Showing papers on "Diffraction published in 1994"

A photorefractive polymer with high optical gain and diffraction efficiency near 100

Abstract: PHOTOREFRACTIVE materials are of considerable interest for the development of all-optical devices1. The photoref ractive effect appears in materials that exhibit an electric-field-dependent refractive index and that are photosensitive, such that the spatial distribution of photogenerated charge carriers is modified on irradiation with light. The diffraction pattern formed by the interference of two coherent light beams within such a material generates a non-uniform internal electric field that in turn modulates the refractive index. The resulting refractive-index pattern forms a grating that can diffract light and thereby give rise to two-beam coupling, whereby one of the writing beams gains energy at the expense of the other—a property that can be exploited in photonic devices. Although the best photorefractive materials currently available are inorganic crystals such as LiNbO3, there is considerable interest in the development of photorefractive polymers2–8, owing to their structural flexibility, ease of processing and lower cost. We describe here a polymer composite with excellent photorefractive properties. We have achieved a diffraction efficiency approaching 100% and a net two-beam coupling gain of more than 200 cm–1, making these polymeric materials suitable for immediate application in areas such as dynamic holographic storage and optical information processing1.

Probing single molecule dynamics.

Abstract: The room temperature dynamics of single sulforhodamine 101 molecules dispersed on a glass surface are investigated on two different time scales with near-field optics. On the 10(-2)- to 10(2)-second time scale, intensity fluctuations in the emission from single molecules are examined with polarization measurements, providing insight into their spectroscopic properties. On the nanosecond time scale, the fluorescence lifetimes of single molecules are measured, and their excited-state energy transfer to the aluminum coating of the near-field probe is characterized. A movie of the time-resolved emission demonstrates the feasibility of fluorescence lifetime imaging with single molecule sensitivity, picosecond temporal resolution, and a spatial resolving power beyond the diffraction limit.

Size Dependence of a First Order Solid-Solid Phase Transition: The Wurtzite to Rock Salt Transformation in CdSe Nanocrystals

Abstract: Measurements of the size dependence of a solid-solid phase transition are presented. High-pressure x-ray diffraction and optical absorption are used to study the wurtzite to rock salt structural transformation in CdSe nanocrystals. These experiments show that both the thermodynamics and kinetics of this transformation are altered in finite size, as compared to bulk CdSe. An explanation of these results in the context of transformations in bulk systems is presented. Insight into the kinetics of transformations in both bulk and nanocrystal systems can be gained.

Patterned condensation figures as optical diffraction gratings

Abstract: Heterogeneous, patterned surfaces comprising well-defined hydrophobic and hydrophilic regions and having micrometer-scale periodicities were prepared by patterning the adsorption of ω-functionalized alkanethiolates in self-assembled monolayers (SAMs) on gold. Condensation of water on such surfaces resulted in drops that followed the patterns in the SAMs. These patterned condensation figures (CFs) acted as optical diffraction gratings for reflected (or transmitted) light from a helium-neon laser (wavelength of 632.8 nanometers). Under an atmosphere of constant relative humidity, the development of the condensation figure was monitored quantitatively, as the temperature of the surface was lowered, by following the change in intensity of a first-order diffraction spot. This experimental technique may be useful in the development of new types of optical sensors that respond to their environment by changing the reflectivity of patterned regions and for studying phenomena such as drop nucleation, contact angle hysteresis, and spontaneous dewetting and break-up of thin liquid films.

Scattering by a random set of parallel cylinders

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.

Method for the simultaneous determination of anisotropic residual stresses and texture by x‐ray diffraction

Abstract: A method is developed that yields the residual stress, the orientation distribution coefficients, the average crystallite dimension, the microstrain, and the crystal structure parameters from x‐ray diffraction data in a single‐step procedure. To this end, a general approach is introduced that combines the equations of micromechanics with the harmonic description of texture. All relationships are cast into a Rietveld‐like format, which incorporates a microstructure model derived from line‐broadening methods. In this manner, data collected over the whole x‐ray‐diffraction pattern at different tilting of the sample can be fitted directly. The associated fitting parameters are the crystal structure and microstructure, the texture coefficients, and the micromechanical properties and fields.

Some simple ideas on X-ray reflection and grazing-incidence diffraction from thin surfactant films

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.

