Showing papers on "Scattering published in 1997"

An entropy based classification scheme for land applications of polarimetric SAR

Abstract: The authors outline a new scheme for parameterizing polarimetric scattering problems, which has application in the quantitative analysis of polarimetric SAR data. The method relies on an eigenvalue analysis of the coherency matrix and employs a three-level Bernoulli statistical model to generate estimates of the average target scattering matrix parameters from the data. The scattering entropy is a key parameter is determining the randomness in this model and is seen as a fundamental parameter in assessing the importance of polarimetry in remote sensing problems. The authors show application of the method to some important classical random media scattering problems and apply it to POLSAR data from the NASA/JPL AIRSAR data base.

Localization of light in a disordered medium

Abstract: ......... 1 phenomenon. This is a disorder-induced phase transition in the electron-transport behaviour from the classical diffusion regime, in which the well-known Ohm’s law holds, to a localized state in which the material behaves as an insulator. The effect finds its origin in the interference of electrons that have undergone multiple scattering by defects in the solid 2‐10 . A similar phenomenon is anticipated for multiple scattering of electromagnetic waves, but with one important simplification: unlike electrons, photons do not interact with one another. This makes transport of photons in disordered materials an ideal model system in which to study Anderson localization 10‐17

Photonic-bandgap microcavities in optical waveguides

Abstract: Confinement of light to small volumes has important implications for optical emission properties: it changes the probability of spontaneous emission from atoms, allowing both enhancement and inhibition. In photonic-bandgap (PBG) materials1,2,3,4 (also known as photonic crystals), light can be confined within a volume of the order of (λ/2n)3, where λ is the emission wavelength and n the refractive index of the material, by scattering from a periodic array of scattering centres. Until recently5,6, the properties of two- and three-dimensional PBG structures have been measured only at microwave frequencies. Because the optical bandgap scales with the period of the scattering centres, feature sizes of around 100 nm are needed for manipulation of light at the infrared wavelength (1.54 µm) used for optical communications. Fabricating features this small requires the use of electron-beam or X-ray lithography. Here we report measurements of microcavity resonances in PBG structures integrated directly into a sub-micrometre-scale silicon waveguide. The microcavity has a resonance at a wavelength of 1.56 µm, a quality factor of 265 and a modal volume of 0.055 µm3. This level of integration might lead to new photonic chip architectures and devices, such as zero-threshold microlasers, filters and signal routers.

Polymers and Neutron Scattering

Abstract: Neutron production and detection - the nuts and bolts sprectrometers and what they measure theoretical basis of scattering labelling with deuterium - how, why, and when to use form factors interacting systems - part 1 zero angle scattering, part 2 finite angle scattering experimental examples of structural studies dynamics neutron reflection for studying surfaces and interfaces.

Seismic Wave Propagation and Scattering in the Heterogeneous Earth

Abstract: Introduction- Heterogeneity in the Lithosphere- Phenomenological Approaches to Seismogram Envelopes in short-periods- Born approximation for Wave Scattering in Random Media- Attenuation of High-Frequency Seismic Waves- Synthesis of Three-Component Seismogram Envelopes for Earthquakes Using Scattering Amplitudes from the Born Approximation- Envelope Synthesis Based on the Radiative Transfer Theory: Multiple Scattering Models- Parabolic approximation and Envelope Synthesis based on the Markov Approximation Summary and Epilogue

Single and multiband modeling of quantum electron transport through layered semiconductor devices

Abstract: Non-equilibrium Green function theory is formulated to meet the three main challenges of high bias quantum device modeling: self-consistent charging, incoherent and inelastic scattering, and band structure. The theory is written in a general localized orbital basis using the example of the zinc blende lattice. A Dyson equation treatment of the open system boundaries results in a tunneling formula with a generalized Fisher-Lee form for the transmission coefficient that treats injection from emitter continuum states and emitter quasi-bound states on an equal footing. Scattering is then included. Self-energies which include the effects of polar optical phonons, acoustic phonons, alloy fluctuations, interface roughness, and ionized dopants are derived. Interface roughness is modeled as a layer of alloy in which the cations of a given type cluster into islands. Two different treatments of scattering; self-consistent Born and multiple sequential scattering are formulated, described, and analyzed for numerical t...

