Multiple-pattern stability in a photorefractive feedback system

We report on the observation of a multiple-pattern stability region in a photorefractive single-feedback system. Whereas hexagonal patterns are predominant for feedback with positive diffraction length we show that a variety of stable non-hexagonal patterns are generated for certain negative diffraction lengths. For the same values of the control parameters square, rectangular, or squeezed hexagonal patterns are found alternating in time. Besides these pure states, we found a number of different mixed-pattern states. We review the linear stability analysis for this system and show that the special shape of the threshold curves in the investigated parameter region gives a first explanation for the occurrence of a multiple-pattern region. Received: 22 June 1999 / Revised version: 30 August 1999 / Published online: 3 November 1999     

The \gamma \gamma \to \gamma \gamma process in the standard and SUSY models at high energies

We study the helicity amplitudes of the process at high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the diagrams involve , charged-quark, and lepton loops, while in SUSY, we also have contributions from chargino-, charged-sfermion, and Higgs-loop diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners; there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant one-loop functions are given which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space at high energies. The various characteristics of a large set of observables are studied in detail. Received: 3 February 1999 / Pulished online: 30 June 1999     

Probing a nucleon spin structure at TESLA by the real polarized gamma beam

The recent proposals concerning the usage of the real polarized gamma beam, obtained by Compton backscattering of the laser photons off the electron beams from either the linear or circular accelerators were considered. The heavy quark photoproduction process, which gives a unique opportunity to measure the polarized gluon distribution, was investigated. Received: 22 March 1999 / Revised version: 17 June 1999 / Published online: 14 October 1999     

Continuous wave laser radiation at 693 nm from LiNbO3:ZnO:Nd3+ nonlinear laser crystal

Red laser radiation at 693 nm has been obtained from a LiNbO:ZnO:Nd crystal by self-frequency-doubling its fundamental laser line at 1386 nm. Pumping was achieved by means of a Ti:sapphire laser. In preliminary experiments a red laser power of 2.3 mW has been obtained for a total absorbed pump power of 800 mW. External second-harmonic experiments (1386–693 nm) are used to investigate some basic non-linear properties such as phase-matching angle, angular and thermal acceptances of this process. Received: 21 June 1999 / Revised version: 15 August 1999 / Published online: 30 November 1999     

Hankel transform of focus wave modes

The Hankel transform generally used to analyse the transverse behaviour of Gaussian beams also supplies new classes of scalar focus wave modes. Received: 4 January 1999 / Revised version: 4 March 1999 / Published online: 2 June 1999     

Quantum dot lasers: recent progress in theoretical understanding and demonstration of high-output-power operation

The field of semiconductor quantum dot (QD) diode lasers is rapidly developing. Important milestones, such as low-threshold operation and room-temperature cw operation, have been achieved in the last years. We review the progress in theoretical understanding and present recent results on high-power QD laser operation (>3 W@1100 nm). Received: 24 June 1999 / Revised version: 23 August 1999 / Published online: 20 October 1999     

Phase conjugation in a continuous-wave diode-pumped Nd:YVO4 laser

We demonstrate phase conjugation and dynamic holography by degenerate four-wave mixing in a continuous-wave diode-pumped Nd:YVO₄ laser for the first time. A phase-conjugate reflectivity of 1.4% is obtained with a 12 W diode power. The experimental results and the optimizing parameters, such as the cavity mirror reflectivities, are compared with results from a steady-state theoretical analysis. Received: 6 April 1999 / Published online: 24 June 1999     

Global analysis of eeqq contact interactions and future prospects for high energy physics

Data from HERA, LEP and the Tevatron as well as from low energy experiments are used to constrain the scale of possible electron–quark contact interactions. Different models are considered, including the most general one, in which all new couplings can vary independently. Limits on couplings and mass scales are extracted and upper limits on possible effects to be observed in future HERA, LEP and Tevatron runnings are estimated. The total hadronic cross section at LEP and the scattering cross section at HERA are strongly constrained by the existing data, whereas large cross section deviations are still possible for Drell–Yan lepton pair production at the Tevatron. Received: 10 May 1999 / Revised version: 24 June 1999 / Published online: 22 October 1999     

The emission of correlated electrons from surfaces

This work offers a theoretical framework for the treatment of the simultaneous many-electron excitation in solids and surfaces. Starting from the transition operator, an expansion is presented that treats all involved interactions on equal footing. The method is applied to the simultaneous two-orbital excitation and the results are compared with recent experimental findings. It is concluded that such studies provide a useful tool for the investigation of inter-electronic coupling. Received: 23 March 1999 / Accepted: 25 June 1999 / Published online: 23 September 1999     

Determination of the Auger upconversion rate in fiber-coupled diode end-pumped Nd:YAG and Nd:YVO4 crystals

An analytical model is developed to study the influence of the Auger upconversion process on the thermal loading under lasing and nonlasing conditions. With the developed model, Auger upconversion rates can be determined by comparing theoretical calculations with experimental results for the ratio of the thermal loading under lasing and nonlasing conditions. The upconversion rates obtained with the present method are compared with the results measured from the fluorescence decay experiment. Received: 7 June 1999 / Revised version: 3 August 1999 / Published online: 30 November 1999     

