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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 1,801 records · Page 100
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Determination of optical conductivity and different optical energy losses for non-crystalline Vanadyl tetra tert-butyl 2,3 Naphthalocyanine thinfilms

Amorphous Vanadyl Tetra Tert Butyl 2, 3 naphthalocyanine thin films (VTTBNc) have been deposited using Physical Vapor Deposition technique. By analyzing the X-ray diffraction, the structure of as deposited films is found to be non-crystalline. Different optical properties of these thin films have been investigated by means of optical absorption and reflection spectra. Various optical constants like band gap energy, E{sub g} the width of band tails of localized states into the gap, E{sub U} and steepness parameter, {beta} gets calculated and the variation of different optical parameters like refractive index, extinction coefficient, dielectric constants, optical conductivity and surface and volume energy losses with photon energy are estimated.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The splitted laser beam filamentation in interaction of laser and an exponential decay inhomogeneous underdense plasma

The splitted beam filamentation in interaction of laser and an exponential decay inhomogeneous underdense plasma is investigated. Based on Wentzel-Kramers-Brillouin (WKB) approximation and paraxial/nonparaxial ray theory, simulation results show that the steady beam width and single beam filamentation along the propagation distance in paraxial case is due to the influence of ponderomotive nonlinearity. In nonparaxial case, the influence of the off-axial of {alpha}{sub 00} and {alpha}{sub 02} (the departure of the beam from the Gaussian nature) and S{sub 02} (the departure from the spherical nature) results in more complicated ponderomotive nonlinearity and changing of the channel density and refractive index, which led to the formation of two/three splitted beam filamentation and the self-distortion of beam width. In addition, influence of several parameters on two/three splitted beam filamentation is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Two-dimensional plasma grating by non-collinear femtosecond filament interaction in air

We experimentally demonstrated that two-dimensional (2D) plasma gratings could be generated in air by nonlinear interaction of three femtosecond filaments. The intensity interference of non-collinearly overlapped filaments was self-projected along a relatively long distance and accompanied with a wavelength-scale periodic change of the refractive index in the encircling air due to periodic plasma density modulation. The 2D plasma gratings supported 2D diffraction of the generated third-harmonic pulses. By using in-line time-resolved holographic imaging and time-delayed diffraction, the 2D plasma gratings were evidenced to last a few tens picoseconds after the excitation pulses.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Computer simulation on spatial resolution of X-ray bright-field imaging by dynamical diffraction theory for a Laue-case crystal analyzer

Recently, dark-field imaging (DFI) and bright-field imaging (BFI) have been proposed and applied to visualize X-ray refraction effects yielded in biomedical objects. In order to clarify the spatial resolution due to a crystal analyzer in Laue geometry, a program based on the Takagi-Taupin equation was modified to be used for carrying out simulations to evaluate the spatial resolution of images coming into a Laue angular analyzer (LAA). The calculation was done with a perfect plane wave for diffraction wave-fields, which corresponded to BFI, under the conditions of 35 keV and a diffraction index 440 for a 2100 {mu}m thick LAA. As a result, the spatial resolution along the g-vector direction showed approximately 37.5 {mu}m. 126 {mu}m-thick LAA showed a spatial resolution better than 3.1 {mu}m under the conditions of 13.7 keV and a diffraction index 220.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Talbot-Lau x-ray interferometry for high energy density plasma diagnostic

High resolution density diagnostics are difficult in high energy density laboratory plasmas (HEDLP) experiments due to the scarcity of probes that can penetrate above solid density plasmas. Hard x-rays are one possible probe for such dense plasmas. We study the possibility of applying an x-ray method recently developed for medical imaging, differential phase-contrast with Talbot-Lau interferometers, for the diagnostic of electron density and small-scale hydrodynamic instabilities in HEDLP experiments. The Talbot method uses micro-periodic gratings to measure the refraction and ultra-small angle scatter of x-rays through an object and is attractive for HEDLP diagnostic due to its capability to work with incoherent and polychromatic x-ray sources such as the laser driven backlighters used for HEDLP radiography. Our paper studies the potential of the Talbot method for HEDLP diagnostic, its adaptation to the HEDLP environment, and its extension of high x-ray energy using micro-periodic mirrors. The analysis is illustrated with experimental results obtained using a laboratory Talbot interferometer.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

