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Showing papers on "Diffraction published in 2008"


Journal ArticleDOI
TL;DR: In this paper, parity-time symmetric periodic potentials are investigated in detail for both one-and two-dimensional lattice geometries, and it is shown that PT periodic structures can exhibit unique characteristics stemming from the nonorthogonality of the associated Floquet-Bloch modes.
Abstract: The possibility of parity-time (PT) symmetric periodic potentials is investigated within the context of optics. Beam dynamics in this new type of optical structures is examined in detail for both one- and two-dimensional lattice geometries. It is shown that PT periodic structures can exhibit unique characteristics stemming from the nonorthogonality of the associated Floquet-Bloch modes. Some of these features include double refraction, power oscillations, and eigenfunction unfolding as well as nonreciprocal diffraction patterns.

1,512 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that electromagnetic waves at the surface of a metal have the enormous bandwidth of a light pulse and can be channeled into circuit components smaller than the diffraction limit.
Abstract: Electromagnetic waves at the surface of a metal have the enormous bandwidth of a light pulse and can be channeled into circuit components smaller than the diffraction limit.

656 citations


Journal ArticleDOI
13 Jun 2008-Science
TL;DR: It is shown that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule.
Abstract: Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.

622 citations


Journal ArticleDOI
TL;DR: In this article, electron backscattering diffraction was used to identify six components of the lattice curvature tensor, and improved lower bounds for the geometrically necessary dislocation content were obtained by linear optimization.

569 citations


Journal ArticleDOI
TL;DR: A broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals) that manifests high diffraction efficiency and high extinction ratio in both theory and experiment.
Abstract: We demonstrate a broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals). This achromatic polarization grating (PG) manifests high diffraction efficiency (~100%) and high extinction ratio (⩾1000:1) in both theory and experiment. We show an operational bandwidth Δλ/λ0~56% (roughly spanning visible wavelength range) that represents more than a fourfold increase of bandwidth over conventional PGs (and significantly larger than any other grating). The diffraction angle and operational region (visible, near-infrared, midwave infrared, and ultraviolet wavelengths) may be easily tailored during fabrication. The essence of the achromatic design is a stack of two chiral PGs with an opposite twist sense and employs the principle of retardation compensation. We fully characterize its optical properties and derive the theoretical diffraction behavior.

344 citations


Journal ArticleDOI
TL;DR: In this paper, photonic crystals were utilized to simulate enhanced light-trapping in a-Si:H thin-film solar cells, where a one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector.
Abstract: We utilize photonic crystals to simulate enhanced light-trapping in a-Si:H thin film solar cells A one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector A two dimensional photonic crystal between the absorber layer and the Bragg reflector diffracts light at oblique angles within the absorber The photonic crystal geometry is optimized to obtain maximum absorption The photonic crystal provides lossless diffraction of photons, increasing the photon path length within the absorber layer The simulation predicts significantly enhanced photon harvesting between 600 and 775nm below the band edge, and an absorption increase by more than a factor of 10 near the band edge The optical path length ratio can exceed the classical limit predicted for randomly roughened scattering surfaces at most wavelengths near the band edge The optical modeling is performed with a rigorous scattering matrix approach where Maxwell’s equations are solved

342 citations


Journal ArticleDOI
TL;DR: It is shown that coherent diffraction patterns recorded from individual nanocrystals are very sensitive to the atomic structure of nanocrystal surfaces, indicating a coordination/facet dependence that markedly differentiates the structural dynamics of nanocystals from bulk crystalline surfaces.
Abstract: Surface atoms have fewer interatomic bonds than those in the bulk that they often relax and reconstruct on extended two-dimensional surfaces. Far less is known about the surface structures of nanocrystals. Here, we show that coherent diffraction patterns recorded from individual nanocrystals are very sensitive to the atomic structure of nanocrystal surfaces. Nanocrystals of Au of 3-5 nm in diameter were studied by examining diffraction intensity oscillations around the Bragg peaks. Both results obtained from modelling the experimental data and molecular dynamics simulations strongly suggest inhomogeneous relaxations, involving large out-of-plane bond length contractions for the edge atoms (approximately 0.2 A); a significant contraction (approximately 0.13 A) for {100} surface atoms; and a much smaller contraction (approximately 0.05 A) for atoms in the middle of the {111} facets. These results denote a coordination/facet dependence that markedly differentiates the structural dynamics of nanocrystals from bulk crystalline surfaces.

