Filamentation of femtosecond laser pulses in turbulent air

Formation and wandering of filaments in air are studied both experimentally and numerically. Filament-center deflections are collected from 1100 shots of 190-fs and 800-nm pulses in the plane perpendicular to the propagation direction. To calculate the filament wandering in air we have developed a model of powerful femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses. Statistical processing of experimental and numerical data shows that filament-center displacements in the transverse plane obey the Rayleigh-distribution law. Parameters of the Rayleigh distribution obtained for numerical and experimental data are close to each other. Received: 23 May 2001 / Revised version: 26 September 2001 / Published online: 29 November 2001     

A novel approach for the development of photoluminescent material

We report on several amorphous compounds based on different metal oxianions with intense photoluminescence at room temperature. These compounds were synthesised by a soft chemical process and deposited on Si (100) by a spin-coating technique. To select these different metal oxianions, a classic concept based on a metal oxide network former is used. We describe a minimum set of requirements to obtain an amorphous metal oxide with photoluminescence emission at room temperature. Received: 27 August 2001 / Accepted: 29 August 2001 / Published online: 20 December 2001     

Luminescence properties of thin films prepared by laser ablation of Nd-doped potassium gadolinium tungstate

Optically active thin films on Si substrates have been produced by laser ablation of a Nd-doped potassium gadolinium tungstate (Nd:KGW) single crystal. Films grown at low oxygen pressures (<0.6 mbar) are potassium-deficient and appear to be mainly disordered. They show a poor photoluminescence (PL) performance that improves upon annealing in air at temperatures in the range 700–1000 °C. Films grown at high oxygen pressure (1 mbar) show instead good stoichiometry and the presence of a dominant textured gadolinium-tungstate phase compared to KGW. These films have low absorption, a refractive index close to that of bulk KGW and good PL performance, the emission lifetimes being longer (τ>150 μs) under certain conditions than those measured in the single-crystal material. Received: 25 July 2001 / Accepted: 26 July 2001 / Published online: 17 October 2001     

Nondestructive remote imaging of ferroelectric domain distributions with high three-dimensional resolution

The domain distribution in the bulk and at the surface of ferroelectric crystals is imaged by second-harmonic microscopy with three-dimensional far-field optical resolution. The nondestructive technique allows us to investigate the success of a poling procedure in a fabrication state of an integrated optical device in which the poling electrodes have not been removed. Additionally, a confocal linear optical imaging technique is introduced, which reveals the surface topography with ±0.7 nm height sensitivity. At the surface of a periodically poled specimen, we detect unexpected and unwanted surface topographies that correspond to the domain structure. The search for improved fabrication parameters that guarantee smooth surfaces could be substantially promoted with our topography-detection technique. Received: 16 May 2001 / Published online: 23 October 2001     

Polarisation-sensitive optical coherence tomography for material characterisation and strain-field mapping

Optical coherence tomography (OCT) is a relatively new imaging technique capable of recording cross-sectional images of transparent and turbid structures with micrometer-scale resolution. Originally developed for biomedical imaging applications, this technique also has a great potential for non-destructive material characterisation and testing. Polarisation-sensitive (PS) OCT is a recent extension of classical OCT that measures and images birefringence properties of a sample, which, however, has not yet been applied to materials science. We present imaging of glass-fibre-enforced epoxy resin compounds and the detection of dry spots, where the epoxy did not properly penetrate the glass-fibre structure. Furthermore, we demonstrate PS-OCT imaging of the birefringence properties of different materials. The mapping of strain fields of samples under uniaxial and non-uniform external stress and the detection of flow patterns in injection-moulded plastic parts could be demonstrated with this technique for the first time. Received: 21 October 2002 / Accepted: 22 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +43-732/9015-5618, E-mail: david.stifter@uar.at     

Detection of NO in a spark-ignition research engine using degenerate four-wave mixing

We report the first application of degenerate four-wave mixing (DFWM) to combustion diagnostics in a methane-fuelled internal combustion research engine. Combustion-generated NO in the spark-ignited engine was detected using scanning narrowband DFWM in a modified forward folded BOXCARS geometry. The resulting spectra of the X²Π-A²Σ⁺(0,0) band at 226 nm display an excellent signal-to-noise ratio. Extension of the technique to different engine operating conditions and to time-resolved multiplex DFWM is discussed. Received: 3 May 2001 / Revised version: 1 October 2001 / Published online: 29 November 2001     

Applications of Kalman filtering to real-time trace gas concentration measurements

