Showing papers on "Doppler broadening published in 1997"

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

A fast scan submillimeter spectroscopic technique

Abstract: A new fast scan submillimeter spectroscopic technique (FASSST) has been developed which uses a voltage tunable backward wave oscillator (BWO) as a primary source of radiation, but which uses fast scan (∼105 Doppler limited resolution elements/s) and optical calibration methods rather than the more traditional phase or frequency lock techniques. Among its attributes are (1) absolute frequency calibration to ∼1/10 of a Doppler limited gaseous absorption linewidth (<0.1 MHz, 0.000 003 cm−1), (2) high sensitivity, and (3) the ability to measure many thousands of lines/s. Key elements which make this system possible include the excellent short term spectral purity of the broadly (∼100 GHz) tunable BWO; a very low noise, rapidly scannable high voltage power supply; fast data acquisition; and software capable of automated calibration and spectral line measurement. In addition to the unique spectroscopic power of the FASSST system, its implementation is simple enough that it has the prospect of impacting a wide range of scientific problems.

Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

Abstract: We have measured the Doppler broadening energy spectra of positron annihilation using a variable monoenergetic positron beam as a function of positron energy and of temperature in polystyrene. The $S$ parameters from the energy spectra versus temperature show that onset temperatures significantly decrease as the positron energy decreases. This result gives direct evidence that the glass-transition temperature is suppressed at a magnitude of 57 \ifmmode^\circ\else\textdegree\fi{}C at the depth of 50 \AA{} from the surface.

Recycling of guided mode light emission in planar microcavity light emitting diodes

Abstract: Results are presented on planar microcavity light emitting diodes with different device diameters. A record external quantum efficiency of 20% is achieved for a 1.5 mm light emitting diode. The strong dependence of the quantum efficiency on current density and device size are compared with theoretical results. A good correspondence is obtained when spectral broadening and photon recycling are taken into account.

Speed-dependent pressure broadening and shift in the soft collision approximation

Abstract: The influence of speed-changing collisions and the correlation between Doppler and collisional broadening on the spectral line profiles is considered in a unified way using the Anderson-Talman classical phase-shift theory for collisional broadening and the Galatry diffusion model for the motion of radiating particles. A general formula for the correlation function is derived in the impact limit, which yields the well known speed-dependent Voigt profile in the case when the speed-changing collisions are neglected, but the Doppler-collision correlations are taken into account. In the opposite case, when the Doppler-collision correlations are omitted, but the speed-changing collisions are included this formula becomes identical to that derived by Galatry. This formula is shown to lead to the results close to those calculated from the line profile formula recently derived by Duggan et al [ Phys. Rev. A 51 , 218 (1995)].

γ-ray spectra from positron annihilation on atoms and molecules

Abstract: Positron annihilation on a wide variety of atoms and molecules is studied. Room-temperature positrons confined in a Penning trap are allowed to interact with molecules in the form of low-pressure gases so that the interaction is restricted to binary encounters between a positron and a molecule. Data are presented for the \ensuremath{\gamma}-ray spectra resulting from positrons annihilating in such interactions. The Doppler broadening of these spectra is a measure of the momentum distribution of the annihilating electron-positron pairs. Consequently, these spectra provide information about the electron and positron wave functions. Systematic studies of annihilation line shapes are discussed for noble gases, a variety of inorganic molecules, alkanes, alkenes, aromatics, and perfluorinated and partially fluorinated hydrocarbons. In the case of molecules, the measurements are used to determine the probability of positrons annihilating at specific locations in the molecule. For example, in the case of partially fluorinated hydrocarbons, we have been able to determine the relative probability of annihilation on the fluorine atoms and on the C-H bonds. Insights that these studies provide in understanding the interaction of low-energy positrons with atoms and molecules are discussed.

