Propagation Of Ultrashort Laser Pulses Research Articles

Self-action of a high-power 10-μm laser radiation in gases: control of the pulse duration and generation of hot electrons

The propagation of ultrashort 10-μm laser pulses of power exceeding the critical self-focusing power in xenon and air is numerically simulated. It is shown that the pulse duration in certain regimes in xenon can be decreased by 3–4 times simultaneously with the increase in the pulse power by 2–3 times. It is found that the average energy of electrons in a filament upon filamentation of 10-μm laser pulses in air can exceed 200 eV. The features of the third harmonic and terahertz radiation generation upon filamentation are discussed.

High-efficiency second-harmonic generation of superintense ultrashort laser pulses

The effect of instantaneous cubic nonlinearity on second-harmonic generation of femtosecond laser pulses is investigated. The possibility of improving generation efficiency and shortening pulse duration is analyzed. Energy conversion efficiency of 60% was obtained in an experiment in a 1 mm thickness KDP crystal at the peak intensity of 0.6 TW/cm(2) and the duration of 60 fs.

Propagation of femtosecond laser pulses through water in the linear absorption regime

We investigate the controversy regarding violations of the Bouguer-Lambert-Beer (BLB) law for ultrashort laser pulses propagating through water. By working at sufficiently low incident laser intensities, we make sure that any nonlinear component in the response of the medium is negligible. We measure the transmitted power and spectrum as functions of water cell length in an effort to confirm or disprove alleged deviations from the BLB law. We perform experiments at two different laser pulse repetition rates and explore the dependence of transmission on pulse duration. Specifically, we vary the laser pulse duration either by cutting its spectrum while keeping the pulse shape near transform-limited or by adjusting the pulses chirp while keeping the spectral intensities fixed. Over a wide range of parameters, we find no deviations from the BLB law and conclude that recent claims of BLB law violations are inconsistent with our experimental data. We present a simple linear theory (based on the BLB law) for propagation of ultrashort laser pulses through an absorbing medium and find our experimental results to be in excellent agreement with this theory.

Two-photon area evolution and optical limiting of ultrashort laser pulses in DBASVP molecule media

Taking the one-dimensional asymmetrical π conjugation molecule material （DBASVP molecules） as medium, with the two-photon resonance, we studied the propagation of ultrashort laser pulses in the medium based on the two-photon area theorem and the numerical solutions of the coupled Bloch and Maxwell's equations. The evolution of two-photon area is thus investigated and the applicability of the two-photon area is then discussed. A theoretical method for simulating the optical limiting properties of molecular medium is pointed out. The molecular electronic structures and dipole moments are calculated by use of density functional theory at ab initio level. The numerical results show that the two-photon area theorem based on the slowly varying envelope and phase approximations cannot accurately describe the evolvution of the two-photon area in the molecular medium. The optical limiting behavior based on the two-photon absorption is related to the width of the molecular medium.

Effect of transverse ponderomotive nonlinearity on the propagation of ultrashort laser pulses in a plasma channel

The present study deals with the propagation of an ultrashort narrow laser beam in a parabolic plasma channel. The effect of transverse ponderomotive nonlinearity on the propagation characteristics of the laser beam is analyzed. Using the variational technique, coupled equations describing the evolution of pulse length and spot size are obtained. The variation of intensity, of mismatched and matched laser pulses, with propagation distance is graphically depicted.

Few-cycle laser-pulse collapse in Kerr media: The role of group-velocity dispersion andX-wave formation

We study ultrashort laser-pulse propagation and filamentation dynamics in dispersive Kerr media. We identify the regime for which the filamentation threshold $({P}_{\mathrm{th}})$ is considerably higher than the critical power $({P}_{\mathrm{cr}})$ for monochromatic beam collapse in pure Kerr media. In particular, we compare the threshold for the formation of filaments with that for the formation of $\mathsf{X}$-waves. At powers ${P}_{\mathrm{cr}}<P<{P}_{\mathrm{th}}$, self-focusing is arrested by group-velocity dispersion, and no filaments or $\mathsf{X}$-waves are formed. At $P\ensuremath{\geqslant}{P}_{\mathrm{th}}$, we observe $\mathsf{X}$-wave formation and a weak filamentation regime. At $P⪢{P}_{\mathrm{th}}$, we observe both $\mathsf{X}$-waves and fully formed filaments.

Nonlinear Propagation of Ultrashort Laser Pulses in Water and Self-Focusing Effect

The propagation of ultrashort laser pulses in water has been investigated with the extended nonlinear Schrodinger equation and the electron density equation. With the model that incorporates the diffraction, normal group-velocity dispersion, multiphoton absorption, self-focusing and defocus of light by the generated free electrons, we examined numerically the spatio-temporal development of the on-axis light intensity along the propagation and the laser-induced electron plasma. Simulation of the underlying dynamics of the ultrashort laser propagation in water reveals that the self-focusing interplays competitively with the laser-induced optical breakdown. Moreover, the laser energy, its duration and focusing geometry determine in combination the structure of plasma filament and the reshaping of energy fluence. The results may provide a better understanding and application of the ultrashort laser pulses in medicine.