Neutron scattering from vitreous silica. V. The structure of vitreous silica: What have we learned from 60 years of diffraction studies?

Abstract: A quantitative assessment is presented of the structural information derived from X-ray and neutron diffraction and small angle scattering investigations of vitreous silica, starting from the classic studies of Warren and co-workers in the 1930s. Particular points that are addressed include the regularity of the SiO4 tetrahedral structural units, the SiOSi angle distribution, the interpretation of the first peak in the diffraction pattern and the extent of the longer range density fluctuations as revealed by X-ray and neutron measurements at small scattering vectors. The use of structural modelling in the interpretation of the diffraction data is discussed and representative models are compared with experiment. The extent of the (dis)agreement of each model with experiment is compared quantitatively and the paper concludes by identifying the most accurate structural model of vitreous silica known to the author at the present time.

Algorithm for the rigorous coupled-wave analysis of grating diffraction

Abstract: Diffraction of light by periodic gratings is analyzed with a characteristic-matrix formalism based on a rigorous coupled-wave approach. This formalism is particularly convenient for modeling the diffraction by nonuniform periodic structures. In order to overcome numerical difficulties that are due to inhomogeneous eigenmodes, we propose a new algorithm that remains stable for gratings of any thickness. We obtain the stability by distinguishing in the computation the growing and the decaying inhomogeneous modes. Numerical examples and comparisons with previous results are given.

Adsorbate structure determination on surfaces using photoelectron diffraction

Abstract: Photoelectron diffraction is the name given to the phenomenon resulting from the coherent interference of the directly emitted component of an electron wavefield, emerging from an atom as a result of core level photoemission, with other components elastically scattered by surrounding atoms. Experimental characterization of this effect provides information which can be used to provide quantitative determinations of the structure of surfaces, and particularly of adsorbed species on surfaces, in an element-specific fashion. Since the initial. Demonstration of the phenomenon in the late 1970s, an extensive methodology for surface structure determination has been developed. In this review the background physics of the process, and the development of the technique is described. A brief discussion of the high energy forward scattering version of the technique (X-ray photoelectron diffraction-XPD), which utilizes zero-order diffraction effects, is included, but the most of the review is concerned with the lower energy backscattering method more relevant to the determination of detailed adsorption sites on surfaces. In addition to the general theoretical, experimental and methodology background, a number of the more recent developments are described including use of 'direct inversion' methods for (approximate) structure determination, including a survey of photoelectron holography, and the realization of chemical shift photoelectron diffraction to allow structure determinations of surfaces including atoms of one element in more than one inequivalent site. All of the developments are illustrated with specific examples, mainly of molecular and atomic adsorbates on metal surfaces.

Guided waves by electromagnetic gratings and non‐uniqueness examples for the diffraction problem

Abstract: We consider the two-dimensional scalar problem of the diffraction of a plane wave by an infinite grating of conducting bodies immersed in a periodical dielectric medium. A Fredholm-type formulation is derived and studied. The existence of a solution is proved and some uniqueness results are established. A detailed description of the guided modes of the grating is carried out. Finally, various non-uniqueness examples for the diffraction problem are exhibited.

Crystal structures and cation distributions in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the temperature dependence of the cation distribution in ZnAl2O4

Abstract: The crystal structure and cation distributions in the spinels MgCr2O4, ZnCr2O4, Fe3O4 and a suite of ZnAl2O4 samples annealed at 900 to 1400° C and then rapidly quenched, have been determined by powder X-ray diffraction, using several different X-ray procedures and both conventional structure-factor refinement and whole-pattern (or Rietveld) refinement methods. The chromite spinels are expected from crystal chemical considerations to have an almost completely normal cation distribution (inversion parameter, x, equal to zero). In agreement with this expectation, three samples of MgCr2O4 annealed at 900, 1100 and 1300° C, and ZnCr2O4 were all found to have x=0 within two estimated standard deviations (esd), suggesting that the accuracy with which cation distributions in spinels may be determined by powder XRD is close to the estimated precision. Slightly better results are obtained assuming neutral-atom scattering curves rather than half-ionized or fully ionized, but the differences are small (within the esd). The results from the Rietveld refinements are similarly in good agreement with those using the conventional structure factor refinement approach (agreement within the combined esd's), although in detail the Rietveld procedure sometimes produces small systematic differences in refined parameters. The suite of ZnAl2O4 spinels show a smooth increase in x from 0.01 at 900° C to 0.05 at 1300° C, and this behaviour is well described by the simple thermodynamic model for disordering in spinels with αZn-Al=89 kJ/mol, assuming β=−20 kJ/mol. The oxygen positional parameters for Fe3O4 are similar to those from published single crystal studies, indicating that the powder method also yields accurate interatomic distances in spinels.

