Design of polarization independent InGaAs/InGaAlAs coupled quantum well structure in 1.55 μm wavelength region

A novel polarization independent InGaAs/InGaAlAs quantum well (QW) structure in the 1.55 m wavelength region is proposed. A coupled QW structure with tensile strain in the QW and/or barrier region is considered for the reduction of the optical gain difference between TE and TM modes in the wide spectral range. A triple-coupled QW structure with alternative strain (tensile/compressive/tensile) is found to be the most effective in reducing the polarization gain difference. This is because the transition strength difference of each polarization is reduced by energy states coupling. The optimized triple-coupled QW structure shows polarization independence for wide carrier density and wavelength range, which is suitable for polarization independent operation of QW based semiconductor devices, such as semiconductor optical amplifiers.     

Spectral asymmetry,condensates and the QCD chiral Lagrangian

A chiral Lagrangian for massive pseudoscalar mesons is derived from the QCD-generating functional by the chiral bosonization method. The low energy region in the fermionic integral where chiral noninvariant quark fluctuations dominate, is described by two mass parameters — an invariant cutoff and a spectral asymmetry M related to the gluon and quark condensates.     

Statistics of velocity gradients in wall-bounded shear flow turbulence

The statistical properties of velocity gradients in a wall-bounded turbulent channel flow are discussed on the basis of three-dimensional direct numerical simulations. Our analysis is concentrated on the trend of the statistical properties of the local enstrophy and the energy dissipation rate with increasing distance from the wall. We detect a sensitive dependence of the largest amplitudes of both fields (which correspond with the tail of the distribution) on the spectral resolution. The probability density functions of each single field as well as their joint distribution vary significantly with increasing distance from the wall. The largest fluctuations of the velocity gradients are found in the logarithmic layer. This is in agreement with recent experiments which observe a bursting of hairpin vortex packets into the logarithmic region.     

Excitation of a singly charged Sr Ion by e-Sr collisions

Excitation of spectral lines of SrII, including laser lines, is studied experimentally in collisions of slow electrons with strontium atoms. Twenty-one excitation cross sections are measured at an energy of electrons of 30 eV. Five optical excitation functions are recorded at an energy ranging from 0 to 200 eV. Direct-excitation cross sections for the 5p ² P°_{1/2, 3/2} levels and the contribution to their population by cascade transitions are calculated. Excitation cross sections of two laser transitions in the IR spectral region are determined using known branching ratios. The results obtained are compared with data of previous experiments.     

The study of photoluminescence spectra depth profiles of epitaxial layers GaP: Zn,O

The method of depth profiling of low temperature photoluminescence spectra was applied to the study of samples GaP: Zn, O prepared by LPE technology. The obtained results were used to calculate the depth profiles of zinc and sulphur concentration in the samples. Several layers showed considerable variations of the red emission band shape with the depth of measured region (atT 85 K) due to the changes in mutual intensity ratio of several bands lying in this spectral region. Primarily the bands corresponding to exciton and pair mechanism of radiative recombination on the complexes (O-Zn) and a shortwave emission band with the peak near the energy of 1·89 eV are concerned.The authors wish to thank J. Waraus and Dr. D. Nohavica for supplying samples of GaP: Zn, O (LPE) layers and Dr. M. Cukr for the implementation of mass spectral analysis of some materials.     

Quantum Chemical Calculation of the Spectral Luminescent and Physicochemical Properties of Anisidines

The influence of the position of a methoxy group in the phenyl radical of aniline on the spectral, geometric, and photophysical characteristics, and also on the proton-donating and proton-accepting properties of the free molecules of o-, m-, and p-anisidines is investigated by the methods of electron spectroscopy and quantum chemistry. The energy levels of the singlet and triplet states of anisidines are calculated. For all the anisidines the band in the intermediate spectral region is shown to be formed by two electron transitions of the ^*- and ^* type of different intensity.     

