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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Reflective mode X-ray microscopy of inclined objects

We propose an optical configuration of a reflective mode X-ray microscope which operates by the reflection of radiation incident on an object at grazing incidence. Primary emphasis is placed on the image recording with minimal aberrations. Numerical simulations are used to estimate the spatial resolution and the field of view, and a comparison is made with the results of investigation of surface-modified objects with a reflective mode X-ray laser microscope operating at a wavelength of 14.9 nm. (laser applications and other topics in quantum electronics)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

On the nonlocality of the reflection of an electromagnetic wave from a flat surface upon excitation of surface waves

Diffraction phenomena arising due to reflection of a plane electromagnetic wave from a metal film, which are caused by a limited size of the incident beam aperture, are considered in the Kretschmann scheme used to excite a surface plasmon wave. It is shown that the wave reflection occurs nonlocally, that is, the region of the surface on which the wave is incident and the region from which it is reflected do not substantially coincide. The localisation of the maximum field is investigated when a surface plasmon is excited on a silver film. The absolute values of the gain in terms of the amplitude of the field on the free surface of the film are calculated. The proposed theory is applied to explain the Goos – Hänchen effect. (integrated optics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The NuSTAR view of reflection and absorption in NGC 7582

NGC 7582 is a well-studied X-ray bright Seyfert 2 with moderately heavy (N{sub H} ∼ 10{sup 23}−10{sup 24} cm{sup −2}), highly variable absorption and strong reflection spectral features. The spectral shape changed around the year 2000, dropping in observed flux and becoming much more highly absorbed. Two scenarios have been put forth to explain this spectral change: (1) the central X-ray source partially “shut off” around this time, decreasing in intrinsic luminosity, with a delayed decrease in reflection features due to the light-crossing time of the Compton-thick material or (2) the source became more heavily obscured, with only a portion of the power law continuum leaking through. NuSTAR observed NGC 7582 twice in 2012, two weeks apart, in order to quantify the reflection using high-quality data above 10 keV. We find that the most plausible scenario is that NGC 7582 has recently become more heavily absorbed by a patchy torus with a covering fraction of ∼80%–90% and an equatorial column density of ∼3×10{sup 24} cm{sup −2}. We find the need for an additional highly variable full-covering absorber with N{sub H} = (4–6) × 10{sup 23} cm{sup −2} in the line of sight, possibly associated with a hidden broad line region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Near total reflection x-ray photoelectron spectroscopy: quantifying chemistry at solid/liquid and solid/solid interfaces

Near total reflection regime has been widely used in x-ray science, specifically in grazing incidence small angle x-ray scattering and in hard x-ray photoelectron spectroscopy (XPS). In this work, we introduce some practical aspects of using near total reflection (NTR) in ambient pressure XPS and apply this technique to study chemical concentration gradients in a substrate/photoresist system. Experimental data are accompanied by x-ray optical and photoemission simulations to quantitatively probe the photoresist and the interface with the depth accuracy of ~1 nm. Together, our calculations and experiments confirm that NTR XPS is a suitable method to extract information from buried interfaces with highest depth-resolution, which can help address open research questions regarding our understanding of concentration profiles, electrical gradients, and charge transfer phenomena at such interfaces. Furthermore, the presented methodology is especially attractive for solid/liquid interface studies, since it provides all the strengths of a Bragg-reflection standing-wave spectroscopy without the need of an artificial multilayer mirror serving as a standing wave generator, thus dramatically simplifying the sample synthesis.

solid/solid interface↗
DOE OSTI2013

NuSTAR SPECTROSCOPY OF GRS 1915+105: DISK REFLECTION, SPIN, AND CONNECTIONS TO JETS

We report on the results of spectral fits made to a NuSTAR observation of the black hole GRS 1915+105 in a 'plateau' state. This state is of special interest because it is similar to the 'low/hard' state seen in other black holes, especially in that compact, steady jets are launched in this phase. The 3-79 keV bandpass of NuSTAR, and its ability to obtain moderate-resolution spectra free from distortions such as photon pile-up, are extremely well suited to studies of disk reflection in X-ray binaries. In only 15 ks of net exposure, an extraordinarily sensitive spectrum of GRS 1915+105 was measured across the full bandpass. Ionized reflection from a disk around a rapidly spinning black hole is clearly required to fit the spectra; even hybrid Comptonization models including ionized reflection from a disk around a Schwarzschild black hole proved inadequate. A spin parameter of a = 0.98 ± 0.01 (1σ statistical error) is measured via the best-fit model; low spins are ruled out at a high level of confidence. This result suggests that jets can be launched from a disk extending to the innermost stable circular orbit. A very steep inner disk emissivity profile is also measured, consistent with models of compact coronae above Kerr black holes. These results support an emerging association between the hard X-ray corona and the base of the relativistic jet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

