Roland A. Sauerbrey

Fission of actinides with tabletop lasers

We report the first laser induced fission of actinide nuclei with high intensity tabletop lasers. The results are discussed in the context of laser plasma diagnostics and nuclear waste handling.

Megavolt discharges triggered and guided with laser filaments

Long ionized channels appear as a result of nonlinear propagation of femtosecond terawatt pulses. We demonstrate the ability of such filament bundles to trigger and guide megavolt discharges across a gap up to 3.8 m.

PHELIX: a petawatt high-energy laser for heavy ion experiments

The unique combination of an intense heavy ion beam accelerator and a high energy laser opens the possibility of exploring new physics taking advantage of the synergy of both facilities. A variety of new fields can be addressed with this combination in plasma physics, atomic physics, nuclear- and astro-physics as well as material research. In addition, using CPA-technology, laser pulses with a pulse power of up to a petawatt opens the door to explore the regime of fully relativistic plasmas. Therefore the Gesellschaft fuer Schwerionenforschung is augmenting the current high intensity upgrade of the heavy ion accelerator facility with the construction of PHELIX. Designed with two pulse-generating front ends and send to multiple experimental areas PHELIX will serve as a highly versatile laser system for various applications. In this report, we present the design of the laser system and some key experiments that can be performed with this combination for the first time.

Preliminary results from key experiments on sources for EUV lithography

Abstract While huge progress has been achieved for EUVL system design and multilayer optics during the last years the decision on the best suited source is still open. In a German basic research cooperation on short-wavelength plasma based sources some key issues also relevant for EUVL are addressed: the comparison of existing sources, investigations on the scalability of source concepts and the demonstration of key features. Preliminary results of investigations on parameters for best conversion efficiency of laser produced plasmas, concepts for high-power lasers and scalability of gas discharge based sources are presented. Comparability of results is assured by calibrated metrology tools which are cross checked with ASML’s flying circus.

Extreme-ultraviolet source development: a comparison of different concepts

We discuss the results of the studies of Z-pinch sources for photolithographic applications developed by Lambda Physik. We also report the results of fundamental investigations pursued by Fraunhofer-Institut fuer Lasertechnik. Friedrich- Schiller Universitat Jena, Max-Born Institut Berlin, and Gustav August Universitat Gottingen. The later efforts are supported by German government and steered by the industrial consortium led by Lambda Physik.

Femtosecond LIDAR: new perspectives of atmospheric remote sensing

High-power femtosecond laser pulses can lead to strong nonlinear interactions during the propagation through a medium. In air the well known self-guiding effect produces long intense and moderately ionized filaments, in which a broad white-light continuum from the near UV to the mid IR is generated. The forward directed white-light can be used to do range resolved broadband absorption measurements, which opens the way to a real multi-component lidar for the simultaneous detection of several trace gases. On the other hand, enhanced nonlinear scattering and characteristic emission from the filament region, as well as from the interaction of intense pulses with aerosols, can be observed. This opens perspectives towards a novel kind of analysis of atmospheric constituents, based upon nonlinear optics. Additionally, the conductivity of the filaments can be used for lightning control. Here we present the basic concepts of the femtosecond lidar, laboratory experiments and recent results of atmospheric measurements.

Optical damage of sputtered gold films irradiated with femtosecond laser pulses

Multi-shot damage tests were performed of gold coated mirrors in the femtosecond and in the nanosecond laser pulse regime. Sputtered gold films from different suppliers of various thicknesses were investigated. Considerable differences in the optical quality and the damage threshold are reported. The best films withstand a maximum fluence of 0.7 J/cm2 for 50-fs Ti:sapphire laser irradiation (804 nm) and 7 J/cm2 for 8-ns Nd:YAG irradiation (1064 nm). For gold films with poor optical quality a permanent surface modification one order of magnitude below the damage threshold was observed.

POLARIS - A compact, diode-pumped laser system in the Petawatt regime

POLARIS is a compact diode pumped Yb-phosphate glass laser system. It is designed to reach the petawatt regime with a possible repetition frequency of 0.1 Hz. The first three amplifiers of this system are operating, and already constitute an all diode pumped terawatt system. The POLARIS system is designed to have an output of about 150 J in 150 fs, and should be completed in 2006.

Time-resolved x-ray diffraction with subpicosecond x-ray pulses

The emission from plasmas created with fs-lasers provides sub-picosecond x-ray pulses in the keV-range. Intense emission of K(alpha) lines as well as quasi continuum x-rays can be used for time-resolved diffraction and spectroscopy, i.e. to study lattice or atomic dynamics with sub-picosecond resolution by using a laser pump x-ray probe technique. The x-ray yield and x-ray pulse duration of the laser plasma source depend on the laser parameters and the target design, such as intensity, laser wavelength, pulse duration and prepulse level. To accumulate as many photons as possible of the isotropic source an efficient large aperture optic has to be used to select an x-ray line or a wavelength range and focus the radiation onto the sample. It is shown that the use of toroidally bent crystals provides the possibility to refocus 10-4 of the photons emitted in the whole solid angel to spot size of around 80 micrometers with a temporal broadening below 100 fs. Combinations of bent focusing crystals with a flat sample crystal for fast x-ray diffraction application are discussed. Experiments showing the temporal response of laser heated crystals are presented and compared with theoretical simulations based on Takagi-Taupin theory.

X-ray optics for study of ultrafast processes in crystalline samples

Non-thermal melting of semiconductor crystals, phase transitions on a sub-picosecond time scale can be studied by optical pump x-ray probe experiments. Powerful femtosecond lasers deliver brilliant ultrashort K(alpha ) pulses on a time scale from 100 fs to 1 ps that can be optimized for these pump-probe experiments. These experiments consist of two diffracting elements: (i) a bent crystal imaging the flash x-ray source in a narrow spectral window; and (ii) the sample crystal diffracting the ultrashort x-ray pulse. As penetration depths of optical pump beam are usually much shorter than x-ray extinction depths, best sensitivity to ultrafast structural changes is obtained for minimum x-ray extinction depths. This can be achieved by selecting samples containing heavy elements, thin crystalline film samples and by using asymmetric Bragg reflections, respectively. Several theoretical codes have been developed to optimize design of the instruments. X-ray topographic cameras and diffractometers were modified for fabrication and characterization of 2D bent crystals. Best practical results were obtained when structurally perfect wafers of Si, Ge, and quartz crystals were prepared while monitored by x-ray topography and diffractometry. After a final check of x-ray imaging and reflection properties of the toroidal crystals, monochromatic x-ray beam and laser pump beam are adjusted spatially to coincide on the sample crystal. Because converging x-rays impinge on the sample crystal, its rocking curve can be registered as a spatial distribution on the detector. In comparison to synchrotron experiments where about 104 pulses must be integrated, in these experiments rocking curves can be recorded in a single or in a few laser shots. Ultrafast processes are studied in Langmuir Blodgett films containing Cd, in bulk semiconductors, such as InSb, and in CdTe semiconductor films. Focused, pulsed monochromatic x-rays have been transmitted through biological samples to register many reflections, which opens the way to ultrafast studies in structural biology.