An airborne far-infrared spectrometer for astronomical observations

A Fabry-Perot spectrometer for airborne observations of astronomical spectral lines has been designed for use in the 50 to 200 m region of the far-infrared. The spectrometer uses a fixed wavelength Fabry-Perot filter cooled to 4.2 K in conjunction with a high order scanning Fabry-Perot at room temperature. This arrangement provides high spectral resolution and also minimizes the thermal background radiation on the detector. The instrument has been successfully used to study all the presently detected fine structure lines between 50 and 120 m.Work supported in part by NASA Grant NGR 05-003-511     

Determining the optimum wavelength pairs to use for molecular absorption thermometry based on the continuous-spectral lower-state energy

A novel technique for selecting wavelengths that optimize the temperature precision of absorption-based thermometry is developed. A single universal objective function considering continuous spectral lower-state energy , spectral absorbance intensities and an experimental noise level is derived and applied in a brute-force approach to analyze two-wavelength, Doppler-broadened H₂O vapor-absorbance thermometry in the 1.33-1.37 μm range. The results reveal the top wavelength pairs and their temperature precisions over a wide temperature range from 280 to 2800 K. Although the spectral database used includes over thousands of transitions in this spectral range, the top pairs are composed of only 12 unique wavelengths. This wavelength selection technique is able to help researchers choose key wavelengths that will perform well in practical applications.     

Systematics in the core level spectra of Th-intermetallics

We have studied the Th 4f core level spectra in a number of related Th-intemetallics, which exhibit interesting trends in terms of the intensity and the energy separation of the satellite with respect to the main signal. With the help of model calculations, we correlate the changes in the core level spectra to the density of states (DOS) in the valence band region. We have studied in detail the effect of changes in the strenght of 5f-conduction band hybridization (), the energy position of the empty 5f level ( _f ) and the DOS in the vicinity ofE _F on the spectral shape of Th 4f region. These parameters are also found to affect the binding energies of the main peaks.     

Distinguishing between dark matter and pulsar origins of the ATIC electron spectrum with atmospheric Cherenkov telescopes

Recent results from the Advanced Thin Ionization Calorimeter (ATIC) balloon experiment have identified the presence of a spectral feature between approximately 300 and 800 GeV in the cosmic ray electron spectrum. This spectral feature appears to imply the existence of a local (1 kpc) source of high energy electrons. Emission from a local pulsar and dark matter annihilations have each been put forth as possible origins of this signal. In this Letter, we consider the sensitivity of ground based atmospheric Cherenkov telescopes to electrons and show that observatories such as HESS and VERITAS should be able to resolve this feature with sufficient precision to discriminate between the dark matter and pulsar hypotheses with considerably greater than 5σ significance, even for conservative assumptions regarding their performance. In addition, this feature provides an opportunity to perform an absolute calibration of the energy scale of ground based, gamma ray telescopes.     

Photoacoustic studies and selective ablation of vascular tissue using a pulsed dye laser

The optical attenuation coefficient for human aorta has been measured in the 440–500 nm spectral region using the transient photoacoustic (thermoelastic) response produced by an excimer-laser-pumped dye laser. It is confirmed that diseased tissue has a significantly higher attenuation than normal samples in this spectral region. The maximum difference occurs at 480 nm and there is a lowering of the ablation threshold fluence for diseased tissue by a factor of 1.5 for dye laser ablation at this wavelength.     

Laser Tunable Source for the Red and Near UV Regions of the Spectrum

The spectral and lasing characteristics of a number of dyes for the red and near IR portions of the lasing spectrum in laser and lamp pumpings are investigated. Smooth tuning of a pulse-periodic lamp-pumped laser in the 660–720 nm range and generation of its second harmonic in the 330–360 nm region were obtained. As a result, the radiation range of the earlier developed tunable laser source has been extended in the red and near UV regions of the spectrum. The use of mixtures of dyes and proton-donating additives in working solutions has substantially improved the energy characteristics of this source.     

