Temporally Resolved Measurement of Electron Densities (>10(23) cm-3) with High Harmonics

High-Brilliance Ultranarrow-Band X Rays via Electron Radiation in Colliding Laser Pulses

A setup of a unique x-ray source is put forward employing a relativistic electron beam interacting with two counterpropagating laser pulses in the nonlinear few-photon regime. In contrast to Compton scattering sources, the envisaged x-ray source exhibits an extremely narrow relative bandwidth of the order of 10^{-4}, comparable with an x-ray free-electron laser. The brilliance of the x rays can be an order of magnitude higher than that of a state-of-the-art Compton source. By tuning the laser intensities and the electron energy, one can realize either a single peak or a comblike x-ray source of around keV energy. The laser intensity and the electron energy in the suggested setup are rather moderate, rendering this scheme compact and tabletop size, as opposed to x-ray free-electron laser and synchrotron infrastructures.

Single-shot extreme-ultraviolet wavefront measurements of high-order harmonics

We perform wavefront measurements of high-order harmonics using an extreme-ultraviolet (XUV) Hartmann sensor and study how their spatial properties vary with different generation parameters, such as pressure in the nonlinear medium, fundamental pulse energy and duration as well as beam size. In some conditions, excellent wavefront quality (up to λ/11) was obtained. The high throughput of the intense XUV beamline at the Lund Laser Centre allows us to perform single-shot measurements of both the full harmonic beam generated in argon and individual harmonics selected by multilayer mirrors. We theoretically analyze the relationship between the spatial properties of the fundamental and those of the generated high-order harmonics, thus gaining insight into the fundamental mechanisms involved in high-order harmonic generation (HHG).

Frequency gating to isolate single attosecond pulses with overdense plasmas using particle-in-cell simulations

We present the isolation of single attosecond pulses for multi-cycle and few-cycle laser pulses from high harmonic generation in overdense plasmas, calculated with particle-in-cell simulations. By the combination of two laser pulses of equal amplitude and a small frequency shift between them, we demonstrate that it is possible to shorten the region in which the laser pulse is most intense, therefore restricting the generation of high harmonic orders in the form of attosecond pulses to a narrower time window. The creation of this window is achieved due to the combination of the laser pulse envelope and the slow oscillating wave obtained from the coherent sum of the two pulses. A parametric scan, performed with particle-in-cell simulations, reveals how the pulse isolation behaves for different input laser pulse lengths and which are the optimal frequency shifts between the two laser pulses in each case, giving the conditions for having a good isolation of an attosecond pulse when working with laser-plasma interaction in overdense targets.

Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources

We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.

Interaction of ultrashort x-ray pulses with B4C , SiC, and Si

The interaction of 32.5 and 6 nm ultrashort x-ray pulses with the solid materials B4C , SiC, and Si is simulated with a nonlocal thermodynamic equilibrium radiation transfer code. We study the ionization dynamics as a function of depth in the material and modifications of the opacity during irradiation, and estimate the crater depth. Furthermore, we compare the estimated crater depth with experimental data, for fluences up to 2.2 J/cm2. Our results show that, at 32.5 nm irradiation, the opacity changes by less than a factor of 2 for B4C and Si and by a factor of 3 for SiC, for fluences up to 200 J/cm2. At a laser wavelength of 6 nm, the model predicts a dramatic decrease in opacity due to the weak inverse bremsstrahlung, increasing the crater depth for high fluences.

Fundamental and harmonic emission from the rear side of a thin overdense foil irradiated by an intense ultrashort laser pulse

The emission of fundamental and harmonic radiation from the rear side of thin foils in the thickness range 50-460 nm irradiated by intense frequency doubled Ti:sapphire laser pulses of the duration of 150 fs and intensities up to a few 10(18) W/cm(2) was investigated. Following up a previous study of the rear side harmonic emission [Teubner, Phys. Rev. Lett. 92, 185001 (2004)], we measured the emission efficiencies, polarization properties, and the spectral shapes of the fundamental frequency and the second harmonic. Rear side emission is only observed when the obliquely incident laser light is p -polarized. Particle-in-cell (PIC) simulations indicate that the foils remain strongly overdense during the interaction with the laser pulse and that the rear side emission is caused by energetic electron bunches which are generated at the front side by resonance absorption. They are accelerated into the foil and drive strong plasma oscillations at the fundamental and higher harmonic frequencies.

Probing hot and dense laser-induced plasmas with ultrafast XUV pulses

In this Letter, we demonstrate the instantaneous creation of a hot solid-density plasma generated by focusing an intense femtosecond, high temporal contrast laser on an ultrathin foil (100 nm) in the 10(18) W/cm2 intensity range. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate such a medium, enables the study of the dynamics of this plasma on the 100 fs time scale. The comparison of the transmission of two successive harmonics permits us to determine the electronic density and the temperature with accuracies better than 15%, never achieved up to this date in the regime of laser pulses at relativistic intensity.

Frequency-sheared, time-delayed extreme-ultraviolet pulses produced by high-harmonic generation in argon

We report the production of frequency-sheared high harmonics in argon by control of the envelope and chirp of the electric field of the femtosecond driving laser pulse. Using the classic three-step model of high-harmonic generation, we established a direct link between the properties of the harmonics and the fully characterized driving pulses. A simulation of the single-atom response in the strong-field approximation confirms the simple picture and shows good agreement with the experimental results.