A Probe of Primordial Gravity Waves and Vorticity

Abstract: A formalism for describing an all-sky map of the polarization of the cosmic microwave background is presented. The polarization pattern on the sky can be decomposed into two geometrically distinct components. One of these components is not coupled to density inhomogeneities. A nonzero amplitude for this component of polarization can only be caused by tensor or vector metric perturbations. This allows unambiguous identification of long-wavelength gravity waves or large-scale vortical flows at the time of last scattering.

Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids

Abstract: Laboratory and in situ measurements show that scattering properties of natural nonspherical particles can be significantly different from those of volume-or surface-equivalent spheres, thus suggesting that Mie theory may not be suitable for interpreting satellite reflectance measurements for dustlike tropospheric aerosols. In this paper we use the rigorous T-matrix method to extensively compute light scattering by shape distributions of polydisperse, randomly oriented spheroids with refractive indices and size distributions representative of naturally occurring dust aerosols. Our calculations show that even after size and orientation averaging, a single spheroidal shape always produces a unique, shape-specific phase function distinctly different from those produced by other spheroidal shapes. However, phase functions averaged over a wide aspect-ratio distribution of prolate and oblate spheroids are smooth, featureless, and nearly flat at side-scattering angles and closely resemble those measured for natural soil and dust particles. Thus, although natural dust particles are, of course, not perfect spheroids, they are always mixtures of highly variable shapes, and their phase function can be adequately modeled using a wide aspect-ratio distribution of prolate and oblate spheroidal grains. Our comparisons of nonspherical versus projected-area-equivalent spherical particles show that spherical-nonspherical differences in the scattering phase function can be large and therefore can cause significant errors in the retrieved aerosol optical thickness if Mie theory is used to analyze reflectance measurements of nonspherical aerosols. On the other hand, the differences in the total optical cross sections, single-scattering albedo, asymmetry parameter of the phase function, and backscattered fraction are much smaller and in most cases do not exceed 10%. This may suggest that for a given aerosol optical thickness the influence of particle shape on the aerosol radiative forcing is negligibly small. Spherical-nonspherical differences in the extinction-to-backscatter ratio are very large and should be explicitly taken into account in inverting lidar measurements of dustlike aerosols.

Predictions and measurements of scattering and absorption over broad wavelength ranges in tissue phantoms.

Abstract: Predictions from Mie theory regarding the wavelength dependence of scattering in tissue from the near UV to the near IR are discussed and compared with experiments on tissue phantoms For large fiber separations it is shown that rapid, simultaneous measurements of the elastic scatter signal for several fiber separations can yield the absorption coefficient and reduced scattering coefficient With this information, the size of the scattering particles can be estimated, and this is done for Intralipid Measurements made at smaller source detector separations support Mie theory calculations, demonstrating that the sensitivity of elastic scatter measurements to morphological features, such as scatterer size, is enhanced when the distance between the source and detector fibers is small

X-ray charge densities and chemical bonding

Abstract: 1. Scattering of X-rays and Neutrons 2. The Effect of Thermal Vibrations on the Intensities of the Diffracted Beams 3. Chemical Bonding and the X-ray Scattering Formalism 4. Least-squares Methods and Their Use in Charge Density Analysis 5. Fourier Methods and Maximum Entropy Enhancement 6. Space Partitioning and the Analysis of the Total Charge Density 7. The Electrostatic Moments of a Charge Distribution 8. X-ray Diffraction and the Electrostatic Potential 9. The Electron Density and the Lattice Energy of Crystals 10. Charge Density Studies of Transition Metal Compounds 11. The Charge Density in Extended Solids 12. Electron Density Studies of Molecular Crystals

Harmonic inversion of time signals and its applications

Abstract: New methods of high resolution spectral analysis of short time signals are presented. These methods utilize the filter-diagonalization approach of Wall and Neuhauser [J. Chem. Phys. 102, 8011 (1995)] that extracts the complex frequencies ωk and amplitudes dk from a signal C(t)=∑kdke−itωk in a small frequency interval by recasting the harmonic inversion problem as the one of a small matrix diagonalization. The present methods are rigorously adapted to the conventional case of the signal available on a sparse equidistant time grid and use a more efficient boxlike filter. Various applications are discussed, such as iterative diagonalization of large Hamiltonian matrices for calculating bound and resonance states, scattering calculations in the presence of narrow resonances, etc. For the scattering problem the harmonic inversion is directly applied to the signal cn=(χf,Tn(Ĥ)χi), generated by the dynamical system governed by a modified Chebyshev recursion, avoiding the usual recasting the problem to the time d...