EIT Based Gas Detector Design by Using Michelson Interferometer

Electromagnetically induced transparency (EIT) is one of the interesting phenomena of light-matter interaction which modifies matter properties for propagation of light. In other words, we can change the absorption and refractive index (RI) in neighborhood of the resonant frequency using EIT. In this paper, we have doped 3-level quantum dots in one of the Michelson Interferometer's mirror and used EIT to change its RI. So, a controllable phase difference between lights in two arms of interferometer is created. Long response time is the main drawback of Michelson interferometer based sensor, which is resolved by this technique.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Formation and Characterization of Au/TiO{sub 2} NanoComposite by Laser Irradiation

Nanosized noble metal particles and dielectric matrix composite materials have attracted extensive attention due to their large third-order nonlinear susceptibility and ultrafast response properties, and have great potential for applications in optical devices. In this study, composite Au/TiO{sub 2} nanoparticles were synthesized by laser irradiation. Characterization of the composite particles by means of UV-Vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) was carried out. We have successfully produced composite Au/TiO{sub 2} nanoparticles by laser ablation of metal gold plate in TiO{sub 2} sol. The red shift of 30 nm in the surface plasmon band of Au is found due to the refractive index of TiO{sub 2}. The surrounding TiO{sub 2} matrix is amorphous, but upon heat treatment becomes a crystalline oxide. Various morphologies of particles are visible, including circular, hexagonal and triangular projection; circular and thus spherical particles being the most common, which were confirm by the TEM images. A large size distribution of particles is visible in TEM images; the average grain size was measured 35 nm, which was confirmed by X-ray diffraction studies.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Experimental Investigation of RF Sheath Rectification in ICRF and LH Heated Plasmas on Alcator C-Mod

Radio frequency (RF) rectification of the plasma sheath is being actively studied on C-Mod as a likely mechanism that leads to prohibitively high molybdenum levels in the plasma core of ion cyclotron RF (ICRF) heated discharges. We installed emissive, ion sensitive, Langmuir, and 3-D B-dot probes to quantify the plasma potentials ({Phi}{sub P}) in ICRF and lower hybrid (LH) heated discharges. Two probe sets were mounted on fixed limiter surfaces and one set of probes was mounted on a reciprocating (along the major radius) probe. Initial results showed that RF rectification is strongly dependent on the local plasma density and not on the local RF fields. The RF sheaths had a threshold-like appearance at the local density of {approx}10{sup 16} m-{sup 3}. Radial probe scans revealed that the RF sheaths peaked in the vicinity of the ICRF limiter surface, agreeing with a recent theory. The highest {Phi}{sub P}'s were observed on magnetic field lines directly mapped to the active ICRF antenna. Measurements in LH heated plasmas showed a strong {Phi}{sub P} dependence on the parallel index of refraction n{sub ||} of the launched LH waves: {Phi}{sub P} is greater at lower n{sub ||}. Little dependence was observed on the local plasma density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Scattering of Radio Frequency Waves by Edge Density Blobs in Tokamak Plasmas

The density blobs and fluctuations present in the edge region of magnetic fusion devices can scatter radio frequency (RF) waves through refraction and diffraction. The scattering can diffuse the rays in space and in wave-vector space. The diffusion in space can make the rays miss their intended target region, while the diffusion in wave-vector space can broaden the wave spectrum and modify the wave damping and current profile.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

