Showing papers on "Doppler broadening published in 2009"

Slow light enhancement of nonlinear effects in silicon engineered photonic crystal waveguides

Abstract: We report nonlinear measurements on 80microm silicon photonic crystal waveguides that are designed to support dispersionless slow light with group velocities between c/20 and c/50. By launching picoseconds pulses into the waveguides and comparing their output spectral signatures, we show how self phase modulation induced spectral broadening is enhanced due to slow light. Comparison of the measurements and numerical simulations of the pulse propagation elucidates the contribution of the various effects that determine the output pulse shape and the waveguide transfer function. In particular, both experimental and simulated results highlight the significant role of two photon absorption and free carriers in the silicon waveguides and their reinforcement in the slow light regime.

Measuring the cosmic-ray acceleration efficiency of a supernova remnant.

Abstract: Cosmic rays are the most energetic particles arriving at Earth. Although most of them are thought to be accelerated by supernova remnants, the details of the acceleration process and its efficiency are not well determined. Here we show that the pressure induced by cosmic rays exceeds the thermal pressure behind the northeast shock of the supernova remnant RCW 86, where the x-ray emission is dominated by synchrotron radiation from ultrarelativistic electrons. We determined the cosmic-ray content from the thermal Doppler broadening measured with optical spectroscopy, combined with a proper-motion study in x-rays. The measured postshock proton temperature, in combination with the shock velocity, does not agree with standard shock heating, implying that >50% of the postshock pressure is produced by cosmic rays.

Identifying defects in multiferroic nanocrystalline BaTiO(3) by positron annihilation techniques.

Abstract: Room temperature ferromagnetism in nanoparticles of otherwise nonmagnetic materials has been attributed to point defects at the surface of the nanoparticles. Here, we have employed positron annihilation spectroscopy to identify the nature of defects in multiferroic BaTiO(3) nanocrystalline materials with varying average particle sizes. Ratio curve analysis of the Doppler broadening profile to a reference profile suggests that the defect is an oxygen vacancy. The decrease of intensity of the intermediate lifetime component with increasing particle size indicates a decrease of surface defect concentration. The large defect concentration in nanocrystalline BaTiO(3) can explain the observed room temperature ferromagnetism.

Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

Abstract: Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (...

Supercontinuum generation by higher-order mode excitation in a photonic crystal fiber

Abstract: Higher-order-mode excitation experiments are interesting because the anomalous dispersion region is shifted towards smaller wavelengths and because the existence of a cut-off wavelength generates spectral broadening only on the blue side of the pump wavelength.

Surface core level shifts of clean and oxygen covered Ir(111)

Abstract: We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin-Rosengren theory. From the phonon- induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at 200meV and +230meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38ML, in good agreement with recent density functional theory calculations.

On-the-fly doppler broadening for Monte Carlo codes

Abstract: A methodology to allow on-the-fly Doppler broadening of neutron cross sections for use in Monte Carlo codes has been developed. The Monte Carlo code only needs to store 0K cross sections for each isotope and the method will broaden the 0K cross sections for any isotope in the library to any temperature in the range 77K-3200K. The methodology is based on a combination of Taylor series expansions and asymptotic series expansions. The type of series representation was determined by investigating the temperature dependence of U238 resonance cross sections in three regions: near the resonance peaks, mid-resonance, and the resonance wings. The coefficients for these series expansions were determined by a regression over the energy and temperature range of interest. Since the resonance parameters are a function of the neutron energy and target nuclide, the ψ and χ functions in the Adler-Adler multi-level resonance model can be represented by series expansions in temperature only, allowing the least number of terms to approximate the temperature dependent cross sections within a given accuracy. The comparison of the broadened cross sections using this methodology with the NJOY cross sections was excellent over the entire temperature range (77K-3200K) and energy range. A Monte Carlo code was implemented to apply the combined regression model and used to estimate the additional computing cost which was found to be less than <1%.

Experimental signature of optical wave thermalization through supercontinuum generation in photonic crystal fiber.

Abstract: We report an experimental, numerical and theoretical study of the incoherent regime of supercontinuum generation in a two zero-dispersion wavelengths fiber. By using a simple experimental setup, we show that the phenomenon of spectral broadening inherent to supercontinuum generation can be described as a thermalization process, which is characterized by an irreversible evolution of the optical field towards a thermal equilibrium state. In particular, the thermodynamic equilibrium spectrum predicted by the kinetic wave theory is characterized by a double peak structure, which has been found in quantitative agreement with the numerical simulations without adjustable parameters. We also confirm that stimulated Raman scattering leads to the generation of an incoherent structure in the normal dispersion regime which is reminiscent of a spectral incoherent soliton.

