Showing papers by "Vienna University of Technology published in 1997"

Compression of high-energy laser pulses below 5 fs

Abstract: High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.

Nonlinear optical pulse propagation in the single-cycle regime

Abstract: A general three-dimensional wave equation first order in the propagation coordinate is derived covering a broad range of phenomena in nonlinear optics. This equation provides an accurate description of the evolution of the wave packet envelope down to pulse durations as short as one carrier oscillation cycle. The concept of envelope equations is found to be applicable to the single-cycle regime of nonlinear optics.

Generation of Coherent X-rays in the Water Window Using 5-Femtosecond Laser Pulses

Abstract: Coherent extreme-ultraviolet radiation extending to wavelengths below the carbon K edge at 4.37 nanometers (nm) has been generated at a repetition rate of 1 kilohertz by focusing 5-femtosecond near-infrared (780 nm) laser pulses into a helium gas jet. The incident light field performs just a few oscillations, which results in the emission of an x-ray supercontinuum rather than discrete harmonics. Owing to the extremely short rise time of the driving pulses, neutral atoms can be exposed to high fields before they are depleted by ionization. As a result, the observed x-ray radiation extends well into the water window and is delivered in a well-collimated beam (divergence less than 1 milliradian). The high repetition rate and spatial coherence result in a brightness of about 5 × 108 photons per square millimeter per square milliradian per second in a 1-percent bandwidth at 4.37 nm, the carbon edge of the water window. The compact laboratory system holds promise as a source for biological holography and nonlinear optics in the x-ray regime.

SIMON-A simulator for single-electron tunnel devices and circuits

Abstract: SIMON is a single electron tunnel device and circuit simulator that is based on a Monte Carlo method. It allows transient and stationary simulation of arbitrary circuits consisting of tunnel junctions, capacitors, and voltage sources of three kinds: constant, piecewise linearly time dependent, and voltage controlled. Cotunneling can be simulated either with a plain Monte Carlo method or with a combination of the Monte Carlo and master equation approach. A graphic user interface allows the quick and easy design of circuits with single-electron tunnel devices. Furthermore, as an example of the usage of SIMON, we discuss the essential problem of random background charge and present possible solutions.

Molecular Precursors in the Dissociative Adsorption of O 2 on Pt(111)

Abstract: Ab initio local-spin-density calculations for the adsorption of ${\mathrm{O}}_{2}$ on Pt(111) are presented. We identify two distinct, but energetically almost degenerate chemisorbed precursors. A superoxolike paramagnetic precursor is formed at the bridge site, with the molecule parallel to the surface. A second peroxolike nonmagnetic precursor is formed in the threefold hollow, with the atom slightly canted in a top-hollow-bridge geometry. The nature of the barrier for dissociation into atoms adsorbed in the hollows is explored.

Ultrasoft pseudopotentials applied to magnetic Fe, Co, and Ni: From atoms to solids

Abstract: We present a study of the accuracy, transferability, and plane-wave convergence properties of ultrasoft Vanderbilt-type pseudopotentials for Fe, Co, and Ni in the context of atomic, molecular, and solid calculations. Special attention has been given to the magnetic properties of these systems. To go beyond the local-spin-density-approximation, generalized gradient approximations for the exchange-correlation functional have been included. All calculations have been performed using a plane-wave basis set, and we show that ultrasoft pseudopotentials allow -- as expected -- for a considerably lower cutoff energy than standard soft norm-conserving pseudopotentials. Lattice properties show very good agreement with all-electron calculations and experiment, while larger discrepancies exist for magnetic structural energy differences (which however remain smaller than 2 mRy/atom). These differences can be traced back to the frozen core approximation which is implicitly assumed in the construction of the pseudopotentials. More accurate results for the magnetization energies of atomic configurations can be obtained by treating the $3p$ semicore states as valence states.

The Personal Interaction Panel - a Two-Handed Interface for Augmented Reality

Abstract: This paper describes the introduction of a new interaction paradigm to augmented reality applications. The everyday tool handling experience of working with pen and notebooks is extended to create a three dimensional two-handed interface, that supports easy-to-understand manipulation tasks in augmented and virtual environments. In the design step we take advantage from the freedom, given by our very low demands on hardware and augment form and functionality to this device. On the basis of examples from object manipulation, augmented research environments and scientific visualization we show the generality of applicability. Although being in the first stages implementation, we consider the wide spectrum of suitability for different purposes.

