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PubMed2003

Creation of solitons and vortices by bragg reflection of Bose-Einstein condensates in an optical lattice.

We study the dynamics of Bose-Einstein condensates in an optical lattice and harmonic trap. The condensates are set in motion by displacing the trap and initially follow simple semiclassical paths, shaped by the lowest energy band. Above a critical displacement, the condensate undergoes Bragg reflection. For high atom densities, the first Bragg reflection generates a train of solitons and vortices, which destabilize the condensate and trigger explosive expansion. At lower densities, soliton and vortex formation requires multiple Bragg reflections, and damps the center-of-mass motion.

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PubMed2005

Spin-dependent quasiparticle reflection and bound States at interfaces with itinerant antiferromagnets.

We find a novel channel of quasiparticle reflection from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. Low-energy quasiparticles in a normal metal (N) experience spin-dependent retroreflection at AF/N interfaces. As a combined effect of antiferromagnetic and Andreev reflections, subgap Andreev states arise at an AF/superconductor (SC) interface. When the antiferromagnetic reflection dominates the specular one, Andreev bound states have almost zero energy on AF/s-wave superconductor (sSC) interfaces, whereas there are no low-energy subgap states on AF/d-wave superconductor (dSC) boundaries. For an sSC/AF/sSC junction, the bound states are found to split, due to the finite width of the AF interlayer, and carry the supercurrent. The theory developed in the present Letter is based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets.

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PubMed1971

Reflectance and transmittance of light by leaves.

Spectrophotometric transmittance and reflectance curves were recorded for wavelengths from 0.45 (in some cases 0.34) to 2.7 micrometers for faces and backs of leaves and for stacked leaves of several plant species. Measurements were made at different angles of illumination. Leaf spectrophotometric curves were compared with curves for leaf extracts, potato tuber tissue, glass beads in water, and frozen leaves to demonstrate the physical bases for the leaf curves. Leaves were infiltrated with liquids of different refractive indices for further comparison of spectrophotometric curves. Goniophotometric reflectance curves were recorded, giving visible reflectance and degree of polarization as functions of viewing angle for two different angles of illumination.No retroreflection was observed, and no phenomena were observed which could be attributed to interference because of similarity between leaf structural sizes and wavelengths used.

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PubMed1987

Far-Red Light Reflection from Green Leaves and Effects on Phytochrome-Mediated Assimilate Partitioning under Field Conditions.

The influence of plant spacing and row orientation on spectral distribution of light received by growing soybean (Gylcine max [L.] Merr.) plants was measured under field conditions. Light absorption, reflection and transmission of individual leaves showed that most of the blue and red was absorbed while most of the far-red was either reflected or transmitted. Plants growing in the field received different ratios of far-red relative to red, depending on nearness and/or orientation of other vegetation. Plants grown in close-spaced rows, or high population densities, received higher far-red/red ratios than did those grown in wide rows, or sparse populations. Heliotropic movements of the leaves also contributed to the far-red reflection patterns associated with row orientation. Under field conditions, differences in far-red/red ratios associated with nearness of competing vegetation became more pronounced with low solar angle near the end of the day. Plants exposed to far-red for 5 minutes at the end of each day in controlled environments, and those grown in close-spaced rows in the field, developed longer internodes and fewer branches. Red, far-red photoreversibility in the controlled environment study indicated involvement of phytochrome. Dry matter partitioning among plant components in the field was related to far-red/red light ratio received during growth and development.

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PubMed1989

Reflected Far-Red Light Effects on Chlorophyll and Light-Harvesting Chlorophyll Protein (LHC-II) Contents under Field Conditions.

