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Results for “RAMAN EFFECT”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Formation and Properties of Locally Tensile Strained Ge Microstructures for Silicon Photonics

The formation and properties of locally tensile strained Ge microstructures (“microbridges”) based on Ge layers grown on silicon substrates are investigated. The elastic-strain distribution in suspended Ge microbridges is analyzed theoretically. This analysis indicates that, in order to attain the maximum tensile strain within a microbridge, the accumulation of strain in all corners of the fabricated microstructure has to be minimized. Measurements of the local strain using Raman scattering show significant enhancement of the tensile strain from 0.2–0.25% in the initial Ge film to ~2.4% in the Ge microbridges. A considerable increase in the luminescence intensity and significant modification of its spectrum in the regions of maximum tensile strain in Ge microbridges and in their vicinity as compared to weakly strained regions of the initial Ge film is demonstrated by microphotoluminescence spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Structure of Se{sub 95}As{sub 5} Chalcogenide Glassy Semiconductor Doped by EuF{sub 3} Impurity

The local structure of film samples of the chalcogenide glassy semiconductors Se{sub 95}As{sub 5} and Se{sub 95}As{sub 5}(EuF{sub 3}){sub x} (x = 0.01–1 at %) are studied by X-ray diffraction and Raman scattering. The “quasiperiod” of the structure, the correlation length, the structural elements, and the chemical bonds, which form the amorphous matrix of the investigated materials, are determined. The obtained results are interpreted within the framework of the Elliott void–cluster model taking into account the chemical activity of europium ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Basic Requirements of Spin-Flip Raman Scattering on Excitonic Resonances and Its Modulation through Additional High-Energy Illumination in Semiconductor Heterostructures

We describe the major requirements to experimentally perform and observe resonant spin-flip Raman scattering on excitonic resonances in low-dimensional semiconductors. We characterize in detail the properties of this resonant light scattering technique and evaluate the criteria, which must be fulfilled by the experimental setup and the semiconductor sample studied to be able to observe a spin-flip scattering process. We also demonstrate the influence of additional unpolarized laser illumination with energies, which exceed considerably the band gap energy of the semiconductor nanostructure under study, on the resonantly excited electron spin-flip scattering in InAs-based quantum dot ensembles as well as on the paramagnetic Mn-ion spin-flip in (Zn,Mn)Se/(Zn,Be)Se quantum wells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

New Nanosized Luminophores Obtained via Evaporation of REE Silicates and Germanates

Nanophosphors in the amorphous state are first obtained via pulsed electron beam evaporation of targets made of polycrystalline phosphors with the compositions Ca{sub 2}M{sub 8}(SiO{sub 4}){sub 6}O{sub 2}:Eu (M = Y and Gd) and Ca{sub 2}La{sub 8}(GeO{sub 4}){sub 6}O{sub 2}:Eu with the structure of oxyapatite. Reduction of ions Eu{sup 3+} → Eu{sup 2+} in the electron beam is found. Modification of the Raman scattering (RS) spectra of the samples in the case of a decrease in the size of the particles from bulk to a nanosized state is found. The change in the forbidden band width E{sub g} of the samples in the case of transition from a bulk powder to an NP is considered. The spectral and luminescence characteristics of the samples in the polycrystalline and nanoamorphous states are studied. It is shown that, in the case of transition to nanosamples, the ligand field around Eu{sup 2+} changes. This may be due to the violation of the translational symmetry in the NP. The bond between the 4f and 5d electrons weakens. Degeneracy of the {sup 2}e{sub g} level appears. Presumably, the reduction of ions Eu{sup 3+} → Eu{sup 2+} in the electron beam due to the breaking of the Si(Ge)–O bond in the process of evaporation of the samples and capture of the released electron by the Eu{sup 3+} ions is found.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Emission of Light by CdMnTe/CdMgTe Heterostructure with Narrow Quantum Wells

