Normal Reflection Research Articles

Optical absorption edge evolution of vanadium pentoxide films during lithium intercalation

The influence of lithium intercalation on the optical absorption of vanadium pentoxide films is reported in this paper. Thin films were prepared by thermal evaporation followed by annealing in air. Lithium was inserted electrochemically so that Li x V 2O 5 (0 ≤ x ≤ 0.54) was obtained. The pristine and lithium inserted films were characterized by X-ray diffraction (XRD), optical spectroscopy, and ellipsometry. The optical absorption coefficient was calculated from measured normal transmittance and reflectance spectra. XRD analysis has shown that the as-grown V 2O 5 films were amorphous, becoming microcrystal after annealing at 350 °C. The optical absorption spectra of all the samples exhibited two distinct regions of behavior: at high-energy where ( αhν) 1/2 varied linearly with the photonic energy hν, and a low-energy tail. The blue shift of the optical absorption edge is attributed to the Fermi level going into the conduction band after lithium insertion, and the relation between the change of the optical gap and the inserted electron concentration is interpreted in terms of a Burstein–Moss shift. The results confirm that the anodic electrochromism of V 2O 5 thin films is due to the blue shift of the absorption edge, and the near-infrared cathodic electrochromism arises from absorption of small polarons in V 2O 5.

Seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the Black Sea

A bottom simulating reflector (BSR), which marks the base of the gas hydrate stability zone, has been detected for the first time in seismic data of the Black Sea. The survey area is in the northwestern Black Sea at 44°–45°N and 31.5°–32.5°E. In this paper, seismic wide-angle ocean bottom hydrophone (OBH) and ocean bottom seismometer (OBS) data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment. An image of the subsurface is computed from wide-angle data by using Kirchhoff depth migration. The image shows the BSR at 205–270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the BSR. The top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals. An analysis of traveltimes and reflection amplitudes leads to 1-D P-wave velocity–depth and density–depth models. An average S-wave velocity of 160 m s−1 between the seafloor and the BSR is determined from the traveltime of the P to S converted wave. The normal incidence PP reflection coefficient at the BSR is −0.11, where the P-wave velocity decreases from 1840 to 1475 m s−1. Velocities and density are used to compute the porosity and the system bulk modulus as a function of depth. The Gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation, which depend on porosity and on the bulk moduli of the dry and saturated sediment. A gas hydrate saturation–depth profile is obtained, which shows that there is 38 ± 10 per cent hydrate in the pore space at the BSR depth, where the porosity is 57 per cent (OBS 24). This value is derived for the case that the gas hydrate does not cement the sediment grains, a model that is supported by the low S-wave velocities. There is 0.9 or 0.1 per cent free gas in the sediment below the BSR, depending on the model for the gas distribution in the sediment. The free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity, which can be identified in the subsurface image.

Integration of Seismic Anisotropy and Reservoir-Performance Data for Characterization of Naturally Fractured Reservoirs Using Discrete-Feature-Network Models

SummaryThis paper proposes a method for quantitative integration of seismic (elastic) anisotropy attributes with reservoir-performance data as an aid in characterizing systems of natural fractures in hydrocarbon reservoirs. This method is demonstrated through application to history matching of reservoir performance using synthetic test cases.Discrete-feature-network (DFN) modeling1 is a powerful tool for developing fieldwide stochastic realizations of fracture networks in petroleum reservoirs. Such models are typically well conditioned in the vicinity of the wellbore through incorporation of core data, borehole imagery, and pressure-transient data. Model uncertainty generally increases with distance from the borehole. Three-dimensional seismic data provide uncalibrated information throughout the interwell space. Some elementary seismic attributes such as horizon curvature and impedance anomalies have been used to guide estimates of fracture trend and intensity (fracture area per unit volume) in DFN modeling2 through geostatistical calibration with borehole and other data. However, these attributes often provide only weak statistical correlation with fracture-system characteristics.The presence of a system of natural fractures in a reservoir induces elastic anisotropy that can be observed in seismic data. Elastic attributes such as azimuthally dependent normal moveout velocity (ANMO), reflection amplitude vs. azimuth (AVAZ), and shear-wave birefringence can be inverted from 3D-seismic data. Anisotropic elastic theory provides physical relationships among these attributes and fracture-system properties such as trend and intensity. Effective-elastic-media models allow forward modeling of elastic properties for fractured media.A technique has been developed in which both reservoirperformance data and seismic anisotropic attributes are used in an objective function for gradient-based optimization of selected fracture-system parameters. The proposed integration method involves parallel workflows for effective elastic and effective permeability media modeling from an initial DFN estimate of the fracture system. The objective function is minimized through systematic updates of selected fracture-population parameters. Synthetic data cases show that 3D-seismic attributes contribute significantly to the reduction of ambiguity in estimates of fracture-system characteristics in the interwell rock mass. The method will benefit enhanced-oil-recovery (EOR) program planning and management, optimization of horizontal-well trajectory and completion design, and borehole-stability studies.