Doppler radar spectral width broadening due to beamwidth and wind shear

Abstract: . The spectral width observed by Doppler radars can be due to several effects including the atmospheric turbulence within the radar sample volume plus effects associated with the background flow and the radar geometry and configuration. This study re-examines simple models for the effects due to finite beamwidth and vertical shear of the horizontal wind. Analytic solutions of 1- and 2-dimensional models are presented. Comparisons of the simple 2-dimensional model with numerical integrations of a 3-dimensional model with a symmetrical Gaussian beam show that the 2-dimensional model is usually adequate. The solution of the 2-dimensional model gives a formula that can be applied easily to large data sets. Analysis of the analytic solutions of the 2-dimensional model for off-vertical beams reveals a term that has not been included in mathematical formulas for spectral broadening in the past. This term arises from the simultaneous effects of the changing geometry due to curvature within a finite beamwidth and the vertical wind shear. The magnitude of this effect can be comparable to that of the well-known effects of beam-broadening and wind shear, and since it can have either algebraic sign, it can significantly reduce (or increase) the expected spectral broadening, although under typical conditions it is smaller than the beam-broadening effect. The predictions of this simple model are found to be consistent with observations from the VHF radar at White Sands Missile Range, NM.

Doppler broadening of energy spectra in Compton cameras

Abstract: Compton cameras utilize the Compton effect to achieve directional localization of gamma rays. The accuracy of the localization depends, in part, on the uncertainties in the measurement of energies. These uncertainties have conventionally been assumed to be due to the effects of finite detector energy resolution alone. There is another source of energy uncertainty that none of the Compton cameras proposed or built thus far accounts for the Doppler broadening of energy spectra that arises from the Compton interaction between gamma rays and moving electrons bound to atoms. The authors have used Monte Carlo simulations and direct calculations to demonstrate that the energy uncertainties due to Doppler broadening are non-negligible. For low-energy gamma rays, these uncertainties are generally more significant than those due to the finite energy resolutions of semiconductor detectors (for example, germanium and silicon). Expressions for estimating the angular uncertainties in Compton cameras due to energy resolution and Doppler broadening are derived. The authors conclude that the accuracy of the calculation of the scatter angle can be improved if the electron pre-collision momentum is known. However, even without such knowledge. Compton cameras still have the potential to achieve spatial resolutions that are comparable to those of mechanically collimated systems.

Iron k emission lines in the energy spectra of low-mass x-ray binaries observed with asca

Abstract: ASCA archive data of 20 low-mass X-ray binaries (LMXBs) were analyzed to study the iron K emis- sion lines. The LMXBs analyzed here include Z sources, atoll sources, dippers, bursters, and ADC (accretion disk corona) sources. We paid particular attention to reducing systematic errors for the esti- mation of line parameters, and the combination of SIS and GIS, which have diUerent energy resolutions, was found to be very useful for this purpose. We detected signi—cant iron lines from about one-half of the sources. The line center was 6.56 keV on average, and a —nite width of D0.5 keV (FWHM) was obtained from six sources; these properties may be common to LMXBs. On the other hand, equivalent width (EW) of the lines showed large scatter among the sources, from less than 10 to 170 eV. These line parameters do not show any correlation with the source categories or luminosities. The iron K lines from LMXBs are likely produced through the radiative recombination of photoionized plasma. We discuss the origin of the line width, which may result from the combination of the line blending, Doppler broadening, and Compton scattering. Subject headings: binaries: closestars: neutronX-rays: stars

Spectroscopic characterization of the DIII-D divertor

Abstract: Radiative losses along a fixed view into the divertor chamber of the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol I, p. 159] have been characterized for attached and partially detached discharges by analyzing line-integrated vacuum ultraviolet (VUV) signals. Essentially all the emission can be ascribed to carbon and deuterium. Because the majority of the most intense lines, which lie at wavelengths above 1100 A, are not accessible to the present instrumentation, extensive use has been made of collisional-radiative (CR) calculations for level populations of the important ions in order to relate the total radiated power to shorter wavelength transitions. In beam-heated plasmas, the fraction of radiation detected from carbon along the VUV spectrometer view is usually between 50% and 80% of the total. Carbon densities are estimated from a simplified approach to modelling the emission using a one-dimensional transport code. For partially detached plasmas the concentrations range from 2%–6% of the electron density; but in attached plasmas it appears that carbon may supply most of the electrons in the divertor region just below the X point. Ion temperatures are measured from Doppler broadening of spectral lines by fitting measured profiles to theoretical lineshapes, which account precisely for atomic sublevel splitting caused by the Zeeman/Paschen-Back effect in the tokamak magnetic field.