Space charge effects in photoemission electron microscopy using amplified femtosecond laser pulses

Abstract: Linear and nonlinear photoemission microscopy is used to study the origin of space charge effects that are frequently observed if amplified femtosecond lasers are used for generation of photoelectrons. Space charge effects are apparent in the width of the photoemission spectra, but also create image blur. The onset threshold for space charge effects is determined by recording the width of photoemission spectra and by finding the conditions under which spectral broadening is just less than the energy resolution of the microscope. The principal findings are independent if harmonics of the fundamental of the fs laser pulses are used, but the space charge effects are found to be more dominant at lower repetition rates. By inserting apertures into the electron path, the place at which space charge effects occur can be localized.

Investigation by slow positron beam of defects in CLAM steel induced by helium and hydrogen implantation

Abstract: Hydrogen and helium ion beams delivering different doses are used in the ion implantation, at room temperature, of China Low Activation Martensitic (CLAM) steel and the induced defects studied by Doppler broadening of gamma-rays generated in positron annihilation. Defect profiles are analysed in terms of conventional S and W parameters, measures of relative contributions of low and high-momentum electrons in the annihilation peak, as functions of incident positron energies E up to 30 keV. The behaviours of the S – E , W – E and S – W plots under different implantation doses indicate clearly that the induced defect size has obvious variation with depth, taking values that interpolate between surface and bulk values, and depend mainly on helium ion fluences. The S – W plot indicates that two types of defects have formed after ion implantation.

A collisional cooling investigation of the pressure broadening of the 110←101 transition of water from 17 to 200 K

Abstract: An investigation of the 1 10 ←1 01 rotational transition of water at 556GHz pressure broadened by hydrogen, helium, nitrogen, oxygen and carbon dioxide has been completed. Using the collisional cooling technique the broadening of this transition by each gas was explored from 200 K down to the condensation point of the broadening gas or 17 K, which ever is lower. This marks the first time such an extensive investigation of the broadening of this rotational transition as a function of temperature has been completed. The results of this investigation will be presented including the exponential temperature dependence of the broadening, the lack of temperature dependence of the broadening observed in helium and the unique behavior of the hydrogen broadening at low temperature (<40K). In addition, the broadening of the 1 10 ←1 01 rotational transition of water by nitrogen and oxygen was recorded at room temperature and a direct comparison of this data with recent work will be discussed.

Neutron Resonance Capture and Transmission Analysis

Abstract: Neutrons can be used as probes to investigate and study properties of materials and objects. The deep penetration of neutrons into matter allows the study of bulk properties. One evolving activity in this field concerns the existence of resonances in neutron-induced reaction cross sections in the neutron energy range from thermal up to a few megaelectronvolts for certain nuclides. Since these resonances appear at neutron energies, which are specific for each nuclide, they are very suitable for the determination of the elemental composition of objects and materials. Neutron resonance capture analysis (NRCA) and neutron resonance transmission analysis (NRTA) are two methods used for exploiting the resonance structure in the neutron-induced reaction cross sections. Both methods are nondestructive, determine the bulk elemental composition, do not require any sample preparation, and result in a negligible residual activity. NRCA is applicable to almost all stable elements, while NRTA is preferred for light elements or nuclei near a closed shell. Quantitative NRCA and NRTA have been used for the analysis of archeological objects and reference materials used for cross-section measurements. Other applications of neutron resonance spectroscopy range from radiography of nuclear fuel (including spent fuel), detection of explosives and drugs, to thermometry and diamond mining. Keywords: neutron; resonances; epithermal; capture; transmission; Doppler broadening; archeology; reference material; nuclear fuel; thermometry; radiography; nondestructive assay

Supercontinuum generation in aqueous colloids containing silver nanoparticles.

Abstract: We numerically study low-threshold supercontinuum generation using the significant enhancement of nonlinearity in aqueous colloids with silver nanoparticles. We predict octave-spanning spectral broadening by femtosecond pulses with an intensity in the range of tens of GW/cm2. The strong frequency dependence of the effective nonlinear coefficient of the composite significantly influences the spectral broadening by self-phase modulation and leads to a large blueshift of the spectra.

OSNR variation of multiple laser lines in Brillouin-Raman fiber laser.

