High-intensity Ultrashort Laser Pulses Research Articles

Small-scale electron density and magnetic-field structures in the wake of an ultraintense laser pulse.

We investigate the interaction of a high intensity ultrashort laser pulse with an underdense collisionless plasma in the regime where the Langmuir wake wave excited behind the laser pulse is loaded by fast particle beams, formed during the wake wave breaking. The beam loading causes the deterioration of the central part of the wake wave near the pulse axis, and the formation of bunches of sharply focalized ultrarelativistic electrons. The bunches of electrons generate a fast propagating magnetic field, which we interpret in terms of the magnetic component of the Lienard-Wiechert potential of a moving electric charge.

Monomode Guiding of1016W/cm2Laser Pulses over 100 Rayleigh Lengths in Hollow Capillary Dielectric Tubes

Monomode guiding over 100 Rayleigh lengths (10 cm) of high intensity ultrashort laser pulses ( ${10}^{16}\mathrm{W}/{\mathrm{cm}}^{2}$, 120 fs) has been demonstrated in hollow dielectric capillary tubes (45--70 \ensuremath{\mu}m internal diameter) without inner wall damage. Analytical predictions for coupling conditions and damping length are confirmed experimentally for tubes under vacuum. With 5 to 40 mbar of He gas in the tube, when laser ionization occurs the energy and duration of the transmitted pulse decrease while its spectrum is broadened.

Electron distributions generated by tunnelling ionization during gas breakdown by high intensity laser radiation

The electron distribution resulting from the breakdown of gas during irradiation by ultra-short high intensity laser pulses is considered using the quasi-static tunnel approximation. A general expression for arbitrary elliptic polarization is derived together with a number of approximations. The role of the pulse shape in modifying the distribution by determining the actual field and its variation during the ionization of a particular stage is considered in some detail. The ionization process is found to be sequential in most practical situations. Relaxation of the distribution in both anisotropy and energy due to collisions is investigated. The application of Monte Carlo methods for simulating these problems is described. A brief discussion of the consequences of the work for x-ray laser design is given.

Summary of Topical Papers on Material Processing by High-Intensity Ultrashort Laser Pulses

Summary of Topical Papers on Material Processing by High-Intensity Ultrashort Laser Pulses

Generation and Control of High-Intensity Ultrashort Laser Pulses.

Intense femtosecond laser pulses have opened new avenues to ultrashort-time and extremely high-field regions. Our research project aims at the further development of high-intensity ultrashort-pulse laser technology and its application to advanced material-control techniques. This paper presents a brief review of the laser technology under development in this project and describes related topics.

Laser induced intrinsic defects: Subpicosecond study of trapping kinetics

We use a time resolved interferometric method to study the kinetics of carriers trapping and defects formation following electronic excitation by high intensity ultrashort laser pulses in alkali halides. A systematic study of initial excitation density dependence of trapping kinetics, as well as the comparison of pure and doped samples, allows to investigate in detail the mechanism of energy relaxation and carriers localization accompanying the lattice rearrangement. The experimental results concern samples of pure and NO − 2 doped KBr, and of pure NaCl. We observe faster trapping in KBrNO − 2 than in pure KBr. In NaCl, the trapping rate increases with excitation density. These results show that in both materials the hole trapping is the primary process towards radiation induced defects. Furthermore, in KBr our results give strong evidence for the existence of an intermediate state with a short lifetime in the relaxation pathway between the initial electron–hole pairs and the final F–H pairs. A simple model describing the modification of the refractive index and rate equations governing free and trapped electron and holes populations are used to quantitatively interpret the experimental results.

Single-shot wave-front measurements of high-intensity ultrashort laser pulses with a three-wave interferometer

A simple new three-wave interferometric technique is used to measure, for what is believed to be the first time, the wave front of femtosecond ultrahigh-peak-power pulses carrying a strong B integral (B = 5.26+/-0.15) in a single shot. Wave-front distortions of a terawatt-class laser system are measured with good accuracy (lambda/50) and discussed. These distortions can significantly reduce the focused peak intensity, emphasizing the necessity of implementing adaptive optics in ultrahigh-intensity chirped-pulse amplification lasers.

Electron acceleration by a laser pulse in vacuum

The acceleration of an electron by a circularly polarized, high-intensity ultrashort laser pulse in vacuum is studied. It appears that the energy of the electron can be accelerated significantly, and the electron moves almost along the propagation direction of the laser pulse.

Magnetic interaction of ultrashort high-intensity laser pulses in plasmas

A review of our theoretical studies and computer simulations with PIC codes of the interaction of ultrashort, high-intensity laser pulses with plasmas is presented. The mutual attraction of the currents inside the self-focused channels, arising from the pulse filamentation, makes them interact magnetically and coalesce into a single channel with strongly enhanced electromagnetic energy density. Pulses of finite length and width propagate in the shape of a `bullet' and produce a wake consisting of magnetic dipoles correlated with an electron vortex row behind the pulse. The vortices evolve into an antisymmetric configuration which is shown to be stable when the distance between its vortices is greater than the electron skin depth.