The influence of various colors of soil cover (mulch) on the farred/red (FR/R) ratio in upwardly reflected light and on concentrations of chlorophyll (Chl) and light-harvesting Chl protein (LHC-II) were measured under field conditions. The FR/R ratios above green surfaces were higher than over white surfaces. Even though plants (Gossyplum hirsutum L. cv PD-1) were grown in full sunlight, those that received higher FR/R ratios in upwardly reflected light were taller and had thinner leaves with higher concentrations of Chl and LHC-II. A controlled environment experiment showed FR/R control of Chl and LHC-II concentrations. The results illustrate the importance of spectral distribution of reflected light on plant growth and a potential means of altering the chemistry of leaf crops under field conditions.

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PubMed2001

Chirality-induced 'forbidden' reflections in X-ray resonant scattering.

It is shown that additional Bragg reflections can appear exclusively owing to the local chirality associated with the left-right asymmetric environment of scattering atoms in non-magnetic crystals. The structure amplitude of these reflections depends on the antisymmetric part of a third-rank tensor describing the spatial dispersion effects. It enhances for resonant near-edge scattering through a mixed multipole transition, which includes a dipole-quadrupole contribution. It is shown that this mechanism works even for centrosymmetric crystals, and some realistic examples are considered in detail (alpha-Fe2O3, LiNbO3 etc.). For instance, the interference between the dipole-quadrupole and quadrupole-quadrupole terms may be responsible for the threefold symmetry of the azimuthal dependence of the hhh, h = 2n + 1, reflections observed recently in hematite.

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PubMed2004

Ab initio structure solution by charge flipping. II. Use of weak reflections.

The original charge flipping algorithm [Oszlanyi & Suto (2004). Acta Cryst. A60, 34-141] is an amazingly simple structure solution method which works ab initio on high-resolution X-ray diffraction data. In this paper, a new version of the algorithm is presented that complements the phase exploration in reciprocal space. Instead of prescribing observed moduli of all structure factors, weak reflections are treated separately. For these reflections, calculated moduli are accepted unchanged and calculated phases are shifted by a constant Deltaphi=pi/2. This means that the observed data of weak reflections are not used in the iteration, except for the knowledge that they are indeed weak. The improvement is drastic, in some cases the success rate is increased by a factor of ten, in other cases a previously unsolvable structure becomes solvable by the modified algorithm.

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PubMed2003

Point-defect-induced forbidden reflections in resonant X-ray scattering.

Point defects in crystals cause displacements of neighbouring atoms and hence become the source of an additional anisotropy of the X-ray resonant scattering amplitude. This anisotropy can induce 'forbidden' Bragg reflections near absorption edges, called point-defect-induced (PDI) reflections, which are absent in non-resonant X-ray scattering and in resonant X-ray scattering by perfect crystals. The deformation of external electron shells is the physical reason for this phenomenon. Some examples of crystals are considered in which the PDI reflections could be observed.

Computer Simulation↗

PubMed1997

On the off-Bragg reflectivity enhancement in dynamical multiple diffraction of X-rays by an In(0.5)Ga(0.5)P/GaAs heteroepitaxial structure.

The effect of dynamical interactions between reflections involved in 000/111/220 multiple diffraction by an In(0.5)Ga(0.5)P/GaAs heteroepitaxial structure was observed, which significantly increases the intensity of the (111) reflection in the region of the total (220) reflection from the substrate during Renninger-like scanning. The amplitude of the peak is roughly five times as great as the peak of detoured excitation for ideal GaAs. Possible applications of the observed feature in synchrotron-radiation-based studies are discussed.

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PubMed1997

Application of imaging-grazing-incidence X-ray diffraction and specular reflectivity to the structural investigation of quantum-confinement semiconductor devices.

X-ray reflectivity and grazing-incidence X-ray diffraction techniques have been employed to investigate the structure of quantum-well and quantum-dot semiconductor devices. This work has been performed using both laboratory and synchrotron radiation sources. The use of synchrotron radiation enabled reflectivity studies to be performed on small samples, and established the feasibility of imaging-grazing-incidence diffraction studies on quantum-confinement structures. Interdiffusion effects in quantum-well and quantum-dot structures, the disordering of overlayers grown on quantum dots, and the variation in diffraction pattern with incident angle have been observed. It is evident that X-ray reflectivity and imaging-grazing-incidence X-ray diffraction yield quite different but complementary information.

