K. Witte

Direct observation of attosecond light bunching.

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1u2009as = 10-18u2009s) timescales. A frequency ‘comb’ of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump–probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.

Correction of strong phase and amplitude modulations by two deformable mirrors in a multistaged Ti:sapphire laser

We describe a novel scheme consisting of two deformable bimorph mirrors that can free ultrashort laser pulses from simultaneously present strong wave-front distortions and intensity-profile modulations. This scheme is applied to the Max-Planck-Institut für Quantenoptik 10-TW Advanced Titanium-Sapphire Laser (ATLAS) facility. We demonstrate that with this scheme the focusability of the ATLAS pulses can be improved from 10(18) to 2x10(19) W/cm(2) without any penalty in recompression fidelity.

10-fs pulse generation from a unidirectional Kerr-lens mode-locked Ti:sapphire ring laser

For the first time to our knowledge, nearly bandwidth-limited 10-fs laser pulses centered at 790 nm are generated in a Ti:sapphire ring resonator. Intracavity dispersion control is provided by dispersive dielectric mirrors together with the insertion of small amounts of intracavity glass path. Mode locking is self-starting without the use of an aperture.

Comparative study of time-resolved K-shell spectra from aluminum plasmas generated by ultrashort laser pulses at 395 and 790 nm

A comparative study of temporally and spectrally resolved K-shell emission from aluminum targets heated with 150 fs Ti:sapphire laser pulses at 790 and 395 nm is presented at an intensity of 5×1017u2009W/cm2. Whereas at 395 nm spectrally broad intense plasma lines and a weak Kα line with durations (full width at half maximum) of 1–2 ps are observed, the spectrum at 790 nm shows weak narrow plasma lines and an intense Kα line with durations of 3–5 ps.

Temporal characterization of short-pulse third-harmonic generation in an atomic gas by a transmission-grating Michelson interferometer

By use of a transmission-grating-based Michelson interferometer, second-order interferometric as well as intensity autocorrelation traces of the third harmonic of a Ti:sapphire 50-fs laser beam produced in Ar have been measured. The duration of the harmonic is found to be that expected from lowest-order perturbation theory. At this wavelength, the performance of the interferometer with respect to pulse-front distortion and dispersion is found to be satisfactory. This result is a first step toward the use of the interferometer for the temporal characterization of higher harmonics or harmonic superposition forming attosecond pulse trains.

New high-power laser facility PALS—prospects for laser–plasma research

In this paper, we report on a new laser facility called PALS (Prague Asterix Laser System), which is currently under construction, and which will house the high-power iodine laser Asterix IV. Upon its completion in late 1999, the PALS facility will be capable of providing single- or multiple-pulse irradiation with a variable pulse duration ranging from 100 to 500 ps. Wavelengths available will be 1.315 μm, 658 nm, and 438 nm. The system will provide one main beam with energy up to 1200 J and two smaller auxiliary beams with a combined energy of up to 100 J. A wide variety of geometries and variable pulse timings is available. We assess PALS potential for investigating the physics of laser plasmas in inertial confinement fusion, the development and applications of X-ray lasers, X-ray spectroscopy, and radiation transport, using multiple-pulse and extended beam capability.

Physics of ultra-intense laser-plasma interaction

The fast-ignition scheme in ICF has instigated research in a new area of laser-plasma interaction physics. This is due to the high laser-pulse intensities involved. At power densities of 1018 W cm-2, the physics becomes essentially kinetic in nature and highly nonlinear and hence requires special tools for its adequate description. Particle-in-cell (PIC) codes meet this challenge. Simulations obtained with the first three-dimensional PIC code VLPL (virtual laser plasma laboratory) developed at MPQ are presented for a broad variety of irradiation and plasma conditions. Particular attention is paid to a detailed comparison of numerical and experimental results and to the mechanism which accelerates the electrons to relativistic energies.

Experiments with ASTERIX and ATLAS

We report on measurements of the equation-of-state of copper and gold in the multi-ten Megabar range, opacity measurements based on K-shell absorption in aluminium, and X-ray laser studies on a large number of neon-like ions applying the prepulse technique. For these investigations, the one-beam iodine laser facility ASTERIX emitting pulses with energies of up to 1000 J at 1315 nm and of up to 420 J at 438 nm was used. We also give a brief account of experimental and theoretical results referring to the propagation of an ultrashort pulse through underdense hydrogen or nitrogen plasmas and X-ray spectra from an optically field-ionized nitrogen plasma generated either by linearly or elliptically polarized light. For these investigations, 150-fs/200-mJ/800-nm pulses emitted from our titanium:sapphire laser ATLAS were employed.

MeV Electrons and Positrons from a Femtosecond Table-Top Laser System

We experimentally demonstrate the feasibility of generating a well colli-mated multi-MeV electron jet utilizing 790nm/120fs/1.2TW pulses from a titanium:sapphire laser running at 10Hz. The method uses the process of relativistic self-channeling in an underdense plasma of almost critical density. In a second step, we have succeeded in producing 2 × 108 positrons/sec by letting the electron jet impinge on a 2 mm thick lead converter. The scheme exhibits a favorable scaling to higher laser intensities.