340 citations


Patent
09 Dec 2008
TL;DR: In this paper, a method for determining overlay between a first grating and a second grating on a substrate (100), where the second and first gratings forming a composite grating (110,120) was presented.
Abstract: A method for determining overlay between a first grating (110) and a second grating (120) on a substrate (100), the second grating (120) on top of the first grating(110), the second grating (120) having substantially identical pitch (P1) as the first grating (110), the second and first gratings forming a composite grating (110,120), the method including providing a first illumination beam (IB) for illuminating the composite grating (110,120) under an angle of incidence (β) along a first horizontal direction (D1) along the surface of the substrate, and measuring a first intensity (i+) of a first order diffracted beam (B+) from the composite grating (110,120); providing a second illumination beam for illuminating the composite grating (11θ!i2O) under the angle of incidence (-β) along a second horizontal direction (D2) along the surface of the substrate, wherein the second horizontal direction (D2) is opposite to the first horizontal direction (DI)1 and measuring a second intensity (i-) of a minus first order diffracted beam (B-) from the composite grating (110,120).

312 citations


Journal ArticleDOI
TL;DR: In this article, the authors address both old, but "renovated" methods and new methods for diffraction line-profile analysis, focusing on distinct anisotropic linebroadening effects, as due to the type, orientation and distribution of dislocations and minute compositional variation.
Abstract: This paper addresses both old, but "renovated" methods and new methods for diffraction line-profile analysis. Classical and even extremely simple single-line methods for separating "size" and "strain" broadening effects have merit for characterization of the material im- perfectness, but it is generally very difficult to interpret the data obtained in terms of microstructure parameters as used in materials science. Developments of recent years, focusing on distinct anisotropic line-broadening effects, as due to the type, orientation and distribution of dislocations and minute compositional variation, will be touched upon. The most promising development may be the synthesis of line profiles on the basis of a microstructure model and application of the (kinematical) diffraction theory without any further assumption, which contrasts with the other methods. This approach can in principle be applied in sin- gle-line and multiple-line variants and also in analyses of the whole diffraction pattern. The advantage is the direct evaluation of microstructure parameters as used in materi- als science. The challenge is to develop microstructure models which are flexible enough to be applicable in more than one case ...

305 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate negative refraction and focusing of ultrasonic waves in two-dimensional phononic crystals made of stainless steel rods assembled in a triangular lattice and immersed in a liquid.
Abstract: We present experimental demonstrations of negative refraction and focusing of ultrasonic waves in two-dimensional phononic crystals made of stainless steel rods assembled in a triangular lattice and immersed in a liquid. Negative refraction is achieved for the range of frequencies in the second band, where the directions of the wave vector and group velocity are antiparallel to each other due to circular equifrequency contours. Negative refraction is unambiguously observed using a prism-shaped crystal. By exploiting the circular equifrequency contours in the second band, focusing of the ultrasonic field emitted by a pointlike source was demonstrated using a flat phononic crystal filled with and immersed in water. During these experiments, the importance of imaging in the regime of all angle negative refraction (AANR) was established for obtaining high-quality images. The regime of AANR was achieved in a similar flat crystal, in which the liquid inside the crystal (methanol) was different from the outside medium (water). This design resulted in matching circular equifrequency contours at the frequency of $0.55\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, implying that a flat ultrasonic lens with an effective refractive index of $\ensuremath{-}1$ was realized. By imaging a subwavelength line source with this crystal, a resolution of $0.55\ensuremath{\lambda}$ was observed, which is just above the diffraction limit.

285 citations


Journal ArticleDOI
TL;DR: In this paper, Johnson et al. proposed a novel technique for nondestructive and simultaneous mapping of the three-dimensional grain and the absorption microstructure of a material using X-ray diffraction contrast tomography.
Abstract: The principles of a novel technique for nondestructive and simultaneous mapping of the three-dimensional grain and the absorption microstructure of a material are explained. The technique is termed X-ray diffraction contrast tomography, underlining its similarity to conventional X-ray absorption contrast tomography with which it shares a common experimental setup. The grains are imaged using the occasionally occurring diffraction contribution to the X-ray attenuation coefficient each time a grain fulfils the diffraction condition. The three-dimensional grain shapes are reconstructed from a limited number of projections using an algebraic reconstruction technique. An algorithm based on scanning orientation space and aiming at determining the corresponding crystallographic grain orientations is proposed. The potential and limitations of a first approach, based on the acquisition of the direct beam projection images only, are discussed in this first part of the paper. An extension is presented in the second part of the paper [Johnson, King, Honnicke, Marrow & Ludwig (2008). J. Appl. Cryst. 41, 310–318], addressing the case of combined direct and diffracted beam acquisition.

