Showing papers on "Diffraction published in 1992"

New principle for optical filters

Abstract: A theoretical characterization of the guided‐mode resonance properties of planar dielectric waveguide gratings is presented. Efficient exchange of energy between forward and backward propagating diffracted waves is shown to be possible with smooth line shapes. The linewidths of the resonances can be controlled by the grating modulation amplitude. Due to the inherent separation between the TE and TM modes associated with the waveguide grating, these elements can provide polarization separation. Applications to polarization sensitive filtering and to electro‐optic switching are suggested. The guided‐mode resonance filter represents a basic new optical element.

Surface X‐Ray Diffraction

Abstract: A general introduction to x-ray diffraction and its application to the study of surfaces and interfaces is presented. The application of x-ray diffractkm to various problems in surface and interface science is illustrated through five different techniques: crystal truncation rod analysis, two-dimensional crystallography, three-dimensional structure analysis, the evanescent wave method and lineshape analysis. These techniques are explained with numerous examples from recent experiments and with the aid of an extensive bibliography.

Structural refinement of superlattices from x-ray diffraction.

Abstract: We present a general procedure for quantitative structural refinement of superlattice structures. To analyze a wide range of superlattices, we derived a general kinematical diffraction formula that includes random, continuous, and discrete fluctuations from the average structure. We show that only the structure factor of one single layer of each material has to be averaged over the random variables and prove that this relation is equivalent to earlier, less general models. Implementing a nonlinear-fitting algorithm to fit the entire x-ray-diffraction profile, refined parameters that describe the average superlattice structure and deviations from this average are obtained. We compare the results of structural refinement to results obtained independently from other measurements. The roughness introduced artificially during growth in Mo/Ni and Nb/Cu superlattices is accurately reproduced by the refinement. The lattice parameters of Ag/Mn obtained from this refinement procedure are in very good agreement with the values obtained from independent extended x-ray-absorption fine-structure and x-ray photoelectron diffraction studies. The relative thicknesses of the layers can be accurately determined, as proved for Cu/Ni in comparison with chemical analysis, for W/Ni compared to the calibrated sputtering rate, and for Mo/Ni compared to the low-angle profile.

Time-reversal of ultrasonic fields. III. Theory of the closed time-reversal cavity

Abstract: For pt.II see ibid., vol.39, no.5, p.567-78 (1992). A theoretical model for time-reversal cavities to optimize focusing in homogeneous and inhomogeneous media is described. The concept of the cavity can be understood as the most realistic approximation to an exact three-dimensional (3-D) time-reversal of ultrasonic fields; it is also a generalization of the time-reversal mirrors realized experimentally in the laboratory. The proposed method is based on an approach in the transient regime that is more general than the monochromatic formalism used in optics to analyze the phase conjugation mirrors efficiency. This method uses impulse diffraction theory to obtain the impulse response of the cavity in any inhomogeneous medium. An original interpretation of the limitations due to diffraction observed in wave field propagation in terms of the different waves generated inside the cavity is also proposed. The time-reversal focusing process using a closed cavity in a weakly inhomogeneous medium is compared with more classical techniques to compensate wavefront distortions, thus illustrating the focusing improvement due to the time-reversal method. >

Screw Dislocations in Light Wavefronts

Abstract: We discuss screw dislocations of a phase surface as the one type of wavefront of a monochromatic wave. The simple method for construction of the optical wavefronts with an isolated screw dislocation is reported. Laser beams with the dislocations of different orders were experimentally achieved by using diffraction on computer-synthesized gratings.

Medium-range order in glasses: Comparison of Raman and diffraction measurements.

Abstract: The correlation lengths of vitreous structures estimated from the low-frequency (boson) peak in Raman spectra and from the width of the first sharp diffraction peak are compared. This first comparison shows that the two estimates, which are based on dynamic and static properties, respectively, are in good quantitative agreement. The present result supports the concepts that (a) a decisive scale for structural order in glasses is \ensuremath{\sim}10 \AA{}, and (b) the low-frequency excitations are defined by the characteristic length in vitreous structures.

Accuracy of pair distribution function analysis applied to crystalline and non-crystalline materials

Abstract: Pair distribution function (PDF) analysis of neutron or X-ray powder diffraction data is a useful technique for analysis of short-range structure in both amorphous and crystalline materials. Errors in PDF determinations may arise from several sources: termination of the Fourier transform, lack of instrument resolution, counting statistics and inaccurate corrections for experimental artifacts. Estimates of the amount of error from termination and instrument resolution are computed using a model structure. A general method for estimating the expected contribution of statistical error to the PDF is introduced for the first time. The effect of termination varies with the type of material and with the amplitude of lattice vibrations but, in general, termination with Q > 30 A produces minimal errors. Broadening of the diffraction pattern produces negligible effect for conventional instrumentation. With moderate data-collection times, the statistical errors can be reduced to reasonable levels. Pulsed-neutron diffraction can provide accurate and precise PDF measurements, as is demonstrated in this work by the agreement between model and experimental results for polycrystalline aluminium.