A Kalman filtering technique is applied to the simultaneous detection of NH₃ and CO₂ with a diode-laser-based sensor operating at 1.53 μm. This technique is developed for improving the sensitivity and precision of trace gas concentration levels based on direct overtone laser absorption spectroscopy in the presence of various sensor noise sources. Filter performance is demonstrated to be adaptive to real-time noise and data statistics. Additionally, filter operation is successfully performed with dynamic ranges differing by three orders of magnitude. Details of Kalman filter theory applied to the acquired spectroscopic data are discussed. The effectiveness of this technique is evaluated by performing NH₃ and CO₂ concentration measurements and utilizing it to monitor varying ammonia and carbon dioxide levels in a bioreactor for water reprocessing, located at the NASA–Johnson Space Center. Results indicate a sensitivity enhancement of six times, in terms of improved minimum detectable absorption by the gas sensor. Received: 13 July 2001 / Revised version: 11 October 2001 / Published online: 29 November 2001     

Effective A-state fluorescence lifetime of formaldehyde in atmospheric pressure CH4/air flames

Using excitation pulses of ∼30-ps duration and a fast photomultiplier detector, effective fluorescence lifetimes of the A-stateof formaldehyde after excitation at 355 and 339 nm have been measured in the preheating zone of an atmospheric pressure, premixed methane/air flame. The fluorescence lifetimes were determined as a function of height above the exit of a slot burner and were thus probed in regions of varying gas temperature and composition. The fluorescence lifetimes were independent of the intensity of the excitation pulse and decreased as a function of height in the burner from ∼18±8 ns at 1.2 mm down to 7±1 ns at 3.8 mm. This trend of the effective fluorescence lifetime with composition and temperature in the flame can qualitatively be reproduced using calculated major species mole fractions and species-specific quenching cross sections for CH from the literature. Received: 13 June 2001 / Revised version: 27 September 2001 / Published online: 29 November 2001     

Detection of small forest fires by lidar

The possibility of detecting small forest fires with the help of a simple and cheap lidar operating at 0.532-μm wavelength up to distances of about 6.5 km is demonstrated. The values of the signal-to-noise ratio (SNR) achieved in the experiments are consistent with theoretical estimations obtained by computational modeling of the lidar detection process, including simulation of the smoke-plume shape and of the laser beam–plume interaction. This model was used to assess the potential of the lidar technique for fire surveillance in large forest areas. In particular, the upper limiting range for effective detection (SNR>5) of small localized fires in dry- and clear-weather conditions is estimated at 7–15 km depending on operation mode, burning rate, and observation geometry. Received: 29 August 2001 / Published online: 29 November 2001     

Holographic recording mechanisms of gratings in indium oxide films using 325 nm helium–cadmium laser irradiation

UV (325 nm) holographic recording of gratings in indium oxide films fabricated by reactive pulsed laser deposition has been investigated as a function of growth temperature, oxygen pressure and angle of incidence of the plasma plume on the substrate. The influence of the ambient environment (air or vacuum) and the film temperature during recording has also been studied. Large steady state refractive index changes up to 6×10^-3 were observed in layers grown at an oblique angle of 75°. About 77% of the magnitude of these changes residues after thermal annealing and is attributed to UV-induced permanent structural rearrangements. In contrast, refractive index changes in films grown at normal incidence were smaller in magnitude and completely reversible. Received: 30 July 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Artifacts in femtosecond transient absorption spectroscopy

The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed. Received: 29 May 2001 / Final version: 15 October 2001 / Published online: 29 November 2001     

Spin-resolved NEXAFS from resonant X-ray scattering (RXS): Application to MnO

Resonantly excited metal K core line spectra of NiO, MnO, CuO and other compounds have been investigated at the beamlines X21 (NSLS/BNL), BW1 and W1.1 (HASYLAB/DESY). A novel technique for quantitative resolution of NEXAFS spectra into spin-up and spin-down components has been developed. Since the method employs spin conservation and local spin references, it needs no circularly polarized radiation and no sample magnetization for taking both the RXS and NEXAFS spectra. Hence antiferromagnetic and paramagnetic materials can be investigated as well.By utilizing linear dichroism with angular-dependent measurements on single-crystal samples, additional resolution of NEXAFS spectra is possible with respect to the orbital symmetry. Application of the method to paramagnetic MnO, for the first time, provides new and unambiguous experimental results confirming modern (LSDA+U) calculations. Received: 29 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Geometrical optical modeling of femtosecond setups having angular dispersion