Journal ArticleDOI
25 Apr 2008-Science
TL;DR: Using a patterned, grating-like plate to control the electromagnetic near field, this work demonstrates focusing well beyond the diffraction limit at ∼ 1 gigahertz.
Abstract: Using a patterned, grating-like plate to control the electromagnetic near field, we demonstrate focusing well beyond the diffraction limit at ∼ 1 gigahertz. The near-field plate consists of only capacitive elements and focuses microwaves emanating from a cylindrical source to a spot of size ≈ λ/20 (half-power beamwidth), where λ is the free-space wavelength. These plates will find application in antennas, beam-shaping devices, nonradiative wireless power-transfer systems, microscopy, and lithography.

Journal ArticleDOI
TL;DR: In this paper, the textured photonic crystal (TPC) backside reflector was applied to thin-film Si solar cells to increase the short-circuit current density.
Abstract: Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 μm thick monocrystalline thin film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.

Journal ArticleDOI
TL;DR: A potential based on 3d reconstruction of the reciprocal space visualising special structural features of materials such as partial disorder is demonstrated and new features implemented into the acquisition part are described.

Journal ArticleDOI
TL;DR: In this paper, a data-oriented approach for diffraction-based image resolution is proposed, which is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data.
Abstract: High resolution imaging is of great value to an interpreter, for instance to enable identification of small scale faults, and to locate formation pinch-out positions. Standard approaches to obtain high-resolution information, such as coherency analysis and structure-oriented filters, derive attributes from stacked, migrated images. Since they are image-driven, these techniques are sensitive to artifacts due to an inadequate migration velocity; in fact the attribute derivation is not based on the physics of wave propagation. Diffracted waves on the other hand have been recognized as physically reliable carriers of high- or even super-resolution structural information. However, high-resolution information, encoded in diffractions, is generally lost during the conventional processing sequence, indeed migration kernels in current migration algorithms are biased against diffractions. We propose here methods for a diffraction-based, data-oriented approach to image resolution. We also demonstrate the different behaviour of diffractions compared to specular reflections and how this can be leveraged to assess characteristics of subsurface features. In this way a rough surface such as a fault plane or unconformity may be distinguishable on a diffraction image and not on a traditional reflection image. We outline some characteristic properties of diffractions and diffraction imaging, and present two novel approaches to diffraction imaging in the depth domain. The first technique is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data. The second technique modifies the migration kernel and consists of a reverse application of stationary-phase migration to suppress contributions from specular reflections to the diffraction image. Both techniques are proposed as a complement to conventional full-wave pre-stack depth migration, and both assume the existence of an accurate migration velocity.

Journal ArticleDOI
TL;DR: In this article, a non-traditional approach based on high-energy X-ray diffraction and atomic pair distribution function data analysis is used instead of traditional Bragg peaks, and its great potential is discussed.

Journal ArticleDOI
TL;DR: In this article, the authors present new observational data on small-angle light scattering properties of natural, random shaped particles, as contrasted with spherical particles, and show that a kernel matrix for random-shaped particles results in improved interpretation of field multiangle scattering observations.
Abstract: [1] We present new observational data on small-angle light scattering properties of natural, random shaped particles, as contrasted with spherical particles. The interest in this ‘‘shape effect’’ on scattering arises from the need for a suitable kernel matrix for use in the laser diffraction method (LD) of particle sizing. LD is now used broadly for measuring size distribution of suspended marine particles. LD involves the measurement of small-angle forward scattering at multiple angles. This data is inverted using the kernel matrix to produce size distribution. In the absence of a suitable matrix for random shaped particles, past practice has been to use a model based on Mie theory, applicable strictly only to homogeneous spheres. The present work replaces Mie theory with empirical data. The work was motivated in part by anomalous field observations of size distribution and settling velocity distributions reported in literature. We show that a kernel matrix for random shaped particles results in improved interpretation of field multiangle scattering observations. In particular, a rising edge at the fine particle end of the size spectrum is shown to be associated with shape effects.

Journal ArticleDOI
TL;DR: In this paper, the interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analyzed in terms of finite element method and experimental study.
Abstract: The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S0) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.