Experimental verification of nondiffracting X waves

Abstract: The propagation of acoustic waves in isotropic/homogeneous media and electromagnetic waves in free space is governed by the isotropic/homogeneous (or free space) scalar wave equation. A zeroth-order acoustic X wave (axially symmetric) was experimentally produced with an acoustic annular array transducer. The generalized expression includes a term for the frequency response of the system and parameters for varying depth of field versus beam width of the resulting family of beams. Excellent agreement between theoretical predictions and experiment was obtained. An X wave of finite aperture driven with realizable (causal, finite energy) pulses is found to travel with a large depth of field (nondiffracting length). >

A detailed analysis of the optical beam deflection technique for use in atomic force microscopy

Abstract: A Michelson interferometer and an optical beam deflection configuration (both shot noise and diffraction limited) are compared for application in an atomic force microscope. The comparison shows that the optical beam deflection method and the interferometer have essentially the same sensitivity. This remarkable result is explained by indicating the physical equivalence of both methods. Furthermore, various configurations using optical beam deflection are discussed. All the setups are capable of detecting the cantilever displacements with atomic resolution in a 10 kHz bandwidth.

A suite of programs for calculating x‐ray absorption, reflection, and diffraction performance for a variety of materials at arbitrary wavelengths

Abstract: One of the most useful characteristics of synchrotron radiation is the wide spectral distribution of the source. For applications involving tuned monochromatic beams it is often helpful to predict the x‐ray optical characteristics of a sample or the beam line optics at a particular wavelength. In contrast to this desire stands the fact that tabulated values for the optical parameters of interest are generally available only at wavelengths corresponding to typical x‐ray tube sources. We have developed a suite of fortran programs which calculate photoabsorption cross sections and atomic scattering factors for materials of arbitrary, uniform composition or for arbitrary layered materials. Further, the suite includes programs for calculation of x‐ray diffraction or reflection from such materials. These programs are of use for experimental planning, data analysis, and predictions of performance of beam line optical elements.

Diffraction Effects in Semiclassical Scattering

Abstract: 1. Critical effects in semiclassical scattering 2. Diffraction and coronae 3. The rainbow 4. The glory 5. Mie solution and resonances 6. Complex angular momentum 7. Scattering by an impenetrable sphere 8. Diffraction as tunnelling 9. The Debye expansion 10. Theory of the rainbow 11. Theory of the glory 12. Near-critical scattering 13. Average cross sections 14. Orbiting and resonances 15. Macroscopic applications 16. Applications to atomic, nuclear and particle physics References Index.

Diffraction Effects in Semiclassical Scattering

Abstract: 1. Critical effects in semiclassical scattering 2. Diffraction and coronae 3. The rainbow 4. The glory 5. Mie solution and resonances 6. Complex angular momentum 7. Scattering by an impenetrable sphere 8. Diffraction as tunnelling 9. The Debye expansion 10. Theory of the rainbow 11. Theory of the glory 12. Near-critical scattering 13. Average cross sections 14. Orbiting and resonances 15. Macroscopic applications 16. Applications to atomic, nuclear and particle physics References Index.

Step-density variations and reflection high-energy electron-diffraction intensity oscillations during epitaxial growth on vicinal GaAs(001).

Abstract: Systematic measurements and simulations are reported for the transition from growth by nucleation and accretion of two-dimensional islands to step advancement on misoriented GaAs(001) surfaces during epitaxial growth. The growth conditions have been chosen in order to satisfy as much as possible the underlying assumptions of a solid-on-solid model of epitaxial growth, namely, that the adatom mobility is isotropic (by using a surface misoriented toward the [010] direction), the effect of the As is not rate determining (by using a As/Ga ratio of 2.5), and that the presence of any surface reconstruction can be subsumed in effective migration parameters, i.e., that no explicit account of surface reconstruction is required if the reconstruction does not change (by maintaining the 2\ifmmode\times\else\texttimes\fi{}4 reconstruction). The diffraction conditions for reflection high-energy electron-diffraction (RHEED) measurements were chosen to eliminate as much as possible the contribution of well-known incoherent features to the specular intensity on GaAs(001). Using these growth and diffraction conditions, the parameters of a solid-on-solid model have been optimized by performing extensive simulations to quantitatively reproduce the measured misorientation-angle dependence and the Ga flux dependence of the growth-mode transition temperature. Since at the chosen diffraction conditions, the kinematic diffraction is insensitive to surface morphologies, we have modeled the growth-induced loss of intensity from the specular beam as being due to the steps on the surface, with each step acting as an individual source of scattering. Direct comparisons between the time-dependent density of surface steps of the simulated surfaces and the RHEED specular intensity profiles measured during growth reveal several qualitative and quantitative similarities. The most striking of these is that the two quantities show the same relative change of magnitude with time and temperature for a given misorientation and Ga flux. The implications of these comparisons are discussed for the growth dynamics of GaAs(001), for the scattering processes in RHEED, and the morphological sensitivity of RHEED for these diffraction conditions.

