. The effective time-dependent nonlinear refractive index containing both Kerr and Raman processes is derived. Linear stability analysis is used to obtain growth rates and phase matching conditions for the SRRS, modulational, and filamentation instabilities. Numerical solutions of the propagation equations in three dimensions show the detailed evolution of the Raman scattering instability for various pulse formats. The dependence of the growth rate of SRRS on pulse duration is examined and under certain conditions it is shown that short (approximately psec) laser pulses are stable to the SRRS instability. The interaction of pulses in a train through the Raman polarization field is also illustrated.
TL;DR: In this article, a criterion for filamentation instability of a circularly polarized Alfven wave train propagating along a strong ambient field in a collisionless plasma is obtained from a transverse modulational analysis of a generalized kinetic derivative nonlinear Schrodinger equation.
Abstract: A criterion for filamentation instability of a circularly polarized Alfven wave train propagating along a strong ambient field in a collisionless plasma is obtained from a transverse modulational analysis of a generalized kinetic derivative nonlinear Schrodinger equation. This equation, which retains Landau damping, results from the Vlasov–Maxwell equations via a long-wave reductive perturbative expansion. For bi-Maxwellian equilibrium distribution functions, it is shown that a moderate anisotropy of the electron temperature and/or a moderate ratio of the electron to ion temperatures can significantly broaden the existence range of this instability, making its occurrence possible in small-β plasmas.
TL;DR: The space-time transformation of a 150-fs pulse undergoing self-focusing and filamentation in water is monitored by means of the nonlinear gating technique and it is observed that pulse splitting and subsequent recombination apply to axial temporal intensity only, whereas the space-integrated pulse profile preserves its original shape.
Abstract: We have monitored the space-time transformation of 150-fs pulse, undergoing self-focusing and filamentation in water, by means of the nonlinear gating tech- nique. We have observed that pulse splitting and subsequent recombination apply to axial temporal intensity only, whereas space-integrated pulse profile preserves its original shape.
Journal Article•
TL;DR: In this paper, the authors focus 42 femtosecond laser pulses in air using three different focal length lenses: f=100, 30 and 5 cm, and show that when the shorter focal length lens is used, a high density plasma is generated near the geometrical focus and coexists with a weak plasma channel of the filament.
Abstract: In this work we present experiments by focusing 42 femtosecond laser pulses in air using three different focal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is a weak plasma column, is observed. When the shorter focal length lens is used, a high density plasma is generated near the geometrical focus and coexists with a weak plasma channel of the filament. Under the tightest focusing condition, filamentation is prevented and only a strong plasma volume appears at the geometrical focus.