High-intensity Ultrashort Laser Research Articles

Modeling of clusters by a molecular dynamics model using a fast tree method

The dynamics of clusters irradiated by a high-intensity ultrashort pulse laser has been studied using a fully relativistic three-dimensional Molecular Dynamics Model. A fast three-dimensional tree algorithm for computing the electrostatic force has been developed and compared with the conventional particle-particle method. The particle-particle method requires computation time, which scales as O(Np2), and it is faster for small number of particles Np 103, the preferred method is the tree algorithm whose computation time scales as O(Np log Np). The tree algorithm has been benchmarked against the particle-particle method for clusters composed of xenon and deuterium atoms and its accuracy and computation time have been analyzed. The optimum free parameter of the tree method has been determined to be θ≈0.5. We addressed the effects of boundary conditions by studying the contribution of adjacent clusters to the total electromagnetic force exerted on individual particles. We found that the adjacent clusters play a minor role in the overall cluster dynamics.

Long plasma channels in segmented capillary discharges

Guided propagation of ultrashort (100fs) high intensity (1016Wcm−2) laser pulses over distances up to 12.6cm using optimized segmented capillary is reported. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. The current waveform was optimized to obtain a long lasting, deep radial profile. Radial profiles with the maximal electron density from 4×1017 to 2×1018cm−3 and up to 25% deep were obtained thereby, whereas longitudinal profiles were found to be remarkably uniform. The potential application of these long channels to the laser wake field accelerator is discussed.

Spectroscopic characterization of an ultrashort-pulse-laser-driven Ar cluster target incorporating both Boltzmann and particle-in-cell models

A model that solves simultaneously both the electron and atomic kinetics was used to generate a synthetic He alpha and satellite x-ray spectra to characterize a high intensity ultrashort laser driven Ar cluster target experiment. In particular, level populations were obtained from a detailed collisional-radiative model where collisional rates were computed from a time varying electron distribution function obtained from the solution of the zero-dimensional Boltzmann equation. In addition, a particle-in-cell simulation was used to model the laser interaction with the cluster target and provided the initial electron energy distribution function (EEDF) for the Boltzmann solver. This study suggests that a high density average, <Na>high, of 3.2 x 10(20) cm(-3) was held by the system for a time, delta tau, of 5.7 ps, and during this time the plasma was in a highly nonequilibrium state in both the EEDF and the ion level populations.

Ion acceleration at the front and rear surfaces of thin foils with high-intensity 40-fs laser pulses

Ion acceleration from the front and rear sides of a foil target is observed by measurements of the ions’ spectral and spatial emission characteristics when irradiating the targets with ultrashort (40-fs) high-intensity laser pulses. The experimental results show that the origin of accelerated ions, from both the front and rear surfaces of the target, strongly depend on the laser energy absorption mechanism. In particular, laser pulse parameters such as pulse duration and contrast are crucial and determine the entire acceleration scenario. Thus, the experimental outcome can be controlled by selection of the irradiation conditions.

Stabilized color centers created by high-intensity ultra-short pulse laser in pure YLF crystals

In this work, we show that it is possible to produce stable color centers inside YLiF 4 crystals with dimensional control, by focusing a high-intensity ultra-short laser pulses in the material. In particular, it is possible to determine the center creation intensity threshold and therefore to discuss the basic formation mechanisms of these centers.

Color center production by femtosecond-pulse laser irradiation in fluoride crystals

Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF4 single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm2 was determined in YLF and LiF, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F2, F +2 , and F +3 . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived F +2 centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.

Production of stabilized color centers in YLiF4 crystals by high-intensity ultrashort laser pulses

In this work we show that is possible to produce stable color centers in YLiF4 crystals, with dimensional control, by focusing high-intensity ultrashort laser pulses in the bulk. In particular, with the spectroscopic characterization of ultrashort laser-irradiated YLF samples, it was possible to discuss the basic formation mechanisms of these centers.

Fabrication of spatial transient-density structures as high-field plasma photonic devices

Fabrication of periodic transient-density structures in a gas jet with a boundary scale length approaching 10μm was demonstrated. This was achieved by passing an ultrashort high-intensity laser pulse through a patterned mask and imaging the mask onto the target plane. Gas/plasma density at the laser-irradiated regions drops as a result of hydrodynamic expansion following ionization and heating by the laser pulse. The fabrication of gas/plasma density structures with such a scheme is an essential step in the development of plasma photonic devices for applications in high-field physics.

