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At least 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Octave-spanning terahertz quarter-wave plates based on over-coupled Fabry–Pérot resonances in reflective metal–dielectric–metal metasurfaces

Compact devices for broadband polarization control in the terahertz (THz) regime remain challenging to realize due to the intrinsic phase dispersion associated with birefringent materials and resonant structures. Here, we demonstrate achromatic THz quarter-wave plates with near-unity efficiency and operating under a wide range of incidence angles, based on over-coupled metal–dielectric–metal reflective metasurfaces. The devices operate as single-port anisotropic Fabry–Pérot cavities, in which the phase dispersion of the over-coupled resonances is engineered to maintain an approximately constant relative phase delay between orthogonal field components over a broad frequency range. Four complementary metasurface designs operating at an incidence angle of 45° collectively cover the 0.25–3 THz frequency range accessible to a typical THz time-domain spectroscopy system. Each device exhibits an approximately octave-wide operational bandwidth, with axial ratios below 3 dB and polarization-conversion efficiencies exceeding 80% across most of the operating band. Systematic structural optimization suppresses coupling to diffraction and guided-wave modes, thereby further extending the usable bandwidth while preserving the required phase relationship between orthogonal polarizations. The demonstrated metasurfaces are compatible with wafer-scale fabrication processes, and experimental results show excellent agreement with simulations. These findings establish over-coupled reflective metasurfaces as a robust, scalable, and versatile platform for broadband THz polarization control.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Analytical model of diffuse reflectance spectrum of skin tissue

We have derived simple analytical expressions that enable highly accurate calculation of diffusely reflected light signals of skin in the spectral range from 450 to 800 nm at a distance from the region of delivery of exciting radiation. The expressions, taking into account the dependence of the detected signals on the refractive index, transport scattering coefficient, absorption coefficient and anisotropy factor of the medium, have been obtained in the approximation of a two-layer medium model (epidermis and dermis) for the same parameters of light scattering but different absorption coefficients of layers. Numerical experiments on the retrieval of the skin biophysical parameters from the diffuse reflectance spectra simulated by the Monte Carlo method show that commercially available fibre-optic spectrophotometers with a fixed distance between the radiation source and detector can reliably determine the concentration of bilirubin, oxy- and deoxyhaemoglobin in the dermis tissues and the tissue structure parameter characterising the size of its effective scatterers. We present the examples of quantitative analysis of the experimental data, confirming the correctness of estimates of biophysical parameters of skin using the obtained analytical expressions. (biophotonics)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Accounting for the instrument function of crystal spectrometers operating in many reflection orders in the diagnostics of laser plasma from its continuum spectrum

By the example of an X-ray spectrometer with a spherically bent mica crystal we consider the diagnostic errors entailed when the observed bremsstrahlung spectrum is attributed to a single spectrum of reflection. As an illustration we choose the X-ray emission spectrum of the plasma produced at the surface of a steel foil by femtosecond laser pulses of relativistic intensity. It is shown that obtaining adequate results in the case of a high-temperature plasma calls for the consistent inclusion of all orders of reflection or diffraction with a dispersing device. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Mid-IR reflectance spectrum of multilayer graphene: Influence of adsorbate at the graphene – substrate interface

Water adsorbate at the interface between graphene and SiO{sub 2}/Si substrate is found to affect the reflectance spectra of (3 – 6)-layer graphene in the mid-IR range (λ = 2 – 16 μm). The sample under study was previously kept in acetone vapour to remove water adsorbate from the graphene – substrate interface. The graphene reflectance in the range of 8 – 16 μm tends to decrease with recovery of water adsorbate layer at the graphene – substrate interface at room temperature and relative air humidity RH = 40 % – 60 %. The observed effect is most likely caused by the blue shift of the absorption edge at interband transitions due to the change in the Fermi level position in the graphene valence band under the influence of the charge induced in graphene by the water adsorbate film. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Holographic reflective Bragg gratings of gain and their role in the operation of high-power pulsed lasers

The influence of reflective Bragg gratings that are formed in an active medium due to the spatial periodic modulation of gain on the spectral properties of lasing are investigated by an example of a pulsed Nd : YAG laser. It is shown that, at a certain choice of the formation regime of reflective gratings of gain, the lasing bandwidth can be reduced by a factor of 3 – 5, with a corresponding increase in the pulse energy and significant improvement of spatial beam quality. (paper)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Derivation of the Two-Exponential Probability Density Function for Rossi-Alpha Measurements of Reflected Assemblies and Validation for the Special Case of Shielded Measurements

