Showing papers on "Scattering published in 1994"
TL;DR: In this article, the authors used the discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, including complex-conjugate gradient algorithms and fast-Fourier transform methods.
Abstract: The discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, is reviewed. Computational considerations, i.e., the use of complex-conjugate gradient algorithms and fast-Fourier-transform methods, are discussed. We test the accuracy of the DDA by using the DDA to compute scattering and absorption by isolated, homogeneous spheres as well as by targets consisting of two contiguous spheres. It is shown that, for dielectric materials (|m| ≲ 2), the DDA permits calculations of scattering and absorption that are accurate to within a few percent.
3,283 citations
Book•
28 Feb 1994
TL;DR: First-order radiative transfer solution passive sensing formulation of the surface scattering problem surface model and special cases ranges validity of the IEM model matrix doubling formulations for scattering and emission scattering and emissions models for snow and sea ice comparisons of model predictions with backscattering and emission measurements from snow and ice.
Abstract: First-order radiative transfer solution first-order radiative transfer solution passive sensing formulation of the surface scattering problem surface model and special cases ranges validity of the IEM model matrix doubling formulations for scattering and emission scattering and emissions models for snow and sea ice comparisons of model predictions with backscattering and emission measurements from snow and ice.
1,334 citations
TL;DR: In this article, the authors investigated the radiative properties of a laser dye dispersed in a strongly scattering medium (a colloidal suspension of titanium dioxide particles), and found that the emissions from such systems can exhibit spectral and temporal properties characteristic of a multimode laser oscillator, even though the systems contain no external cavity.
Abstract: THE radiative properties of an atomic or molecular system may be altered significantly in the presence of coherent optical scattering1,2. In the course of investigating the radiative properties of a laser dye dispersed in a strongly scattering medium (a colloidal suspension of titanium dioxide particles), we have found that the emissions from such systems can exhibit spectral and temporal properties characteristic of a multimode laser oscillator, even though the systems contain no external cavity. The threshold excitation energy for laser action is surprisingly low. We suggest that these composite systems might find applications in laser instrumentation and photonics.
1,104 citations
TL;DR: It is concluded that noninvasive measurements of optically thick tissue require a rigorous treatment of the tissue boundary, and a unified partial-current--extrapolated boundary approach is suggested.
Abstract: Using the method of images, we examine the three boundary conditions commonly applied to the surface of a semi-infinite turbid medium. We find that the image-charge configurations of the partial-current and extrapolated-boundary conditions have the same dipole and quadrupole moments and that the two corresponding solutions to the diffusion equation are approximately equal. In the application of diffusion theory to frequency-domain photon-migration (FDPM) data, these two approaches yield values for the scattering and absorption coefficients that are equal to within 3%. Moreover, the two boundary conditions can be combined to yield a remarkably simple, accurate, and computationally fast method for extracting values for optical parameters from FDPM data. FDPM data were taken both at the surface and deep inside tissue phantoms, and the difference in data between the two geometries is striking. If one analyzes the surface data without accounting for the boundary, values deduced for the optical coefficients are in error by 50% or more. As expected, when aluminum foil was placed on the surface of a tissue phantom, phase and modulation data were closer to the results for an infinite-medium geometry. Raising the reflectivity of a tissue surface can, in principle, eliminate the effect of the boundary. However, we find that phase and modulation data are highly sensitive to the reflectivity in the range of 80-100%, and a minimum value of 98% is needed to mimic an infinite-medium geometry reliably. We conclude that noninvasive measurements of optically thick tissue require a rigorous treatment of the tissue boundary, and we suggest a unified partial-current--extrapolated boundary approach.
998 citations
TL;DR: Comparison of trapping forces for latex and gold spheres demonstrates that the gradient force is the major determinant of trapping strength and that competing effects, such as scattering or radiometric forces, are relatively minor.
Abstract: Metallic objects reflect light and have generally been considered poor candidates for optical traps, particularly with optical tweezers, which rely on a gradient force to provide trapping. We demonstrate that stable trapping can occur with optical tweezers when they are used with small metallic Rayleigh particles. In this size regime, the scattering pictures for metals and dielectrics are similar, and the larger polarizability of metals implies that trapping forces are greater. The latter fact makes the use of metal particles attractive for certain biological applications. Comparison of trapping forces for latex and gold spheres demonstrates that the gradient force is the major determinant of trapping strength and that competing effects, such as scattering or radiometric forces, are relatively minor.
738 citations
TL;DR: In this article, a novel technique, based on optical coherence tomography, was described for enhanced optical sectioning in confocal microscopy, which was demonstrated and compared with the predictions of a single-backscatter theory.