Diffraction of femtosecond pulses; nonparaxial regime

We present a systematic study of linear propagation of ultrashort laser pulses in media with dispersion, dispersionless media, and vacuum. The applied method of amplitude envelopes makes it possible to estimate the limits of the slowly varying amplitude approximation and to describe an amplitude integrodifferential equation governing propagation of optical pulses in the single-cycle regime in solids. The well-known slowly varying amplitude equation and the amplitude equation for the vacuum case are written in dimensionless form. Three parameters are obtained defining different linear regimes of optical pulse evolution. In contrast to previous studies we demonstrate that in the femtosecond region the nonparaxial terms are not small and can dominate over the transverse Laplacian. The normalized amplitude nonparaxial equations are solved using the method of Fourier transforms. Fundamental solutions with spectral kernels different from those according to Fresnel are found. Exact unidirectional analytical solution of the nonparaxial amplitude equations and the 3D wave equations with initial conditions compatible with Gaussian light bullets are obtained also. One unexpected new result is the relative stability of light bullets (pulses with spherical and spheroidal spatial form) when we compare their transverse enlargement with paraxial diffraction of light beams in air. It is important to emphasize here the case of light disks, i.e., pulses whose longitudinal size is small with respect to the transverse one, which in some partial cases are practically diffractionless over distances of a thousand kilometers. A new formula that calculates the diffraction length of optical pulses is suggested. Finally, propagation of single-cycle pulses in air and vacuum was investigated, and a coronal (semispherical) form of diffraction at short distances was observed.

Magnetic-Dipole Vortex Generation by Propagation of Ultraintense and Ultrashort Laser Pulses in Moderate-Density Plasmas

A magnetic-dipole vortex is generated in the behind of an ultraintense and ultrashort laser pulse in a near critical density plasma. The vortex is self-sustained by its magnetic field pressure which expels background electrons, and resulting sheath field accelerates electrons to drive high amplitude electric current inside the vortex. The electron energy spectra shows nonthermal distribution with relatively high energy. The vortex is stable for a long period since it is in the electromagnetic equilibrium, whose structure and characteristics are explained by a simple analytical model.

Propagation Technique for Ultrashort Pulses I: Propagation Equation for Ultrashort Pulses in a Kerr Medium

In this work we investigated propagation of ultrashort laser pulses in dispersive nonlinear media. We derived a general propagation equation of pulses which includes the linear and nonlinear effects to all orders. We studied in the specific case of Kerr media and obtained an ultrashort pulse propagation equation called a Generalized Nonlinear Schrodinger Equation. The impact of the third order dispersion, the higher-order nonlinear terms self-steepening, and stimulated Raman scattering are explicitly analyzed.

Carrier-Envelope Phase Dependence of Few-Cycle Ultrashort Laser Pulse Propagation in a Polar Molecule Medium

We theoretically investigate carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium. Our results show that a soliton pulse can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse duration and intensity, depend crucially on the carrier-envelope phase of the incident pulse, which could be utilized to determine the carrier-envelope phase of a few-cycle ultrashort laser pulse from a mode-locked oscillator.

Femtosecond filament and supercontinuum generated in cobalt phthalocyanine chloroform solution

Filamentation and supercontinuum generation in cobalt phthalocyanine chloroform solution by 800 nm 50-fs Ti: sapphire laser pulse is investigated for the first time to the best of our knowledge. It is found that filamentation is triggered by self-focusing and defocusing. The main mechanism of supercontinuum and conical emission generation is self-phase modulation (SPM) enhanced by self-steepening of the pulse and group velocity dispersion (GVD). This investigation of organic dye solutions by fs laser is a good complementarity to study the propagation of powerful ultrashort laser pulses in liquid media.

Supercontinuum generation of ultrashort laser pulses in air at different central wavelengths

Supercontinuum generation by femtosecond filaments in air is investigated for different laser wavelengths ranging from ultraviolet to infrared. Particular attention is paid on the role of third-harmonic generation and temporal steepening effects, which enlarge the blue part of the spectrum. A unidirectional pulse propagation model and nonlinear evolution equations are numerically integrated and their results are compared. Apart from the choice of the central wavelength, we emphasize the importance of the clamped intensity reached by self-guided pulses, together with their temporal duration and propagation length as key players acting on both supercontinuum generation of the pump wave and emergence of the third harmonic. Maximal broadening is observed for large wavelengths and long filamentation ranges.