Harmonic emission from the rear side of thin overdense foils irradiated with intense ultrashort laser pulses

The harmonic emission from thin solid carbon and aluminum foils, irradiated by 150 fs long frequency-doubled Ti:sapphire laser pulses at lambda=395 nm and peak intensities of a few 10(18) W/cm(2), has been studied. In addition to the harmonics emitted from the front side in the specular direction, we observe harmonics up to the 10th order, including the fundamental from the rear side in the direction of the incident beam, while the foil is still strongly overdense. The experimental observations are well reproduced by particle-in-cell simulations. They reveal that strong coupling between the laser-irradiated side and the rear side occurs via the nonlocal electron current driven by the laser light.

Generation and focusing of submilliwatt-average-power 50-nm pulses by the fifth harmonic of a KrF laser

We demonstrate generation and focusing of 49.7-nm pulses with an average power of 0.1 mW at 200 Hz and with a pulse energy of >1 microJ at 10 Hz by the fifth harmonic of a femtosecond KrF laser. The fifth harmonic is selected and focused with a concave Sc/Si multilayer mirror to a diameter of 2microm, resulting in a peak intensity of 0.5 TW/cm(2), which will make extreme-ultraviolet nonlinear optics feasible. A novel single-shot linear in situ method of spot-size measurement by use of self-trapped exciton luminescence is also demonstrated.

Dynamics of the critical surface in high-intensity laser-solid interactions: modulation of the XUV harmonic spectra

The generation of harmonics from the interaction of an intense (I>or=10(18) W cm(-2)) laser with a solid surface is investigated. Modulation of the harmonic emission spectrum with a periodicity of 2 to 4 harmonics is observed at higher laser intensities. A similar modulation is predicted by a particle-in-cell simulation. The modulation is shown to be caused by the higher modes of oscillation of the critical surface during the interaction. As a result, the dynamics of the critical surface can be inferred from the shape of the harmonic spectrum.

Harmonics generation in electron-ion collisions in a short laser pulse

Anomalously high generation efficiency of coherent higher field harmonics in collisions between oppositely charged particles in the field of femtosecond lasers is predicted. This is based on rigorous numerical solutions of a quantum kinetic equation for dense laser plasmas that overcomes limitations of previous investigations.

Nonlinear relaxation field in charged systems under high electric fields

The influence of an external electric field on the current in charged systems is investigated. The results beyond linear response from the classical hierarchy of density matrices are compared with the results from quantum kinetic theory. It is found that even an infinitesimal friction with the background changes the results in a noncontinuous way. The kinetic theory yields a systematic treatment of the nonlinear current beyond linear response. To this end the dynamically screened and field-dependent Lenard-Balescu equation is integrated analytically and the nonlinear relaxation field is calculated. The classical linear response result known as the Debye-Onsager relaxation effect is obtained only if asymmetric screening is assumed. When considering the kinetic equation of one species, the other species have to be screened dynamically while the screening with the same species itself has to be performed statically. Other different approximations are discussed and compared.

Transport coefficients for dense metal plasmas

Thermoelectric transport coefficients of metal plasmas are calculated within the linear response theory applied previously to determine the electrical conductivity of Al and Cu plasmas [R. Redmer, Phys. Rev. E 59, 1073 (1999)]. We consider temperatures of 1-3 eV and densities of 0.001-1 g/cm(3) as relevant in rapid wire evaporation experiments. The plasma composition is calculated considering higher ionization stages of atoms up to 5+, and solving the respective system of coupled mass action laws. Interactions between charged particles are treated on T matrix level. Results for the electrical conductivity of various metal plasmas are in reasonable agreement with experimental data. Thermal conductivity and thermopower are also given. In addition, we compare with experimental data for temperatures up to 25 eV and liquidlike densities.

Self-induced parametric amplification of high-order harmonics

A theoretical analysis of self-induced parametric amplification of high-order harmonics is given. This mechanism permits the elimination of limitations on the harmonic-generation efficiency that are imposed by absorption.

Real-time kadanoff-baym approach to plasma oscillations in a correlated electron Gas

A nonequilibrium Green's functions approach to the collective response of correlated Coulomb systems at finite temperatures is presented. It is shown that solving Kadanoff-Baym-type equations of motion for the two-time correlation functions including the external perturbing field allows one to compute the plasmon spectrum with collision effects in a systematic and consistent way. The scheme has a "built-in" sum-rule preservation and is simpler to implement numerically than the equivalent equilibrium approach based on the Bethe-Salpeter equation.

Extreme ultraviolet interferometry measurements with high-order harmonics

We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2-10(20)electrons/cm>(3) are measured.

Quantum kinetic theory of plasmas in strong laser fields

A kinetic theory for quantum many-particle systems in time-dependent electromagnetic fields is developed based on a gauge-invariant formulation. The resulting kinetic equation generalizes previous results to quantum systems and includes many-body effects. It is, in particular, applicable to the interaction of strong laser fields with dense correlated plasmas.

Beam-quality measurement of a focused high-order harmonic beam

Using a multilayer spherical mirror, we focus the high-order harmonic radiation produced near 55 nm by the nonlinear interaction of an intense femtosecond laser pulse and a xenon gas jet. The focused XUV beam is characterized by a knife-edge technique in the focal region and by far-field imaging. We show that good-quality beams, nearly two times diffraction limited, can be generated, a conclusion that is at variance with recent predictions of harmonic phase-front distortion. Spot sizes close to 10 microm are obtained, resulting in a high XUV intensity. Increasing the gas density and the length of the generating medium results in a large increase in the divergence in and degradation of the beam quality.