Abstract: We describe a novel technique, based on optical coherence tomography, for enhanced optical sectioning in confocal microscopy. Confocal imaging deep into highly scattering media is demonstrated and compared with the predictions of a single-backscatter theory.
720 citations
TL;DR: In this paper, the authors measured the water density profile perpendicular to a silver surface at two applied voltages and found that the water molecules are ordered in layers extending about three molecular diameters from the electrode, and that the spacing between the electrode and first water layer indicates an oxygen-up (oxygen-down) average orientation for negative (positive) charge.
Abstract: THE arrangement of water molecules at charged, aqueous interfaces is an important question in electrochemistry, geochemistry and biology. Theoretical studies1–11 suggest that the molecules become arranged in several layers adjacent to a solid interface, with densities similar to that in the bulk, and that the molecules in the first layer are reoriented from oxygen-up to oxygen-down as the electrode charge changes from negative to positive. Few of these predictions have been verified experimentally12–16, however. Using X-ray scattering, we have measured the water density profile perpendicular to a silver (111) surface at two applied voltages. We find that the water molecules are ordered in layers extending about three molecular diameters from the electrode, and that the spacing between the electrode and first water layer indicates an oxygen-up (oxygen-down) average orientation for negative (positive) charge. Contrary to current models, however, we find that the first layer has a far greater density than that in bulk water. This implies that the hydrogen-bonding network is disrupted in this layer, and that the properties of the water in the layer are likely to be very different from those in the bulk.
534 citations
TL;DR: In this article, the triple pomeron coupling was shown to have a (−t) − 1 2 2 singularity at t = 0 when one of the pomerons' momenta is zero while the other two have momentum transfer.
507 citations
TL;DR: In this paper, the authors calculated the electron mobility in GaN and InN as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter.
Abstract: Electron mobilities in GaN and InN are calculated, by variational principle, as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter. Both GaN and InN have maximum mobilities between 100 and 200 K, depending on the electron density and compensation ratio, with lower electron density peaking at lower temperature. This is due to the interplay of piezoelectric acoustic phonon scattering at low carrier concentrations and ionized impurity scattering at higher carrier concentrations. Above 200 K, polar mode optical phonon scattering is the mobility limiting process. The 300 and 77 K electron and Hall mobilities as functions of carrier concentration in the range of 1016–1020 cm−3 and compensation ratio are also calculated. The theoretical maximum mobilities in GaN and InN at 300 K are about 1000 and 4400 cm2 V−1 s−1, respectively, while at 77 K the limits are beyond 6000 and 30 000 cm2 V−1 s−1, respectively. We compare the results with experimental data and find reasonable correlation, but with evidence that structural imperfection and heavy compensation play important roles in the material presently available. Only phonon limited scattering processes are considered in the calculation of the mobility in AlN since it is an insulator of extremely low carrier concentration. We find a phonon limited electron mobility of about 300 cm2 V−1 s−1 at 300 K.
479 citations
Book•
01 Jun 1994
TL;DR: In this paper, a survey of wave scattering from rough surfaces is presented, with the roughness being both deterministic and statistical in nature, along with some new results which were up to now only available in the general literature.
Abstract: Wave Scattering From Rough Surfaces is devoted to analytical methods for calculating wave scattering from reasonably plane but rough surfaces, with the roughness being both deterministic and statistical in nature. Along with the classical concepts, the monograph includes some new results which were up to now only available in the general literature. A systematic development of the various methods allows the results to be presented in simple and universal forms. For the benefit of the reader, all calculations are given in detail and can easily be followed. The necessary mathematical background is summarized, and further details on well-known issues, for example, the Rayleigh hypothesis, are listed.
466 citations
TL;DR: In this article, both in-plane and vertical correlations of the roughness profiles have been considered and it has been demonstrated that the vertical roughness correlation substantially affects the nonspecular scattering.
Abstract: X-ray reflection from periodical multilayers with randomly rough interfaces has been described within the distorted-wave Born approximation. The method is suitable for calculating both specular x-ray reflection and nonspecular (diffuse) scattering. In this paper, both in-plane and vertical correlations of the roughness profiles have been considered and it has been demonstrated that the vertical roughness correlation substantially affects the nonspecular scattering. The theory can explain resonant effects observed in the beam scattered nonspecularly from a periodical multilayer. The theoretical approach has been used for the study of interfacial roughness in a long-periodic AlAs/GaAs multilayer and good agreement has been achieved between the experimental results and the theory.
Journal Article•
TL;DR: In this paper, the authors used the discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, including complex-conjugate gradient algorithms and fast-Fourier transform methods.