Journal ArticleDOI
TL;DR: In this article, a simple hybrid design was developed to produce practically scatterless aperture slits for small-angle X-ray scattering and high-resolution X -ray diffraction.
Abstract: A simple hybrid design has been developed to produce practically scatterless aperture slits for small-angle X-ray scattering and high-resolution X-ray diffraction The hybrid slit consists of a rectangular single-crystal substrate (eg Si or Ge) bonded to a high-density metal base with a large taper angle (> 10°) The beam-defining single-crystal tip is oriented far from any Bragg peak position with respect to the incident beam and hence produces none of the slit scattering commonly associated with conventional metal slits It has been demonstrated that the incorporation of the scatterless slits leads to a much simplified design in small-angle X-ray scattering instruments employing only one or two apertures, with dramatically increased intensity (a threefold increase observed in the test setup) and improved low-angle resolution

Journal ArticleDOI
TL;DR: In this article, an analytical method for deriving the thermomechanical properties of polycrystalline materials under high-pressure (P) and high-temperature (T) conditions is presented.
Abstract: An analytical method is presented for deriving the thermomechanical properties of polycrystalline materials under high-pressure (P) and high-temperature (T) conditions. This method deals with non-uniform stress among heterogeneous crystal grains and surface strain in nanocrystalline materials by examining peak-width variation under different P–T conditions. Because the method deals directly with lattice d spacing and local deformation caused by stress, it can be applied to process any diffraction profile, independent of detection mode. In addition, a correction routine is developed using diffraction elastic ratios to deal with severe surface strain and/or strain anisotropy effects related to nano-scale grain sizes, so that significant data scatter can be reduced in a physically meaningful way. Graphical illustration of the resultant microstrain analysis can identify micro/local yields at the grain-to-grain interactions resulting from high stress concentration, and macro/bulk yield of the plastic deformation over the entire sample. This simple and straightforward approach is capable of revealing the corresponding micro and/or macro yield stresses, grain crushing or growth, work hardening or softening, and thermal relaxation under high-P–T conditions, as well as the intrinsic residual strain and/or surface strain in the polycrystalline bulk. In addition, this approach allows the instrumental contribution to be illustrated and subtracted in a straightforward manner, thus avoiding the potential complexities and errors resulting from instrument correction. Applications of the method are demonstrated by studies of α-SiC (6H, moissanite) and of micro- and nanocrystalline nickel by synchrotron X-ray and time-of-flight neutron diffraction.

Journal ArticleDOI
TL;DR: A small gold particle was illuminated with a hard x-ray nanobeam and is reconstructed from its coherent diffraction pattern and a resolution of about 5 nm is achieved in 600 s exposure time.
Abstract: Coherent x-ray diffraction imaging is an x-ray microscopy technique with the potential of reaching spatial resolutions well beyond the diffraction limits of x-ray microscopes based on optics. However, the available coherent dose at modern x-ray sources is limited, setting practical bounds on the spatial resolution of the technique. By focusing the available coherent flux onto the sample, the spatial resolution can be improved for radiation-hard specimens. A small gold particle (size <100 nm) was illuminated with a hard x-ray nanobeam (E=15.25 keV, beam dimensions approximately 100 x 100 nm2) and is reconstructed from its coherent diffraction pattern. A resolution of about 5 nm is achieved in 600 s exposure time.

Journal ArticleDOI
TL;DR: Evidence of the temporal fluctuations of the phase modulation property of a liquid crystal on silicon (LCoS) display is provided, and its effect when the device is used for displaying a diffractive optical element is analyzed.
Abstract: In this paper we provide evidence of the temporal fluctuations of the phase modulation property of a liquid crystal on silicon (LCoS) display, and we analyze its effect when the device is used for displaying a diffractive optical element. We use a commercial twisted nematic LCoS display configured to produce a phase-only modulation, and we provide time resolved measurements of the diffraction efficiency that show rapid fluctuations of the phase modulation, in the millisecond order. We analyze how these fluctuations have to be considered in two typical methods for the characterization of the phase modulation: two beam interference and diffraction from a binary grating. We finally provide experimental results on the use of this device for displaying a computer generated hologram. A reduction of the modulation diffraction efficiency results from the phase modulation fluctuation.