Rossi-alpha measurements of fissionable assemblies are used to estimate the prompt neutron decay constant, α, of a system, which can be used to infer reactivity. Previous works have demonstrated that two-exponential fits for Rossi-alpha measurements of reflected assemblies are better than one exponential fits; however, the two-exponential probability density function that is needed to obtain α from the fit parameters has not been derived. This paper derives the two-region point kinetics model (two-exponential fit) for Rossi-alpha measurements, a generalization of the current, one region model (one-exponential fit). The new model is validated with measurements of reflected, fissionable assemblies (highly enriched uranium with k eff ≈ 0.95 and weapons-grade plutonium with k eff > 0.77) with α held constant. Here, the results show that the twoexponential model can (i) predict the constant α, and (ii) deconvolve α and the time a neutron spends in the reflector, neither of which is possible with the one-exponential model.

42 ENGINEERING↗
DOE OSTI2014

First ultraviolet reflectance measurements of several Kuiper Belt objects, Kuiper Belt object satellites, and new ultraviolet measurements of A Centaur

We observed the 2600-3200 Å (hereafter, mid-UV) reflectance of two Kuiper Belt Objects (KBOs), two KBO satellites, and a Centaur, using the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS). Other than measurements of the Pluto system, these constitute the first UV measurements obtained of KBOs, and KBO satellites, and new HST UV measurements of the Centaur 2060 Chiron. We find significant differences among these objects, constrain the sizes and densities of Haumea's satellites, and report the detection of a possible spectral absorption band in Haumea's spectrum near 3050 Å. Comparisons of these objects to previously published UV reflectance measurements of Pluto and Charon are also made here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANALYTIC MODELS FOR ALBEDOS, PHASE CURVES, AND POLARIZATION OF REFLECTED LIGHT FROM EXOPLANETS

New observational facilities are becoming increasingly capable of observing reflected light from transiting and directly imaged extrasolar planets. In this study, we provide an analytic framework to interpret observed phase curves, geometric albedos, and polarization of giant planet atmospheres. We compute the observables for non-conservative Rayleigh scattering in homogeneous semi-infinite atmospheres using both scalar and vector formalisms. In addition, we compute phase curves and albedos for Lambertian, isotropic, and anisotropic scattering phase functions. We provide analytic expressions for geometric albedos and spherical albedos as a function of the scattering albedo for Rayleigh scattering in semi-infinite atmospheres. Given an observed geometric albedo our prescriptions can be used to estimate the underlying scattering albedo of the atmosphere, which in turn is indicative of the scattering and absorptive properties of the atmosphere. We also study the dependence of polarization in Rayleigh scattering atmospheres on the orbital parameters of the planet-star system, particularly on the orbital inclination. We show how the orbital inclination of non-transiting exoplanets can be constrained from their observed polarization parameters. We consolidate the formalism, solution techniques, and results from analytic models available in the literature, often scattered in various sources, and present a systematic procedure to compute albedos, phase curves, and polarization of reflected light.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Emergent symmetry in TbTe 3 revealed by ultrafast reflectivity under anisotropic strain

Here, we report ultrafast reflectivity measurements of the dynamics of the order parameter of the charge density wave (CDW) in TbTe 3 under anisotropic strain. We observe an increase in the frequency of the amplitude mode with increasing tensile strain along the a-axis (which drives the lattice into a > c, with a and c the lattice constants), and similar behavior for tensile strain along c (c > a). This suggests that both strains stabilize the corresponding CDW order and further support the near equivalence of the CDW phases oriented in a- and c-axis, in spite of the orthorhombic space group. The results were analyzed within the time-dependent Ginzburg–Landau framework, which agrees well with the reflectivity dynamics. Our study presents an ultrafast approach to assess the stability of phases and order parameter dynamics in strained systems.

Kim, Soyeun↗
DOE OSTI2019

Neutrino vertex reconstruction with in-ice radio detectors using surface reflections and implications for the neutrino energy resolution

Ultra high energy neutrinos (E ν > 10 16.5 eV) are efficiently measured via radio signals following a neutrino interaction in ice. An antenna placed1Script O(15 m) below the ice surface will measure two signals for the vast majority of events (90% at E ν =10 18 eV): a direct pulse and a second delayed pulse from a reflection off the ice surface. This allows for a unique identification of neutrinos against backgrounds arriving from above. Furthermore, the time delay between the direct and reflected signal (D'n'R) correlates with the distance to the neutrino interaction vertex, a crucial quantity to determine the neutrino energy. In a simulation study, we derive the relation between time delay and distance and study the corresponding experimental uncertainties in estimating neutrino energies. We find that the resulting contribution to the energy resolution is well below the natural limit set by the unknown inelasticity in the initial neutrino interaction. We present an in-situ measurement that proves the experimental feasibility of this technique. Continuous monitoring of the local snow accumulation in the vicinity of the transmit and receive antennas using this technique provide a precision of Script O(1 mm) in surface elevation, which is much better than that needed to apply the D'n'R technique to neutrinos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY OBSERVATIONS OF A REFLECTING LONGITUDINAL WAVE IN A CORONAL LOOP