Abstract: The discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, is reviewed. Computational considerations, i.e., the use of complex-conjugate gradient algorithms and fast-Fourier-transform methods, are discussed. We test the accuracy of the DDA by using the DDA to compute scattering and absorption by isolated, homogeneous spheres as well as by targets consisting of two contiguous spheres. It is shown that, for dielectric materials (|m| ≲ 2), the DDA permits calculations of scattering and absorption that are accurate to within a few percent.
TL;DR: In this article, the transport coefficients of the Anderson model are calculated by extending Wilson's numerical renormalization group method to finite-temperature Green functions, and accurate results for the frequency and temperature dependence of the single-particle spectral densities and transport time tau ( omega, T) are obtained and used to extract the temperature dependence.
Abstract: The transport coefficients of the Anderson model are calculated by extending Wilson's numerical renormalization group method to finite-temperature Green functions. Accurate results for the frequency and temperature dependence of the single-particle spectral densities and transport time tau ( omega , T) are obtained and used to extract the temperature dependence of the transport coefficients in the strong-correlation limit of the Anderson model. Results are obtained for values of the local level position ranging from the Kondo regime to the mixed valency and empty orbital regimes. The low-temperature anomalies in the resistivity, rho (T), thermopower, S(T), thermal conductivity, kappa (T), and Hall coefficient, RH(T), are discussed in terms of the behaviour of the spectral densities. At low temperature all quantities exhibit the expected Fermi liquid behaviour, rho (T)= rho 0(1-c(T/TK)2), S(T) approximately gamma T, kappa (T)/ alpha T=1+ beta (T/TK)2, RH(T)=-Rinfinity (1- delta (T/TK)2). Analytic results based on Fermi liquid theory are derived here for the first time for beta and the numerical results are shown to be consistent with this coefficient. The range of temperatures over which universal behaviour extends is also discussed. Scattering of conduction electrons in higher-angular-momentum, l>0, channels is also considered and an expression is derived for the corresponding transport time and used to discuss the influence of the interference terms between the resonant l=0 and non-resonant l=1 channels on the transport properties. The presence of non-resonant scattering is shown to be particularly important for the thermopower at half filling, where the sign of the thermopower can depend sensitively on the non-resonant phase shift. Finally the relation of the results to experiment is discussed.
TL;DR: In this article, the problem of unitarity in high energy onium-onium scattering can be solved in a purely perturbative context, with a non-running coupling if the onium is heavy enough.
Abstract: {
arrower\smallskip High Energy onium-onium scattering is calculated as a function of impact parameter in the one and two pomeron exchange approximation. Difficulties with using the multiple scattering series to unitarize single pomeron exchange at high energy are noted. An operator formalsim which sums all numbers of pomeron exchange is given. A toy model which has a similar operator structure at high energy as QCD is presented and the S-matrix is evaluated. Estimates of the energies and impact parameters at which blackness occurs in onium-onium scattering are given. It is emphased that the problem of unitarity in high energy onium-onium scattering can be solved in a purely perturbative context, with a non-running coupling if the onium is heavy enough.\smallskip}
TL;DR: In this paper, consistent sets of Ar++Ar and Ar+++Ar differential and integral cross sections for modeling ion scattering that take into account differential scattering data and the fact that symmetric charge transfer collisions are one aspect of elastic scattering collisions are presented.
Abstract: We suggest consistent sets of Ar++Ar and Ar+++Ar differential and integral cross sections for modeling ion scattering that take into account differential scattering data and the fact that symmetric charge transfer collisions are one aspect of elastic scattering collisions. These suggestions make possible a considerable improvement in the accuracy of future Monte Carlo calculations of the angular, energy, and temporal distributions of Ar+ and Ar++ ions passing through the electrode sheaths of low‐pressure, rf, and dc discharges in Ar. The cross sections necessary for a proper modeling of the energy dissipation in the gas and at the electrodes by fast neutral Ar atoms formed in symmetric‐charge‐transfer collisions of Ar+ and Ar++ with Ar are also reviewed.
TL;DR: Numerically, using the Mie theory, light transmission through a multiply scattering medium composed of a collection of uncorrelated, optically inactive spherical particles is studied, finding the characteristic length of depolarization for incident linearly polarized light is found to exceed that for incident circularly polarized light.