Temporal self-action and compression of intense ultrashort laser pulses in hollow photonic-crystal waveguides

Hollow-core waveguides with a periodic (photonic-crystal) cladding are shown to allow efficient temporal compression of high-intensity ultrashort laser pulses and formation of megawatt soliton-like features in the regime of robust isolated guided modes. We numerically analyze the temporal envelope evolution and spectral transformation of the light field in air-guided modes of gas-filled hollow coaxial periodic Bragg waveguides. Based on this analysis, we define optimal compression regimes, permitting high compression ratios (of about six) and high compression efficiencies (up to 73%) to be achieved for microjoule laser pulses with an initial pulse length of 80–400 fs.

Enhancement of a 2477-nm line emitted by the plasma of a boron nitride capillary discharge irradiated by a high-intensity ultrashort laser pulse

Investigations of plasma produced by a boron nitride capillary discharge irradiated with a guided 20-TW Ti: sapphire laser pulse at a peak intensity of 4 x 10(18) W/cm2 are presented. The guided laser radiation in the plasma channel generated He-like ions that, subject to suitable plasma temperature, recombined into Li-like nitrogen ions. Intense radiation at a wavelength of 24.77 nm was observed, indicating possible lasing at the 3d(5/2) - 2p(3/2) transition in Li-like nitrogen.

Numerical Modeling of Laser Pulse Behavior in Nonlinear Crystal and Application to the Second Harmonic Generation

We propose an efficient numerical strategy for the full wave integration of the nonlinear Maxwell equations by a finite difference approach (finite-difference time-domain). We compare it to the asymptotic limit described by an extended nonlinear Schrodinger equation. This is achieved by considering the nonlinear behavior of high intensity ($\geq 5GW/cm^2$) ultrashort ($ \leq 100 fs$ FWMH) laser beams in a KDP crystal (type I). We assume that this crystal can be characterized by two linear dispersion Lorentz resonances, as well as instantaneous quadratic and cubic nonlinearities. Investigations are performed, for different optical regimes, in the context of the second harmonic generation for one-dimensional and two-dimensional models.

Phase-matched four-wave mixing of sub-100-TW∕cm2femtosecond laser pulses in isolated air-guided modes of a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to allow propagation and nonlinear-optical frequency conversion of high-intensity ultrashort laser pulses in the regime of isolated guided modes confined in the hollow gas-filled fiber core. With a specially designed dispersion of such modes, the 3omega=2omega+2omega-omega four-wave mixing of fundamental (omega) and second-harmonic (2omega) sub-100- TW/ cm(2) femtosecond pulses of a Cr:forsterite laser can be phase matched in a hollow photonic-crystal fiber within a spectral band of more than 10 nm, resulting in the efficient generation of femtosecond pulses in a well-resolved higher-order air-guided mode of 417-nm radiation.

Source-size measurements and charge distributions of ions accelerated from thin foils irradiated by high-intensity laser pulses

We report on measurements of source sizes and charge state distributions of ions accelerated from thin foils irradiated by ultrashort (100–300 fs) high-intensity (1-6×1019 W/cm2) laser pulses. The source sizes of proton and carbon ion beams originating from hydrocarbon contaminants on the surfaces of 5 μm thick aluminum foils were investigated using the knife-edge method. For low-energy protons and low-carbon charge states, the source area was found to exceed the focal spot area by a factor of 104. For the determination of charge state distributions, sandwich targets consisting of a 25 μm thick tungsten layer, a 2-nm thin beryllium layer, and again a tungsten layer whose thickness was varied were used. These targets were resistively heated to remove the light surface contaminants. Peaked energy spectra of oxygen and argon ions corresponding to the equilibrium distribution after propagation through matter were observed.

Kinetics of ultrashort relativistic electron pulses emitted from solid targets

Interaction of ultrashort high-intensity laser pulses with solid targets generates relativistic electrons which escape from the target. The kinetics of these ultrashort electron pulses is governed by self-fields generated by the charge of the electron cloud. In this paper an analytical theory is developed which allows calculation of electron trajectories, electron fluxes, and electron spectra at any distance from the target. The theory is exact for two limiting cases: (a) a monoenergetic electron pulse with an arbitrary temporal shape; (b) an infinitely short electron pulse with an arbitrary energy spectrum. These results have applications in high-intensity irradiation experiments, e.g., in experiments irradiating samples with ultrashort electron or x-ray pulses, in developing optics for fourth-generation light sources, and in work relating to x-ray lasers.

