Diffraction Fringes Research Articles

X-ray characterization of In xGa 1−xAs/GaAs quantum wells

X-ray diffraction patterns have been measured for pseudomorphic In x Ga 1− x As/GaAs quantum well (QW) structures of widths 183, 121 and 61 ∢, and composition x of about 0.28 grown by molecular beam epitaxy. The measured patterns show Pendellösung fringes due to the 513 ∢ GaAs barriers. These Pendellösung fringes are modulated by a broad weak peak mostly coming from the 183 Å In x Ga 1− x As well layer. The X-ray diffraction fringe modulating patterns between 32° and 33° are fairly symmetric for a non-interrupted growth procedure. The diffraction pattern for the same structures with interrupted growth is less symmetric, since there is further modulation by another weaker peak. This shows that the In x Ga 1− x As well layers have less well controlled In composition if grown with interruption times of tens of seconds at each interface. The thickest In x Ga 1− x As well layer exhibits the highest In composition. Layer thickness and In composition in thin layers (around 100 ∢ in the thickness) can be estimated by X-ray modeling and good agreement obtained between the measured and the calculated X-ray diffraction patterns.

Geometrical optics approach to the nematic liquid crystal grating: numerical results

The problem of the grating action of a periodically distorted nematic liquid crystal layer, in the geometrical optics ray approximation is considered, and a theory for the calculation of the fringe powers is proposed. A nonabsorbing nematic phase is assumed, and the direction of incidence is taken to be normal to the layer. The powers of the resulting diffraction fringes are related to the spatial and angular deviation of the rays propagating across the layer and to the perturbation of the phase of the wave associated with the ray. The theory is applied to the simple case of a harmonically distorted nematic layer. In the case of a weakly distorted nematic layer the results agree with the predictions of Carroll's model, where only even-order fringes are important. As the distortion becomes larger, odd-order fringes (with the exception of the first order) become equally important, and particularly those at relatively large orders (e.g., seven and nine) exhibit maxima greater than that of the even-order neighbors. Finally, the dependence of the powers of odd-order fringes on the distortion angle is quite different from that of the even-order fringes.

The use of wire chambers in structural biology

Position-sensitive neutron detectors with high detection efficiency and high spatial resolution are essential for neutron protein crystallography as well as for small-angle neutron scattering on large biological assemblies. Here we report on the performance of two wire chambers recently developed by the Brookhaven Instrumentation Division. Both detectors have a spatial resolution of about 1.3 mm, a detection efficiency of about 80%, and excellent differential and integral linearities. Using small-angle data as an example, we show that the new detectors, furthermore, have excellent temporal stability and can provide difference spectra of unparalleled accuracy. The Biology small-angle spectrometer, because of its location on the constrained reactor floor, has a short secondary flight path of 2 m. High detector resolution is therefore essential to preserve the ability to measure at small angles and to record narrowly spaced diffraction fringes. In addition, we demonstrate in detail that the state-of-the-art detector makes it possible to extend the precision and resolution of a protein single-crystal data set. By measuring beam divergence, wavelength spread, and the crystal mosaic in auxilliary experiments, a set of parameters is obtained for the calculation of a spatial filter for each diffraction spot.

An atomic resolution study of precipitates in an Al-Ge Alloy

The difficulty of precipitating germanium in dilute aluminum-germanium alloys is due to a large difference in crystal structures (face-centered cubic and diamond cubic) accompanied by a substantial volume expansion of 36%. A great variety of precipitate morphologies and orientation relationships are observed. A frequently found morphology is that of <100> needles. By selected area diffraction and Moire fringes it has been established that <100> Al and <110> Ge are parallel along the needle axis. The typical aspect ratio of about 100 has made it difficult in the past to investigate the cross-sectional shape and internal structure of these needles, although some indications of internal twinning were found in plates. in the present work, the Berkeley Atomic Resolution Microscope was used to examine needles in cross section by imaging along the <110> Ge <100> Al needle axis.

Voidites in diamond Evidence for a crystalline phase containing nitrogen

Abstract Small octahedral bubbles (′voidites′), 10–100 A in diameter, occur in some type Ia diamonds. In a study of one such specimen, electron diffraction and moire fringe analysis have been used to demonstrate that the bubbles contain a crystalline cubic phase. Energydispersive X-ray microanalysis on the 20 A scale provides evidence that the phase contains nitrogen. The cell constants derived from the electron diffraction data are consistent with X-ray and compressibility data for solid ammonia, although the possibility of a hitherto unreported phase of solid N2 cannot be ruled out.