UNAMBIGUOUS DETECTION OF REFLECTION IN MAGNETIC CATACLYSMIC VARIABLES: JOINT NuSTAR–XMM-NEWTON OBSERVATIONS OF THREE INTERMEDIATE POLARS

In magnetic cataclysmic variables (CVs), X-ray emission regions are located close to the white dwarf surface, which is expected to reflect a significant fraction of intrinsic X-rays above 10 keV, producing a Compton reflection hump. However, up to now, a secure detection of this effect in magnetic CVs has largely proved elusive because of the limited sensitivity of non-imaging X-ray detectors. Here we report our analysis of joint NuSTAR–XMM-Newton observations of three magnetic CVs, V709 Cas, NY Lup, and V1223 Sgr. The improved hard X-ray sensitivity of the imaging NuSTAR data has resulted in the first robust detection of Compton hump in all three objects, with amplitudes of ∼1 or greater in NY Lup, and likely <1.0 in the other two. We also confirm earlier reports of a strong spin modulation above 10 keV in V709 Cas, and we report the first detection of small spin amplitudes in the others. We interpret this as due to different height of the X-ray emitting region among these objects. A height of ∼0.2 white dwarf radii provides a plausible explanation for the low reflection amplitude of V709 Cas. Since emission regions above both poles are visible at certain spin phases, this can also explain the strong hard X-ray spin modulation. A shock height of ∼0.05 white dwarf radii can explain our results on V1223 Sgr, while the shock height in NY Lup appears negligible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

In situ laser reflectivity to monitor and control the nucleation and growth of atomically thin 2D materials*

The growth of atomically thin two-dimensional (2D) layered and other quantum materials is typically performed without in situ monitoring or control. Here, a simple laser reflectivity approach is demonstrated to provide in situ control over sub-monolayer thickness and growth kinetics during pulsed laser deposition (PLD) of MoSe 2 layers. First, the general technique is presented with emphasis on designing the maximum sensitivity of the optical contrast through consideration of Fresnel's equations with proper choice of layer thickness, substrate, and laser monitoring wavelength, incidence angle, and laser polarization. Then the 632.8 nm optical reflectivity of MoSe 2 layers on SiO 2 /Si substrates was predicted and compared with in situ monitoring of MoSe 2 growth by PLD under actual growth conditions using a probe HeNe laser beam. The measurements showed high sensitivity and excellent agreement with MoSe 2 surface coverages calculated from atomic resolution STEM analysis of 2D layers deposited in arrested growth experiments. Finally, growth kinetics revealed by these measurements showed sigmoidal nucleation and growth stages in the formation of the 2D MoSe 2 layers that are described by a simple model, indicating the promise of the laser reflectivity technique for in situ monitoring and control of 2D materials deposition.

2D materials↗
DOE OSTI2022

Deep-learning-guided high-resolution subsurface reflectivity imaging with application to ground-penetrating radar data

Subsurface reflectivity imaging is one of the most important geophysical characterization methods for revealing subsurface structures. In many cases, accurate subsurface reflectivity imaging is challenging because of, for example, random or coherent noise in the data and sparse source-receiver observation geometry. Here, we develop a deep-learning-guided iterative imaging method to improve subsurface structure imaging. Specifically, we train a supervised neural network to infer a noise-free, high-resolution image from a noisy, low-resolution image and use this estimated image as guidance to regularize least-squares imaging. We develop a systematic method to generate high-quality synthetic training data (data-label pairs) to train the guidance neural network. The trained neural network can provide high-fidelity predictions even for field-data images that are not in the training data. We validate our new imaging method using one synthetic and two field ground-penetrating radar data examples, and find that our method can produce clean, high-resolution subsurface reflectivity images where existing single-pass and least-squares imaging methods fail due to noise and insufficient data coverage.