Study of the Van Cittert and Gold iterative methods of deconvolution and their application in the deconvolution of experimental spectra of positron annihilation

Abstract: The study of deconvolution by Van Cittert and Gold iterative algorithms and their use in the processing of experimental spectra of Doppler broadening of the annihilation line in positron annihilation measurement is described. By comparing results from both algorithms it was observed that the Gold algorithm was able to eliminate linear instability of the measuring equipment if one uses the 1274 keV 22Na peak, that was measured simultaneously with the annihilation peak, for deconvolution of annihilation peak 511 keV. This permitted the measurement of small changes of the annihilation peak (e.g. S-parameter) with high confidence. The dependence of γ-ray-like peak parameters on the number of iterations and the ability of these algorithms to distinguish a γ-ray doublet with different intensities and positions were also studied.

Kinetic to thermal energy transfer and interpenetration in the collision of laser-produced plasmas

Abstract: An experimental and numerical analysis of the collision of two plasmas produced from laser-exploded Al/Al and Al/Mg pairs of foils is presented. Various imaging and spectroscopic x-ray techniques have been used to diagnose the collision over a broad range of intertarget distances and laser intensities. Ion temperatures in the 10 keV range have been measured from Doppler broadening. Electron temperatures and densities have been deduced from line ratios and interpenetration distances have been determined by the spatial extent of Mg and Al x-ray lines. Eulerian multifluid simulations have been developed and coupled to atomic physics postprocessing. The comparison of the measurements with these simulations shows that interpenetration prevails at large intertarget distances and high laser intensities; kinetic to thermal energy transfer then takes place on a ∼200-μm wide region and during ∼150 ps.

A modeling study of the effect of drizzle on cloud optical depth and susceptibility

Abstract: This paper examines the impact of drop spectral broadening, generated by the collection process, on the optical depth, cloud albedo, and susceptibility of marine stratocumulus clouds. The results are arrived at using (1) the output from a simple box model calculation of collection and (2) the output from an eddy-resolving model of stratocumulus clouds that explicitly represents the size distribution of the drops. It is shown that commonly used relationships for cloud optical properties developed for narrow spectra do not generally apply to spectra undergoing spectral broadening. The optical depth dependence on the drop number concentration to the one-third power is shown to be an overestimate of the optical depth when spectra broaden through collection. In addition, the cloud susceptibility dependence on drop number is shown to be larger for spectra experiencing broadening than for narrow spectra.

Line mixing sum rules for the analysis of multiplet spectra.

Abstract: In the impact approximation, the spectrum of transitions overlapped by collisional broadening is given by a sum over Lorentzian absorption and dispersion profiles about each transition. The dispersion components arise from the interference among lines coupled by the collisions, and their magnitudes and signs are given by the line mixing parameters. The line mixing parameters, as well as the transition intensities, widths and shifts, are obtained from inversion of the relaxation matrix and may be highly non-linear in pressure when the coupled lines are strongly overlapped. For any overlap, however, the line mixing parameters summed over all coupled lines is zero and the total integrated intensities, widths and shifts are conserved. These sum rules may be used as constraints in least-squares fitting of multiplet spectra to reduce the number of free parameters and their correlation. Also for the fitting of spectra at atmospheric pressures, a prescription is given for incorporating Doppler broadening and Dicke narrowing.

Photoionization of hydrogen in atmospheres of magnetic neutron stars

Abstract: The strong magnetic fields (B ~ 1012-1013 G) characteristic of neutron stars make all the properties of an atom strongly dependent on the transverse component K⊥ of its generalized momentum. In particular, the photoionization process is modified substantially: (1) threshold energies are decreased as compared with those for an atom at rest, (2) cross section values are changed significantly, and (3) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calculate the photoionization cross sections, we employ, for the first time, exact numerical treatment of both initial and final atomic states. This enables us to take into account the quasi-bound (autoionizing) atomic states as well as coupling of different ionization channels. We extend the previous consideration, restricted to the so-called centered states corresponding to relatively small values of K⊥, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substantially from those of atoms at rest. In particular, the photoionization edges are strongly broadened by the thermal motion of atoms; this "magnetic broadening" exceeds the usual Doppler broadening by orders of magnitude. The decentered states of the atoms give rise to the low-energy component of the photoionization cross section. This new component grows significantly with increasing temperature above 105.5 K and decreasing density below 1 g cm-3, i.e., for the conditions expected in atmospheres of middle-aged neutron stars.

