Showing papers on "Diffraction published in 1987"

Diffraction-free beams.

Abstract: It was recently predicted that nondiffracting beams, with beam spots as small as a few wavelengths, can exist and propagate in free space. We report the first experimental investigation of these beams.

Spatial dissipative structures in passive optical systems

Abstract: We consider a nonlinear, passive optical system contained in an appropriate cavity, and driven by a coherent, plane-wave, stationary beam. Under suitable conditions, diffraction gives rise to an instability which leads to the emergence of a stationary spatial dissipative structure in the transverse profile of the transmitted beam.

X-ray diffraction studies of the structure of nanometer-sized crystalline materials

Abstract: Recently, nanometer-sized crystalline materials have been proposed to represent a new solid-state structure which exhibits neither long-range order (like crystals) nor short-range order (like glasses). It was the purpose of this study to test this idea by x-ray diffraction experiments. Nanometer-sized crystalline materials are polycrystals in which the size of the crystallites is a few (1--10) nanometers. Structurally, these materials consist of the following two components, the volume fraction of which is about 50% each: a crystalline component, formed by all atoms located in the lattice of the crystallites, and an interfacial component comprising the atoms situated in the interfaces. It is this interfacial component which was proposed to exhibit an atomic arrangement without short- or long-range order. In order to test this idea, the interference function of nanometer-sized crystalline iron (6-nm crystal size) was measured by x-ray diffraction. The measured interference function was compared with the interference function computed by assuming the interfacial component to be short-range ordered or to consist of randomly displaced atoms (no short- or long-range order). It was found that the experimental interference function can only be matched by the computed one if the interfacial component is assumed to have no short- or long-range order.

Static compression of H2O-ice to 128 GPa (1.28 Mbar)

Abstract: The high-pressure behaviour of H2O is of fundamental importance in both condensed matter and planetary physics1,2. The hydrogen bonding in this system gives rise to a variety of phases at low pressures and temperatures (that is, 100 GPa) where a variety of stable phases, including the metallic form, have been proposed9. Information on the properties of H2O at these pressures has been limited to the results of shock-wave experiments, which probe the fluid phase at high pressures and temperatures10, and to theoretical statistical electron calculations11–15. In this study we have compressed ice in a diamond-anvil cell to 128 GPa and measured the molar volume as a function of pressure by synchrotron X-ray diffraction techniques. The diffraction data are consistent with the body-centred cubic (b.c.c.) oxygen sublattice of ice VII persisting to the highest pressures of our measurements. The measured equation of state indicates that ice is less compressible at very high pressures than is suggested by recent experiments in the 30–50 GPa range, but more compressible than statistical electron and recent pair-potential models predict.

A tunnel in the large ribosomal subunit revealed by three-dimensional image reconstruction

Abstract: A better understanding of the molecular mechanism of protein biosynthesis depends on the availability of a reliable model for the ribosome particle. The application of a diffraction technique, namely, three-dimensional image reconstruction from two-dimensional sheets of the large ribosomal subunits of Bacillus stearothermophilus at a resolution of 30 angstroms is described. The resulting three-dimensional model shows at least four projecting arms, arranged radially near the presumed interface with the 30S subunit. The projecting arms are positioned around a cleft, which turns into a tunnel with a length of 100 to 120 angstroms and a diameter of up to 25 angstroms. This tunnel spans the particle and may provide the path taken by the nascent polypeptide chain.

X-ray diffraction computed tomography

Abstract: Coherent scattering of x-ray photons leads to the phenomenon of x-ray diffraction, which is widely used for determining atomic structure in materials science. A technique [x-ray diffraction computed tomography (CT)] is described, analogous to conventional CT, in which the x-ray diffraction properties of a stack of two-dimensional object sections may be imaged. The technique has been investigated using a first generation (single pencil beam) CT scanner to measure small angle coherent scatter, in addition to the customary transmitted radiation. Diffraction data from a standard CT performance phantom obtained with this new technique and with an x-ray diffractometer are compared. The agreement is satisfactory bearing in mind the poor momentum resolution of our apparatus. The dose and sensitivity of x-ray diffraction CT are compared with those of conventional transmission CT. Diffraction patterns of some biological tissues and plastics presented in a companion paper indicate the potential of x-ray diffraction CT for tissue discrimination and material characterization. Finally, possibilities for refinement of the technique by improving the momentum resolution are discussed.

X-ray diffraction studies of organic monolayers on the surface of water.