Soft-x-ray imaging from an ultrashort-pulse laser-produced plasma using a multilayer coated optic

Measurements are presented of soft-x-ray images from a plasma produced by a high-intensity ultrashort-pulse laser. For the intensity range of 1015–1016 W/cm2 the soft-x-ray source appears to follow the spatial profile of the driving laser. A curved multilayer coated optic is used to collect 13.5-nm light and form a magnified image of the plasma. Knife-edge scans have been performed in the image plane and show a geometrically limited spot size of 280 μm.

Optical third-harmonic generation at interfaces.

Optical third-harmonic generation (THG) is generally a weak process but is dipole allowed, therefore it occurs in all materials, including dielectric materials with inversion symmetry. We report that when using focused high-intensity ultrashort laser pulses, this normally weak THG process becomes highly operative at a simple air-dielectric interface and is much stronger than the bulk of most dielectric materials. We characterized this nonlinear optical response at interfaces as a phenomenological surface-enhanced THG in transmission and/or in reflection. This surface THG is further cascaded in transmission or reflection from layered composite dielectric materials of a high-low index of refraction, resulting in a marked increase in photon conversion efficiency than that of a single interface. Although the present THG efficiency is lower than that of a typical phase-matching harmonic crystal, it is important to note that the surface-enhanced optical THG is a fundamental physical process occurring at all interfaces and is relatively free from the constraint of a phase-matching condition and wavelength restriction. Using optical THG at an interface, it becomes possible to generate wavelengths at which harmonic crystals are unavailable. These findings may lead to a new development of surface-enhanced studies and prompt a reexamination of the processes of high-harmonic generation at interfaces using a focused beam.

Pedestal Reduction by Temporal and Spectral Filterings in Chirped Pulse Amplification

We present numerical calculations on pedestal reduction of high-intensity ultrashort laser pulses generated by chirped pulse amplification. When an optical fiber is used as a pulse expander to generate a chirped pulse, spectral filtering is not effective in reducing the pedestal arising from the nonlinear chirp near the central frequency. The temporal filtering, in contrast, is effective in reducing the pedestals arising from various chirp nonlinearities and amplified spontaneous emission. It is shown that a contrast ratio exceeding 1010 is achievable with appropriate filtering techniques.

Kinetic description of plasmas produced in multiphoton-ionization processes.

A kinetic description of inhomogeneous plasmas produced by irradiating an atomic gas by a well-focused ultrashort high-intensity laser pulse is presented. The Vlasov-Maxwell equation is integrated by representing the distribution function f(r,v;t) in the form ${\mathit{f}}_{0}$(Dr-Bv,-Cr+Av), with ${\mathit{f}}_{0}$ the initial distribution and the A,B,C,D functions depending on r, v, and t. On the basis of this representation the evolution of some charge distributions is examined, by focusing the attention on the transport of ion packets, the effect of the ponderomotive potential, and the capture of electrons by the ion potential well.

Ultrashort pulse laser for high density plasma experiments

The use of high intensity ultrashort laser pulses in the study of hot, high density plasmas requires pulses of extreme temporal fidelity. We have developed a system that provides continuously variable independent tuning of the higher order frequency dependent phase of ultrashort laser pulses. Numerical results are presented indicating that higher order phase terms that lead to temporal wings can be compensated by a properly adjusted air spaced doublet lens within the pulse stretcher.

High dynamic range third-order correlation measurement of picosecond laser pulse shapes

A third-order correlation system has been developed, which enables the contrast ratio of high-intensity ultra-short laser pulses to be measured on a single shot, with a dynamic range of better than 108.

Interaction of free electrons with an intense focused laser pulse in Gaussian and conical axicon geometries

We calculate the dynamics and the radiation emitted from free electrons in an ultrashort pulsed laser focus. In a Gaussian focus, at low electron densities for which space charge is negligible, we find that ponderomotive forces limit the effective volume of radiation emission to a region that is much smaller than the focal waist and even significantly smaller than the fundamental wavelength. Constructive interference produces significant enhancements in the radiation emitted by the plasma for the lower harmonics, whereas destructive interference limits the radiation for the higher harmonics. We also investigate the radiation emitted by electrons in the focus of a conical axicon, an optical geometry that was proposed for laser-based electron accelerators [ J. Appl. Phys.54, 4285 ( 1983)]. This radiation may be a useful diagnostic in such accelerators. These studies are intended as a prelude to experiments with high-intensity ultrashort laser pulses.

Picosecond-gated optical amplifier

A picosecond-gated optical amplifier (PGOA) characterized by high gains (≳102), large extinction ratios (∼105), and short aperture times (∼10 psec) is described. The active component of the system is a saturable absorbing dye which is irradiated by a high-intensity ultrashort laser pulse. The technique is illustrated by simultaneously gating and amplifying a weak picosecond continuum pulse propagating through a scattering medium. Performance characteristics of the PGOA and ultrafast shutters are compared.