Efficient generation of nearly diffraction-free beams using an axicon

Abstract: ZnSe axicon is used to efficiently generate approximations to diffraction-free Bessel function beams at λ = 10.6 μm. The central spot radius of ~ 170 μm generated when the axicon is illuminated by a plane wave is maintained for distances of up to 11 cm with sufficient laser power to allow drilling of plexiglass to be demonstrated.

Diffraction anomalous fine structure: A new x-ray structural technique.

Abstract: A new x-ray structural technique, diffraction anomalous fine structure (DAFS), which combines the long-range order sensitivity of diffraction techniques with the short-range order sensitivity of absorption techniques, is described. We demonstrate that synchrotron DAFS measurements for the Cu(111) and Cu(222) Bragg reflections provide the same local atomic structural information as x-ray absorption fine structure and describe how DAFS can be used to provide enhanced site and spatial sensitivities for polyatomic and/or spatially modulated structures

Measurement-induced diffraction and interference of atoms

Abstract: The position of an atom passing through a standard light wave is localized by making a quadrature phase measurement on the light field. This localization can be thought of as the creation of a virtual slit (or slits) for the atom by the field measurement. Diffraction and interference behavior may be observed in the far field.

Fourier-transform holographic microscope

Abstract: We describe a holographic microscope with a spatial resolution approaching the diffraction limit. The instrument uses a tiny drop of glycerol as a lens to create the spherically diverging reference illumination necessary for Fourier-transform holography. Measurement of the point-spread function, which is obtained by imaging a knife edge in dark-field illumination, indicates a transverse resolution of 1.4 μm with wavelength λ = 514.5 nm. Longitudinal resolution is obtained from the holograms by the numerical equivalent of optical sectioning. We describe the method of reconstruction and demonstrate the microscope’s capability with selected biological specimens. The instrument offers two unique capabilities: (1) it can collect three-dimensional information in a single pulse of light, avoiding specimen damage and bleaching; and (2) it can record three-dimensional motion pictures from a series of light pulses. The conceptual design is applicable to a broad range of wavelengths and we discuss extension to the x-ray regime.

Two-Dimensional Crystallography of Amphiphilic Molecules at the Air–Water Interface

Abstract: The advent of well-collimated, high-intensity synchrotron X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air–liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, α-amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X-ray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.

Electron diffraction techniques

Abstract: S. Amelinckx & D. van Dyck: EM imaging and diffraction contrast K. Yagi: Rheed and REM J. Gjonnes: Disorder and defect scattering S. Amelinckx & D. van Dyck: Electron diffraction effects due to modulated structures M. Carr & C. Lyman: Identifications of unknowns Index.

Laser diffraction determination of the crystalline structure of an electrorheological fluid.

Abstract: The electric-field-induced structures of small glass spheres in silicone oil produce laser diffraction patterns which verify, for the first time, the body-centered-tetragonal lattice structure predicted by theory. By n mechanism totally unlike that of conventionnl x-ray scattering, the incident laser beam propagates through the lattice of close-packed spheres via stable optic modes along regular arrays of transparent spheres, and then produces diffraction patterns after exiting from the lattice

Polymer dispersed liquid crystal panel with diffraction grating

Abstract: A layer having an irregular sectional surface is formed on the surface of at least one of a pixel electrode and confronting electrode of a liquid crystal layer so as to confront a liquid crystal layer. The liquid crystal layer is formed of a polymer dispersed liquid crystal. If the layer having an irregular sectional surface is formed in a convex and concave pattern regularly and cyclically, the thus formed layer serves as a diffraction grating. When the liquid crystal is in an OFF condition, the refractive index n of the diffraction grating is different from the refractive index of the liquid crystal layer. As a result, an incident light to the liquid crystal panel is scattered and at the same time, diffracted. When the liquid crystal in in an ON condition, there is no difference in the refractive index between the diffraction grating and the liquid crystal, and the incident light passes straight through. Using the liquid crystal panel as a light valve, the light from a light source is incident to the liquid crystal panel, and the image formed on the liquid crystal panel is projected through a projection lens onto a screen. In the case of a black display, the incident light is scattered or diffracted in the liquid crystal panel according to the projected image. The scattered or diffracted light is stopped by a light shielding part of a aperture, and thus does not arrive at the screen.