Stark Broadening of the B III 2s-2p Lines

Abstract: We present a quantum-mechanical calculation of Stark linewidths from electron-ion collisions for the ${2s}_{1/2}{\ensuremath{-}2p}_{1/2,3/2},$ $\ensuremath{\lambda}=2066$ and 2067 \AA{}, resonance transitions in B III. The results confirm previous quantum-mechanical $R$-matrix calculations, but contradict recent measurements and semiclassical and some semiempirical calculations. The differences between the calculations can be attributed to the dominance of small $L$ partial waves in the electron-atom scattering, while the large Stark widths inferred from the measurements would be substantially reduced if allowance is made for hydrodynamic turbulence from high-Reynolds-number flows and the associated Doppler broadening.

Positron Annihilation with Inner-Shell Electrons in Noble Gas Atoms

Abstract: Measurements are presented of the Doppler broadening of the 511-keV {gamma} -ray line from thermalized positrons annihilating with Ar, Kr, and Xe. The experiments are conducted at low pressures using positrons stored in a Penning-Malmberg trap, which ensures that only two-body interactions are involved. The contributions from annihilation on inner-shell electrons are studied quantitatively in an isolated atomic system for the first time. The spectra are compared with theoretical calculations using a simple Hartree-Fock model to identify the inner-shell contributions. {copyright} {ital 1997} {ital The American Physical Society}

Method and apparatus for all-optical data regeneration

Abstract: The specification relates to a method and apparatus for all-optical regeneration of return-to-zero (RZ) data streams. The effect of self-phase modulation (SPM) of a data signal passing through a nonlinear medium (NLM) creates spectral broadening in individual data pulses which are then subsequently filtered to pass a selected bandwidth centered at a frequency, ω f , which is shifted with respect to the input data carrier frequency, ω 0 . Since the degree of broadening of a pulse passing through the NLM is a function of the initial intensity of the pulse, noise in data "zeros" (null values) possess insufficient intensity (providing incidental distortion pulse intensity attributable to accumulated process noise is less than a critical value) to cause the requisite amount of spectral broadening to encompass the selected filter bandwidth centered around ω f , and the noise is subsequently suppressed. Conversely, noise (amplitude fluctuations) in data "ones" (set values) possess sufficient intensity (providing input pulse intensity, including incidental distortion pulse intensity, is greater than a critical value) to cause the requisite amount of spectral broadening to encompass the selected filter bandwidth centered around ω f , and that portion of the spectrally broadened pulse contained within the bandwidth centered around ω f is subsequently passed.

A study of the spectral broadening of simulated Doppler signals using FFT and AR modelling

Abstract: Doppler ultrasound is used clinically to detect stenosis in the carotid artery. The presence of stenosis may be identified by disturbed flow patterns distal to the stenosis that cause spectral broadening in the spectrum of the Doppler signal around peak systole. This paper investigates the behaviour of the spectral broadening index (SBI) derived from wide-band spectra obtained using autoregressive modelling (AR), compared with the SBI based on the fast-Fourier transform (FFT) spectra. Simulated Doppler signals were created using white noise and shaped filters to analyse spectra typically found around the systolic peak and to assess the magnitude and variance of AR and FFT-SBI for a range of signal-to-noise ratios. The results of the analysis show a strong correlation between the indices calculated using the FFT and AR algorithms. Despite the qualitative improvement of the AR spectra over the FFT, the estimation of SBI for short data frames is not significantly improved using AR.

Transport of positrons in the electrically biased metal-oxide-silicon system

Abstract: This paper describes a study of the effect of an external electric field on the behavior of positrons in metal-oxide-silicon (MOS) systems. Doppler broadening measurements of the annihilation radiation were performed on capacitors with identical thermally grown SiO2 layers and with Al, W and Au layers as a gate. The data were analyzed by the combined use of the shape- and wing-parameters of the photo peak. The observed effects of the electric field are due to the field-driven transport of positrons through the SiO2, silicon and the interfaces. By applying a field of the order of 1 MV/cm the positrons can be efficiently transported through the approximately 100 nm thick SiO2 layer. From the transport behavior of the positrons it is concluded that the positron affinity is higher for SiO2 than for silicon and for the gate metal. By properly choosing the direction of the field, the positrons implanted into the SiO2 layer are collected either at the Si/SiO2 interface or at the SiO2/gate interface. For negative...