Electronic properties of Cu(In,Ga)Se2 heterojunction solar cells–recent achievements, current understanding, and future challenges

The recent achievements of high-efficiency Cu(In,Ga)Se₂ heterojunction solar cells are reviewed with a special focus on the understanding of the electronic transport properties of the devices. We discuss the basic limitations of the device performance, the present understanding of electronic device analysis, as well as the role of intrinsic defects and of the interfaces for the performance of the solar cells. Received: 12 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Photorefractive fixed optical correlator

A photorefractive optical correlator stored and fixed in lithium niobate is presented. The device shows good correlation characteristics together with very high output efficiency and insensitivity to optical erasure during read-out. Received: 10 May 1999 / Revised version: 21 June 1999 / Published online: 16 September 1999     

Laser mode manipulation by intracavity dynamic holography: Application to mode selection

Intra-cavity dynamic holography is proposed as a self-adaptive, easy to set up technique for efficient laser mode manipulation. A first implementation is presented here as a longitudinal mode selector providing a spectacular spectral narrowing in linear laser cavities operating both in the pulsed and the cw regimes. Received: 12 April 1999 / Published online: 24 June 1999     

Laser ablation of phenylazide in an argon matrix: direct observation and chemical reactivity of ablated fragments

Ablation of pentafluorophenylazide (FPA) in an Ar matrix at 8–10 K was carried out upon irradiation with ns-pulsed UV lasers in a vacuum. The plume of ablated products was monitored by a time-resolved imaging/spectroscopic technique using a gated and intensified CCD camera system. A large amount of pentafluorophenylnitrene (FPN) having a high kinetic energy (≈6 eV) was ejected as fragments from the matrix film during ablation. A quantitative formation of triplet FPN from the photolysis of the FPA was observed by spectroscopic measurements in the IR and UV-visible regions, and was confirmed by a theoretical IR spectrum calculated according to density functional theory. A FPN beam is useful for chemical surface modification of organic materials, such as aromatic polyester and alkylthiol. A surface analysis of these materials by X-ray photoelectron spectroscopy and Fourier transform infrared reflection absorption spectroscopy showed that the FPN was immobilized onto the surface through chemical bonds. This technique for the chemical surface modification of materials is made possible by a pulsed beam of reactive fragments with a high density in the laser ablation process. Received: 14 June 1999 / Accepted: 18 June 1999 / Published online: 21 October 1999     

Stability of localized structures in degenerate wave mixing

We give experimental proof of the stability of (supercritical) spatial phase solitons in degenerate wave mixing. Received: 16 June 1999 / Published online: 25 August 1999     

Radiative effects in the processes of hadron electroproduction

An approach to calculate radiative corrections to the unpolarized cross section of semi-inclusive electroproduction is developed. Explicit formulae for the lowest order QED radiative correction are presented. A detailed numerical analysis is performed with the kinematics of experiments with fixed targets. Received: 17 March 1999 / Revised version: 18 June 1999 / Published online: 28 September 1999     

On particle multiplicities in three-jet events

A thorough verification of the distinct differences in the properties of quark and gluon jets is considered as one of the most instructive tests of the basic ideas of QCD. In the real life experiments such a comparison appears to be quite a delicate task and various subtle issues require further theoretical efforts. In this paper we discuss in detail the possibility to extract the theoretically adequate information from the particle multiplicity patterns in three-jet events in annihilation. Received: 8 June 1999 / Published online: 14 October 1999     

Comment on “a unified scheme for flavored mesons and baryons”

We indicate the inconsistencies and mistakes in the paper [1] and conclude that their numerical results should be recalculated. Received: 1 June 1999 / Revised version: 24 September 1999     

Surface-state linewidth from scanning tunnelling spectroscopy

The STM can be used to investigate lifetimes of hot holes in a surface state at low temperatures. We analysed dI/dV data from Ag(111) using detailed tunnelling calculations and a simple model and found an electron self-energy of Σ=4.9±0.6 meV. The corresponding lifetime τ= 67±8 fs is considerably higher than those determined by angle-resolved photoemission, although it remains below theoretical predictions. Spatially resolved dI/dV spectra reveal that the lifetime decreases drastically in proximity to defects such as surface steps. Received: 22 March 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Synthesis of carbon nitride films by low-power inductively coupled plasma-activated transport reactions from a solid carbon source

Thin films of carbon nitride were prepared by low-power inductively coupled plasma chemical vapor deposition from a solid carbon source by utilizing transport reactions. The maximum deposition rate achieved was 10 nm/min and depended mainly on the substrate position in the reactor. The nitrogen fraction in the films was not so sensitive to the process parameters and was at about 0.5 for all experiments as measured by Auger electron spectroscopy (AES) and elastic recoil detection (ERD) analysis. The chemical bonding structure studied by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed the presence of triple, double and single bonds between carbon and nitrogen atoms. Received: 12 May 1999 / Accepted: 12 May 1999 / Published online: 24 June 1999