A Tunable picosecond dye laser based on cavity quenching and spectro-temporal selection

The development of a tunable picosecond dye laser, based on quenching cavity (QC) in an oscillator and spectro-temporal selection (STS) before amplification, is described. A theoretical model was constructed for the calculation of temporal and spectral evolutions of dye laser emission using a series of equations representing population inversions. The pulsewidth, linewidth, and pulse energy, obtained experimentally, were 90 ps, 0.2 nm, and 10 J, respectively. The tunable range, in which a single picosecond pulse can be obtained, was expanded from 4 to 20 nm for a single dye solution by applying QC to STS. Thus, the spectral region was covered from 320 to 360 nm using two laser dyes, i.e., BM-terphenyl and BBQ. Experimental data obtained using the instrument were in reasonably good agreement with values predicted from theory. PACS 42.55.Mv; 42.60.By     

Theoretical Analysis of the Conformation of the Skeletal Base of Epoxysaccharide Molecules by the Vibrational Spectra

To explain the influence of the conformation of the skeletal base of the epoxysaccharide molecule on the frequencies and forms of normal vibrations, we have made a comparative theoretical analysis of the frequencies, forms, and potential energy distributions of normal vibrations in the 4001500 cm^–1 spectral range of five conformers of the epoxysaccharide methyl3,4anhydroD,Lallopyranoside molecule that approximate conformations of the hexapyranose rings close to the halfchair and boat forms. It is shown that a change in the structure of the molecule leads to frequency shifts and distortions of the forms of normal vibrations practically throughout the spectral range under consideration. The character of these changes is specific for different spectral ranges. The most sensitive to conformational changes in the pyranose ring are normal vibrations localized mainly within this ring, and the least sensitive ones are those localized on the bonds lying on the molecule's periphery. The frequency shift of normal vibrations comparable in form in the lowfrequency region in the series of conformers can reach 100 cm^–1. It is shown that the 650800 cm^–1 spectral range is the most convenient for identifying the form of the epoxypyranose ring. The changes in the force field associated with the conformation transitions of the pyranose ring are analyzed.     

Hyperfine structure in the atomic spectrum of niobium

We report on hyperfine structure measurements in 21 lines of atomic niobium in the spectral region from 640 nm to 870 nm by means of optogalvanic laser spectroscopy and laser induced fluorescence spectroscopy using a hollow cathode discharge and a tunable single-mode cw ring laser. Hyperfine structure constants A and B of altogether 29 excited energy levels were determined, 18 of them for the first time. Received 18 July 2002 / Received in final form 8 November 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: sk@kalium.physik.tu-berlin.de     

Harmonic generation in the discrete spectral region of xenon using broadband femtosecond laser pulses

The conversion of 34-fs Ti:sapphire laser pulses into the wavelength region 105–210 nm has been studied in xenon for laser intensities up to 5×10¹³ W/cm². A strongly structured, pressure-dependent emission spectrum is observed. Radiation is detected in regions expected for the 5th and 7th harmonics but also in regions in between. In the resonance region (<147 nm), self-phase-modulation processes in the conversion medium together with phase-matched, efficient harmonic generation in negative-dispersive spectral regions explain the observations. Broadband emission is detected in the resonance-free, positive-dispersive spectral region 155–210 nm. Emission from the xenon dimer is superimposed on a direct 5th-harmonic signal. PACS 32.70.-n; 32.80.-t; 42.65.Ky; 42.65.Re     

The Spectral Gap of the Ferromagnetic XXZ-Chain

We prove that the spectral gap of the spin- ferromagnetic XXZ-chain with HamiltonianH=–_x S^{(1)}_xS^{(1)}_{x+1}+S^{(2)}_xS^{(2)}_{x+1}+\Delta S^{(3)}_xS^{(3)}_{x+1}, is given by -1 for all 1. This is the gap in the spectrum of the infinite chainin any of its ground states, the translation invariant ones as well asthe kink ground states, which contain an interface between an up and a down region.In particular, this shows that the lowest magnon energy is not affected by the presence of a domain wall. This surprising fact is a consequence of the SU _q (2)quantum group symmetry of the model.     