58 GEOSCIENCES↗
DOE OSTI2018

Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity

Extreme ultraviolet (XUV) transient reflectivity around the germanium M 4,5 edge (3d core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of 3.2±0.2ps attributed to the X-L intervalley scattering and a hot hole relaxation rate of 600±300fs ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4-5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of 66° with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment (66°-70° for the range of XUV energies used. Finally, this work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Extending the capillary wave model to include the effect of bending rigidity: X-ray reflectivity and diffuse scattering

The surface roughness of a thin film at a liquid interface exhibits contributions of thermally excited fluctuations. This thermal roughness depends on temperature (𝑇), surface tension (𝛾), and elastic material properties, specifically the bending modulus (𝜅) of the film. A nonzero 𝜅 suppresses the thermal roughness at small length scales compared to an interface with zero 𝜅, as expressed by the power spectral density of the thermal roughness. The description of the x-ray scattering of the standard capillary wave model (CWM), which is valid for zero 𝜅, is extended to include the effect of 𝜅. The extended CWM (eCWM) provides a single analytical form for both the specular x-ray reflectivity (XRR) and the diffuse scattering around the specular reflection, and recovers the expression of the CWM at its zero 𝜅 limit. This theoretical approach enables the use of single-shot grazing incidence x-ray off-specular scattering (GIXOS) measurements for characterizing the structure of thin films on a liquid surface. The eCWM analysis approach decouples the thermal roughness factor from the surface scattering signal, providing direct access to the intrinsic surface-normal structure of the film and its bending modulus. Moreover, the eCWM facilitates the calculation of reflectivity at any desired resolution (pseudo-XRR approach). The transformation into pseudo-XRR provides the benefit of using widely available XRR software to perform GIXOS analysis. The extended range of the vertical scattering vector (𝑄 𝑧 ) available with the GIXOS pseudo-XRR approach allows for a higher spatial resolution than with conventional XRR. Experimental results are presented for various lipid systems, showing strong agreement between conventional specular XRR and pseudo-XRR methods. This agreement validates the proposed approach and highlights its utility for analyzing soft, thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Improving experimental phases for strong reflections prior to density modification

A genetic algorithm has been developed to optimize the phases of the strongest reflections in SIR/SAD data. This is shown to facilitate density modification and model building in several test cases. Experimental phasing of diffraction data from macromolecular crystals involves deriving phase probability distributions. These distributions are often bimodal, making their weighted average, the centroid phase, improbable, so that electron-density maps computed using centroid phases are often non-interpretable. Density modification brings in information about the characteristics of electron density in protein crystals. In successful cases, this allows a choice between the modes in the phase probability distributions, and the maps can cross the borderline between non-interpretable and interpretable. Based on the suggestions by Vekhter [Vekhter (2005 ▶), Acta Cryst. D61, 899–902], the impact of identifying optimized phases for a small number of strong reflections prior to the density-modification process was investigated while using the centroid phase as a starting point for the remaining reflections. A genetic algorithm was developed that optimizes the quality of such phases using the skewness of the density map as a target function. Phases optimized in this way are then used in density modification. In most of the tests, the resulting maps were of higher quality than maps generated from the original centroid phases. In one of the test cases, the new method sufficiently improved a marginal set of experimental SAD phases to enable successful map interpretation. A computer program, SISA, has been developed to apply this method for phase improvement in macromolecular crystallography.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Study of interface reaction in a B 4 C/Cr mirror at elevated temperature using soft X-ray reflectivity

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Å thickness deposited with an adhesive layer of 20 Å Cr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr–SiO2 region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiOx). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

A Total Internal Reflection Photoconductive Switch

We demonstrate a novel illumination technique for extrinsic photoconductive switches based on the creation of a total internal reflection trap to increase the optical path length. The optimal geometry is a square substrate illuminated from one of the corners along the diagonal, ensuring that the total internal reflection condition is maintained after any reflection off the device lateral sides. The optical absorption uniformity throughout the device bulk was improved by designing the entry window to have a cylindrical shape and by using a square core optical fiber as an illumination source. In conclusion, the concept is experimentally validated on a vanadium-doped silicon carbide device that shows approximately four-fold improvement in responsivity compared to normal illumination.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