Loss of Andreev backscattering in superconducting quantum point contacts

We study effects of magnetic field on the energy spectrum in a superconducting quantum point contact. The supercurrent induced by the magnetic field leads to intermode transitions between the electron waves that pass and do not pass through the constriction. The latter experience normal reflections which couple the states with opposite momenta inside the quantum channel and create a minigap in the energy spectrum that depends on the magnetic field.

Optimization techniques for the estimation of the thickness and the optical parameters of thin films using reflectance data

The present work considers the problem of estimating the thickness and the optical constants (extinction coefficient and refractive index) of thin films from the spectrum of normal reflectance R. This is an ill-conditioned highly underdetermined inverse problem. The estimation is done in the spectral range where the film is not opaque. The idea behind the choice of this particular spectral range is to compare the film characteristics retrieved from transmittance T and from reflectance data. In the first part of the paper a compact formula for R is deduced. The approach to deconvolute the R data is to use well-known information on the dependence of the optical constants on photon energy of semiconductors and dielectrics and to formulate the estimation as a nonlinear optimization problem. Previous publications of the group on the subject provide the guidelines for designing the new procedures. The consistency of the approach is tested with computer-generated thin films and also with measured R and T spectral data of an a-Si:H film deposited onto glass. The algorithms can handle satisfactorily the problem of a poor photometric accuracy in reflectance data, as well as a partial linearity of the detector response. The results on gedanken films and on a-Si:H indicate a very good agreement between expected and retrieved values.

A concept to determine the true temperature of opaque materials using a tricolor pyroreflectometer

Despite the existence of several possible optical pyrometric methodologies, the determination of the true temperature has still not received a universal and perfect solution. We present here an analysis of our theoretical simulations and experimental measurements that have been carried out in order to validate a three-color method. Our goal is to test a pyroreflectometer (working wavelengths 0.84, 1.3, and 1.55μm) equipped with an optical fiber probe. This apparatus has been developed for in situ measurements in severe conditions (high temperature, difficult accessibility, controlled atmosphere, vacuum,…) to determine the true temperature of opaque materials. The method is based on three monochromatic measurements proportional to the emitted radiance; three monochromatic measurements proportional to normal reflectivities; the hypothesis of the same bidirectional reflectivity distribution function (BRDF) for the instrumented sample at the three working wavelengths. Using the six simultaneous measurements, the diffuse factor η and the apparent diffuse factor η∗ are introduced to determine the convergence temperature T∗ that is assumed to be equal to the true temperature T. After theoretical simulation, the method has been experimentally verified on several materials and an analysis of the causes of errors and their impact on the method accuracy and the operator modus, is discussed. The quality of the results obtained demonstrates the utility of the tricolor pyroreflectometer in a situation where fundamental parameters, such as the temperature and radiative properties, are difficult to measure.

The effect of interfacial roughness on the normal incidence bandgap of one-dimensional photonic crystals

As discussed previously, interfacial roughness in one-dimensional photonic crystals (1DPCs) can have a significant effect on their normal reflectivity at the quarter-wave tuned wavelength. We report additional finite-difference time-domain (FDTD) simulations that reveal the effect of interfacial roughness on the normal-incidence reflectivity at several other wavelengths within the photonic bandgaps of various 1DPC quarter-wave stacks. The results predict that both a narrowing and red-shifting of the bandgaps will occur due to the roughness features. These FDTD results are compared to results obtained when the homogenization approximation is applied to the same structures. The homogenization approximation reproduces the FDTD results, revealing that this approximation is applicable to roughened 1DPCs within the parameter range tested (rms roughnesses < 20% and rms wavelengths < 50% of the photonic crystal periodicity) across the entire normal incidence bandgap.