In Situ Determination of the NiAs Phase of FeO at High Pressure and Temperature

Abstract: In situ synchrotron x-ray diffraction measurements of FeO at high pressures and high temperatures revealed that the high-pressure phase of FeO has the NiAs structure (B8). The lattice parameters of this NiAs phase at 96 gigapascals and 800 kelvin are a = 2.574(2) angstroms and c = 5.172(4) angstroms (the number in parentheses is the error in the last digit). Metallic behavior of the high-pressure phase is consistent with a covalently and metallically bonded NiAs structure of FeO. Transition to the NiAs structure of FeO would enhance oxygen solubility in molten iron. This transition thus provides a physiochemical basis for the incorporation of oxygen into the Earth's core.

Rigorous justification of the localized approximation to the beam-shape coefficients in generalized Lorenz–Mie theory. I. On-axis beams

Abstract: Generalized Lorenz–Mie theory describes electromagnetic scattering of an arbitrary light beam by a spherical particle. The computationally most expensive feature of the theory is the evaluation of the beam-shape coefficients, which give the decomposition of the incident light beam into partial waves. The so-called localized approximation to these coefficients for a focused Gaussian beam is an analytical function whose use greatly simplifies Gaussian-beam scattering calculations. A mathematical justification and physical interpretation of the localized approximation is presented for on-axis beams.

Pattern formation in a passive Kerr cavity

Abstract: Analytic and numerical investigations of a cavity containing a Kerr medium are reported. The mean field equation with plane-wave excitation and diffraction is assumed. Stable hexagons are dominant close to threshold for a self-focusing medium. Bistable switching frustrates pattern formation for a self-defocusing medium. Under appropriate parametric conditions that we identify, there is coexistence of a homogeneous stationary solution, of a hexagonal pattern solution and of a large (in principle infinite) number of localized structure solutions which connect the homogeneous and hexagonal state. Further above threshold, the hexagons show defects, and then break up with apparent turbulence. For Gaussian beam excitation, the different symmetry leads to polygon formation for narrow beams, but quasihexagonal structures appear for broader beams.

Crystal Structure Analysis for Chemists and Biologists

Abstract: Introduction to Crystal Structure Analysis Crystals Diffraction by Crystals Symmetry in Crystals and their Diffraction Patterns Physical Properties of Crystals Combining Waves to Obtain an Image Measurement of Structure Amplitudes Estimation of the Phase Angles The Electron Density Map Least--Squares Refinement of the Structure Interpreting x, y, z, (Atomic Coordinates) Conformation Atomic and Molecular Displacements Chirality and Absolute Structure Packing in Crystals Comparisons of Structures Recognition and Receptors Structure--Activity Results Index

Nanometer spatial resolution achieved in hard x-ray imaging and Laue diffraction experiments

Abstract: Tapered glass capillaries have successfully condensed hard x-ray beams to ultrasmall dimensions providing unprecedented spatial resolution for the characterization of materials. A spatial resolution of 50 nanometers was obtained while imaging a lithographically prepared gold pattern with x-rays in the energy range of 5 to 8 kiloelectron volts. This is the highest resolution scanning x-ray image made to date with hard x-rays. With a beam 360 nanometers in diameter, Laue diffraction was observed from the smallest sample volume ever probed by x-ray diffraction, 5 x 10(-3) cubic micrometers.

Biomedical ultrasound beam forming

Abstract: The principles of biomedical ultrasound beam forming control the quality of diagnostic imaging. Beam parameters associated with imaging quality are: (1) lateral and axial resolutions; (2) depth of field; (3) contrast and (4) frame rate. In this paper, we review some of the current beam forming techniques and their principles. We focus on trade-offs among the above four aspects of beam forming and relate them to system parameters such as aperture size, f-number (the ratio between focal length and aperture diameter), central frequency (wavelength), system bandwidth and sidelobes. Methods for steering conventional and limited diffraction beams with array transducers are also reviewed.