Abstract: We study numerically, using the Mie theory, light transmission through a multiply scattering medium composed of a collection of uncorrelated, optically inactive spherical particles The characteristic length over which a plane-wave field is depolarized depends on whether it is initially linearly or circularly polarized and on the size of the particles In a medium containing particles small compared to the wavelength (Rayleigh regime), the characteristic length of depolarization for incident linearly polarized light is found to exceed that for incident circularly polarized light, while the opposite is true in a medium composed of particles large compared to the wavelength (Mie regime) A comparison of numerical results with the data from measurements on suspensions of polystyrene latex spheres in water is made Agreement between these simulations and experiment is good for the range of sizes considered in this paper We also discuss the relevance of the helicity-flip model to the analysis of these data
TL;DR: A frequency-domain near-infrared tissue spectrometer capable of measuring the reduced scattering coefficient of tissue with enough precision to detect changes in glucose levels in the physiological and pathological range is designed and constructed.
Abstract: Tissue glucose levels affect the refractive index of the extracellular fluid. The difference in refractive index between the extracellular fluid and the cellular components plays a role in determining the reduced scattering coefficient (micro(s)') of tissue. Hence a physical correlation may exist between the reduced scattering coefficient and glucose concentration. We have designed and constructed a frequency-domain near-infrared tissue spectrometer capable of measuring the reduced scattering coefficient of tissue with enough precision to detect changes in glucose levels in the physiological and pathological range.
TL;DR: In this paper, a theory of scattering by a finite number of cylinders of arbitrary cross section is presented, based on a self-consistent approach that identifies incident and scattered fields around each cylinder and then uses the notion of a scattering matrix in order to get a linear system of equations.
Abstract: A theory of scattering by a finite number of cylinders of arbitrary cross section is presented. This theory is based on a self-consistent approach that identifies incident and scattered fields around each cylinder and then uses the notion of a scattering matrix in order to get a linear system of equations. Special attention is paid to the simplified case of a sparse distribution of small cylinders for low frequencies. Surprisingly, it is found that the classical rules of homogenization must be modified in that case. The phenomenon of enhanced backscattering of light is investigated from numerical data for a dense distribution of cylinders.
TL;DR: In this article, the authors measured the Ramaman spectra of n-type gallium nitride with different carrier concentrations and found that the phonon band shifted towards the high frequency side and broadened with an increase in carrier concentration.
Abstract: Raman spectra of n‐type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high‐frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line‐shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation‐potential and electro‐optic mechanisms.
TL;DR: In this article, the authors derived the conductance response to oscillating external chemical potentials, and determined the screening potential for slowly oscillating potentials from the condition of local charge neutrality.
Abstract: In the presence of a static potential drop a carrier stream incident at a contact of the sample is partitioned into the other contacts according to the transmission probabilities of the sample. The bare response to oscillating potentials, on the other hand, violates current conservation due to the piling up of unscreened charges in the sample, and has to be modified by taking the induced screening potential into account. We present a novel derivation of the conductance response to oscillating external chemical potentials, find the response to an arbitrary internal potential in terms of functional derivatives with respect to the local potential of the scattering matrix of the conductor, and determine the screening potential for slowly oscillating potentials from the condition of local charge neutrality. We find that the current partitioning depends on ratios of local densities of states which reflect the injection and emission properties of the contacts of the sample.
TL;DR: Results from in vivo studies find that whilst the 820 nm feature is measured with lower accuracy for a given light flux than the 975 nm feature, it is intrinsically a more accurate differential path estimator.
Abstract: We describe a simple method for measuring the 'differential pathlength' of photons in a scattering medium utilizing the spectral absorption features of water. Determination of this differential pathlength is a prerequisite for quantifying chromophore concentration changes measured by near-infrared spectroscopy (NIRS), and the method proposed here achieves this without the need for time- or frequency-resolved measurements. The quantification of tissue chromophore concentration measurements is a major goal in NIRS research, allowing, for example, the non-invasive measurement of blood flow and volume in the brain and other organs. We present the results of validation experiments performed on tissue phantoms comparing the differential path estimates yielded by water absorption and time-resolved measurements, finding that the weak water absorption feature at 820 nm can yield a differential path estimate in addition to the main feature at 975 nm. We also present results from in vivo studies in which we find that whilst the 820 nm feature is measured with lower accuracy for a given light flux than the 975 nm feature, it is intrinsically a more accurate differential path estimator. Studies on the adult forearm showing differences between time-resolved and water absorption differential path estimates suggest that the measurement of both could aid in quantifying NIRS signals in heterogeneous tissues.
TL;DR: Theoretical expressions for the phonon thermal conductivity of single crystals of graphite in the basal plane at room and at elevated temperatures were derived in this paper, where the phonons were treated by a two-dimensional Debye model in the frequency range from 4 to 46 THz.
Book•
01 Jan 1994
TL;DR: Theoretical description of charge transfer in Atom-Surface Collisions (H. Rabalais et al. as mentioned in this paper ) and charge-transfer dynamics in Low and Hyperthermal-Energy Alkali-Ion-surface collisions (B. Cooper & E. Behringer).