Abstract: We have used synchrotron radiation to study organic monolayers on water (''Langmuir films''). At high monolayer pressures, lead stearate (Pb(C/sub 17/H/sub 35/COO)/sub 2/) shows a powder peak at 1.60 A/sup -1/, implying an area per unit cell of 17.8 A/sup 2/ if the lattice is triangular. The correlation length is about 250 A. Lignoceric acid (C/sub 23/H/sub 47/COOH) shows a similar peak even though no heavy ions are attached. When the pressure is reduced, the peak in lead stearate does not observably move or broaden; below the ''knee'' in the isotherm, however, the peak height decreases slowly with increasing area, implying a first-order melting transition.

Composition and lattice‐mismatch measurement of thin semiconductor layers by x‐ray diffraction

Abstract: X‐ray diffraction methods for determining alloy composition and mismatch have been used for many years by measuring the separation of peaks in a high‐resolution diffractogram. This method can still be used, but not for layer thicknesses below 1–2 μm. The diffractogram may appear simple to interpret, for example, two diffraction peaks from a single layer on a substrate, but simple measurement of peak separation can lead to significant errors in determining the mismatch and hence alloy composition. This paper gives examples of peak shifting with thin layers and shows how, by simulating the diffraction profiles, a reliable determination of mismatch, and hence alloy composition can be made.

Laser Diffraction Spectrometry: Fraunhofer Diffraction Versus Mie Scattering

Abstract: Laser diffraction spectrometry (LDS) is often claimed to operate on the principle of Fraunhofer diffraction. This is only true, however, if particles are large compared to the wavelength of light or if the ratio of the refractive indices of the disperse and continuous phases, m, is clearly different from unity. In this study it has been established that LDS, as applied to particle and droplet sizing in suspensions and emulsions, is based on Miescattering. Scattering patterns of single particles may be calculated if the refractive indices of both phases are known. Thus, a theoretical basis has been provided for the application of LDS to size-measurement in suspensions and emulsions, and for extension of this method to the lower size ranges and those cases in which the refractive indices of the disperse and continuous phases are similar. Extension of the work presented in this paper will enable the calculation of scattering matrices so that calibration of the apparatus with standard materials may be avoided.

Multilayer Optical Learning Networks

Abstract: A new approach to learning in a multilayer optical neural network based on holographically interconnected nonlinear devices is presented. The proposed network can learn the interconnections that form a distributed representation of a desired pattern transformation operation. The interconnections are formed in an adaptive and self-aligning fashioias volume holographic gratings in photorefractive crystals. Parallel arrays of globally space-integrated inner products diffracted by the interconnecting hologram illuminate arrays of nonlinear Fabry-Perot etalons for fast thresholding of the transformed patterns. A phase conjugated reference wave interferes with a backward propagating error signal to form holographic interference patterns which are time integrated in the volume of a photorefractive crystal to modify slowly and learn the appropriate self-aligning interconnections. This multilayer system performs an approximate implementation of the backpropagation learning procedure in a massively parallel high-speed nonlinear optical network.

a Method for Detection of Diffracted Waves on Common-Offset SECTIONS*

Abstract: A method of detection of diffracted waves on common-offset sections is proposed. The method utilizes the main kinematic and dynamic properties of the diffracted waves. The detection algorithm is defined by an automatic procedure including phase correlation of the diffracted waves and the application of certain statistical criteria. This procedure enables one to make decisions with regard to the presence of the diffracted waves and also to estimate parameters of the scattering objects. The method is applied to synthetic and field data and, even for a relatively low signal-to-noise ratio, it gives reliable results.

Determination of critical layer thickness in InxGa1−xAs/GaAs heterostructures by x‐ray diffraction

Abstract: The critical thickness hc of strained InxGa1−xAs layers grown by molecular beam epitaxy on GaAs(100) substrates is determined by double‐crystal x‐ray diffraction for 0.07≤x≤0.25. The experimental results are in good agreement with critical thicknesses calculated from the energy balance model of R. People and J. C. Bean [Appl. Phys. Lett. 47, 322 (1985)] but differ from prior photoluminescence measurements of hc for this material. Beyond the critical thickness the transition from the strained to the relaxed state occurs more rapidly as the In concentration and hence the lattice mismatch increases.

Electromagnetic theory of diffraction by a circular aperture in a thick, perfectly conducting screen

Abstract: A rigorous electromagnetic theory of the diffraction of radiation by a circular aperture in a thick screen is developed. In particular, the case of an incident plane wave is considered, and the effects of varying the thickness of the screen and of varying the wavelength, polarization, and angle of incidence of the incident wave on the reflection and transmission properties of the screen are investigated.