The optical properties of the Cd{sub 0.4}Mn{sub 0.4}Te/Cd{sub 0.5}Mg{sub 0.5}Te heterostructure with quantum wells of 7 monolayers in thickness are studied. The temperature dependences of the integral intensity of luminescence of the barrier layer and quantum well and intracenter luminescence of manganese ions are determined, and the luminescence excitation spectra are measured. Numerous components corresponding to LO phonons of the CdTe, MgTe, and MnTe types are observed in the Raman scattering spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis of Nano-Crystalline Graphite Films in Hollow Cathode Discharge

The results of experiments on the production of nano-crystalline graphite films in the hollow cathode discharge on Ni(111)/sapphire substrate are presented. The characteristics of the discharge for different gases and pressures are given. The emission spectra of the plasma have been measured. The layers of the deposited graphite have been characterized by the methods of Raman scattering and atomic force microscopy. The field emission of nano-crystalline graphite was also measured. The produced layers have good homogeneity and high value of the electron field emission. The presence of vertically growing graphene is revealed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Frequency Conversion of High-Power Gyrotron Radiation under Conditions of Raman Backscattering on an Auxiliary Electron Beam

The regime of Raman backscattering of a pumping wave that is cocurrent with an electron beam into the counterpropagating wave can be used for the radiation frequency conversion in high-power gyrotrons. The absolute instability developing in this system ensures generation of a scattered signal in the absence of external resonators, which allows the frequency of scattered radiation to be smoothly controlled within 20–40% by varying the electron energy in the case of high-power (megawatt) gyrotrons used as sources of pumping radiation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Modeling of Raman-Scattering Signals in Biological Tissues by Direct and Two-Step Approaches

A search for an effective method of modelling of the Raman-spectroscopy problem in turbid (scattering) media has been performed taking into account the corresponding parameters of the detector and sample volume. A solution of the radiative-transfer equation by Monte-Carlo method underlies the proposed model. Two fundamental approaches to numerical modeling of Raman scattering are considered: the direct transport problem of Rayleigh and Raman photons at each point of the medium and the two-step model, in which a photon flux in the medium is calculated in the first stage, followed by generation of the corresponding number of Raman photons at each point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of an Optical Breakdown on the Stimulated Raman Scattering in Water in the Field of Picosecond Laser Pulses

Experiments on nonstationary stimulated Raman scattering (SRS) of laser pulses and an optical breakdown are performed to study water samples with gas nanobubbles (bubstons) and without the bubston phase. The SRS excitation threshold in water with the bubston phase turns out to be higher than that in bubston-free water, and the optical breakdown threshold in bubston-containing water is lower than that in bubston-free water. The spectral composition of the SRS signal from bubston-free water samples is found to differ from that of bubston-containing water samples. This effect is caused by the formation of long-lived high-molecular H{sub 2}O complexes near the bubston surface, which are stabilized by the dispersion forces acting from this surface. Since these complexes are located inside 1–2 monolayers adjacent to the surface, the volume density of these complexes is negligibly small as compared to the density of “free” (not bound to bubstons) molecules. To interpret the experimental results, we advance a hypothesis that the detected effect is related to the giant enhancement caused by the generation of a resonance plasmon inside a bubston during an optical breakdown. As follows from our estimates, the bubston-related plasma frequency falls in the range 2600–3200 cm{sup –1}; it is this range that contains the SRS spectral lines of water containing the bubston phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Effect of ion interactions on the Raman spectrum of NO 3 − : Toward monitoring of low-activity nuclear waste at Hanford

Raman spectroscopy is a valuable in-situ technique for many applications. The concentrations of species in complex ionic mixtures of nuclear waste can be estimated using Raman measurements. However, it has been experimentally observed that ion interactions can cause a modification of Raman peak intensities and positions. The present work explores the nonlinear behavior associated with the nitrate anion, NO 3 − , which is present in abundance in low activity nuclear waste. We examine changes in the main Raman peak of the nitrate anion in the presence of other ions. A wide range of concentrations are covered, including those expected during direct-feed low-activity waste (DFLAW) processing at the Hanford site in the State of Washington. The experiments showed that the ions interact and associate to form ion pairs, which results in a blue shift (i.e., a shift towards higher wavenumbers) in the main Raman peak of nitrate. The results indicate that cation concentration is a better predictor of the peak shift, compared to ionic strength, both for binary and multicomponent mixtures. These findings have direct implications on the development of spectra-to-composition models for the DFLAW system, since they show deviations from the linearity assumptions used in common chemometric models.