Spectral broadening measurements in poly(phenylene vinylene) polymer channel waveguides

Abstract: In a polyconjugated main chain polymer strip waveguide the nonlinear, nonresonant refractive index n2 was measured by monitoring the signal spectrum broadening due to self-phase modulation. The two photon absorption coefficient α2 was obtained by calibrating the inverse transmission measurement. The nonlinear coefficients were determined to be n2=0.85×10−14 cm−2/W andα2=0.08 cm/GW at a wavelength λ=885.6 nm. The used polymer was poly[1,4-phenylene1,2-di(phenoxyphenyl)vinylene]. The result is in good agreement with interferometer measurements. The material is suitable for all-optical switching.

Crystallization of silica studied by positron annihilation

Abstract: Crystallization of SiO 2 glass was studied by positron lifetime spectroscopy and Doppler broadening. Heat treatments were performed in the temperature range between 700 and 1600°C to investigate the crystallization process isothermal and isochronal. Lifetime spectra were analyzed by two lifetime components. The decrease of the short lifetime (300 ps) was attributed to the increasing volume fraction of the crystalline phase. The long lifetime (1500 ps) was related with the pick-off annihilation of the ortho-positronium state in the free volume. Long lifetime increase, with the onset of crystallization, has been explained by expanding cavities at the interface between the crystalline phase and amorphous matrix. Positronium formation decreases with devitrification, which is in agreement with measurements of Doppler broadening. Positron annihilation was shown to be especially suited to investigate surface crystallization.

Enhancement and spectral shift of optical gain in semiconductors from non-Markovian intraband relaxation

Abstract: We will investigate some aspects of the phenomenological treatment of transition (line shape) and level broadening arising from phase and energy relaxation of Bloch states, respectively. Calculating the absorption/gain via the spontaneous emission formula and performing the broadening within the latter circumvents certain artifacts for both level and transition broadening. When using k-independent relaxation times, Gaussian (non-Markovian) broadening functions are superior to Lorentzian (Markovian) ones. In contrast to transition broadening, level broadening may even enhance the gain over its whole spectral width. In contrast to Lorentzian transition broadening, Gaussian transition broadening yields a blue shift of the gain maximum. The direction and magnitude of the spectral shift arising from Gaussian level broadening depends on the degree of degeneracy of the electron and hole bands involved. The level broadening can have a significant influence on the carrier statistics, which, consequently, has to be included into a consistent treatment. Thus, the phenomenological model functions depend distinctly on which kind of relaxation process is faster, energy or phase relaxation. For GaAs-like semiconductors, the application of transition broadening-even when using the spontaneous emission formula-to cases of dominant intraband relaxation yields significant numerical deviations from the correct treatment of level broadening. Broadening the energy levels requires an additional convolution integration. We will present an approximation, which yields excellent results for the gain in GaAs-like semiconductors. This enables one to include the significant effects of level broadening without increasing the numerical effort and leads to favorable formulae for experimental data fitting and device modeling.

Neutron time-of-flight signals from expanding or contracting spherical sources

Abstract: The width of the energy distribution of fusion-produced neutrons is often used as an indication of the temperature of the reacting ions. The Doppler broadening of the neutron energy is due to the center-of-mass velocity of reacting ion pairs and is characterized by the ion temperature for a Maxwellian distribution of ions with zero collective velocity. If there is bulk fluid motion or turbulence characterized by a velocity on the order of the ion thermal speed, a significant additional broadening may be introduced. Suggestions of this phenomenon have been observed for two classes of laser targets. The first is a “gas bag” target, in which a deuterated hydrocarbon gas is contained in a thin spherical membrane and illuminated uniformly. The second target is an ICF capsule with a deuterated plastic inner layer. In both cases, measured neutron energy distributions were wider than expected from theoretical ion temperatures and if interpreted as indicative of the ion temperature, are inconsistent with the neutron yields observed.