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PubMed1998

Focusing and reflection by a bent crystal for high-energy synchrotron radiation.

The focusing properties and resolution of a doubly bent crystal in the Bragg case have been analytically studied from a geometrical viewpoint. Simulation using the Takagi-Taupin equations was also performed for singly bent crystal reflections to study the reflectivity. The critical radius of curvature for changing from dynamical to kinematical diffraction is calculated to be of the order of a few tens of metres for an Si 400 reflection of 110 keV X-rays.

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PubMed1996

Input impedance and reflection coefficient in fractal-like models of asymmetrically branching compliant tubes.

A mathematical model is described, based on linear transmission line theory, for the computation of hydraulic input impedance spectra in complex, dichotomously branching networks similar to mammalian arterial systems. Conceptually, the networks are constructed from a discretized set of self-similar compliant tubes whose dimensions are described by an integer power law. The model allows specification of the branching geometry, i.e., the daughter-parent branch area ratio and the daughter-daughter area asymmetry ratio, as functions of vessel size. Characteristic impedances of individual vessels are described by linear theory for a fully constrained thick-walled elastic tube. Besides termination impedances and fluid density and viscosity, other model parameters included relative vessel length and phase velocity, each as a function of vessel size (elastic nonuniformity). The primary goal of the study was to examine systematically the effect of fractal branching asymmetry, both degree and location within the network, on the complex input impedance spectrum and reflection coefficient. With progressive branching asymmetry, fractal model spectra exhibit some of the features inherent in natural arterial systems such as the loss of prominent, regularly-occurring maxima and minima; the effect is most apparent at higher frequencies. Marked reduction of the reflection coefficient occurs, due to disparities in wave path length, when branching is asymmetric. Because of path length differences, branching asymmetry near the system input has a far greater effect on minimizing spectrum oscillations and reflections than downstream asymmetry. Fractal-like constructs suggest a means by which arterial trees of realistic complexity might be described, both structurally and functionally.

Arteries↗

PubMed2002

Arterial pulse wave reflection as feedback.

Traditionally, input impedance (Z(in)) has been used to characterize the global dynamic properties of an arterial system independent of properties of the heart. Defined as the relationship of pressure and flow at the entrance of an arterial system, it describes the ability of an arterial system to dynamically impede blood flow. Recently, a new description has been developed that also characterizes the arterial system independent of properties of the heart. Apparent arterial compliance (C(app)) is defined as the dynamic relationship of input pressure and volume stored in an arterial system, and describes the ability of the arterial system to dynamically store blood. Both Z(in) and C(app) are influenced by pulse wave propagation and reflection. However, the functional form of Capp lends itself to describing the arterial system in terms of negative feedback. Pulse wave reflection decreases the pulsatile volume stored (gain) at low frequencies, but increases the range of frequencies (bandwidth) in which the pulsatile volume is determined by total arterial compliance. This paper illustrates, by simple analytical formula, large-scale arterial system modeling, and direct analysis of data, how this conceptualization of reflection offers a new means to interpret changes in arterial system dynamics resulting from changes in arterial compliance.

Animals↗

PubMed2001

Combined FEM and Green's function analysis of periodic SAW structure, application to the calculation of reflection and scattering parameters.

Because of more and more stringent requirements on SAW filter performances, it is important to compute, with very good accuracy, the SAW propagation characteristics, which include the calculation of reflection and scattering parameters. For that reason, the analysis of periodic structures on a semi-infinite piezoelectric substrate is one of the most important problems being investigated by SAW researchers. For infinite periodic grating modeling, we developed numerical mixed FEM/BEM (finite element method-boundary element method) models using an efficient interpolation basis function that takes into account the singularity at both edges of each electrode. In this paper, a review of the numerical program that has been developed during the past few years will be presented. For an infinite periodic grating, it is convenient to solve the propagation problem in the Fourier domain (wave number space and harmonic excitation), and important efforts have been spent to properly integrate the so-called periodic harmonic Green function. Using this numerical model together with the general P- matrix formalism, it is possible to compute all of the basic parameters with a very good accuracy. These consist of the single strip reflectivity, acoustic wave-phase velocity, and position offset between reflection and transduction centers. Simulations and comparisons with experiments are shown for each model.