Journal ArticleDOI
TL;DR: The observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-micros pulses is reported.
Abstract: We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-� s pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments. We report the observation of large, two-dimensional regular arrays of plasma filaments generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW 110-GHz gyrotron with 3-� s pulse length. These filamentary arrays consist of a large number of regularly spaced elements, more than 100 in some cases. They are observed both involume breakdown and in breakdown at surfaces. The filamentary arrays are notably different from the well-known filamentation observed in laserinduced plasmas. Laser generated filaments are aligned along the direction of propagation, while the filaments observed here are perpendicular to the direction of propagation. Each filament is aligned parallel to the direction of the electric field. There is an extensive literature on gas breakdown at microwave frequencies [1‐3] and at optical frequencies [4,5]. Recent research has also addressed a number of important issues in breakdown at surfaces [6]. However, due to the scarcity of powerful sources in the millimeter wavelength range, there have been relatively few studies on gas breakdown at these wavelengths. Studies have been carried out with gyrotrons at frequencies of 35 to 86 GHz [7‐10] at power levels up to 250 kW and pulse lengths of 0.05 to 20 ms. The available power in these studies was insufficient to achieve breakdown in air at atmospheric pressure, but careful studies were conducted either by using lower pressure (where the breakdown threshold is reduced), using alternate gases such as helium, or using initiators such as a collection of fine wires placed near the beam focus. These studies yielded some interesting evidence of filamentary structures in the breakdown volume. However, none of these studies produced the large, twodimensional, filamentary regular arrays seen in the present experiments. Also, the present study differs substantially from these studies in terms of time scales (3 � s vs 0.05‐ 20 ms) as well as intensities (on the order of 10 6 vs 10 4 W=cm 2 ) used for initiating and sustaining the breakdown plasma. A number of theoretical studies have also been reported to explain the structures such as ‘‘snakes’’ and ‘‘chains’’ that have been observed in high-pressure microwave breakdown research [11‐13]. These explanations emphasize the role of plasma kinetics and instabilities. In this study, we explain the array as developing from a sequential process of electromagnetic wave diffraction. Incident microwave power is diffracted from plasma filaments formed early in the breakdown process. As the technology of high power millimeter wave sources matures and their applications broaden, breakdown will inevitably be an issue in transporting through a gas, or on the surface of a window through which such radiation is transmitted. This was our motivation for conducting volume and surface breakdown experiments in this relatively unexplored frequency band. Figure 1 shows the experimental setup for the breakdown studies. The 110 GHz gyrotron, operating at 96-kV beam voltage and 40-A beam current, can produce 1.5 MW of output power in 3-� s pulses [14]. The TE22;6 mode

Journal ArticleDOI
TL;DR: In this article, the beamline BL10XU at SPring-8, designed for X-ray diffraction experiments using diamond anvil cells at high pressure and low/high temperature, is continuously upgraded.
Abstract: Beamline BL10XU at SPring-8, designed for X-ray diffraction experiments using diamond anvil cells at high pressure and low/high temperature, is continuously upgraded. The X-ray source, optics, and attractive experimental equipment such as simultaneous measurement systems have been optimized over the past years. The high energy and high intensity monochromatic X-ray beams emitted by an undulator source, focused using a characteristic X-ray refractive lens, have enabled us to obtain excellent counting statistics and high-resolution X-ray diffraction data even from a highly compressed sample under multi-megabar pressure. At BL10XU, low- and high-temperature conditions are achieved using a cryostat (10–300 K) and double-sided laser-heating system (1000–4000 K), respectively. Numerous results have been obtained in the fields of high-pressure materials science and mineral physics: for instance, the structural information on novel materials under pressure, including new pressure-induced structural phase transiti...

Journal ArticleDOI
TL;DR: Car-Parrinello simulation proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions and bulk structure of solutions and the capabilities and limitations of the methods in describing the solution structure are discussed.
Abstract: To determine the structure of aqueous sodium hydroxide solutions, results obtained from x-ray diffraction and computer simulation (molecular dynamics and Car-Parrinello) have been compared. The capabilities and limitations of the methods in describing the solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in describing the hydration spheres of the sodium ion and yield structural information on the anion’s hydration structure. Classical molecular dynamics simulations were not able to correctly describe the bulk structure of these solutions. However, Car-Parrinello simulation proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions and bulk structure of solutions. The results of Car-Parrinello simulations were compared with the findings of diffraction experiments.