Modelling of the energy density deposition profiles of ultrashort laser pulses focused in optical media

The propagation of ultrashort laser pulses in dense optical media is investigated theoretically by solving numerically the nonlinear Schrödinger equation. It is shown that the maximum energy density deposition as a function of the pulse energy presents a well-defined threshold that increases with the pulse duration. As a consequence of plasma defocusing, the maximum energy density deposition is generally smaller and the size of the energy deposition zone is generally larger for shorter pulses. Nevertheless, significant values of the energy density deposition can be obtained near threshold, i.e., at lower energy than for longer pulses.

Propagation of ultrashort laser pulses through water

In this paper, propagation of ultrashort pulses through a long 3.5 meter water channel was studied. Of particular interest was the attenuation of the beam at various lengths along the variable path length and to find an explanation of why the attenuation deviates from typical Beer Lambert law around 3 meters for ultrashort laser pulse transmission. Laser pulses of 10 fs at 75 MHz, 100 fs at 80 MHz and 300 fs at 1 KHz were employed to investigate the effects of pulse duration, spectrum and repetition rate on the attenuation after propagating through water up to 3 meters. Stretched pulse attenuation measurements produced from 10 fs at a frequency of 75 MHz were compared with the 10 fs attenuation measurements. Results indicate that the broad spectrum of the ultrashort pulse is the dominant reason for the observed decrease in attenuation after 3 meters of travel in a long water channel. The repetition rate is found not to play a significant role at least for the long pulse scenario in this reported attenuation studies.

Waves in Nonlinear Lattices: Ultrashort Optical Pulses and Bose-Einstein Condensates

The nonlinear Schrödinger equation i (partial differential)(z)A(z,x,t)+(inverted Delta)(2)(x,t)A+[1+m(kappax)]|A|2A=0 models the propagation of ultrashort laser pulses in a planar waveguide for which the Kerr nonlinearity varies along the transverse coordinate x, and also the evolution of 2D Bose-Einstein condensates in which the scattering length varies in one dimension. Stability of bound states depends on the value of kappa=beamwidth/lattice period. Wide (kappa>>1) and kappa=O(1) bound states centered at a maximum of m(x) are unstable, as they violate the slope condition. Bound states centered at a minimum of m(x) violate the spectral condition, resulting in a drift instability. Thus, a nonlinear lattice can only stabilize narrow bound states centered at a maximum of m(x). Even in that case, the stability region is so small that these bound states are "mathematically stable" but "physically unstable."

Nonlinear propagation of ultraintense and ultrashort laser pulses in a plasma channel limited by metallic walls

The nonlinear propagation of an ultraintense and ultrashort (UIUS) laser pulse in a metallic capillary is investigated using a classical model which takes into account the inverse bremsstrahlung absorption (IBA) in the formed plasma. The attenuation of the laser pulse due to the IBA in the plasma and to the laser energy dissipation in the metallic walls is shown. The guiding length and the twist of the laser pulse temporal envelope are presented for several values of the parameters of the plasma, the laser pulse and the metal. The numerical treatment shows that the guiding length increases when the pulse duration becomes shorter. This calculus shows also that in the case of moderate electronic densities, ne<1017m−3, the formed plasma has a negligible effect compared to that of the metallic walls.

Experimental determination of radiation scattering and absorption coefficients in a homogeneous layer of highly dispersive biological medium

A method for experimental determination of optical characteristics of a highly-dispersive medium (radiation scattering and absorption coefficients) is described. The method is based on two mathematical models of ultrashort laser pulse propagation through a highly-dispersive medium (HDM), an axial model and a diffusion model. Milk dissolved in water was used as HDM. Dependences of optical characteristics of HDM on the concentration of milk in water are obtained. The limits of applicability of the axial and diffusion models to media with different scattering and absorption characteristics are determined.

Efficient 20TW Ti:sapphire laser with two stage amplifiers

Propagation of ultrashort intense laser pulses in a plasma Bragg grating induced by two intersecting laser pulses is studied. Such a plasma grating exhibits an ultrawide photonic bandgap, near which the strong dispersion appears. Particle-in-cell (PIC) simulations show the light speed reduction, pulse stretching, chirped pulse compression, and fast compression of Bragg grating solitons in the plasma grating. The nonlinear coupled-mode theory agrees well with the PIC results. Since the plasma grating can support an ultrahigh damage threshold, it can be a novel photonic device to manipulate extremely intense femtosecond laser pulses.

Manipulating intense laser pulses by plasma Bragg gratings

Propagation of ultrashort intense laser pulses in a plasma Bragg grating induced by two intersecting laser pulses is studied. Such a plasma grating exhibits an ultrawide photonic bandgap, near which the strong dispersion appears. Particle-in-cell (PIC) simulations show the light speed reduction, pulse stretching, chirped pulse compression, and fast compression of Bragg grating solitons in the plasma grating. The nonlinear coupled-mode theory agrees well with the PIC results. Since the plasma grating can support an ultrahigh damage threshold, it can be a novel photonic device to manipulate extremely intense femtosecond laser pulses.