We report high resolution observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) of intensity oscillations in a hot, T ∼ 8-10 MK, loop. The AIA images show a large coronal loop that was rapidly heated following plasma ejection from one of the loop's footpoints. A wave-like intensity enhancement, seen very clearly in the 131 and 94 Å channel images, propagated ahead of the ejecta along the loop, and was reflected at the opposite footpoint. The wave reflected four times before fading. It was only seen in the hot, 131 and 94 Å channels. The characteristic period and the decay time of the oscillation were ∼630 and ∼440 s, respectively. The phase speed was about 460-510 km s{sup –1} which roughly matches the sound speed of the loop (430-480 km s{sup –1}). The observed properties of the oscillation are consistent with the observations of Dopper-shift oscillations discovered by the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement of Emitted Radiation and with their interpretation as slow magnetoacoustic waves. We suggest that the impulsive injection of plasma, following reconnection at one of the loop footpoints, led to rapid heating and the propagation of a longitudinal compressive wave along the loop. The wave bounces back and forth a couple of times before fading.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Infrasonic directivity of monopole, dipole and bipole ground-surface reflected sources

Infrasound (acoustic waves below 20 Hz) can be used to detect, locate and quantify activity in the atmosphere such as volcanic eruptions and anthropogenic explosions. Attempts to quantify volcanic eruption parameters such as exit velocity, plume height and mass flow rate using infrasound data depend strongly on assumptions of the acoustic source type. Infrasonic sources may produce omnidirectional or directional wavefields, while propagation effects, such as interaction with topography, can induce further wavefield directivity that is measured by field instrumentation. Limited sampling of these wavefields can hinder our ability to infer the underlying source, and thus our understanding of the eruption characteristics. Equivalent sources are often used to represent acoustic source mechanisms and resultant wavefields. In this study, we review equivalent acoustic sources as they pertain to infrasonic scale and wavelengths commonly encountered in very local (⁠<5 km range) geophysical field deployments. We highlight the equivalent infrasonic bipole source that can be induced by ground-reflection of an elevated monopole; we are not aware of any prior infrasound studies that use the bipole source concept. We use analytical and numerical methods to explore source directivity of monopole, dipole and bipole ground-reflected sources at infrasonic frequencies as well as the additional directivity complications introduced by interactions with topography. We illustrate that for typical volcano-infrasound wavelengths, increasing height above the ground as well as increasing source frequency leads to increased wavefield directivity. Numerical modelling using a simple omnidirectional monopole source embedded in topography further illustrates that both horizontal and vertical infrasound directionality can be induced by topography at the distance scales appropriate for local volcano infrasound monitoring. Information summarized in this analytical and numerical exploration of infrasound directivity may be used to help guide future volcano-infrasound field deployments intended to estimate source parameters or quantify wavefield directivity. Analytic solutions for simple whole-space or half-space atmospheres provide useful formulations for planning or initially analysing geophysical field-scale experimental data; however, especially at very local distances from the source (⁠<5 km), 3-D simulations are necessary to account for complex topography commonly encountered in volcano-infrasound applications.

Infrasound↗
DOE OSTI2023

Second-Phase Hepatitis C Plasma Viral Kinetics Directly Reflects Reduced Intrahepatic Burden of Hepatitis C Virus

Mathematical models explain how antivirals control viral infections. Hepatitis C virus (HCV) treatment results in at least 2 phases of decline in viremia. Here, the first phase reflects clearance of rapidly produced virions. The second phase is hypothesized to derive from loss of infected cells but has been challenging to prove. Using single-cell methods, we quantified the number of hepatitis C virus (HCV)-infected hepatocytes in liver biopsies taken before and within 7 days of initiating direct-acting antivirals (DAAs) in a double-blinded randomized controlled trial testing 2 (sofosbuvir-velpatasvir) versus 3 (sofosbuvir-velpatasvir-voxilaprevir) DAAs. We employed thousands of intrahepatic measurements in 10 persons with chronic genotype 1a HCV infection: median proportion of infected hepatocytes declined from 11.3% (range, 1.3%–59%) to 0.6% (range, <0.3%–5.8%), a loss of 75%–95% infected hepatocytes. Plasma viremia correlated with numbers of HCV-infected hepatocytes (r = 0.77; P < .0001). Second-phase plasma dynamics and changes in infected hepatocytes were indistinct (P = .16), demonstrating that second-phase viral dynamics derive from loss of infected cells. DAAs led to a decline in intracellular HCV RNA and interferon-stimulated gene expression (P < .05 for both). We proved that second-phase viral dynamics reflect decay of intrahepatic burden of HCV, partly due to clearance of HCV RNA from hepatocytes.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