Computing Maximum Task Execution Times — A Graph-BasedApproach

Abstract: The knowledge of program execution times is crucial for the development and the verification of real-time software. Therefore, there is a need for methods and tools to predict the timing behavior of pieces of program code and entire programs. This paper presents a novel method for the analysis of program execution times. The computation of MAximum eXecution Times (MAXTs) is mapped onto a graph-theoretical problem that is a generalization of the computation of a maximum cost circulation in a directed graph. Programs are represented by T-graphs, timing graphs, which are similar to flow graphs. These graphs reflect the structure and the timing behavior of the code. Relative capacity constraints, a generalization of capacity constraints that bound the flow in the edges, express user-supplied information about infeasible paths. To compute MAXTs, T-graphs are searched for those execution paths which correspond to a maximum cost circulation. The search problem is transformed into an integer linear programming problem. The solution of the linear programming problem yields the MAXT. The special merits of the presented method are threefold: It uses a concise notation to characterize the static structure of a program and its possible execution paths. Furthermore, the notation allows for a description of the feasible paths through the program code that characterizes the behavior of the code sufficiently to compute the exact maximum execution time of the program – not just a bound thereof. Finally, linear program solving does not only yield maximum execution times, but also produces detailed information about the execution time and the number of executions of every single program construct in the worst case. This knowledge is valuable for a more comprehensive analysis of the timing of a program.

Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate.

Abstract: A compact all-solid-state femtosecond Ti:sapphire oscillator–amplifier system using no grating-based pulse stretcher produces 20-fs, 1.5-mJ pulses at a 1-kHz repetition rate. The pulses are subsequently compressed in a hollow-fiber chirped-mirror compressor. The system delivers bandwidth-limited 5-fs, 0.5-mJ pulses at 780 nm in a diffraction-limited beam.

Center dominance and Z 2 vortices in SU(2) lattice gauge theory

Abstract: We find, in close analogy to Abelian dominance in the maximal Abelian gauge, the phenomenon of center dominance in the maximal center gauge for SU(2) lattice gauge theory. The maximal center gauge is a gauge-fixing condition that preserves a residual ${Z}_{2}$ gauge symmetry; ``center projection'' is the projection of SU(2) link variables onto ${Z}_{2}$ center elements, and ``center dominance'' is the fact that the center-projected link elements carry most of the information about the string tension of the full theory. We present numerical evidence that the thin ${Z}_{2}$ vortices of the projected configurations are associated with ``thick'' ${Z}_{2}$ vortices in the unprojected configurations. The evidence also suggests that the thick ${Z}_{2}$ vortices may play a significant role in the confinement process.

Complexity and expressive power of logic programming

Abstract: This paper surveys various complexity results on different forms of logic programming. The main focus is on decidable forms of logic programming, in particular propositional logic programming and datalog, but we also mention general logic programming with function symbols. Next to classical results on plain logic programming (pure Horn clause programs), more recent results on various important extensions of logic programming are surveyed. These include logic programming with different forms of negation, disjunctive logic programming, logic programming with equality, and constraint logic programming. The complexity of the unification problem is also addressed.

Robust heteroclinic cycles

Abstract: One phenomenon in the dynamics of differential equations which does not typically occur in systems without symmetry is heteroclinic cycles. In symmetric systems, cycles can be robust for symmetry-preserving perturbations and stable. Cycles have been observed in a number of simulations and experiments, for example in rotating convection between two plates and for turbulent flows in a boundary layer. Theoretically the existence of robust cycles has been proved in the unfoldings of some low codimension bifurcations and in the context of forced symmetry breaking from a larger to a smaller symmetry group. In this article we review the theoretical and the applied research on robust cycles.