The Numerical Computation of Clear-Sky Transmittance and Reflectance of Solar Radiation for the Modeling of PV System Performance

Modeling of photovoltaics (PV) system performance requires numerical computation of plane-of-array (POA) irradiances that are influenced by atmospheric absorption and scattering. Clear-sky transmittance and reflectance of diffuse irradiance are essential for the understanding of solar radiation related to multiple reflection between land surface and clouds. This study analytically solves the radiative transfer equation for clear-sky conditions and numerically computes the transmittance and reflectance of diffuse irradiance as functions of the optical thickness of the atmosphere. The computation will be implemented in all-sky radiative transfer models to improve their computational efficiency.

14 SOLAR ENERGY↗
DOE OSTI2014

Characterization of zinc oxide thin film using atomic force microscopy and optimized X-ray reflectivity by genetic algorithm

For the nanostructured thin films, the study of surface physical parameters is often of crucial importance. For example, the transport properties of ZnO thin film are strongly affected by the geometry of this film. In this study sol—gel ZnO thin film was spin coated on glass substrate. The inverse problem of thin film characterization is formulated as a parameter identification problem in which a set of parameters corresponding to the thin film property can be found by minimizing fitness functions formulated using theoretical and experimental X-ray reflectivity. Atomic force microscopy (AFM) measurements used as an independent procedure for extracting structural parameters of the film and accurate interpretation of X-ray reflectivity data. After applying Fractal calculations on the AFM measured data the Genetic algorithm (GA) method was applied for extracting other important structural parameters. Optimization of specular X-ray reflectivity by GA method gives information along the depth such as thickness, roughness, and electron density profile of layers in the film.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Effect of Specular Reflectances on the Interaction of an Electromagnetic E Wave with a Thin Metal Film Placed between Two Dielectric Media

The interaction of an electromagnetic E wave with a thin metal film placed between two dielectric media is calculated in the case of different specular reflectances q{sub 1} and q{sub 2} for the reflection of electrons from the surface of the thin metal layer, in the case of variations in the values of dielectric permittivities ε{sub 1} and ε{sub 2} of the media, and in the case of different values of angle of incidence θ of the electromagnetic wave. The behavior of reflection coefficient R, transmission coefficient T, and absorption coefficient A in relation to the frequency of the external field is analyzed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Andreev reflection in rotating superfluid {sup 3}He-B

Andreev reflection of quasiparticle excitations from quantized line vortices is reviewed in the isotropic B phase of superfluid {sup 3}He in the temperature regime of ballistic quasiparticle transport at T ≤ 0.20T{sub c}. The reflection from an array of rectilinear vortices in solid-body rotation is measured with a quasiparticle beam illuminating the array mainly in the orientation along the rotation axis. The result is in agreement with the calculated Andreev reflection. The Andreev signal is also used to analyze the spin-down of the superfluid component after a sudden impulsive stop of rotation from an equilibrium vortex state. In a measuring setup where the rotating cylinder has a rough bottom surface, annihilation of the vortices proceeds via a leading rapid turbulent burst followed by a trailing slow laminar decay, from which the mutual friction dissipation can be determined. In contrast to the currently accepted theory, it is found to have a finite value in the zero-temperature limit: α(T→0) = (5 ± 0.5) × 10{sup −4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Reflection and backscattering of microwaves under doubling of the plasma density and displacement of the gyroresonance region during electron cyclotron resonance heating of plasma in the l-2M stellarator

Reflection and backscattering of high-power (400 kW) gyrotron radiation creating and heating plasma at the second harmonic of the electronic cyclotron frequency in the L-2M stellarator have been investigated experimentally. The effect of the displacement of the gyroresonance region from the axis of the plasma column under doubling of the plasma density on the processes of reflection and backscattering of microwave radiation has been examined. A near doubling of short-wavelength (k{sub ⊥} ≈ 30 cm{sup –1}) turbulent density fluctuations squared is observed. The change in the energy confinement time under variations of plasma parameters and characteristics of short-wavelength turbulence is discussed. A discrepancy between the measured values of the reflection coefficient from the electron cyclotron resonance heating region and predictions of the one-dimensional model is revealed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