Coherent laser addition using binary phase gratings

Abstract: Binary phase diffraction gratings are shown to couple light coherently from a laser array into a single on-axis beam. The diffraction grating, designed to split a single beam into a specific number of equal intensity diffraction orders, is placed inside the cavity formed by the laser array and a common output mirror. The grating superimposes the light beams from the lasers in the array and produces a far-field pattern with the same divergence as that of a single laser. Six GaAlAs lasers from an antireflection-coated linear array were combined with a coupling efficiency of 68.4%. The far field of the combined GaAlAs lasers consisted of a single on-axis Gaussian beam.

On the validity of the ray approximation for interpreting delay times

Abstract: This chapter deals with the influence of diffracted waves on tomographic observations. Probably most workers in tomography are aware of the existence of such waves and of their potential to interfere with the direct wave on which the tomographic interpretation is based. Strong diffraction effects are expected when the scale of heterogeneity is in the order of the seismic wavelength, but the smallest detail one wants to resolve with seismic tomography is typically one or two orders of magnitude larger. Can we safely neglect diffraction in this case?

Vacuum Structure and Diffraction Scattering

Abstract: A simple model of the vacuum is developed in order to explain the properties of the pomeron observed in experiments. In the model, pomeron exchange corresponds to two-gluon exchange. We explain how it can be that when the pomeron couples to a hadron, the two gluons couple predominantly to the same quark in the hadron, and how together their coupling is similar to that of aC=1 isoscalar photon.

X-ray diffraction measurements of some plastic materials and body tissues

Abstract: X‐ray diffraction allows the investigation of the atomic or molecular structure of materials. The combination of diffractometry with computerized tomography enables spatially resolved imaging of the diffraction properties of extended objects as described in more detail in a companion article [Harding e t a l., Med. Phys. 1 4, 515 (1987)]. We present measured diffraction patterns of some plastics and several biological materials, which allow further optimization of our method and the selection of suitable application areas.

Scattering of plane harmonic P, SV, and Rayleigh waves by dipping layers of arbitrary shape

Abstract: Diffraction of plane harmonic P, SV , and Rayleigh waves by dipping layers of arbitrary shape is investigated by using an indirect boundary integral equation approach. The layers are of finite length perfectly bonded together. The material of the layers is assumed to be homogeneous, weakly anelastic, and isotropic. The displacement field is evaluated throughout the elastic medium so that the continuity of the displacement and the traction fields along the interfaces between the layers is satisfied in a least-squares sense. Presented numerical results show that the surface strong ground motion amplification effects depend upon a number of parameters present in the problem, such as, type, frequency, and angle of the incident wave, the impedance contrast between the layers, the component of the displacement field being observed, and the location of the observation point at the surface of the half-space. The results demonstrate that the presence of soft alluvial deposits in form of dipping layers may cause locally very large amplification or reduction of the surface ground motion when compared with corresponding free-field motion.

Quantitative X-Ray Powder Diffraction Method Using the Full Diffraction Pattern

Abstract: A new quantitative X-ray powder diffraction (QXRPD) method has been developed to analyze polyphase crystalline mixtures. The unique approach employed in this method is the utilization of the full diffraction pattern of a mixture and its reconstruction as a weighted sum of diffraction patterns of the component phases. To facilitate the use of the new method, menu-driven interactive computer programs with graphics have been developed for the VAX series of computers. The analyst builds a reference database of component diffraction patterns, corrects the patterns for background effects, and determines the appropriate reference intensity ratios. This database is used to calculate the weight fraction of each phase in a mixture by fitting its diffraction pattern with a least-squares best-fit weighted sum of selected database reference patterns.The new QXRPD method was evaluated using oxides found in ceramics, corrosion products, and other materials encountered in the laboratory. Experimental procedures have been developed for sample preparation and data collection for reference samples and unknowns. Prepared mixtures have been used to demonstrate the very good results that can be obtained with this method.

Simple x-ray standing-wave technique and its application to the investigation of the Cu(111) ( √3 √3 )R30 ° -Cl structure

Abstract: A new variant of the usual x-ray standing-wave experiment, scanning the Bragg reflection in energy at normal incidence, is shown to be applicable to metal crystals without special precautions being taken to ensure high crystalline perfection. The structure of the Cu(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-Cl chemisorption phase is found to be compatible with the results of a previous surface extended x-ray-absorption fine structure and photoelectron diffraction study.