Crushing of axially compressed steel tubes filled with aluminium foam

Abstract: This study, with the emphasis on experiments, investigates the applicability of aluminium foam as filler material in tubes made of mild steel having square or circular cross sections, which are crushed axially at low loading velocities. In addition to the experiments finite element studies are performed to simulate the crushing behaviour of the tested square tubes, were a crushable foam material model is shown to be suitable for describing the inelastic response of aluminium foam with respect to the considered problems. The experimental results for the square tubes reveal efficiency improvements with respect to energy absorption of up to 60%, resulting from changed buckling modes of the tubes and energy dissipation during the compression of the foam material itself. The principal features as well as the changes of the crushing process due to filling can also be studied by the numerical simulations. A global failure mechanism due to a high foam density can be observed for filled circular tubes. Aluminium foam is shown to be a suitable material for filling thin-walled tubular steel structures, holding the potential of enhancing the energy absorption capacity considerably, provided the plastic buckling remains characterized by local modes.

High-resolution 3-D direction-of-arrival determination for urban mobile radio

Abstract: The in-depth knowledge of the mobile radio channel is particularly important for radio communication modeling and advanced technology system design. We propose an accurate method to determine jointly the azimuth and elevation angle and the delay of waves incoming at the receiver. The method is applied to measurements of the complex impulse response of the mobile radio channel, performed on a planar array placed on a mobile in an urban cellular environment. The directions-of-arrival (DOA) were obtained by the means of a direction finding algorithm-two-dimensional (2-D) unitary ESPRIT. Two-dimensional spatial smoothing as an extension of ordinary spatial smoothing is utilized to decorrelate coherent waves. The application of 2-D unitary ESPRIT increases the angular resolution over conventional Fourier analysis or the scattering function by an order of magnitude and overcomes difficulties due to secondary lobes. The time delay is determined from wideband channel sounder measurements. The results confirm some assumptions on propagation mechanisms: (1) the wave-guiding property of streets (canyon effect), which is especially pronounced for long-delayed paths; (2) the variation of the number of incoming waves with their excess delay-the larger the excess delay, the lower the number of paths comprising an echo in the power delay profile; (3) if a single path remains, the privileged DOA is the direction of the street; (4) the exponential part of the power delay profile due to scatterers all around the receiver; and (5) the elevation dependence or the impinging power. In the tested receiver locations, paths with elevations between 0/spl deg/ and 40/spl deg/ dominate, containing about 90% of the received power.

Few-Cycle THz Emission from Cold Plasma Oscillations

Abstract: We report on THz emission from plasma oscillations in semiconductors excited by femtosecond optical pulses. Time-resolved correlation measurements are performed on p-i-n and n -doped GaAs structures. In p-i-n structures coherent oscillations of the hot photogenerated carrier plasma emit THz radiation. A fundamentally new emission process is proposed in n -doped GaAs structures. Here, the screening of the surface field starts plasma oscillations of the cold electrons in the GaAs bulk leading to an efficient emission of few-cycle THz radiation. {copyright} {ital 1997} {ital The American Physical Society}

A novel-high energy pulse compression system: generation of multigigawatt sub-5-fs pulses