Elastic wave propagation simulation in the presence of surface topography

Abstract: SUMMARY We present a 2-D numerical modelling algorithm based on a pseudo-spectral method which accounts for surface topography. The modelling scheme uses Fourier derivative operators for spatial differencing in the horizontal direction, whereas a Chebyshev operator is used for vertical derivatives. The incorporation of surface topography is achieved by mapping a rectangular grid onto a curved grid. Modelling of surface topography is important to study near-surface effects of wave propagation in field seismic situations, since diffraction and scattering at rough surfaces are non-ray effects and can only be understood as wave phenomena. Static corrections cannot always account for these effects and some care has to be taken in their interpretation. The presented modelling algorithm can serve as a powerful tool for the study of wave propagation phenomena in the vicinity of non-planar surfaces or interfaces.

A modal analysis of lamellar diffraction gratings in conical mountings

Abstract: A rigorous modal analysis of lamellar grating, i.e., gratings having rectangular grooves, in conical mountings is presented. It is an extension of the analysis of Botten et al. which considered non-conical mountings. A key step in the extension is a decomposition of the electromagnetic field in the grating region into two orthogonal components. A computer program implementing this extended modal analysis is capable of dealing with plane wave diffraction by dielectric and metallic gratings with deep grooves, at arbitrary angles of incidence, and having arbitrary incident polarizations. Some numerical examples are included.

Design and first applications of a post-column imaging filter

Abstract: We have designed and built an imaging filter which can be attached to most standard TEMs, and is capable of operating at primary energies of up to 400 keV. The filter uses a 90° magnetic sector prism, a piezoelectrically controlled energy-selecting slit, and 6 quadrupole and 5 sextupole lenses. It fully corrects second-order aberrations and distortions in images formed with electrons of selected energies, and it also produces second-order aberration-corrected spectra of variable dispersion. We show the first applications of the filter at 200 keV primary energy, which indicate that the filter will excel in chemical mapping and spectroscopy, in improving contrast of electron images and diffraction patterns, and in making high resolution electron microscopy and diffraction more quantitative. We conclude the paper with a discussion about atomic resolution image formation using inelastically scattered electrons, and the relationship between energy-filtered diffraction patterns and images formed with electrons of the same energy.

Diffraction of cylindrical and plane waves by an array of absorbing half-screens

Abstract: Multiple forward diffraction past an array of many absorbing half-screens whose separation is large compared to wavelength is examined. Starting with the physical optics approximation for half-planes that are equally spaced and of equal height, the field incident on successive edges is represented by a multidimensional Fresnel integral, which is then expanded into a series of functions studied by Boersma (1978). When the angle of incidence with respect to the plane containing the edges is small, each edge is in the transition region of the previous edge, which precludes the use of the geometrical theory of diffraction and related asymptotic theories. The solution obtained applies for incidence either from above or below the plane containing the edges, and is especially suited to the case of near-grazing incidence. This method of solution allows for numerical evaluation of a large number of half-screens and shows how the multiple diffracted fields are influenced by the physical parameters. Both incident plane waves and incident cylindrical waves can be treated. >

Diffraction optics of complex-structured periodic media

Abstract: This work discusses the interaction of waves (which may represent x-rays, gamma rays, electrons or neutrons) with various kinds of ordered media. Aspects covered include exact and approximate solutions to scattering problems, diffraction of Moessbauer radiation in magnetically ordered crystals, the optics of chiral liquid crystals, the radiation of fast particles in regular media (Cherenkov radiation), nonlinear optics of periodic media, neutron scattering in magnetically ordered media, polarization phenomena in x-ray optics, magnetic x-ray scattering and Moessbauer filtration of synchrotron radiation.

Spatial solitons in photorefractive media

Abstract: We present a new type of spatial soliton, generated by the photorefractive (PR) effect of the medium. The shape of the soliton modulates the refractive index via the PR effect, which results in an exact compensation for the effects of diffraction, and causes the light beam to propagate the unvarying profile. This index modulation is represented in the formalism as a distribution of index gratings, each one of them induced by the interference between two spatial (frequency) plane wave components of the light beam. Since the efficiency of this effect is independent of absolute light intensity, these new solitons can be generated even at very moderate light intensities. Moreover, a given soliton waveform can propagate unchanged in the medium, at very high or very low light intensities (and at all levels in between).