Geometrical Theory of Diffraction

Abstract: Details the ideas underlying geometrical theory of diffraction (GTD) along with its relationships with other EM theories.

High-performance x-ray spectroscopic devices for plasma microsources investigations

Abstract: X-ray spectroscopy with high spectral (up to Δλ/λ = 10−4) and spatial resolution (up to 1 μm) is discussed. Devices based on crystals, diffraction and Bragg-Fresnel elements and their applications in Z- and X-pinches and laser plasma experiments are described.

Limits of scalar diffraction theory for diffractive phase elements

Abstract: The range of validity and the accuracy of scalar diffraction theory for periodic diffractive phase elements (DPE’s) is evaluated by a comparison of diffraction efficiencies predicted from scalar theory to exact results calculated with a rigorous electromagnetic theory. The effects of DPE parameters (depth, feature size, period, index of refraction, angle of incidence, fill factor, and number of binary levels) on the accuracy of scalar diffraction theory is determined. It is found that, in general, the error of scalar theory is significant (∊ > ±5%) when the feature size is less than 14 wavelengths (s < 14λ). The error is minimized when the fill factor approaches 50%, even for small feature sizes (s = 2λ); for elements with an overall fill factor of 50% the larger period of the DPE replaces the smaller feature size as the condition of validity for scalar diffraction theory. For an 8-level DPE of refractive index 1.5 analyzed at normal incidence the error of the scalar analysis is greater than ±5% when the period is less than 20 wavelengths (Λ < 20λ). The accuracy of the scalar treatment degrades as either the index of refraction, the depth, the number of binary levels, or the angle of incidence is increased. The conclusions are, in general, applicable to nonperiodic as well as other periodic (trapezoidal, two-dimensional) structures.

Homogeneous layer models for high-spatial-frequency dielectric surface-relief gratings: conical diffraction and antireflection designs

Abstract: The validity of various homogeneous layer models for high-spatial-frequency rectangular-groove (binary) dielectric surface-relief gratings is examined for both nonconical and conical diffraction. In each model the grating is described by a slab of uniaxial material with its optic axis parallel to the grating vector. The ordinary and principal extraordinary indices of the slab depend on the grating filling factor, the substrate and cover refractive indices, and the ratio of the wavelength to the grating period. These indices can be determined by solving two transcendental equations. Higher-order indices are defined as the exact solution to these equations. Second-order indices (second-order dependence on the wavelengthto- period ratio) and first-order indices (no dependence on the wavelength-to-period ratio) are defined by approximate solutions to these equations. Layer models using higher-order and second-order indices are shown to be accurate for high-spatial-frequency gratings, even at wavelength-to-period ratios near the onset of higher-order propagating diffracted waves. These models are used to design example antireflecting gratings on silicon substrates, including designs for conical incidence. All designs are evaluated and optimized by exact rigorous coupled-wave analysis.

Inverse problems in the mechanics of materials : an introduction

Abstract: Elasticity and plasticity fracture and damage conservation laws dynamic fracture inverse problems in vibrations diffraction of elastic waves diffraction of acoustic waves photothermal detection tomography microgravity identification of materials residual stresses. (Part contents).

Quasicrystals as a hierarchy of clusters

Abstract: Diffraction approaches suggest that some quasicrystal structures can be described as hierarchical packing of atomic clusters. When treated as spherical potentials, these clusters may contain ``magic numbers'' of electrons which allow exact filling of the electronic shells of the hierarchy of wells. Physical propeties such as high resistivity and diamagnetism are consistent with the model. Formation of quasicrystals at very critical composition is also explained.

Loss of spatial coherence by a single spontaneous emission.

Abstract: We have demonstrated the loss of transverse spatial coherence of an atomic wave function after a single spontaneous emission. ${\mathrm{He}}^{*}$ atoms were both diffracted and excited by a standing light wave with a variable period. After the interaction, the excited atoms decay by a single spontaneously emitted photon. By changing the period of the standing light wave, we have mapped the loss of spatial coherence as a function of the transverse coordinate. By detecting the emitted photon one could "erase" the position information available and recover the transverse coherence in a correlation experiment, or realize a Heisenberg microscope.