42 ENGINEERING↗
DOE OSTI2020

Impact of moderate pump–Stokes chirp on femtosecond coherent anti-Stokes Raman scattering spectra

The effects of inducing moderate chirp in the pump and Stokes beams in chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy are discussed. This is an important issue for measurements in high-pressure systems where the CARS beams must be transmitted through glass of significant thickness to reach the probe volume of interest. The effects were investigated both experimentally, by inserting disks of SF-11 glass into the pump and Stokes beam paths, and theoretically, by incorporating pulse chirp into our time-dependent density matrix (TDDM) simulations. Experimentally, we found that inducing moderate pump and Stokes chirp produced significant spectral narrowing of the nonresonant four-wave mixing contribution to the CARS signal, and this allowed us to control the resonant excitation bandwidth to eliminate interferences from neighboring species. The temperature measurement accuracy and precision were essentially unchanged with respect to the unchirped pump and Stokes case. The effect of moderate pump and Stokes chirp on the narrowing of the Raman excitation efficiency envelope was investigated by solving the full set of TDDM through direct numerical integration. The numerical efficiency of the solutions was enhanced dramatically by developing a parallel version of the TDDM code. The calculated Raman excitation efficiency profile was incorporated in our phenomenological spectral fitting codes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Raman spectroscopy of neutron irradiated silicon carbide: Correlation among Raman spectra, swelling, and irradiation temperature

The effects of neutron irradiation on microstructural evolution and the resultant changes in physical and mechanical properties are of critical importance for the development of silicon carbide (SiC) materials for nuclear applications. This study neutron–irradiated β–SiC under a wide range of conditions at temperatures between 235 and 750°C and neutron doses of 0.01–11.8 displacements per atom, and then evaluated the effects on the SiC structure using Raman spectroscopy. The SiC optical phonon lines were shifted to lower wavenumbers by irradiation. Correlations were found among the wavenumber of the longitudinal optical phonon line, irradiation–induced swelling, and irradiation temperature. The peak shift also correlated indirectly with decreasing thermal conductivity of irradiated SiC. The irradiation–induced peak shift is explained by combinations of lattice strain, reduction of the elastic modulus, and other factors including decreasing coherent domain size. Lastly, these findings bridge irradiation–induced microstructural changes and property changes and illustrate how Raman spectroscopy is a useful tool for nondestructively assessing irradiated SiC materials for nuclear applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Modeling of Atom Interferometer Accelerometer

This report presents the theoretical effort to model and simulate the atom-interferometer accelerometer operating in a highly mobile environment. Multitudes of non-idealities may occur in such a rapidly-changing environment with a large acceleration whose amplitude and direction both change quickly. We studied the undesired effect of high mobility in the atom-interferometer accelerator in a detailed model and a simulator. The undesired effects include the atom cloud's movement during Raman pulses, the Doppler effect due to the relative movement between the atom-cloud and the supporting platform, the finite atom cloud temperature, and the lateral movement of the atom cloud. We present the relevant feed-forward mitigation strategies for each identified non-ideality to neutralize the impact and obtain accurate acceleration measurements.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Effect of the Plasma Functionalization of Carbon Nanotubes on the Formation of a Carbon Nanotube–Nickel Oxide Composite Electrode Material