Limiting of microjoule femtosecond pulses in air-guided modes of a hollow photonic-crystal fiber

Self-phase-modulation-induced spectral broadening of laser pulses in air-guided modes of hollow photonic-crystal fibers (PCFs) is shown to allow the creation of fiber-optic limiters for high-intensity ultrashort laser pulses. The performance of PCF limiters is analyzed in terms of elementary theory of self-phase modulation. Experiments performed with 100 fs microjoule pulses of $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $\mathrm{Ti}$:sapphire laser radiation demonstrate the potential of hollow PCFs as limiters for $10\phantom{\rule{0.3em}{0ex}}\mathrm{MW}$ ultrashort laser pulses and show the possibility to switch the limiting level of output radiation energy by guiding femtosecond pulses in different PCF modes.

Generation of transient electric field structures in femtosecond high intensity ionizing laser pulses

A model for the generation of electric field structures in ultrashort high intensity laser pulses in air is developed. A collisionless plasma fluid model is developed to compute the transient electron–ion displacement generated by a high intensity laser pulse in air. We show that the electric fields associated with the electron–ion dipolar displacement are in good agreement with recent observations.

Ab initiocalculation of band-gap renormalization in highly excited GaAs

We present ab initio quasiparticle self-energy calculations in crystalline GaAs for cases of intense electronic excitation (~ 10% of valence electrons excited into conduction band), relevant for high-intensity ultra-short pulsed laser experiments. Calculations are performed using an out-of-equilibrium generalization of the GW approximation based on the Keldysh Green's function approach. Our results indicate that while the band gap is a sensitive function of the amount of excitation, it is not possible to induce complete band gap closure in this system by purely electronic means.

Electron acceleration by surface plasma waves in the interaction between femtosecond laser pulses and sharp-edged overdense plasmas

The relativistic acceleration of electrons by the field of surface plasma waves created in the interaction between ultrashort high-intensity laser pulses with sharp-edged overdense plasmas has been investigated. It is shown that the initial phase of the wave experienced by the electrons play a leading part by yielding a well-defined peaked structure in the energy distribution function. This study suggests that resonant excitation of surface plasma waves could result in quasi-monokinetic energetic electron bunches. When the space charge field becomes too strong, this mechanism can evolve toward a true absorption process of the surface wave energy via an enhanced “vacuum heating” mechanism generalized to the case of surface plasma waves.

MeV ? proton emission from ultrafast laser-driven microparticles

Propagation of a high intensity (∼1019 W/cm2) ultrashort (∼35 fs) laser pulse through a cloud of water spheres (150 nm diameter) results in hot electron driven proton acceleration up to 1 MeV. It is suggested that during the propagation of the short pulse through the low density wing of the cloud, the leading pulse pedestal is reduced owing to absorption by the preplasma created. Then, the high-intensity peak of the pulse propagates through this underdense plasma and interacts with the high-density inner part of the cloud, which has not been transformed into an underdense plasma so that a sheath acceleration process at each individual microsphere can take place. The observed proton spectra show strong modulations, which are interpreted within the framework of a known fluid-expansion model incorporating two hot-electron populations with significantly different densities and temperatures.

Laser acceleration of electrons in vacuum up to energies of ∼ 109 eV

A new mechanism of laser acceleration of charged particles is investigated in detail. Upon irradiation by tightly focused high-intensity ultrashort laser pulses, the acceleration of electrons travelling along the laser beam axis is determined by the longitudinal ponderomotive force and the longitudinal component of the electric field of the laser wave. It is found that the action of the longitudinal field on an electron may be unidirectional during many optical cycles, i.e., the phase slip effect is overcome. Lasers with currently highest possible parameters are shown to enable electron acceleration up to energies ε ∼ 1 GeV, which is comparable to the energies attainable on `large' accelerators of the SLAC type (ε ∼ 30 — 50 GeV). Unlike the schemes considered in the literature, the acceleration in this case is insensitive to the initial field phase (the effect of electron bunching is absent), it is possible to accelerate slow (nonrelativistic) electrons, and the problem of accelerated electron extraction from the field does not exist.