Anomalous Transmission Effect Research Articles

Anomalous transmission through periodic resistive sheets

This work investigates anomalous transmission effects in periodic dissipative media, which is identified as an acoustic analogue of the Borrmann effect. For this, the scattering of acoustic waves on a set of equidistant resistive sheets is considered. It is shown both theoretically and experimentally that at the Bragg frequency of the system, the transmission coefficient is significantly higher than at other frequencies. The optimal conditions are identified: one needs a large number of sheets, which induce a very narrow peak, and the resistive sheets must be very thin compared to the wavelength, which gives the highest maximal transmission. Using the transfer matrix formalism, it is shown that this effect occurs when the two eigenvalues of the transfer matrix coalesce (i.e., at an exceptional point). Exploiting this algebraic condition, it is possible to obtain similar anomalous transmission peaks in more general periodic media. In particular, the system can be tuned to show a peak at an arbitrary long wavelength.

On the Possibility of Observing the Effect of Anomalous Neutron Transmission in Predominantly Resonance Absorption by Perfect Crystals of InSb

Dynamic neutron diffraction on perfect InSb crystals is analyzed theoretically both under conditions of predominantly potential scattering and under conditions of predominantly resonant scattering. Attention is given primarily to the effect of anomalous neutron transmission. A project of an experiment aimed at studying this effect in predominantly resonant neutron scattering via a simultaneous detection of reflected and transmitted beams and secondary gamma rays is proposed. It is shown that such an experiment may be performed in beams from high-flow reactors.

Prospects for application of X-ray anomalous transmission effect tomonochromatization of broadband spectrum.

The characteristics of spectra obtained by monochromatization of a broadband spectrum using the effect of X-ray anomalous transmission (Bormann effect) have been experimentally studied. Optimization of the crystal thickness allows one to obtain the spectral band with a controlled width of ΔE/E = 1-5 10-3 and with a contrast of more than 250 with a peak reflectivity not less than 4% in the transmitted Bormann beam. It was found that due to the wave field compression, the width of the transmitted Bormann beam decreased three times as the thickness of the crystal increased. The value of parallel displacement of the transmitted Bormann beam relative to the primary one, which depends on crystal thickness, was measured. This value was shown to be sufficient for the beams to be separated by the slit. Monitoring the transmitted Bormann beam by the shape of 2d-curves of the diffracted Laue-beam enables the precise control of characteristics of the primary source with a complex shape of angle and energy distributions of photons.

Experimental demonstration of quenched transmission effect of an ultrathin metallic grating.

We report on the experimental study of an anomalous transmission effect in ultrathin metallic gratings, where the metal thickness is much thinner than the skin depth. In particular, incident transverse magnetic polarized waves are reflected while incident transverse electric polarized waves are transmitted. This anomalous effect is strongly dependent on the metal thickness and metal width. We systematically investigate and demonstrate the anomalous effect and find the optimized nanostrip thickness and width by introducing a shadow-mask fabrication approach. Our results demonstrate the possibility of developing ultrathin nanostrip based planar metasurfaces with low loss.

Anomalous transmission of disordered arrays of silver nanoclusters in the near- and mid-IR regions

The results of studies of optical properties of single-layer and double-layer arrays of silver clusters separated by a ZnO film are presented. The arrays of clusters are fabricated via vacuum evaporation and subsequent annealing. All annealed samples exhibit both the surface plasmon resonance and the anomalous transmission effect in the near and middle IR regions.

Using angular dispersion and anomalous transmission to shape ultramonochromatic x rays

Optical spectrometers, instruments that work with monochromatic light, are commonly rated by the spectral bandwidth, which defines the ability to resolve closely spaced spectral components. The ability to detect faint objects among these components, spectral contrast, is another desired aspect. Here we demonstrate that a combined effect of angular dispersion (AD) and anomalous transmission (AT) of x rays in Bragg reflection from asymmetrically cut crystals can shape spectral distributions of x rays to profiles with record high contrast and small bandwidths. The AD&AT x-ray optics is implemented as a five-reflection three-crystal arrangement featuring a combination of the above mentioned attributes, so much desirable for x-ray monochromators and analyzers: a spectral contrast of ~ 500, a bandwidth of ~ 0.46 meV and a remarkably large angular acceptance of ~ 107 urad. The new optics can become a foundation for the next generation inelastic x-ray scattering spectrometers for studies of atomic dynamics.