Abstract: We report experimental results demonstrating the variation of optical signal-to-noise ratio (OSNR) of laser lines in Brillouin-Raman fiber laser against Raman pump power (RPP) variation. The reduction of OSNR is attributed to the spectral broadening of laser lines depending on the RPP. The spectral broadening is owing to the effect of the interaction between laser lines and turbulent waves (nonlinear interaction between longitudinal cavity modes).In our experiment, the worst OSNR is obtained at 650 mW RPP as a result of maximum spectral broadening when the Brillouin pump wavelength is fixed at 1555 nm. On the other hand, the OSNR improvement is obtained for RPP beyond 650 mW due to the effect of red-shift, the Raman peak gain is shifted away from the laser lines generated around 1555 nm thus reduces the spectral broadening effect.

Cosmological Hydrogen Recombination: influence of resonance and electron scattering

Abstract: In this paper we consider the effects of resonance and electron scattering on the escape of Lymanα photons during cosmological hydrogen recombination. We pay particular attention to the influence of atomic recoil, Doppler boosting and Doppler broadening using a Fokker-Planck approximation of the redistribution function describing the scattering of photons on the Lymanα resonance of moving hydrogen atoms. We extend the computations of our recent paper on the influence of the 3d /3s-1s two-photon channels on the dynamics of hydrogen recombination, simultaneously including the full time-dependence of the problem, the thermodynamic corrections factor, leading to a frequency-dependent asymmetry between the emission and absorption profile, and the quantummechanical corrections related to the two-photon nature of the 3d/3s-1s emission and absorption process on the exact shape of the Lymanα emission profile. We show here that due to the redistribution of photons over frequency hydrogen recombination is sped up

Spectral broadening of mid-infrared femtosecond pulses in GaAs

Abstract: We achieved efficient spectral broadening for mid-IR pulses of few-microjoule energy. The spectral bandwidth of the femtosecond pulses at the center wavelength of 5000 nm increased from 540 nm to 2060 nm (from 220 to 910 cm−1 in frequency) by nonlinear propagation in a gallium arsenide single crystal. The spectral broadening was accompanied by nonlinear absorption loss of 25%. The demonstrated scheme should be available at any operation wavelength within the material transparency range and provides a useful tool in nonlinear vibrational spectroscopy.

Pressure, Temperature, and Velocity Measurements in Underexpanded Jets Using Laser-Induced Fluorescence Imaging

Abstract: We report measurements of pressure, temperature, and velocity in an underexpanded jet using planar laser-induced fluorescence of nitric oxide Ultraviolet transitions near 44, 0975 cm -1 in the A-X (0,0) system of nitric oxide were excited using narrow linewidth laser radiation generated from an optical parametric system, injection seeded using a distributed feedback diode laser Planar laser-induced fluorescence images with excellent spatial resolution and signal-to-noise ratio were acquired by tuning the frequency of the laser radiation over the nitric oxide absorption line The images were corrected on a shot-to-shot basis for fluctuations in the laser spatial profile Line shapes constructed from the corrected planar laser-induced fluorescence images were used to determine pressure and temperature values along the centerline of the jet Good agreement between laser-induced fluorescence and previously reported N 2 coherent anti-Stokes Raman scattering measurements was observed The laser-induced fluorescence measurements also compared well with calculations of pressure and temperature using computational fluid dynamics codes Velocity was measured in supersonic regions of the flowfield on the basis of the Doppler shift in the nitric oxide absorption lines Planar laser-induced fluorescence images were acquired using laser sheets propagating at 90 and 45 degrees with respect to the flow direction; velocity was determined from the frequency shift of the absorption lines for these two shifts The laser-induced fluorescence technique can potentially be applied to obtain instantaneous measurements of thermodynamic properties and multiple velocity components in high-speed turbulent flows

Spectral resolution of molecular ensembles under ambient conditions using surface plasmon coupled fluorescence emission

Abstract: We report the ability to spectrally resolve excited-state ensembles of pyranine (Py) utilizing nanometer-thick metal films as a low-cost analytical tool Surface plasmon coupling allows to mitigate the effect of spectral broadening that is responsible for blurring the emission spectrum at room temperature, a situation common in conventional fluorescence spectroscopy The approach is especially useful in the case when several excited-state species are present Fluorescence emission from closely located protonated, deprotonated, and excimer species of Py couple into surface plasmons and are easily separated and observed with 11-14 fold intensity enhancements Furthermore, the ultranarrowband photon-sorting of emission from microenvironments in a multispecies system is performed in this study using instruments that are readily available in most laboratories without employing any deconvolution procedure and/or additional dispersive optics

Realization of Coherent Optically Dense Media via Buffer-Gas Cooling

Abstract: We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool $^{87}\mathrm{Rb}$ atoms to below $7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and slow atom diffusion to the walls. Electromagnetically induced transparency in this medium allows for 50% transmission in a medium with initial optical depth $Dg70$ and for slow pulse propagation with large delay-bandwidth products. In the high-$D$ regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing.