Journal ArticleDOI
TL;DR: In this paper, a combination of synchrotron X-ray diffraction techniques have been applied to resolve ambiguities between experimental and theoretical studies of LiBH4 and to reveal its complex structural evolution as a function of temperature.
Abstract: A combination of synchrotron X-ray diffraction techniques have been applied to resolve ambiguities between experimental and theoretical studies of LiBH4 and to reveal its complex structural evolution as a function of temperature. Crystal structures of the low- and high-temperature polymorphs of LiBH4 have been determined from diffraction on single-crystals. In contrast to recent theoretical conjectures, we find that the high-temperature structure is hexagonal, space group P63mc. Experimental data suggest a nearly isotropic disorder of the rigid tetrahedral BH4 groups as one of the factors stabilizing the hexagonal structure. Tetrahedral BH4 anions are undistorted and geometrically very similar in both polymorphs. The first order phase transition at 381 K is preceded by highly anisotropic lattice expansion and is accompanied by a negative volume change. Disorder phenomena and strong lattice anharmonicity, being ignored, lead to the failure of theoretical predictions of the structural stability of LiBH4 pub...

Patent
07 Dec 2008
TL;DR: In this paper, a first diffractive optical element (DOE) is configured to diffract an input beam so as to generate a first pattern on a first region of a surface, the first diffraction pattern including a zero order beam.
Abstract: Apparatus for projecting a pattern includes a first diffractive optical element (DOE) configured to diffract an input beam so as to generate a first diffraction pattern on a first region of a surface, the first diffraction pattern including a zero order beam. A second DOE is configured to diffract the zero order beam so as to generate a second diffraction pattern on a second region of the surface such that the first and the second regions together at least partially cover the surface.

Journal ArticleDOI
TL;DR: A digital three-color holographic interferometer was designed to analyze the variations in refractive index induced by a candle flame, finding zero order fringe can be easily extracted from the experimental data.
Abstract: A digital three-color holographic interferometer was designed to analyze the variations in refractive index induced by a candle flame. Color holograms are generated and recorded with a three layer photodiode stack sensor allowing a simultaneous recording with a high spatial resolution. Phase maps are calculated using Fourier transform and spectral filtering is applied to eliminate parasitic diffraction orders. Then, the contribution along each color is obtained with the simultaneous three wavelength measurement. Results in the case of the candle flame are presented. Zero order fringe, meaning zero optical path difference, can be easily extracted from the experimental data, either by considering a modeled colored fringe pattern or the wrapped phases along the three wavelengths.

Journal ArticleDOI
13 Feb 2008-Langmuir
TL;DR: This work was able to optimize the diffraction intensity and the tuning range through studying their dependence on variables such as the size distribution and concentration of the Fe(3)O(4) colloids or ionic strength of the solutions.
Abstract: Superparamagnetic Fe(3)O(4) colloids with highly charged surfaces have been assembled into ordered structures in water in response to external magnetic fields. The colloids form chainlike structures with regular interparticle spacings of a few hundred nanometers along the direction of the external field so that the system strongly diffracts visible light. The balance between attractive (in this case, magnetic) and repulsive (electrostatic) forces dictates interparticle spacing and therefore optical properties. By changing the relative strength of these two forces, one can tune the peak diffraction wavelength over the entire visible spectrum. We were able to optimize the diffraction intensity and the tuning range through studying their dependence on variables such as the size distribution and concentration of the Fe(3)O(4) colloids or ionic strength of the solutions. The fast, reversible response and the feasibility for miniaturization impart these photonic materials great potential in applications such as optoelectronic devices, sensors, and color displays.

Journal ArticleDOI
TL;DR: A comprehensive account of the fundamental processes involved in electron pulse propagation is presented, and comparisons with experimental results are made with comparison with recent developments in ultrafast electron diffraction and imaging techniques.
Abstract: Pulsed electron beams allow for the direct atomic-scale observation of structures with femtosecond to picosecond temporal resolution in a variety of fields ranging from materials science to chemistry and biology, and from the condensed phase to the gas phase. Motivated by recent developments in ultrafast electron diffraction and imaging techniques, we present here a comprehensive account of the fundamental processes involved in electron pulse propagation, and make comparisons with experimental results. The electron pulse, as an ensemble of charged particles, travels under the influence of the space–charge effect and the spread of the momenta among its electrons. The shape and size, as well as the trajectories of the individual electrons, may be altered. The resulting implications on the spatiotemporal resolution capabilities are discussed both for the N-electron pulse and for single-electron coherent packets introduced for microscopy without space–charge.