Abstract: Partial table of contents: Quantitative Intensity Analysis of Low-Energy Scattering and Recoiling from Crystal Surfaces (R. Williams). Ion Scattering and Recoiling for Elemental Analysis and Structure Determination (P. Bertrand & J. Rabalais). Theoretical Description of Charge Transfer in Atom-Surface Collisions (H. Shao, et al.). Electron Emission from Slow Ion-Solid Interactions (R. Baragiola). Scattering and Charge-Transfer Dynamics in Low- and Hyperthermal-Energy Alkali-Ion-Surface Collisions (B. Cooper & E. Behringer). The Interaction of Molecular Ions with Surfaces (W. Heiland). Sputtering of Metals and Insulators with Hyperthermal Singly and Doubly Charged Rare-Gas Ions (P. Varga & U. Diebold). Film Deposition from Low-Energy Ion Beams (D. Marton). Indexes.
TL;DR: In this paper, an analytical solution of the radiosity equation is applied to compute vegetation indices, reflectance spectra, and the spectral bidirectional reflectance distribution function for simple canopy geometries.
TL;DR: The ultrasonic pulses that are generated when a picosecond light pulse is absorbed in a metallic film are studied and it is shown that this method can be used to test theories of the electron-phonon and electron-electron scattering rates.
Abstract: We have studied the ultrasonic pulses that are generated when a picosecond light pulse is absorbed in a metallic film. The pulse shape is influenced by the distance that the hot electrons diffuse before losing their energy to the lattice. We show that this method can be used to test theories of the electron-phonon and electron-electron scattering rates. The results are in excellent agreement with a calculation in which the nonthermal character of the electron distribution is taken into account.
TL;DR: In this article, the small-slope approximation (SSA) for wave scattering at the rough interface of two homogeneous half-spaces is developed, which is applicable irrespective of the wavelength of radiation, provided that the slopes of roughness are small compared with the angles of incidence and scattering.
Abstract: The small-slope approximation (SSA) for wave scattering at the rough interface of two homogeneous half-spaces is developed. This method bridges the gap between two classical approaches to the problem: the method of small perturbations and the Kirchhoff (or quasi-classical) approximation. In contrast to these theories, the SSA is applicable irrespective of the wavelength of radiation, provided that the slopes of roughness are small compared with the angles of incidence and scattering. The resulting expressions for the SSA are given for the entries of an S-matrix that represents the scattering amplitudes of plane waves of different polarizations interacting with the rough boundary. These formulae are quite general and are valid, in fact, for waves of different origins. Apart from the shape of the boundary, some functions in these formulae are coefficients of the expansion of the S-matrix into a power series in terms of elevations. These roughness independent functions are determined by a specific s...
TL;DR: A multilevel algorithm is applied to the solution of an integral equation using the conjugate gradient method and shows that the complexity of a matrix-vector multiplication is proportional to N (log(N))2.
Abstract: In the solution of an integral equation using the conjugate gradient (CG) method, the most expensive part is the matrix-vector multiplication, requiring O(N2) floating-point operations. The fast multipole method (FMM) reduced the operation to O(N15). In this article we apply a multilevel algorithm to this problem and show that the complexity of a matrix-vector multiplication is proportional to N (log(N))2. © 1994 John Wiley & Sons, Inc.
TL;DR: By assuming that the scattering properties of the tissue are the same at the excitation and emission wavelengths, a simple relation between the fluorescence and the scatter signals is derived.
Abstract: In general it is not possible to write an analytic expression for the fluorescence signal generated by a fluorophore distributed in a scattering medium such as tissue. However, by assuming that the scattering properties of the tissue are the same at the excitation and emission wavelengths, we have derived a simple relation between the fluorescence and the scatter signals. Along with diffusion theory, this was used to write expressions for the fluorescence signal detected at the tissue surface in both the time and the frequency domains. Experiments using the fluorophore aluminum chlorosulfonated phthalocyanine in tissue-simulating materials confirmed the accuracy of the model. Applications to in vivo spectroscopy are discussed.
TL;DR: In this paper, a slide rule is used to evaluate the horizontal and vertical components of the scattering angle for Langmuir films of long linear amphiphilic molecules at the air-water interface.
Abstract: For Langmuir films of long linear amphiphilic molecules at the air-water interface, grazing-incidence diffraction data resolved in terms of both the horizontal and vertical components of the scattering angle can be evaluated to a fair level of detail by means of a slide rule. Specular reflection data require more sophisticated means although some rules of thumb can be formulated.