Microbeam electron diffraction and lattice fringe studies of defect structures in enamel apatites

A microbeam diffraction method in transmission electron microscopy has been applied to lattice structural studies of human and shark enamel apatite crystals. The technique allows diffraction patterns to be obtained from very small regions of the specimen (40 nm in diameter) and transfers minimal energy to the sample during analysis. The presence of crystal defects, nonisolated dislocations with the dislocation line positioned perpendicular to the 100-type planes, was observed in the central part of the crystals. Further experiments with acid-etching of crystals containing such defects showed an initial preferential dissolution at the site of these atomic imperfections. The relative abundance of the crystal defects was approximately the same in both human enamel and shark enameloid.

Spatial and temporal modulation of light obliquely incident on an ultrasonic beam of rectangular cross-section

The modulation wave forms of the Fraunhofer diffraction fringes obtained in the oblique crossing of a collimated light beam with an ultrasonic beam of stationary waves are calculated in the frame of the Raman-Nath theory. The discrepancies with the experimental results (showing minima of the diffraction effects at incidence angles, for which the theory foresees complete vanishing of any diffraction, and unexpected marked asymmetries) are discussed.

Thomas Young's ideas on light diffraction in the context of electromagnetic theory

According to Young, diffraction fringes occur as a result of interference between the incident wave and a wave arising from the edge of a diffracting aperture or body. This approach had even been the one adopted by Fresnel himself at the beginning of his research on diffraction. However, Fresnel was forced to abandon the concept of an edge wave because of two major difficulties which he perceived and felt unable to overcome at that time. We shall examine here these two "difficulties" of Young's theory through their historical development and with regard to some recent results that we have obtained from diffraction experiments as much as from electromagnetic theory. Thus we shall demonstrate that these two "weaknesses" have become two strong evidences for the validity of Young's ideas.

Noise reduction in high‐magnification micrographs by soft focus printing and digital image processing

AbstractSoft focus printing of micrographs, using screens of opaque material with 10–15 lines/cm, reduced contrast due to noise but preserved all other contrasts and all structural details. Appropriate screens generated diffraction fringes of a spacing similar to the distance of the noise structures. The superimposition of fringes and image produced a diffusion effect which reduced only the contrast of the noise background. Blurring, screening or fogging were not effective or suitable for photographic micrograph processing. Soft focus printing improved recognition of weak high magnification contrasts in SEM micrographs of uncoated, gold decorated, and chromium coated cell surfaces of glomerular endothelium in mouse kidney slices. Effects similar to the optical diffusion were obtained by digital image processing using weighted low pass filters.

LASER FOURIER IMAGING OF CHEMICAL REACTION. OBSERVATION OF THE DIFFUSION PROCESS OF HEAT EMITTED FROM DYE MOLECULES BY STRIPED LASER EXCITATION

Abstract Excitation of Eosin Y in ethanol solution with irradiated stripes of chopped argon ion laser beam gives thermal fringes which diffract helium-neon laser beam; the resulting diffraction fringes correspond to the Fourier transformation of the thermal fringes. By following the growth and decay of the transient diffraction signals the diffusivity of heat evolved by the photoexcitation in the solution was determined.

Study of Fresnel diffraction fields with the use of television technique

A modified method for the study of Fresnel diffraction fields during lectures in physics is proposed. Television technique is applied. The outputs of a television camera are connected simultaneously to a television set and an oscilloscope. The oscilloscope permits the viewing of the light intensity distributions in the individual diffraction fringes of the patterns observed on the television screen.

The rings of Uranus: Occultation profiles from three observatories

Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15-16 August 1980 are compared. The alpha ring shows a 'double-dip' structure; the eta ring shows a broad and narrow component (similar to Saturn's F ring); and the epsilon ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.

Diffraction Fringes of Light Formed by Semi-transparent Membrane

Diffraction Fringes of Light Formed by Semi-transparent Membrane

Optimal pinhole radius for fundamental mode operation

In order to prevent higher-order transversal mode oscillation, a pinhole has to be inserted into the resonator. The pinhole radius must be adapted to the sizes of the fundamental gaussian beam. This paper deals with the optimal pinhole radius, which minimizes diffraction fringes and delivers the smallest far field divergence. Different laser systems were investigated experimentally. The range of the optimal value s = a/ w, with a = pinhole radius and w = gaussian beam radius, is 1 < s opt < 1.5. It is shown, that the optimal value depends on the unsaturated gain G 0 of the oscillator.