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PubMed2006

Recovery of spectral reflectances of objects being imaged without prior knowledge.

Prior knowledge of the noise present in a color image acquisition device is very important in estimating colorimetric values or in recovering the spectral reflectances of pixels of objects being imaged, since these values are greatly influenced by the noise. In this paper, a new model is proposed for the determination of the noise variance of a multispectral color image acquisition system and experimental results to demonstrate its accuracy are presented. It is demonstrated that the noise variance of an actual multispectral color image acquisition system computed by the proposal agrees fairly well with the variance which minimizes the mean-square error of the recovered reflectances by the Wiener filter. As an application of the proposal, it is shown that spectral reflectances of an art painting are recovered accurately by the use of sensor responses without prior knowledge of objects being imaged and noise present in an image acquisition system.

Algorithms↗

PubMed2005

Feasibility of half-data image reconstruction in 3-D reflectivity tomography with a spherical aperture.

Reflectivity tomography is an imaging technique that seeks to reconstruct certain acoustic properties of a weakly scattering object. Besides being applicable to pure ultrasound imaging techniques, the reconstruction theory of reflectivity tomography is also pertinent to hybrid imaging techniques such as thermoacoustic tomography. In this work, assuming spherical scanning apertures, redundancies in the three-dimensional (3-D) reflectivity tomography data function are identified and formulated mathematically. These data redundancies are used to demonstrate that knowledge of the measured data function over half of its domain uniquely specifies the 3-D object function. This indicates that, in principle, exact image reconstruction can be performed using a "half-scan" data function, which corresponds to temporally untruncated measurements acquired on a hemi-spherical aperture, or using a "half-time" data function, which corresponds to temporally truncated measurements acquired on the entire spherical aperture. Both of these minimal scanning configurations have important biological imaging applications. An iterative reconstruction method is utilized for reconstruction of a simulated 3-D object from noiseless and noisy half-scan and half-time data functions.

Algorithms↗

PubMed2004

Determination of the acoustic pressure at and the reflection coefficient of a target through measurements of the absorbed power and the emitter voltage.

A harmonic acoustic wave, fed back by a reflecting target, modifies the electric impedance of the emitter. This effect is studied using a tightly focused beam and various flat targets with known reflection coefficients, which are placed at pressure maxima or minima of the standing wave in the focal zone. It is possible to establish relationships that allow one to determine, for flat targets with unknown acoustic properties, the acoustic impedance and reflection coefficient of the target as well as the acoustic pressure present at the target, only from measurements of the absorbed power and the emitter voltage.

Acoustics↗

PubMed2004

Design considerations on surface acoustic wave resonators with significant internal reflection in interdigital transducers.

This paper discusses, both qualitatively and quantitatively, the operation and the design principle of current surface acoustic wave (SAW) resonators in which the internal reflection within interdigital transducers (IDTs) is not negligible and lower capacitance ratio is necessary. For the purpose, the p-matrix expression is used with the help of the coupling-of-modes theory. The internal reflection causes: deformation of the IDT passband shape, frequency dependence of the effective SAW velocity within IDTs, and suppression of higher-order resonances. It is shown that these features can be effectively used to enhance performances of both one-port SAW resonators and two-port double-mode SAW (DMS) filters. In addition, under proper designs accounting for the internal reflection, most of all structural discontinuities can be removed, and is most preferable for the reduced scattering loss at the discontinuity. Design criteria also are presented for DMS filters of wide bandwidth, and it is demonstrated how device performances are improved by proper design accounting for the criteria.

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