Abstract: Powerful techniques for spectral broadening and ultrabroadband dispersion control, which allow the compression of high-energy femtosecond pulses to a duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow-core fused silica fiber filled with atomic and molecular gases is studied under two excitation regimes with high-energy input pulses of 140 fs and 20 fs duration respectively. Conditions for optimum pulse compression are outlined considering the role of self-phase modulation and gas dispersion in the two regimes. With 20 fs input pulses and under optimum compression conditions we demonstrate a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high-throughput prism-chirped-mirror delay line. These pulses are the shortest generated to date at multigigawatt peak power. PACS: 42.65.Re; 42.65.Vh Ultrashort-pulse lasers are the most important experimental tools for investigating fast-evolving atomic and molecular dynamics in physics, chemistry, and biology. In the last few years, great technological advances have been made in the field of ultrafast pulse generation. New mode-locking techniques such as additive-pulse mode-locking and Kerr-lens mode-locking have been successfully used for femtosecond pulse generation from a wide range of solid-state laser oscillators [1]. Using chirped mirrors [2] for intracavity dispersion control, pulses down to 7.5 fs have been directly generated by a Kerr-lens mode-locked Ti:sapphire oscillator [3] and, more recently, 6.5-fs pulses have been obtained using broadband semiconductor saturable absorbers for self-starting [4]. Ti:sapphire amplifiers seeded by femtosecond laser oscillators can now generate pulses of 20–30 fs with gigawatt [5, 6] or terawatt [7–9] peak power at repetition rates in the kHz and 10 Hz regimes, respectively. Ultrashort pulses can also be generated by extracavity compression techniques, in which the pulses are spectrally broadened upon propagation in a suitable nonlinear waveguide and subsequently compressed in a carefully designed optical dispersive delay line. Spectral broadening of laser pulses by self-phase modulation (SPM) in a single-mode optical fiber is a well-established technique: pulses down to 6 fs were obtained in 1987 from 50-fs pulses from a mode-locked dye laser [10]. More recently 13-fs pulses from a cavity-dumped Ti:sapphire laser were compressed to 5 fs with the same technique [11]. However, the use of single-mode fibers limits the pulse energy to a few nanojoules. A powerful pulse compression technique based on spectral broadening in an hollow fiber filled with noble gases has demonstrated the capability of handling highenergy pulses (sub-mJ range) [12]. This technique presents the advantages of a guiding element with a large diameter mode and of a fast nonlinear medium with high threshold for multiphoton ionization. New concepts in the construction of dispersive delay lines have been applied in the development of specially designed chirped mirrors for fine control of cubic and quartic phase dispersion terms over a large spectral bandwidth [3]. The implementation of the hollow-fiber technique using 20-fs seed pulses from a Ti:sapphire system [5] and a high-throughput broadband dispersive delay line consisting of prisms and chirped mirrors has recently permitted the generation of multigigawatt sub-5 fs pulses [13]. In this paper we present a comprehensive analysis of compression experiments with high-energy femtosecond pulses performed using gas-filled hollow fibers. Spectral broadenings obtained in different gases are compared for 140-fs and 20-fs input pulses generated by Ti:sapphire laser systems, and the optimum conditions for pulse compression are outlined considering the role of SPM and gas dispersion. A new ultrabroadband prism-chirped-mirror dispersive delay line, characterized by a high throughput and dispersion control up to the fourth order, is described in detail. The paper is organized as follows. In Sect. 1 we provide a description of hollow fiber modes and discuss the major advantages of this device compared to optical fibers. Sect. 2 reports on typical spectral broadenings achieved under different excitation conditions. In Sect. 3 we report on the characteristics of the prism-chirped-mirror compressor and discuss the experimental results obtained with 20-fs input pulses. Under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ. These pulses are the

Oxozirconium Methacrylate Clusters: Zr6(OH)4O4(OMc)12 and Zr4O2(OMc)12 (OMc = Methacrylate)

Abstract: The crystalline oxozirconium methacrylate clusters Zr6(O-H)4O4(OMc)12 and Zr4O2(OMc)12 were obtained by reaction of Zr(OnPr)4 with an excess of methacrylic acid and analysed by X-ray diffraction. The oxide and hydroxide groups are in a μ3- bridging mode in both structures, and the methacrylate ligands are chelating or bridging. The zirconium atoms in Zr6(OH)4O4(OMc)12 form an octahedron the cluster having crystalographic C3v symmetry. Each metal atom is square-antiprismatically coordinated by eight oxygen atoms. In Zr4O2(OMc)12, the zirconium atoms have a distorted butterfly arrangement; their coordination geometry is square-antiprismatically coordinated by eight oxygen atomes. In Zr4O2)OMc)12, the zirconium atoms have a distorted butterfly arrangement; their coordination geometry is square-antiprismatic or capped octahedral.

Process controls on regional flood frequency: Coefficient of variation and basin scale

Abstract: The coefficient of variation (CV) of maximum annual floods is examined to understand the effects of process controls and catchment size. A derived flood frequency model is used to interpret data from 489 catchments in Austria. At the core of process controls appears to be the interaction of catchment response time and storm duration, but the magnitude is not large, and often this interaction is hidden by other processes. The dependence of rainfall intensity and duration is clearly very important and reduces CV significantly. Increasing channel travel times with catchment scale tend to translate into decreasing CVs with area for small catchments while they tend to translate into increasing CVs with area for larger catchments. Nonlinear runoff processes, including threshold effects, is the main mechanism for increasing CV. They give rise to complex patterns in the relationship between CV and area. Base flow has been used as a surrogate for a number of processes, such as seasonality of streamflow. It always decreases CV and, in particular, leads to a significant decrease of CV with area. Both the observed tendency of CV to decrease with area and the scatter in the data are the result of a complex interplay of a number of processes which allows various alternative interpretations. Depending on which processes dominate under a particular hydrologic regime, different patterns arise. It appears that the explanations of the relationship between CV and catchment scale suggested in the literature are too simplistic. The case is made for using the concept of hydrologic regimes and process studies of the type presented here to help delineate homogeneous regions for regional flood frequency analyses in a physically consistent way.