Attributed scattering centers for SAR ATR

Abstract: High-frequency radar measurements of man-made targets are dominated by returns from isolated scattering centers, such as corners and flat plates. Characterizing the features of these scattering centers provides a parsimonious, physically relevant signal representation for use in automatic target recognition (ATR). In this paper, we present a framework for feature extraction predicated on parametric models for the radar returns. The models are motivated by the scattering behaviour predicted by the geometrical theory of diffraction. For each scattering center, statistically robust estimation of model parameters provides high-resolution attributes including location, geometry, and polarization response. We present statistical analysis of the scattering model to describe feature uncertainty, and we provide a least-squares algorithm for feature estimation. We survey existing algorithms for simplified models, and derive bounds for the error incurred in adopting the simplified models. A model order selection algorithm is given, and an M-ary generalized likelihood ratio test is given for classifying polarimetric responses in spherically invariant random clutter.

Statistics of resonance poles, phase shifts and time delays in quantum chaotic scattering: Random matrix approach for systems with broken time-reversal invariance

Abstract: Assuming the validity of random matrices for describing the statistics of a closed chaotic quantum system, we study analytically some statistical properties of the S-matrix characterizing scattering in its open counterpart. In the first part of the paper we attempt to expose systematically ideas underlying the so-called stochastic (Heidelberg) approach to chaotic quantum scattering. Then we concentrate on systems with broken time-reversal invariance coupled to continua via Mopen channels; a=1,2,…,M. A physical realization of this case corresponds to the chaotic scattering in ballistic microstructures pierced by a strong enough magnetic flux. By using the supersymmetry method we derive an explicit expression for the density of S-matrix poles (resonances) in the complex energy plane. When all scattering channels are considered to be equivalent our expression describes a crossover from the χ2 distribution of resonance widths (regime of isolated resonances) to a broad power-like distribution typical for the regime of overlapping resonances. The first moment is found to reproduce exactly the Moldauer–Simonius relation between the mean resonance width and the transmission coefficient. Under the same assumptions we derive an explicit expression for the parametric correlation function of densities of eigenphases θa of the S-matrix (taken modulo 2π). We use it to find the distribution of derivatives τa=∂θa/∂E of these eigenphases with respect to the energy (“partial delay times”) as well as with respect to an arbitrary external parameter. We also find the parametric correlations of the Wigner–Smith time delay τw(E)=(1/M)∑a ∂θa/∂E at two different energies E−Ω/2 and E+Ω/2 as well as at two different values of the external parameter. The relation between our results and those following from the semiclassical approach as well as the relevance to experiments are briefly discussed.

Ultraviolet and visible spectroscopies for tissue diagnostics: fluorescence spectroscopy and elastic-scattering spectroscopy

Abstract: We review the application of fluorescence spectroscopy and elastic-scattering spectroscopy, over the ultraviolet-to-visible wavelength range, to minimally invasive medical diagnostics. The promises and hopes, as well as the difficulties, of these developing techniques are discussed.

Estimation of multiple scattering by iterative inversion, Part I: Theoretical considerations

Abstract: A review has been given of the surface-related multiple problem by making use of the so-called feedback model. From the resulting equations it has been concluded that the proposed solution does not require any properties of the subsurface. However, source-detector and reflectivity properties of the surface need be specified. Those properties have been quantified in a surface operator and this operator is estimated as part of the multiple removal problem. The surface-related multiple removal algorithm has been formulated in terms of a Neumann series and in terms of an iterative equation. The Neumann formulation requires a nonlinear optimization process for the surface operator; while the iterative formulation needs a number of linear optimizations. The iterative formulation also has the advantage that it can be integrated easily with another multiple removal method. An algorithm for the removal of internal multiples has been proposed as well. This algorithm is an extension of the surface-related method. Removal of internal multiples requires knowledge of the macro velocity model between the surface and the upper boundary of the multiple generating layer. In Part II (also published in this issue) the success of the proposed algorithms has been demonstrated on numerical experiments and field data examples.

Ring effect studies: Rayleigh scattering, including molecular parameters for rotational Raman scattering, and the Fraunhofer spectrum.