High frequency fast wave current drive for DEMO

A steady-state tokamak reactor (SSTR) requires a high efficiency current drive system, from plug to driven mega-amps. RF systems working in the ion-cyclotron range of frequencies (ICRF) have high efficiency from plug to antenna but a limited current drive (CD) efficiency and centrally peaked CD profiles. The latter feature is not adequate for a SSTR where the current should be sufficiently broad to keep the central safety factor (possibly significantly) above 1. In addition, the fact that the fast wave (FW) is evanescent at the edge limits coupling, requiring high voltage operation, which makes the system dependent on plasma edge properties and prone to arcing, reducing its reliability. A possible way to overcome these weaknesses is to operate at higher frequency (10 times or more the cyclotron frequency). The advantages are: (1) The coupling can be much better (waves propagate in vacuum) if the parallel refractive index n{sub ||} is kept below one, (2) The FW group velocity tends to align to the magnetic field, so the power circumnavigates the magnetic axis and can drive off-axis current, (3) Due to the latter property, n{sub ||} can be upshifted along the wave propagation path, allowing low n{sub ||} launch (hence good coupling, large CD efficiency) with ultimately good electron absorption (which requires higher n{sub ||}. Note however that the n{sub ||} upshift is a self-organized feature, that electron absorption is in competition with {alpha}-particle absorption and that uncoupling of the FW from the lower hybrid resonance at the edge requires n{sub ||} slightly above one. The latter possibly counterproductive features might complicate the picture. The different aspects of this potentially attractive off-axis FWCD scheme are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Optical and structural characterization of thermal oxidation effects of erbium thin films deposited by electron beam on silicon

Thermal oxidation effects on the structural, compositional, and optical properties of erbium films deposited on silicon via electron beam evaporation were analyzed by x-ray diffraction, x-ray photoelectron spectroscopy, Auger electron spectroscopy, and spectroscopic ellipsometry. A gradual rise in oxidation temperature from 700 to 900 deg. C resulted in a transition from ErO- to Er{sub 2}O{sub 3}-rich phase. Additional increase in oxidation temperature above 1000 deg. C led to the formation of erbium silicate due to further oxygen incorporation, as well as silicon out-diffusion from the substrate. A silicon oxide interfacial layer was also detected, with its thickness increasing with higher oxidation temperature. Additionally, film refractive index decreased, while its Tauc bandgap value increased from {approx}5.2 eV to {approx}6.4 eV, as the oxidation temperature was raised from 700 deg. C to above 900 deg. C. These transformations were accompanied by the appearance of an intense and broad absorption band below the optical gap. Thermal oxidation effects are discussed in the context of film structural characteristics and defect states.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Influence of plasma-generated negative oxygen ion impingement on magnetron sputtered amorphous SiO{sub 2} thin films during growth at low temperatures

Growth of amorphous SiO{sub 2} thin films deposited by reactive magnetron sputtering at low temperatures has been studied under different oxygen partial pressure conditions. Film microstructures varied from coalescent vertical column-like to homogeneous compact microstructures, possessing all similar refractive indexes. A discussion on the process responsible for the different microstructures is carried out focusing on the influence of (i) the surface shadowing mechanism, (ii) the positive ion impingement on the film, and (iii) the negative ion impingement. We conclude that only the trend followed by the latter and, in particular, the impingement of O{sup -} ions with kinetic energies between 20 and 200 eV, agrees with the resulting microstructural changes. Overall, it is also demonstrated that there are two main microstructuring regimes in the growth of amorphous SiO{sub 2} thin films by magnetron sputtering at low temperatures, controlled by the amount of O{sub 2} in the deposition reactor, which stem from the competition between surface shadowing and ion-induced adatom surface mobility.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Tunable silica shell and its modification on photoluminescent properties of Y{sub 2}O{sub 3}:Eu{sup 3+}-SiO{sub 2} nanocomposites