Accurate laser wavelength measurement with a precision two-beam scanning Michelson interferometer

This paper gives the details of a precision two-beam scanning Michelson interferometer, designed and perfected for accurate comparison of an unknown laser wavelength and the precisely calibrated wavelength of a reference laser. An iodine Lamb-dip stabilized He-Ne 633-nm laser (calibrated with respect to a Kr standard) is used as the reference. The design incorporates features to minimize instrumental errors and the effect of fringe shifts caused by diffraction (in the IR). It is applied to accurate measurements of a stable CO(2) laser wavelength tuned to the centers of its various transitions. Measurements are done by simultaneous fringe counting and relative fringe-phase comparison at the two wavelengths using on-line data storage and processing with an electronic digital computer. The accuracy in the 10-microm region is several parts in 10(9) and is limited by correction for diffraction fringe shifts. Because of its low-Q and broadband operating characteristics, it can be applied to rapid accurate laser wavelength measurements over the entire wavelength range permitted by its transmitting optics. In the visible range where the diffraction correction is small, the interferometer can be used to perform measurements to within several parts in 10(11). The paper gives theoretical derivation of various diffraction corrections, the design and construction of the interferometer, the alignment procedures, detailed analysis of various error sources, and data processing. It also gives the details of a previously reported accurate measurement of the speed of light using the measured wavelength of the CO(2) R(14) line and its known frequency.

Orbits of nine Uranian rings

Observations of a stellar occultation by Uranus and its nine rings are presented and used to examine the structures and kinematics of the rings. The observations of the occultation of the K giant star KM 12 were obtained in the K band with the 4-m CTIO telescope at a signal-to-noise ratio higher than any previously obtained. Ring occultation profiles reveal the alpha ring to possibly have a double structure and less abrupt boundaries than the gamma ring, which exhibits diffraction fringes, while the eta ring is a broad ring with an unresolved narrow component at its inner edge. The present timing data, as well as previous occultation timings, are fit to a kinematic model in which all nine rings are treated as coplanar eclipses of zero inclination, precessing due to the zonal harmonics of the Uranian gravitational potential to obtain solutions for the ring orbits. Analysis of the residuals from the fitted orbits reveals that the proposed model is a good representation of ring kinematics. The reference system defined by the orbit solutions has also been used to obtain a value of 0.022 + or - 0.003 for the ellipticity of Uranus and a Uranian rotation period of 15.5 h.

A study on SmCo&amp;lt;inf&amp;gt;5&amp;lt;/inf&amp;gt;and Sm&amp;lt;inf&amp;gt;0.5&amp;lt;/inf&amp;gt;MM&amp;lt;inf&amp;gt;0.5&amp;lt;/inf&amp;gt;Co amorphous films

The magnetic and structural changes of amorphous films, which are prepared by DC diode sputtering on to liquid nitrogen-cooled substrates, during crystallization have been investigated. The targets are made of hard magnetic materials SmCo 5 , Sm 0.5 MM 0.5 Co. The study of crystallization process on the SmCo 5 amorphous films by X-ray and electron diffraction shows that the B-Co phase appears at 550°C, but it has only two B-Co diffraction fringes and the original amorphous wide band until 750°C, which shows that the crystalline phase of SmCo 5 intermatallic compound has not yet formed. It differs from the thick amorphous films as reported in the references. It is possible that the films have different crystallization process for larger surface energy. The measurements of the magnetic balance, vibrating sample magnetometer, Hall voltages and magnetic torque show that those amorphous films are soft magnetic and possess in-plane magnetic anisotropy. After the heat treatment of crystalline, the magnetizations have increased distinctly, but still have not hard magnetic property. It coincides the consequence of the structure study which has not formed the intermetallic compound.

Determination of the Autocorrelation Function of Woven Fabrics Using Laser Speckle

When a rough surface is irradiated by a laser light and reflected or transmitted light is observed, a diffraction fringe with speckle is obtained at the observation plane. This diffraction fringe is called the speckle pattern. In the present study the static states of woven fabric surfaces were obtained as the autocorrelation functions from the speckle patterns. The functions indicate the following: 1) By the reflection method the surface configuration of a woven fabric can be determined; by the transmission method the state of plane arrangement of yarn can be determined. 2) Three funda mental weaves (plain weave, twill weave, and satin weave) can be discriminated. 3) The presence and sizes of periodic defects in the woven fabric can be detected.

Fresnel diffraction in a coherent convergent electron beam

The Fresnel diffraction fringes observed in high magnification shadow images obtained in a scanning transmission electron microscope with a field emission source are strikingly different from those observed with parallel incident radiation. The intensities and spacing of the fringes often vary very little instead of dying away rapidly with distance from the shadow edge. Calculations based on a simple wave-optics formulation show good agreement with observed intensity distributions.

Fresnel diffraction in a coherent convergent electron beam

The Fresnel diffraction fringes observed in high magnification shadow images obtained in a scanning transmission electron microscope with a field emission source are strikingly different from those observed with parallel incident radiation. The intensities and spacing of the fringes often vary very little instead of dying away rapidly with distance from the shadow edge. Calculations based on a simple wave-optics formulation show good agreement with observed intensity distributions.