Atom optics with Bose–Einstein condensates: quantum reflection and interferometry

We present recent results on atom optics with Bose–Einstein condensates obtained at MIT. These results demonstrate the flexibility of micro-fabricated atom traps (atom-chips) and the interaction of Bose–Einstein condensates with surfaces through normal incidence quantum reflection.

Microstrip Double-Periodic Grating of Continuous Curvilinear Metal Strips as a High-Impedance Surface

ABSTRACT: Frequency dependence of phase reflection coefficient of a microstrip periodic gratingof undulated strips is numerically investigated by means of the method of moments. The possibility of using of such structure as a high-impedance surface is shown. The influence of change in shape of the strips and in the parameters of dielectric substrate on the frequency characteristics of the grating is analyzed for the case on normal incidence of a plane electromagnetic wave. The possibility of electronic control of the grating characteristics is demonstrated. INTRODUCTION As is known, at normal reflection of a plane electromagnetic wave by perfectly conductive metal plane the vectors of the incident and reflected electric field strength have the same values but opposite directions on this plane owing to equality to zero of the strength of the total electric field E G in a perfect conductor. Therefore the reflection coefficient of such plane (which is called also “electric wall”) is equal to –1, and the surface impedance, as follows from its definition

The Deep Space 1 Encounter with Comet 19P/Borrelly

On 22 Sept 2001 at 22h 29m 33s UTC the Deep Space 1 (DS1) spacecraft encountered the comet 19P/Borrelly at a distance of 2171 km. Measurements were undertaken by two dedicated science instruments on the spacecraft and by the spacecraft’s ion propulsion system diagnostic sensors (IDS). The images are the highest resolution ever obtained of a cometary nucleus and show topography and albedo differences on the surface at a resolution of 47m. The disk averaged geometric albedo is less than 0.03, among the lowest of any solar system object observed. The surface reflectance varies by a factor of three; the most absorbing areas having a normal reflectance of less than 0.01. Material is being emitted of from localized regions on the nucleus generally in the sunward direction. These regions are cylindrically shaped about 0.5 km in diameter and about 5 km in length. The cometary bow shock and other features associated with the cometary environs are asymmetrically offset from the sun-Borrelly axis. This suggests that the localized emission regions seen in the images may cause the coma to be asymmetrically offset from the sun-nucleus line. Such an offset has not been reported in previous spacecraft encounters with other comets.

Diminished normal reflectivity of one-dimensional photonic crystals due to dielectric interfacial roughness

Dielectric reflectors that are periodic in one dimension, also known as one-dimensional photonic crystals (1DPCs), have become extremely useful tools in the optics industry due to the presence of wavelength-tunable photonic bandgaps. However, little is known about the practical effects of manufacturing defects, such as interfacial roughness, on this technologically useful property of 1DPCs. We employ a finite-difference time-domain code to gain further insight into the effect of interfacial roughness on the reflectivity of quarter-wave-tuned 1DPCs in the center of the bandgap at normal incidence. This provides an estimate of the magnitude of the effect of the roughness for even the most-robust incidence conditions.

Sign of the crossed conductances at a ferromagnet/superconductor/ferromagnet double interface

Crossed conductance in hybrid Ferromagnet / Superconductor / Ferromagnet (FSF) structures results from the competition between normal transmission and Andreev reflection channels. Crossed Andreev reflection (CAR) and elastic cotunneling (EC) between the ferromagnets are dressed by local Andreev reflections, which play an important role for transparent enough interfaces and intermediate spin polarizations. This modifies the simple result previously obtained at lowest order, and can explain the sign of the crossed resistances in a recent experiment [D. Beckmann {\sl et al.}, cond-mat/0404360]. This holds both in the multiterminal hybrid structure model (where phase averaging over the Fermi oscillations is introduced ``by hand'' within the approximation of a single non local process) and for infinite planar interfaces (where phase averaging naturally results in the microscopic solution with multiple non local processes).