The unique properties of carbon nanotubes (CNTs) make these systems promising for the production of composite electrode materials based on a combination of CNTs and transition metal oxides. The effect the parameters of the functionalization of vertically aligned CNT arrays in mixed argon–oxygen high-frequency plasma have on the structural characteristics of CNTs is studied by Raman spectroscopy. The effect the duration of treatment at higher partial argon consumption and low total gas consumption of the working mixture has on the distribution of nickel oxide over a CNT array deposited via SILAR is determined. Results from scanning electron microscopy show that lengthening the duration of functionalization from 30 to 1200 s allows the depth of coating a CNT surface with a nickel oxide layer to be increased from 300 nm to 2.5 μm. Composite structures produced in this work can be used as electrodes of supercapacitors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Direct evidence of the effect of a moderate external magnetic field on stimulated Raman scattering in the kinetic regime

We present results from an experiment carried out at the OMEGA-EP laser facility that investigated the effect of a perpendicular magnetic field on stimulated Raman scattering (SRS) and report the first direct measurement of magnetic mitigation of SRS. A 13-T magnetic field generated by pulsed-power coils was imposed on a gas jet plasma, and a novel three-picket interaction beam was used to explore SRS reflectivity for several plasma conditions within single shots. The time-resolved backscattered light shows that SRS was mitigated by the external 13-T magnetic field in the kinetic regime (kλ D ∼ 0.3, where k is the electron plasma wave's wavenumber and λ D is electron Debye length) and at a density of n e /n cr ∼ 0.10. On the other hand, we also measured an enhancement of SRS reflectivity at lower density (n e /n cr < 0.08). We discuss experimental results in the context of magnetohydrodynamic and particle-in-cell simulations that scan SRS dynamics for a variety of plasma conditions. While the experimental evidence of SRS mitigation validates prior work on the kinetic simulation of SRS in an external magnetic field, we also find that other mechanisms such as SRS rescatter can lead to an enhancement of measured SRS reflectivity in simulations of parameters relevant to our experiment.

Gas jet↗
DOE OSTI2025

Gas Hydrate Film Growth in Microfluidic Channels for Carbon Dioxide Capture and Sequestration Applications

Gas or clathrate hydrates are a solid, crystalline compound composed of water and guest molecules that typically form at high pressure and low temperature conditions. Carbon dioxide (CO2) hydrates may be involved in several carbon dioxide capture and sequestration (CCS) applications, including CO2 pipeline transportation and CO2 offshore sequestration. Within these applications, the formation mechanism and kinetics must be well understood to manage the CCS processes, either by preventing or promoting hydrate formation. In this work, a high-pressure glass microfluidic reactor is used in tandem with visual microscopy and in-situ Raman spectroscopy to study both the morphological and kinetic behavior of gas hydrate crystals. Subcooling, pressure, and CO2 flow rate are investigated for their impact on the thickening behavior of pure CO2 hydrates, with flow rate being the only parameter to have a significant effect. Visual and Raman spectroscopy evidence show that both a dense hydrate layer and a porous hydrate layer form, and the latter may provide a path for mass transfer to continue hydrate crystallization. A first principles mass transfer model is developed to describe CO2 hydrate crystal thickening at the interface between gas and water. The impacts of gas impurities and channel wettability are also studied. This method is further applied to investigate the conversion of methane hydrate to CO2 hydrate for combined energy recovery and methane hydrate formation. The authors acknowledge the US Department of Energy Basic Energy Science award # DE-SC0022162.

Wadsworth, Lindsey [Colorado School of Mines, Gold↗
DOE OSTI2019

Quantum Isotope Effect in Silicon at Low Temperatures

High-precision studies of Raman scattering in isotopically pure {sup 28}Si, {sup 29}Si, {sup 30}Si single crystals have been performed in the temperature range from 8 to 300 K. It has been found that a difference of about (0.4 ± 0.1) cm{sup –1} observed at temperatures T ≤ 100 K between the normalized Raman frequencies of the {sup 28}Si and {sup 30}Si isotopes measured with an accuracy of 0.1 cm{sup –1} is certainly due to quantum effects damping with increasing temperature. The possibility of observing quantum isotope effects in germanium has been estimated using experimental data on isotope effects in diamond and silicon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