The reflection of a blast wave by a very intense explosion

We demonstrate that the geometric similarity of Taylor’s blast wave persists beyond reflection from an ideal surface. Upon impacting the surface, the spherical symmetry of the blast wave is lost but its cylindrical symmetry endures. As the flow acquires dependence on a second spatial dimension, an analytic solution of the Euler equations becomes elusive. However, the preservation of axisymmetry, geometric similarity and planar symmetry in the presence of a mirror-like surface causes all flow solutions to collapse when scaled by the height of burst (HOB) and the shock arrival time at the surface. The scaled blast volume for any yield, HOB and ambient air density follows a single universal trajectory for all scaled time, both before and after reflection.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗
DOE OSTI2024

Andreev reflection of quantum Hall states through a quantum point contact

Here, we investigate the interplay between the quantum Hall (QH) effect and superconductivity in InAs surface quantum well (SQW)/NbTiN heterostructures using a quantum point contact (QPC). We use QPC to control the proximity of the edge states to the superconductor. By measuring the upstream and downstream resistances of the device, we investigate the efficiency of Andreev conversion at the InAs/NbTiN interface. Our experimental data is analyzed using the Landauer Büttiker formalism, generalized to allow for Andreev reflection processes. We show that by varying the voltage of the QPC, V QPC , the average Andreev reflection, A, at the QH-SC interface can be tuned from 50% to ~ 10%. The evolution of A with V QPC extracted from the measurements exhibits plateaus separated by regions for which A varies continuously with V QPC . The presence of plateaus suggests that for some ranges of V QPC the QPC might be pinching off almost completely from the QH-SC interface some of the edge modes. Our work shows a new experimental setup to control and advance the understanding of the complex interplay between superconductivity and QH effect in two-dimensional gas systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Polarization-dependent infrared reflectivity study of Sr 2.5 Ca 11.5 Cu 24 O 41 under pressure: Charge dynamics, charge distribution, and anisotropy

We present a polarization-dependent infrared reflectivity study of the spin-ladder compound Sr₂̣₅Ca₁₁̣₅Cu₂₄O₄₁ under pressure. The optical response is strongly anisotropic, with the highest reflectivity along the ladders/chains ( E ∥c) revealing a metallic character. For the polarization direction perpendicular to the ladder plane, an insulating behavior is observed. With increasing pressure the optical conductivity for E ∥c shows a strong increase, which is most pronounced below 2000cm⁻¹. According to the spectral weight analysis of the E ∥c optical conductivity the hole concentration in the ladders increases with increasing pressure and tends to saturate at high pressure. At ~7.5 GPa the number of holes per Cu atom in the ladders has increased by Δδ=0.09(±0.01), and the Cu valence in the ladders has reached the value +2.33. Thus, the optical data suggest that Sr₂̣₅Ca₁₁̣₅Cu₂₄O₄₁ remains electronically highly anisotropic up to high pressure, also at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Reflection thermal diffuse x-ray scattering for quantitative determination of phonon dispersion relations

Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations (h) over bar omega(j) (q), phonon densities of states g((h) over bar omega), and isochoric temperature-dependent vibrational heat capacities c v (T). We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities c v (T), computed within the harmonic approximation from (h) over bar omega(j) (q) values, increase with temperature from 0.4 x 10 -4 eV/atom K at 100 K to 1.4 x 10 -4 eV/atom K at 200 K and 1.9 x 10 -4 eV/atom K at 300 K, in excellent agreement with isobaric heat capacity values c p (T) between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence (θ ≲ θ c where θ c is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to ≲ 10 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Including contributions from entanglement islands to the reflected entropy

Recent work has demonstrated the need to include contributions from entanglement islands when computing the entanglement entropy in quantum field theory states coupled to regions of semiclassical gravity. We propose a new formula for the reflected entropy that includes additional contributions from such islands. We derive this formula from the gravitational path integral by finding additional saddles that include generalized replica wormholes. We also demonstrate that our covariant formula satisfies all the inequalities required of the reflected entropy. We use this formula in various examples that demonstrate its relevance in illustrating the structure of multipartite entanglement that are invisible to the entropies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