Abstract: Improved parameters for the description of Rayleigh scattering in air and for the detailed rotational Raman scattering component for scattering by O2 and N2 are presented for the wavelength range 200 ‐1000 nm. These parameters enable more accurate calculations to be made of bulk molecular scattering and of the Ring effect for a variety of atmospheric radiative transfer and constituent retrieval applications. A solar reference spectrum with accurate absolute vacuum wavelength calibration, suitable for convolution with the rotational Raman spectrum for Ring effect calculations, has been produced at 0.01-nm resolution from several sources. It is convolved with the rotational Raman spectra of O2 and N2 to produce an atmospheric Ring effect source spectrum. © 1997 Optical Society of America

Resistivity due to Domain Wall Scattering

Abstract: Domain walls in ferromagnetic metals are known to be a source of resistance since the early experiments on iron whiskers. Recently it has been possible to identify this contribution from data on cobalt and nickel films which display stripe domains in which the current is driven normal to the domain walls. With the same Hamiltonian as used to explain giant magnetoresistance in structures with collinear magnetic alignments we have determined the spin flip, as well as nonflip, scattering present in domain walls. We calculate the resistivity in zero field, i.e., in the presence of striped domains, and at saturation to show the amount of magnetoresistance that is attributable to domain wall scattering.

Optical polarization imaging

Abstract: The temporal profiles of the parallel and perpendicular polarization components of a light pulse backscattered from a scattering medium are different. The depth of penetration into the tissue and depolarization of the backscattered light depend on the scattering and absorption characteristics of the tissue. Based on these facts, a novel technique is demonstrated for noninvasive surface and beneath-the-surface imaging of biological systems.

Size and Interface Effects on Thermal Conductivity of Superlattices and Periodic Thin-Film Structures

Abstract: Superlattices consisting of alternating layers of extremely thin films often demonstrate strong quantum size effects that have been utilized to improve conventional devices and develop new ones. The interfaces in these structures also affect their thermophysical properties through reflection and transmission of heat carriers. This work develops models on the effective thermal conductivity of periodic thin-film structures in the parallel direction based on the Boltzmann transport equation. Different interface conditions including specular, diffuse, and partially specular and partially diffuse interfaces, are considered. Results obtained from the partially specular and partially diffuse interface scattering model are in good agreement with experimental data on GaAs/AlAs superlattices. The study shows that the atomic scale interface roughness is the major cause for the measured reduction in the superlattice thermal conductivity. This work also suggests that by controlling interface roughness, the effective thermal conductivity of superlattices made of bulk materials with high thermal conductivities can be reduced to a level comparable to those of amorphous materials, while maintaining high electrical conductivities. This suggestion opens new possibilities in the search of high efficiency thermoelectric materials.

Thermal conductivity of germanium crystals with different isotopic compositions

Abstract: We have measured the thermal conductivity of seven germanium crystals with different isotopic compositions in the temperature range between 2 K and 300 K. These samples, including one made of highly enriched ${}^{70}\mathrm{Ge}(99.99%)$, show intrinsic behavior at room temperature with the exception of a $p$-type sample with $|{N}_{d}\ensuremath{-}{N}_{a}|\ensuremath{\cong}2\ifmmode\times\else\texttimes\fi{}{10}^{16} {\mathrm{cm}}^{\ensuremath{-}3}$. The ``undoped'' samples exhibit a ${T}^{3}$ dependence at low temperatures, basically determined by boundary scattering. The maximum value of $\ensuremath{\kappa}$ (which falls in the range between 13 K and 23 K) is found to be a monotonically decreasing function of the isotopic mass variance parameter $g$. The maximum ${\ensuremath{\kappa}}_{m}$ measured for the most highly enriched ${}^{70}\mathrm{Ge}(99.99%)$ sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the full Callaway theory, modified by treating transverse and longitudinal modes separately, using three free adjustable parameters for each set of modes to represent anharmonic effects plus the calculated contributions from isotopic and boundary scattering. For the isotopically purest ${}^{70}\mathrm{Ge}(99.99%)$ sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effect of various surface treatments on the thermal conductivity in the low temperature region. The highest values of $\ensuremath{\kappa}$ are found after polish etching with a SYTON suspension.