H 2 dissociative adsorption on Pd(111)

Abstract: The dissociative adsorption of hydrogen molecules on the (111) surface of palladium is studied by total-energy calculations using the density-functional theory within the framework of the generalized gradient approximation. A variety of dissociation pathways is investigated. Both activated and nonactivated pathways are found. We carry out a detailed analysis of the electronic structure along the different pathways to understand the microscopic mechanism of bond breaking and bond formation. This analysis allows us to account for the energetic ordering of the transition states. Precursor states are also identified for the system considered here.

An analytical thermodynamic model of laser welding

Abstract: An earlier model of deep-penetration laser welding has been simplified in order to provide a useful model of process analysis. This work involves the modelling of the various energy-absorption mechanisms which determine the keyhole shape and thus the dimensions of the melt pool. The penetration depth and weld width (top and bottom) predicted by the model are shown to be in close agreement with experimental results. The widening of the top of the weld seam as a result of Marangoni flow is accurately modelled by introducing an artificially enhanced value for the workpiece's thermal conductivity towards the top of the weld. The model allows analysis of the dependence of the weld profile on the process parameters.

Ab initio density functional studies of transition-metal sulphides: I. Crystal structure and cohesive properties

Abstract: The structural and cohesive properties of more than thirty transition-metal sulphides of various stoichiometries and crystal structures have been investigated using density functional theory, with the aim of establishing a correlation between the strength of the metal - sulphur bond and the catalytic activities of these materials. It is shown that the local density approximation has a tendency to overestimate the strength of the bonding. The overbinding manifests itself in the prediction of too small atomic volumes and too large cohesive energies. Non-local corrections to the local exchange - correlation functional in the form of a generalized-gradient approximation correct the overbinding (albeit with a certain tendency to overcorrect, especially for the sulphides of the heavy transition metals) and result in accurate structural prediction and cohesive energies. A correlation between the sulphur - metal bond strength and the catalytic activities is established.

Ab initio density functional studies of transition-metal sulphides: II. Electronic structure

Abstract: A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).

Halo Properties of the First 1 / 2 + State in F 17 from the O 16 ( p , γ ) F 17 Reaction

Abstract: The capture cross section of the reaction ${}^{16}\mathrm{O}(p,\ensuremath{\gamma}{)}^{17}\mathrm{F}$ was measured in the energy range from ${E}_{\mathrm{c}.\mathrm{m}.}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}200--3750\mathrm{keV}$ using the windowless gas target facility Rhinoceros. The low-energy $S$ factor that is dominated by the transition to the $1/{2}^{+}$ first excited state in ${}^{17}\mathrm{F}$ increases strikingly with decreasing energy. This behavior is explained by the halo properties of this $1/{2}^{+}$ state within the framework of the direct capture model.

Revision of Trichoderma sect. Longibrachiatum including related teleomorphs based on analysis of ribosomal DNA internal transcribed spacer sequences.

Abstract: Variation within the internal transcribed spacer (ITS-1 and ITS-2) regions of ribosomal DNA of 103 strains was studied to examine relationships within Trichoderma sect. Longibrachiatum and related teleomorphs, Hypocrea schweinitzii and H. jecorina. The four species, T longibrachiatum, T. pseudokon- ingii, T parceramosum and T citrinoviride, that were originally assigned by morphological criteria to this section were recognized by sequence analysis. Trich- oderma reesei, which was previously considered to be synonymous with T longibrachiatum, and T saturnis- porum which was placed originally in sect. Saturnis- porum because of its warted conidia, are assigned to the section. ITS sequences of T ghanense, originally assigned to sect. Saturnisporum, could not be distin- guished from the sequence of T parceramosum, in- dicating that the two may be synonymous. Trichoder- ma "todica", an unpublished name based on the an-