Showing papers on "Diffraction published in 1998"

Extraordinary optical transmission through sub-wavelength hole arrays

Abstract: The desire to use and control photons in a manner analogous to the control of electrons in solids has inspired great interest in such topics as the localization of light, microcavity quantum electrodynamics and near-field optics1,2,3,4,5,6. A fundamental constraint in manipulating light is the extremely low transmittivity of apertures smaller than the wavelength of the incident photon. While exploring the optical properties of submicrometre cylindrical cavities in metallic films, we have found that arrays of such holes display highly unusual zero-order transmission spectra (where the incident and detected light are collinear) at wavelengths larger than the array period, beyond which no diffraction occurs. In particular, sharp peaks in transmission are observed at wavelengths as large as ten times the diameter of the cylinders. At these maxima the transmission efficiency can exceed unity (when normalized to the area of the holes), which is orders of magnitude greater than predicted by standard aperture theory. Our experiments provide evidence that these unusual optical properties are due to the coupling of light with plasmons — electronic excitations — on the surface of the periodically patterned metal film. Measurements of transmission as a function of the incident light angle result in a photonic band diagram. These findings may find application in novel photonic devices.

High-Resolution X-Ray Scattering from Thin Films and Multilayers

Abstract: Part 1 Experimental realization: basic elements of an equipment resolution elements diffractometers and reflectometers. Part 2 The theory of X-ray diffraction and its realization by the experiment: kinematical X-ray scattering from ideal crystals kinematical X-ray diffraction from deformed thin layers kinematical X-ray diffraction from randomly disturbed layers dynamical X-ray diffraction in perfect layers dynamical X-ray diffraction in slightly deformed layers optical reflection of X-rays from ideal layers optical reflection of X-rays from layers with rough interfaces dynamical X-ray diffraction in strongly asymmetric cases grazing incidence diffraction (GID). Appendices: elements of the formal theory of scattering structure factors, dispersion corrections and extinction length.

Estimating grain-size distributions in nanocrystalline materials from X-ray diffraction profile analysis

Abstract: It is well known that the Fourier analysis of X-ray diffraction peak profiles (as implemented by Warren and Averbach) can accurately determine the areaweighted average grain size of a fine-grained sample. Less well known is the fact that this method simultaneously yields a volume-weighted average grain size. Under certain circumstances, knowledge of these two weighted average grain sizes is sufficient to permit reliable estimation of the grain-size distribution, even when the distribution cannot be calculated directly from the Fourier coefficients, as is usually the case. We demonstrate this for a nanocrystalline Pd sample prepared by inert-gas condensation; average grain sizes and the grain-size distribution are estimated by X-ray diffraction profile analysis and compared with the same quantities measured directly by transmission electron microscopy (TEM). Very good agreement between the resulting average grain sizes is achieved only after compensating for the fact that diffraction profile analy...

Atomic beam diffraction from solid surfaces

Abstract: Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.

An approach for recording and readout beyond the diffraction limit with an Sb thin film

Abstract: A technique for recording and retrieving small marks beyond the optical diffraction limit was proposed. The basic experiment with this technique was also carried out at a constant linear velocity of 2.0 m/s, rotating a disk with a multi-layered structure of Sb and GeSbTe, which were separated by a thin film of SiN. By use of the optically nonlinear property of the Sb thin film, carrier to noise ratio of more than 10 dB was obtained from recorded marks of 90 nm, using an optical system with the laser wavelength of 686 nm and a numerical aperture of 0.6.

Optical Bloch oscillations in waveguide arrays

Abstract: We show that optical Bloch oscillations can emerge in waveguide arrays with linearly varying propagation constants. The existence of localized modes (Wannier-Stark states) with equidistant wave-number spacing (Wannier-Stark ladder) that do not undergo diffraction is analytically proved. The evolution of arbitrary initial excitations is described, and potential applications are suggested.

Melting of Pb nanocrystals

Abstract: The size-dependent melting and surface melting of Pb nanocrystals is demonstrated by x-ray powder diffraction in ultrahigh vacuum. Whereas some prior studies have measured the size-dependent melting temperature via the diffraction intensity, it is shown here that crystallite reorientation makes the diffraction intensity an unreliable indicator of melting. Instead of the diffraction intensity, the diffraction peak shape reveals the size-dependent melting via changes in the crystallite size distribution. Measurements showed that the melting temperature varies inversely with the crystallite size and quantitatively favors the liquid-skin melting model over the homogeneous melting model. Surface melting is demonstrated via the reversible growth of a 0.5 nm liquid skin on 50 nm crystallites just below the size-dependent melting temperature.