Electromagnetic inverse scattering: Retrievable information and measurement strategies

Abstract: With reference to inverse scattering from an unknown object of limited extension embedded in a homogeneous background at a fixed frequency, we show that only a finite-dimensional representation of the unknown contrast can be hopefully retrieved. Exploiting the quasi-band-limitedness property of scattered fields, an accurate upper bound to the dimension of such a space is evaluated in both the single incidence and multiview cases. Moreover, effective schemes are given to collect all the information available from the scattering experiments in a nonredundant manner. As a by-product, an optimal (minimally redundant) sampling strategy for the monostatic radar cross section is also provided. Finally, we briefly discuss how the requirement for a globally effective and reliable solution scheme can lead to a reduction of the actually retrievable information.

Phonon-boundary scattering in thin silicon layers

Abstract: Temperature fields in microdevices made from silicon-on-insulator (SOI) wafers are strongly influenced by the lateral thermal conductivity of the silicon overlayer, which is diminished by phonon scattering on the layer boundaries. This study measures the thermal conductivity of single-crystal silicon layers in SOI substrates at temperatures between 20 and 320 K using Joule heating and electrical-resistance thermometry in microfabricated structures. Data for layers of thickness between 0.4 and 1.6 μm demonstrate the large reduction resulting from phonon-boundary scattering, particularly at low temperatures, and are consistent with predictions based on the phonon Boltzmann transport equation.

Diffusing-wave spectroscopy

Abstract: Since its invention about a decade ago, dynamic multiple light scattering has found many applications in various areas of soft condensed matter science. It has become a particularly important quantitative tool in colloid physics because of its applicability to systems containing very high concentrations of scatterers, and its extreme sensitivity to small motions. Recent advances of this technique, currently called diffusing-wave spectroscopy because of the diffusive transport of the light waves, include remote optical measurements of frequency-dependent viscoelasticity, studies of the microscopic dynamical processes in flowing sand and aging foam, a theoretical description of dynamic scattering from orientational fluctuations in liquid crystals and imaging of dynamic heterogeneities buried inside a turbid background medium.

Light scattering study of tissues

Abstract: Tissue optics is a rapidly expanding field of great interest to those involved in the development of optical medical technologies. In the present review both strongly (multiple) scattering tissues, such as skin, brain tissues, and vessel walls, and weakly scattering high-transparent tissues, such as eye tissues (cornea and lens), are discussed. For the former, radiation transport theory or Monte Carlo simulation are used to describe the propagation of light (laser beams). For weakly scattering ordered tissues, ensembles of close-packed Rayleigh or Mie scatterers are employed. Methods for solving the inverse problem of finding biotissue optical parameters are discussed. The propagation of photon-density diffusion waves in scattering and absorbing media is analyzed and the prospects of these waves for optical tomography are discussed. Polarization phenomena in both strongly and weakly scattering biotissues are discussed.

A solid tissue phantom for photon migration studies.

Abstract: A solid tissue phantom made of agar, Intralipid and black ink is described and characterized. The preparation procedure is fast and easily implemented with standard laboratory equipment. An instrumentation for time-resolved transmittance measurements was used to determine the optical properties of the phantom. The absorption and the reduced scattering coefficients are linear with the ink and Intralipid concentrations, respectively. A systematic decrease of the reduced scattering coefficient dependent on the agar content is observed, but can easily be managed. The phantom is highly homogeneous and shows good repeatability among different preparations. Moreover, agar inclusions can be easily embedded in either solid or liquid matrixes, and no artefacts are caused by the solid-solid or solid-liquid interfaces. This allows one to produce reliable and realistic inhomogeneous phantoms with known optical properties, particularly interesting for studies on optical imaging through turbid media.

Ultrasonic Techniques for Fluids Characterization

Abstract: Introduction. Water: Measurement of Sound Velocity. The Dependence of Velocity of Sound on Density and Compressibility. The Relationship Between Velocity and Attenuation--Conditions of High Attenuation. The Compressibility of Solute Molecules. Multiphase Media: Apparatus. Determining Composition in the Absence of Phase Changes. Following Phase Transitions. Determination of Solid Fat Content. Crystal Nucleation. The Solution-Emulsion Transition and Emulsion Inversion. Determination of Emulsion Stability by Ultrasound Profiling. Summary. Scattering of Sound: Theories of Sound. A Comparison of Electromagnetic and Acoustic Propagation. Scattering Theory. Scattering from Bubbles. Advanced Techniques: ParticleSizing. Propagation in Viscoelastic Materials. Bubbles and Foams. Automation and Computer Tools. Appendix: Basic Theory. MathCad Solutions of the Explicity Scattering Expressions. Glossary. Bibliography. Index. Table of Tables. Table of Figures.