Electron Density and Temperature Measurement by Stark Broadening in a Cold Argon Arc-Plasma Jet at Atmospheric Pressure

Abstract: Determination of both the electron density and temperature simultaneously in a cold argon arc-plasma jet by analyzing the Stark broadening of two different emission lines is presented. This method is based on the fact that the Stark broadening of different lines has a different dependence on the electron density and temperature. Therefore, a comparison of two or more line broadenings allows us to diagnose the electron density and temperature simultaneously. In this study we used the first two Balmer series hydrogen lines Hα and Hβ for their large broadening width. For this purpose, a small amount of hydrogen was introduced into the discharge gas. The results of the Gigosos–Cardenoso computational model, considering more relevant processes for the hydrogen Balmer lines, is used to process the experimental data. With this method, we obtained reliable electron density and temperature, 1.88 × 1015 cm−3 and 13000 K, respectively. Possible sources of error were also analyzed.

Spectral broadening in femtosecond laser written waveguides in chalcogenide glass

Abstract: Nonlinear spectral broadening to 200 nm, from an initial width of 50 nm, has been demonstrated in gallium lanthanum sulphide glass waveguides from 1540 nm, 200 fs pulses at 30 nJ/pulse. A formation mechanism is presented for these femtosecond laser written waveguides, based on optical characterization and comparisons to previous work. Two different types of waveguide are identified. One has a characteristic long narrow structure and is formed through filamentation caused by self-focusing. The other has a characteristic “teardrop” structure, which is formed by a type IIA photosensitivity mechanism and cumulative heating of glass around a central laser-exposed region.

Electromagnetically induced transparency and slow light in three-level ladder systems: Effect of velocity-changing and dephasing collisions

Abstract: We present a theory of electromagnetically induced transparency (EIT) and slow light in a Doppler broadened three-level ladder (cascade) system incorporating residual Doppler broadening of two-photon coherence and velocity-changing and dephasing collision effects of a buffer gas. Both regimes of wave-vector mismatch occurring when either the probe frequency is higher than that of the (strong) control field or vice versa are considered. It is found that the velocity-changing collisions in general cause narrowing of EIT resonance linewidths which, in a particular wave-vector mismatch regime, can lead to large transparency and slow light generation at relatively low control field intensities. Large collisional dephasing of two-photon coherence in a ladder system, however, tends to mask these effects. A reduction in group velocity by more than 3 orders of magnitude is predicted in the regime where the probe frequency is lesser than that of the control field and in the absence of collisions.

A CAA Based Approach to Tone Haystacking

Abstract: The far-field noise spectra of jet engines show for certain jet configurations and turbine tones a characteristic spectral broadening effect, causing a reduction of tone peaks in favor of a more distributed spectral hump around each tone frequency. This haystacking effect likely occurs due to the interaction of the turbine tones with the unsteady turbulent jet shear layer. A better understanding of this effect may help to utilize it for noise reduction purposes. Furthermore, the effect is of interest for the measurement of tone sources in an open acoustic wind tunnel test section, since the tone will be scattered in the open jet shear layer. A correction for this measured broadening effect is desirable. A non-empirical computational approach to predict tone haystacking as a function of Reynolds number/jet shear layer characteristics is currently missing. This paper reports about ongoing work to utilize Computational Aeroacoustics (CAA) methods for the prediction of haystacking. In a first step CAA techniques are applied to simulate the propagation of tones through the time averaged steady exhaust of a jet engine. To simulate the haystacking effect with CAA, the unsteady turbulent base-flow is modeled with a 4D synthetic turbulence method. The employed RPM approach generates turbulence with all local statistical features as predicted by time-averaged RANS. To study the spectral broadening effect computationally, the experimental set-up of Candel is considered first, which involves an omnidirectional sound source located on the axis of a round jet. The analytical predictions show very good agreement with the general trends as measured by Candel for an observer position normal to the jet axis. The computations reveal a spectral shape, which is in good agreement with those found in the experiments. In a next step the methodology is combined with the exhaust problem to simulate sound propagation through the unsteady turbulent exhaust.