How to make femtosecond pulses overlap

Abstract: Normally, femtosecond light pulses that cross at a nonzero angle overlap over only a small region in space. This limitation can be overcome by the use of diffraction orders of a grating. We consider an arrangement in which, on diffraction of a femtosecond pulse by a grating, two beams that correspond to the first-order diffraction maxima are recombined at the image plane by a system of two confocal lenses. In this arrangement the beams overlap over the their full aperture, with the short duration of the pulses being preserved. We demonstrate the use of this setup as a simple autocorrelator and discuss a possible application to time-resolved vibrational spectroscopy.

Free-space beam propagation between arbitrarily oriented planes based on full diffraction theory: a fast Fourier transform approach

Abstract: A fast and accurate numerical method for free-space beam propagation between arbitrarily oriented planes is developed. The only approximation made in the development of the method was that the vector nature of light was ignored. The method is based on evaluating the Rayleigh–Sommerfeld diffraction integral by use of the fast Fourier transform with a special transformation to handle tilts and offsets of planes. The fundamental aspects of a software package based on the developed method are presented. A numerical example realized with the software package is presented to establish the validity of the method.

Seismic monitoring of diffraction images for detection of local heterogeneities

Abstract: Diffracted waves contain valuable information regarding both the structure and composition of the media they are in In seismic data processing, however, these waves are usually regarded as noise In this paper, we present an attempt to use scattered/diffracted waves for the detection of local heterogeneities The method is based on the detection of diffracted waves by concentrating the signal amplitudes from diffracting points on the seismic section This is done using a correlation procedure that enhances the amplitude of the seismic signal at the location of the diffractors on the common‐diffraction‐point section (D-section) The new local time correction for diffraction traveltime curve parameterization is based on the radius of curvature of the diffracted wavefront and near‐surface velocity We use the idea of seismic monitoring for detection and delineating local objects which may occur within the subsurface resulting from human activity or fast geological processes The method consists of continuou

PHASE SEPARATION IN InGaN/GaN MULTIPLE QUANTUM WELLS

Abstract: Evidence is presented for phase separation in In0.27Ga0.73N/GaN multiple quantum wells. After annealing for 40 h at a temperature of 950 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of In-rich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase. A diffraction peak corresponding to an In-rich phase is also present in the annealed material. Nanoscale In-rich InGaN precipitates are observed by transmission electron microscopy and energy dispersive x-ray chemical analysis.

Chapter 3 Structural Transitions in the Group IV, III-V, and II-VI Semiconductors under Pressure

Abstract: Publisher Summary This chapter discusses structural transitions in the group IV, III–V, and II–VI semiconductors under pressure. The most preferred technique of structural transitions is energy-dispersive diffraction (EDX) in which the full white beam of the synchrotron falls on the sample. The diffraction pattern is recorded as a function of X-ray energy at a fixed scattering angle. The resolution of EDX powder patterns is limited to that of the solid-state detector. The patterns are unavoidably contaminated with fluorescence lines from the sample. The tight collimation of the diffracted beam required to define the scattering angle often leads to poor-powder averaging with the effect that peak intensities are unreliable—for example, the ability to detect and characterize sample microstructure effects, which are known to cause serious problems in high-pressure work. Preferred orientation (PO) can alter peak intensities dramatically leading to a strong reflection that is apparently absent from a powder pattern.