A reliable and efficient method based on a geometrical optical approach is presented to model the propagation of ultrashort pulses in optical systems. It is shown that the commonly used method to determine the group delay of the spectral components of a pulse from their geometrical optical path lengths is only correct for aberration-free optical systems. In the case of systems with angular dispersion and imaging errors, a correction to the path values obtained from ray-tracing calculations must be applied, since for specific systems neglect of the correction causes significant errors. A technique for performing this correction is presented. Two optical arrangements used for the generation and detection of tunable THz radiation by the femtosecond tilted-pulse-front excitation technique are analyzed to demonstrate the indispensability of the correction. Received: 29 July 2002 / Revised version: 22 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +36-72/501-571, E-mail: kozma@fizika.ttk.pte.hu     

Dynamic hysteresis in ferroelectric systems: experiment and Monte Carlo simulation

A Monte Carlo algorithm for dynamic hysteresis simulation in ferroelectric spin systems is developed based on a DIFFOUR model in which the local spontaneous polarization is defined by the double-well potential energy and the nearest-neighbor spin interaction as well as an external electrical field of variable amplitude and frequency. A direct measurement of the hysteresis loop for ferroelectric Pb(Zr_0.52Ti_0.48)O₃ thin film capacitors using the Sawyer–Tower technique is performed. Significant dependence of the hysteresis shape and pattern on the external field is revealed. Direct imaging of the simulated domain pattern indicates serious suppression of the domain switching over the high-frequency range. The evaluated scaling relations from the simulation for remanence, coercivity, and the area of the hysteresis over the low-frequency range are supported by theoretical predictions and experiments, but the high-frequency scaling behaviors as derived are different from one and another. Received: 23 January 2001 / Accepted: 17 August 2001 / Published online: 20 December 2001     

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded. Received: 26 June 2001 / Revised version: 3 September 2001 / Published online: 29 November 2001     

Optical limiting in cobalt-doped polyvinylpyrrolidone

The results of optical limiting investigations of cobalt-doped polyvinylpyrrolidone solutions are presented. The optical limiting studies have shown that this process is due to self-defocusing at the wavelength of 1064 nm and also due to reverse saturable absorption and self-defocusing at the wavelength of 532 nm. The results of measurements on the non-linear optical characteristics of organometallic complexes are presented. Received: 10 July 2001 / Revised version: 15 October 2001 / Published online: 29 November 2001     

Single-mode hollow optical fibres for atom guiding

We present a novel design for a single-mode, hollow optical fibre, which is suitable for use as a waveguide for atomic de Broglie waves. The design, development and characterisation of such a fibre are discussed, as well as an optimised method for coupling light into the fibre. Received: 22 May 2001 / Revised version: 5 September 2001 / Published online: 29 November 2001     

High harmonic generation at 1 kHz repetition rate with a pulsed valve

High harmonic generation at 1 kHz repetition rate is reported. A piezoelectric pulsed valve together with a femtosecond Ti:Sapphire laser both operating at 1 kHz are employed to generate high harmonic radiation in xenon and argon. The characteristics of the valve have been analyzed by using a fast ion gauge and a time-of-flight mass-spectrometer. Received: 28 May 2001 / Revised version: 29 June 2001 / Published online: 18 July 2001     

Quantitative oxygen imaging in an engine

Simultaneous imaging of laser-induced fluorescence of toluene and 3-pentanone was used to determine the local absolute oxygen and residual gas concentrations present within an engine. The technique utilizes the different sensitivities of the laser-excited molecules to quenching by molecular oxygen as a means to determine quantitative images of in-cylinder oxygen concentrations. The difference in the amount of oxygen available between two operating conditions was investigated. Results are in agreement with measurements in the exhaust gas. Received: 4 June 2002 / Published online: 8 August 2002     

Electric field of a buried interfacial region measured by positron-lifetime spectroscopy

Positrons from a radioactive source are implanted into a reverse-biased metal–semiconductor contact and are drifted back towards the contact by the internal electric field where they trap into voids and annihilate. The electric field dependent interface annihilation fraction is monitored by way of the intensity of the long (∼400–500 ps) void lifetime component using positron-lifetime spectroscopy. Unlike previous analyses of such systems a numerical model involving positron drift, annihilation and trapping into the interfacial state has been constructed to describe the positron dynamics in the presence of the non-uniform junction electric field. The use of the positron-lifetime technique in probing the internal electric field at buried contacts is thus demonstrated. Results obtained using the numerical method for the Au, Al and Ni/Semi-Insulating (SI)-GaAs contact systems are found to be consistent with the findings of previous studies on the Au/SI-GaAs system. Received: 29 November 2000 / Accepted: 26 February 2001 / Published online: 25 July 2001