Annealed Scaling for a Charged Polymer

This paper studies an undirected polymer chain living on the one-dimensional integer lattice and carrying i.i.d. random charges. Each self-intersection of the polymer chain contributes to the interaction Hamiltonian an energy that is equal to the product of the charges of the two monomers that meet. The joint probability distribution for the polymer chain and the charges is given by the Gibbs distribution associated with the interaction Hamiltonian. The focus is on the annealed free energy per monomer in the limit as the length of the polymer chain tends to infinity. We derive a spectral representation for the free energy and use this to prove that there is a critical curve in the parameter plane of charge bias versus inverse temperature separating a ballistic phase from a subballistic phase. We show that the phase transition is first order. We prove large deviation principles for the laws of the empirical speed and the empirical charge, and derive a spectral representation for the associated rate functions. Interestingly, in both phases both rate functions exhibit flat pieces, which correspond to an inhomogeneous strategy for the polymer to realise a large deviation. The large deviation principles in turn lead to laws of large numbers and central limit theorems. We identify the scaling behaviour of the critical curve for small and for large charge bias. In addition, we identify the scaling behaviour of the free energy for small charge bias and small inverse temperature. Both are linked to an associated Sturm-Liouville eigenvalue problem. A key tool in our analysis is the Ray-Knight formula for the local times of the one-dimensional simple random walk. This formula is exploited to derive a closed form expression for the generating function of the annealed partition function, and for several related quantities. This expression in turn serves as the starting point for the derivation of the spectral representation for the free energy, and for the scaling theorems. What happens for the quenched free energy per monomer remains open. We state two modest results and raise a few questions.     

Evidence by spectroscopic ellipsometry of optical property change in pulsed laser deposited NiO films when heated in air at Neel temperature

A spectroscopic ellipsometry (SE) study of Nickel oxide (NiO) films, an anti-ferromagnetic insulator, deposited by pulsed laser ablation technique (PLD) on quartz substrate has been undertaken in the region of its Neel temperature (TN=523 K), when its structure changes from rock-salt to a rhombohedral one, owing to a slight contraction along the 111 direction. The films were grown under optimal conditions in a MECA 2000 process chamber so as to reduce surface roughness to a minimum as shown by Atomic Force Microscope (AFM) and SE characterization. The optical properties have been investigated as a function of temperature in an ultra high vacuum (UHV) process chamber (in situ) and in air, under standard atmospheric conditions (ex situ). We report in this study a change in the ellipsometric parameters of NiO in the visible spectral range from 550 nm to 700 nm when the film is heated in air in the region of TN differently from that observed in ultrahigh vacuum. On the contrary, as expected, SmFeO₃ studied under the same conditions in the same temperature range in air shows no such change in ellipsometric parameters in this spectral region which corresponds to the spectral signature of NiO. Results are discussed in terms of spectral lines involving vibrational states of NiO molecule, when transitions occur between its electronic ground state X^3- and the excited states [16.0]³₀^- and [16.0]³₁^-. These results suggest that NiO ought to be classed as a charge-transfer insulator rather than a Mott–Hubbard one. PACS 07.60.Ds; 78.20.Ci; 81.15Fg     

Comparisons between direct and inverse approaches in problems of recovering the true profile of a spectral line

Computer-supported techniques are introduced in the evaluation of experimental data and obtaining the real profile of spectral lines. The direct and inverse approaches were used. The MINUIT program from the packets of CERN's library was used to solve direct problems. Tikhonov's regularization method was also applied to solve the same problems in an inverse manner. Model functions were introduced to check the applicability limitation of these methods and make a comparison between them as well. The advantages and disadvantages of these approaches were shown. The procedures were applied to the measured profiles of He II's spectral lines in a pulsed low-pressure arc. The chosen lines are He II Paschen-alpha (468.6 nm) in the visible region and Balmer-beta () in the VUV spectral region. The range of experimental errors was determined where both approaches have given reliable results. It was found that we can obtain the real profile of He II and He II spectral lines, using the regularization method, when the estimates of errors are less than 15% and 10%, respectively.