The core-shell rare earth (RE) doped nanophosphors are attracting current interests owing to their potential application in bio-images. Understanding the modification of core-shell structure on photoluminescent (PL) properties of RE ions is important in obtaining efficient nanophosphors. In this paper, Y{sub 2}O{sub 3}:Eu{sup 3+}-SiO{sub 2} composites with fixed cubic Y{sub 2}O{sub 3}:Eu{sup 3+} core ({approx}80 nm) and controllable thickness of silica shell ({approx}3-21 nm) were synthesized via a modified Stoeber method and characterized by x-ray diffraction patterns, transmission electronic microscope images, and Fourier-transform infrared absorption spectra. The dependences of PL properties of Y{sub 2}O{sub 3}:Eu{sup 3+} on the thickness of the silica shell and excitation power density were systemically studied under charge transfer excitation (254 nm) and {sup 7}F{sub J}-{sup 5}D{sub 2} inner-shell excitation (460-486 nm). It is interesting to observe that under {sup 7}F{sub J}-{sup 5}D{sub 2} excitation the intensity ratio of {sup 5}D{sub 0}-{sup 7}F{sub 2}/{sup 5}D{sub 0}-{sup 7}F{sub J} (J {ne} 2) for Eu{sup 3+} increases considerably with the increase of shell thickness and decreases with the increasing excitation power density, which confirms the variation of long-scale radiative energy transfer (RET) among Eu{sup 3+} ions and redistribution of {sup 5}D{sub 0}-{sup 7}F{sub J}(J = 0-4) transitions. And more, the radiative decay time of {sup 5}D{sub 0}-{sup 7}F{sub J} as a function of shell thickness demonstrates an optimum, which are assigned to the co-impacts of effective refractive index (n{sub eff}) and RET among Eu{sup 3+} ions.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Multi-chord fiber-coupled interferometry of supersonic plasma jets andcomparisons with synthetic data

A multi-chord fiber-coupled interferometer [Merritt et al., Rev. Sci. Instrum. 83, 033506 (2012)] is being used to make time-resolved density measurements of supersonic argon plasma jets on the Plasma Liner Experiment [Hsu et al., Bull. Amer. Phys. Soc. 56, 307 (2011)]. The long coherence length of the laser (> 10 m) allows signal and reference path lengths to be mismatched by many meters without signal degradation, making for a greatly simplified optical layout. Measured interferometry phase shifts are consistent with a partially ionized plasma in which an initially positive phase shift becomes negative when the ionization fraction drops below a certain threshold. In this case, both free electrons and bound electrons in ions and neutral atoms contribute to the index of refraction. This paper illustrates how the interferometry data, aided by numerical modeling, are used to derive total jet density, jet propagation velocity ({approx} 15-50 km/s), jet length ({approx} 20-100 cm), and 3D expansion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Ultrafast laser-induced changes in optical properties of semiconductors

We study the effect of laser radiation on optical properties of semiconductors of industrial interest. The material is pumped with a laser of chosen central frequency, for which the absorption is maximal, thus inducing electron dynamics, which modifies the optical properties. By using an improved theoretical model, we study ultrafast dynamic changes in the refraction index and reflectivity corresponding to a wide frequency-interval of probing radiation and identify that interval where these optical changes are most significant.

36 MATERIALS SCIENCE↗
DOE OSTI2012

X-ray bang-time and fusion reaction history at picosecond resolution using RadOptic detection

We report recent progress in the development of RadOptic detectors, radiation to optical converters, that rely upon x-ray absorption induced modulation of the optical refractive index of a semiconductor sensor medium to amplitude modulate an optical probe beam. The sensor temporal response is determined by the dynamics of the electron-hole pair creation and subsequent relaxation in the sensor medium. Response times of a few ps have been demonstrated in a series of experiments conducted at the LLNL Jupiter Laser Facility (JLF). This technology will enable x-ray bang-time and fusion burn-history measurements with {approx} ps resolution.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Analysis of the radiative lifetime of Pr{sup 3+} d-f emission

The radiative lifetime of excited states is governed by Fermi's Golden Rule. For many applications, the radiative decay rate is an important parameter. For example, for scintillators materials in PET scanners, a short response time is crucial and it has been realized that the d-f emission of Pr{sup 3+} is faster than for the widely applied d-f emission from Ce{sup 3+}. In this paper, the radiative decay rate of d-f emission from Pr{sup 3+} is systematically investigated in a wide variety of host lattices, including scintillators materials. The variation in the decay rate is analyzed based on Fermi's Golden Rule. The trend observed is best described using a full cavity model to correct for local-field effects and a {lambda}{sup 3} factor to account for the energy of the transition. Still, there is a considerable scatter of the experimental data around the best fit to these data. The variation is explained by uncertainties in the refractive indices and a variation in the transition dipole moment of the d-f transition for Pr{sup 3+}. Based on the results, the shortest radiative lifetime that can be achieved for Pr{sup 3+} d-f emission is predicted to be {approx}6 ns.

74 ATOMIC AND MOLECULAR PHYSICS↗

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