Manifestation of surface phonons in far infrared reflectivity of diamond‐type semiconductors

The coupling of surface phonons with light at (001) surfaces of diamond-structure crystals and its manifestation in far-infrared anisotropy spectra are theoretically studied. We apply the adiabatic bond charge model to describe short-range mechanical interactions together with long-range Coulomb forces and radiation fields, and we solve the corresponding system of coupled equations for the electromagnetic field and the lattice vibrations. We calculate far-infrared normal reflectance spectra of (001) surfaces of semi-infinite diamond-type crystals. In particular, we analyse reflectance spectra for the Si(001) (2 × 1) surface, which exhibit a resonance structure associated with the excitation of surface phonon modes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Seismic stratigraphy of the southern Rockall Basin: a comparison between wide-angle seismic and normal incidence reflection data

A wide-angle reflection/refraction profile from the RAPIDS 3 (Rockall and Porcupine Irish Deep Seismic) experiment is compared to a coincident high-quality normal incidence reflection line, both of which traverse the southern part of the Rockall Basin on the Atlantic continental margin, offshore Ireland. The positions and geometries of the independently-derived layers show a generally close comparison between the two data sets. The normal incidence seismic reflection data show a greater degree of detail on seismic facies while the wide-angle seismic profile displays greater penetration of the entire sedimentary succession and of the crust. The wide-angle geometries and the normal incidence seismic stratigraphy provide complementary data that help constrain the geology and evolution of the lightly explored Rockall region. A seismic stratigraphy is presented, based on correlation between the wide-angle and normal incidence data, shallow seismic site surveys, shallow borehole cores and tectonostratigraphic studies. Within the central part of the Rockall Basin, the sedimentary layers are generally flat-lying and have a maximum thickness of 7 km. The succession is interpreted as Jurassic to Recent in age. Some of the layers are confined to the basin whilst others extend onto the Rockall and Porcupine highs. Marginal basins at the western (Conall Basin) and eastern (Macdara Basin) edges of the Rockall Basin contain up to 4 km of pre-Tertiary sedimentary rocks, some possibly as old as Permo-Triassic.

A comparison of laboratory reflection coefficient data with predictions from a time domain model of sea floor scatter using the small slope approximation (BoRIS-SSA)

Reflection coefficient data has the potential for sediment model verification and remote sensing of sediment parameters. However, a significant component of the bistatic acoustic response is scattering from the rough interface. For example, in typical ocean conditions, surface roughness can alter the apparent normal reflection coefficient by 15 dB or more over a frequency range from 10 to 100 kHz. Therefore, it is important to quantify the contribution from scattering to determine the smooth surface reflection coefficient from bistatic data. The Bottom Response from Inhomogeneities and Surface (BoRIS-SSA) code uses the small slope approximation to calculate bistatic response from rough surfaces. BoRIS-SSA can incorporate both stochastic realizations of the ocean bottom or data from stereo-photographic equipment. In this study, bistatic laboratory data taken at 50–150 kHz from a smoothed and roughened interfaces will be compared to BoRIS-SSA calculations. The model will be considered for its ability to predict both the average value of the bistatic response and the distribution of the data.

Imaging low-velocity anomalies with the aid of seismic tomography

Theoretical considerations (Snell's law) suggest that low-velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray-tracing methods. A synthetic study considering a 40×20 m low-velocity anomaly (300 m/s) placed at the center of a 400×50 m block with gradient background velocity model (from 3000 m/s at the surface to 4000 m/s at the base) indicates that the low ray density in ray-tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low-velocity anomalies. Combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first-arrival travel times form an efficient scheme to resolve low-velocity anomalies such as fracture zones. Furthermore, the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface (weathering) layer and provide seismic reflection images of fracture zones (low-velocity anomaly) within a granitic pluton.