Six-coordinated silicon in glasses

Abstract: It has been accepted since 1932 that the building block of silicate glasses consists of silicon tetrahedrally coordinated to 4 oxygen atoms1 Although the existence of 6-coordinated silicon in a few crystalline materials, such as stishovite, and SiP2O7 is known2,3, the presence of (SiO6) units has not yet been observed experimentally in glasses Recent X-ray diffraction and magic-angle-spinning (MAS) nuclear magnetic resonance investigations4,5 of SiO2-P2O5 glasses found only 4-coordinated silicon although SiP2O7 was produced by devitrification Raman studies6 of Na2O-SiO2-P2O5 glasses were interpreted as indicating the presence of a structural unit containing Si-O-P bonding but otherwise unidentifiable We have applied MAS nuclear magnetic resonance spectroscopy to similar glasses and here we report the first observation of [SiO6] units in glasses

Crystallinity of rf-sputtered MoS2 films

Abstract: The crystallinity and morphology of thin, radio-frequency (rf) -sputtered MoS2 films deposited on 440C stainless steel substrates at both ambient (∼70°C) and high temperatures (245°C) were studied by scanning electron microscopy (SEM) and by x-ray diffraction (Read thin-film photography and 0−20 scans). Under SEM the films exhibited a “ridgelike” (or platelike) formation region for thicknesses between 0.18 and 1.0 μm MoS2. X-ray diffraction was shown to give more detailed and accurate information than electron defraction, previously used for elucidating the structure of sputtered lubricant films. Read thin-film x-ray diffraction photographs revealed patterns consistent with the presence of polycrystalline films and strong orientation of the MoS2 crystallites. Correlation of those patterns with 0−20 scans of the films indicated that the basal planes of the MoS2 crystallites [i.e., the (001) planes] were perpendicular to the substrate surface plane, and that various edge planes [i.e., the (h k 0) planes] in the individual crystallites were parallel to the surface plane, in agreement with previous observations of thinner films. Sliding wear caused the crystallites to orient with their basal planes parallel to the surface plane. The crystallite lattices in all films in this study were shown to exhibit compressive stress (∼ 3%–5% with respect to natural molybdenite) in the direction perpendicular to the (h k 0) planes, and the worn films were expanded (i.e., exhibited tensile stress) perpendicular to the (001) plane. In addition, the shapes of the x-ray diffraction peaks were strongly influenced by the presence of oxygen impurities and/or sulfur vacancies in the MoS2 lattice, indicating that x-ray diffraction may provide a simple quality-control test for the production of a film with optimum lubricating properties.

Method for aligning first and second objects, relative to each other, and apparatus for practicing this method

Abstract: According to this invention, a method and apparatus for aligning a mask and a wafer arranged to oppose each other, in a direction along their opposing surfaces, relative to each other, are arranged as follows. A first diffraction grating as a one-dimensional diffraction grating, bars of which extend in a direction perpendicular to an alignment direction, is formed on the mask. A second diffraction grating which has a checkerboard-like pattern, is formed on the wafer. The first diffraction grating is irradiated with laser beam emitted from a light source. Light beams diffracted and transmitted through the first diffraction grating is transferred to the second diffraction grating. Light beams diffracted and reflected by said second diffraction grating are transferred to said first diffraction grating. The light beams are diffracted by and transmitted through said first diffraction grating, again. Is detected, one of the diffracted light beams, which do not propagate along a predetermined plane. The light beams reflected and diffracted by the surface of the first diffraction grating, is transferred only in the predetermined plane. For this reason, the detected diffracted light beam will not interfere with reflected diffracted light beams. The mask and wafer can be precisely aligned relative to each other, in accordance with the intensity of the detected diffracted light beam.

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

Abstract: The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates. We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth. The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.

Diffraction immunoassay apparatus and method

Abstract: A diffraction immunoassay method in which a diffraction grating design of non-light disturbing primary binding reagent on an insoluble surface is conjugated with analyte in a sample. If the primary binding reagent-analyte conjugate is light disturbing, a diffraction grating is formed. If the primary binding reagent-analyte conjugate is non-light disturbing, the analyte is further conjugated with a secondary binding reagent which is labeled with a light disturbing material to form a diffraction grating. Light from a narrow band light source is then applied to the surface, and the intensity of beams of diffracted light formed by the diffraction grating is determined. The diffraction immunoassay plate for the method comprises a smooth insoluble surface having on the surface thereof, a diffraction grating design of lines of the primary binding reagent. The diffraction immunoassay apparatus comprises a light source, a platform for supporting the diffraction immunoassay plates in the path of a beam of light from the light source, and at least one light detector positioned to detect light diffracted by the diffraction grating.