Study of structural defects in ZnGeP2crystals by X-ray topography based on the Borrmann effect

The first study of structural defects in a ZnGeP2semiconducting nonlinear optical crystal has been carried out by X-ray topography, based on the Borrmann effect, and the effect of anomalous transmission of X-rays on ZnGeP2crystals has been examined. It is shown that the rosette technique of defect study under conditions of the Borrmann effect, developed earlier for elementary semiconductors, can be applied to the study and identification of defects in ZnGeP2. Features of contrast from individual edge and screw dislocations, microdefects, and coherent and semi-coherent microinclusions were considered. Defect identification was carried out by comparison of the experimental intensity contrast with simulated images of the defects.

Electron Channeling: A Problem for X-Ray Microanalysis in Materials Science

Electron channeling effects can create measurable signal intensity variations in all product signals that result from the scattering of the electron beam within a crystalline specimen. Of particular interest to the X-ray microanalyst are any variations that occur within the characteristic X-ray signal that are not directly related to a specimen composition variation. Many studies have documented the effect of crystallographic orientation on the local X-ray yield; however, the vast majority of these studies were carried out on thin foil specimens examined in transmission. Only a few studies have addressed these effects in bulk specimen materials, and these analyses were generally carried out at common scanning electron microscope microanalysis overvoltages (>1.5). At these overvoltage levels, the anomalous transmission effect is weak. As a result, the effect of electron channeling on the characteristic X-ray signal intensity has traditionally been overlooked in the field of quantitative electron probe microanalysis. The present work will demonstrate that electron channeling can produce X-ray variations of up to 26%, between intensity maxima and minima, in low overvoltage X-ray microanalyses of bulk specimens. Intensity variations of this magnitude will significantly impact the accuracy of qualitative and quantitative X-ray microanalyses at low overvoltage on engineering structural materials.

Transmission of gamma rays through the curved single crystals of gadolinium–gallium garnet

The effect of anomalous transmission of gamma rays ( E γ =0.662 MeV) through the bent single crystals of Gd 3Ga 5O 12 (GGG) has been experimentally studied. A simple model has been proposed, which explains the observed phenomenon by the gamma ray focusing.

X-ray standing wave as a result of only the imaginary part of the atomic scattering factor.

The X-ray standing wave has been studied when the real part of the scattering factor is zero. In the symmetric Laue case, the phase of the standing wave advances by pi when the deviation parameter W changes from -1 to 1, which is the same variation as in the usual symmetric Bragg case when only the real part of the scattering factor exists. However, the phase in the former case is different from that in the latter by pi/2. By using the standing waves, the origins of the anomalous transmission and anomalous absorption effects reported by Fukamachi & Kawamura [Acta Cryst. (1993), A49, 384-388] have been analysed. The standing wave in the Laue case can give rise to a more accurate method of site determination of a specified impurity atom as well as a wider range of applications than a conventional standing-wave approach.

The use of the X-ray anomalous transmission effect in the structure investigation of high-temperature superconductors

The effect of the anomalous transmission of X-rays (the Borrmann effect) was observed in high quality Nd 2CuO 4, (LaSr) 2CuO 4, EuBa 2Cu 3O 7− x and YBa 2Cu 3O 7− x single crystals. With the help of it we have shown that the EuBa 2Cu 3O 7− x crystals grown in an alundum container are noncentrosymmetrical. They have the polarity axes along [100] and are divided into antiphase domains. The space group of such crystals is Pmm2. We also obtained the result of the existence of the X-ray anomalous transition effect in strongly distorted crystals. It shows that the theory of the Borrmann effect demands future development.

On the theory of six-beam X-ray spherical-wave diffraction

A general formula is obtained for the intensity distribution in the film behind a single crystal in the case of spherical-wave X-ray multiple diffraction. The theory takes into account the phase shift of the waves not only inside the crystal but also in the vacuum before and after the crystal along the wave path source-crystal-film of length L. The topographic images are calculated in the case of (220/242/044/\bar 244/\bar 202) six-beam diffraction of Cu Kα radiation in a germanium crystal of thickness 0.2 mm for different values of L. The enhancement of the anomalous transmission effect is weakly displayed on the topographs because of strong scattering of the radiation inside the crystal. The intensity distribution depends on L. The possibility is shown of focusing X-rays to a considerable extent.