Anomalous Reflection of a Longitudinal Ultrasonic Wave from a Strongly Dissipative Medium

Normal reflection of a longitudinal acoustic wave from the plane interface between a solid acoustic line and a strongly dissipative medium (an epoxide resin compound in the process of solidification) has been experimentally investigated. A 14‐fold change in the reflection coefficient of pulsed signals and a decrease in their duration have been detected and the point of phase transmission has been reliably determined by the minimum of the reflection coefficient. The coefficient of reflection of continuous acoustic waves with frequencies of 1–10 MHz from the interface between a plexiglas (or aluminum) and an epoxide resin compound in the process of solidification has been measured. The influence of the amplitude‐frequency characteristic of an ultrasonic piezoelectric transducer on the measurement data obtained has been analyzed. The change in the viscosity coefficient of an epoxide resin compound in the process of its solidification has been calculated by the spectral transform method with the use of computer programs and experimental data on the reflection coefficient dynamics.

Impurity scattering effect on charge transport in high-Tc cuprate junctions

It is known that the zero-bias conductance peak (ZBCP) is expected in tunneling spectra of normal-metal/high-Tc cuprate junctions because of the formation of the midgap Andreev resonant states (MARS) at junction interfaces. In the present review, we report the recent theoretical study of impurity scattering effects on the tunneling spectroscopy. In the former part of the present paper, we discuss impurity effects in the normal metal. We calculate tunneling conductance for diffusive normal metal (DN)/high-Tc cuprate junctions based on the Keldysh Green function technique. Besides the ZBCP due to the MARS, we can expect ZBCP of a different origin, i.e., caused by coherent Andreev reflection (CAR) assisted by the proximity effect in DN. Their relative importance depends on the angle α between the interface normal and the crystal axis of high-Tc superconductors. At α=0 we find the ZBCP by the CAR for low-transparency junctions with small Thouless energies in DN; this is similar to the case of diffusive normal metal/insulator/s-wave superconductor junctions. Under increase of α from zero to π/4, the contribution of MARS to ZBCP becomes more prominent and the effect of the CAR is gradually suppressed. Such complex spectral features would be observable in conductance spectra of high-Tc junctions at very low temperatures. In the latter part of our paper, we study impurity effects in superconductors. We consider impurities near the junction interface on the superconductor side. The conductance is calculated from the Andreev and the normal reflection coefficients, which are estimated by using the single-site approximation in an analytic calculation and by the recursive Green function method in a numerical simulation. We find splitting of the ZBCP in the presence of the time reversal symmetry. Thus the zero-field splitting of ZBCP in the experiment does not perfectly prove the existence of a broken time reversal symmetry state.

KCaF(CO3) from X-ray powder data

Numerical analysis of the conductance of quasi-particles (QPs) is presented for two-dimensional (2D) L-shaped bent quantum waveguide (LBQW) with the application of inhomogeneous magnetic fields and a semiconductor-superconductor (Sm-S) junction at the exit arm. A delta-function scattering potential is placed at the Sm-S interface to simulate the imperfectness of the Sm-S interface. The conductance of the QPs as a function of magnetic field strongly depends on normal or superconducting state of the material at Sm-S junction. The conductance displays increasing behavior at the beginning and then it exhibits oscillatory decay. This implies that the Andreev reflection (AR) plays an important role on the conductance. As increasing the magnetic field, the conductance is decreased accordingly as the normal reflection is significantly enhanced and meanwhile the AR becomes imperfect. We also reveal the effects of various physical and geometrical parameters of system on the conductance in detail. (C) 2004 Elsevier B.V. All rights reserved.

Andreev scattering in the L-shaped bent quantum waveguide with inhomogeneous magnetic fields and a semiconductor–superconductor junction at exit arm

Numerical analysis of the conductance of quasi-particles (QPs) is presented for two-dimensional (2D) L-shaped bent quantum waveguide (LBQW) with the application of inhomogeneous magnetic fields and a semiconductor–superconductor (Sm–S) junction at the exit arm. A δ-function scattering potential is placed at the Sm–S interface to simulate the imperfectness of the Sm–S interface. The conductance of the QPs as a function of magnetic field strongly depends on normal or superconducting state of the material at Sm–S junction. The conductance displays increasing behavior at the beginning and then it exhibits oscillatory decay. This implies that the Andreev reflection (AR) plays an important role on the conductance. As increasing the magnetic field, the conductance is decreased accordingly as the normal reflection is significantly enhanced and meanwhile the AR becomes imperfect. We also reveal the effects of various physical and geometrical parameters of system on the conductance in detail.