The integrated intensities of the Laue-diffracted X-rays for monocrystals containing macroscopically homogeneously distributed defects

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OBSERUATION AND CALCULATION OF FRINGE IMAGES OF OVERLAPPING MICRO-TWIN BOUNDARIES

Fringe images of overlapping micro-twin boundaries in a f.c.c. Ni-Cr alloy have been observed by trausmission electron microscopy. It is shown that the average intensity of the fringe images in the overlapping boundary is either much higher or much lower than that of the single boundary depending on the sign and value of the deviation parameters. The contrast of fringes will reverse when one changes the sign of deviation parameters for the matrix and/or the twin. Comparing with the well-known α-fringe and δ-fringe, overlapping twin fringes have special features which may arise from the anomalous absorption-anomalous transmission effects and the dynamic multiple beam interference among the matrix-twin-matrix or twin-matrix-twin. Using Van Dyck's iterative method, simulation images were calculated which are in good agreement with experimental observations.

Symmetrical cases of simultaneous X-ray diffraction and the Borrmann effect. I. The analysis of photoelectric absorption

The strong enhancement of the anomalous transmission (Borrmann) effect in the symmetrical cases of simultaneous diffraction of three, four and six waves is considered. The analytical solution of the diffraction problem has been obtained when the Bragg conditions are satisfied exactly. The effect of the suppression of photoelectric absorption is investigated with consideration of both dipole and quadrupole interactions.

X-ray topographic observations of magnetic domains in Czochralski-grown nickel single crystals in anomalous transmission geometry

The anomalous transmission effect has been observed in nickel single crystals grown by the Czochralski technique. Sample crystals prepared for x-ray topography are 1.5–2.5 cm in diameter and thicker than 0.4 mm; the product of the ordinary linear absorption coefficient and thickness ranges from 17 to 45 for Cu Kα radiation. Topographs taken with an asymmetric-crystal-topographic (ACT) camera have revealed extremely straight line images and rectangular arrangements of short line-segment images, along with the images of crystal imperfections. The line images change their arrangement upon application of magnetic fields. These images are attributed to magnetic domains in the interior of crystals. The formation of domain walls near the crystal surface is also observed and distinguished clearly from magnetic domains in the bulk with the use of the ACT camera.

X-ray topographic observation of dark-line defects in GaAs-Ga1−xAlxAs double-heterostructure wafers

Optically induced dark-line defects (DLD’s) in GaAs-Ga1−xAlxAs double-heterostructure (DH) wafers have been observed by x-ray topography for the first time using the x-ray anomalous transmission effect. This topography, which is sensitive to defects in the thin epitaxial layers, has been successfully applied to the observation. As a result, it has been observed that DLD’s oriented along 〈110〉 directions are composed of dislocations with Burgers vectors of (1/2) a 〈110〉 parallel to the growth surface. It is also shown that the DLD’s originate from both substrate dislocations and crystal edges.

Some Topographic Observations of the Effects of Dynamical Diffraction in Imperfect Metal Crystals

X-ray diffraction topographs obtained in the anomalous transmission geometry contain images which are quite different from those obtained from thin crystals (μL<=l). In this paper various topographic images which are unique to thick crystals (μL>10) will be presented and discussed in terms of dynamical diffraction in imperfect crystals. First, it is observed in topographs that images of crystal imperfections caused by disruption of the anomalous transmission effect are slightly broader or more diffuse in the H-diffracted (Bragg-diffracted) beam than in the O-diffracted (transmitted beam). These observations have been made in both copper and nickel crystals. Such broadening can be explained by the presence of unique extinction terms in the expression for the intensity of the dynamically diffracted beam in an imperfect crystal.

Borrmann effect in the three-wave case of X-ray diffraction

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A multiple reflection effect of anomalous transmission in germanium

A multiple reflection effect of anomalous transmission in germanium