Experimental study of high frame rate imaging with limited diffraction beams

Abstract: Limited diffraction beams have a large depth of field and have many potential applications. Recently, a new method (Fourier method) was developed with limited diffraction beams for image construction. With the method and a single plane wave transmission, both 2D (two-dimensional) and 3D (three-dimensional) images of a very high frame rate (up to 3750 frames/s for a depth of 200 mm in biological soft tissues) and a high signal-to-noise ratio (SNR) can be constructed with relatively simple and inexpensive hardware. If limited diffraction beams of different parameters are used in both transmission and reception and transducer aperture is shaded with a cosine function, high-resolution and low-sidelobe images can be constructed with the new method without montage of multiple frames of images [the image quality is comparable to that obtained with a transmit-receive (two-way) dynamically focused imaging system]. In this paper, the Fourier method was studied with both experiment and computer simulation for 2D B-mode imaging. In the experiment, two commercial broadband 1D array transducers (48 and 64 elements) of different aperture sizes (18.288 and 38.4 mm) and center frequencies (2.25 and 2.5 MHz) were used to construct images of different viewing sizes. An ATS539 tissue-equivalent phantom of an average frequency-dependent attenuation of 0.5 dB/MHz/cm was used as a test object. To obtain high frame rate images, a single plane wave pulse (broadband) was transmitted with the arrays. Echoes received with the arrays were processed with both the Fourier and conventional dynamic focusing (delay-and-sum) methods to construct 2D B-mode images. Results show that the quality (resolution and contrast) of constructed images is virtually identical for both methods, except that the Fourier method is simpler to implement. Both methods have also a similar sensitivity to phase aberration distortions. Excellent agreement among theory, simulation, and experiment was obtained.

Water confined in Vycor glass. I. A neutron diffraction study

Abstract: Neutron diffraction experiments with isotopic substitution on water confined in porous Vycor glass at two hydration states are presented and analyzed in terms of the experimentally accessible site-site distribution functions. The bias on these functions as well as their limitations, due to the presence of regions where water molecules are not allowed (excluded volume effects), and to the contribution of water–Vycor interference to the measured cross sections, are discussed. In particular the relative weight of these cross correlation terms is estimated for the first time. It is shown that the traditional analysis of diffraction data of these kinds which ignore cross correlations may be erroneous. A full account of the excluded volume effects is reported in paper II [A. Soper, F. Bruni, and M. A. Ricci, J. Chem. Phys. 109, 1486 (1998), following paper].

High-efficiency subwavelength diffractive element patterned in a high-refractive-index material for 633 nm.

Abstract: We propose the use of high-index materials for the fabrication of subwavelength diffractive components operating in the visible domain. This approach yields a reduction of fabrication constraints and an improvement of theoretical performance. A blazed grating with subwavelength binary features and with a period of 5.75 wavelengths is designed and fabricated in a TiO2 layer coated upon a glass substrate. The first-order diffraction efficiency measured with a He–Ne laser beam is 83%, which is slightly larger than that achieved theoretically by the best standard (continuous profile) blazed grating fabricated in glass with the same period.

Ultrashort pulsed gaussian light beams

Abstract: We find a family of solutions of the paraxial wave equation that represents ultrashort pulsed light beams propagating in free space. These pulsed beams have an arbitrary temporal form and a nearly Gaussian cross section, while modeling for the pulses emitted by mode-locked lasers with stable two-mirror resonators. We also study the effects arising from their spatiotemporal coupled behavior, such as pulse time delay, distortion, and a frequency shift toward the beam periphery. Time-varying diffraction (with diffraction reduction at the first instants of arrival of the pulsed beam at a given distance) and the dependence of the spatial distribution of energy on the pulse form are also described. These effects become important for pulsed beams with a few optical oscillations within the pulse envelope.

Characterization of surface cracks with Rayleigh waves: a numerical model

Abstract: In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the crack's geometrical features is significant. A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.

Diffraction from Polarization Holographic Gratings with Surface Relief in Side-chain Azobenzene Polyesters

Abstract: We investigate the polarization properties of holographic gratings in side-chain azobenzene polyesters in which an anisotropic grating that is due to photoinduced linear and circular birefringence is recorded in the volume of the material and a relief grating appears on the surface. A theoretical model is proposed to explain the experimental results, making it possible to understand the influence of the different photoinduced effects. It is shown that at low intensity the polarization properties of the diffraction at these gratings are determined by the interaction of the linear and circular photobirefringences, and at larger intensity the influence of the surface relief dominates the effect of the circular anisotropy. Owing to the high recording efficiency of the polyesters, the ±1-order diffracted waves change the polarization interference pattern during the holographic recording, resulting in the appearance of a surface relief with doubled frequency.

MSLS, a Least-Squares Procedure for Accurate Crystal Structure Refinement from Dynamical Electron Diffraction Patterns

Abstract: In X-ray crystallography, a least-squares structure refinement is used to two purposes: to prove the correctness of the proposed model and to improve it. In electron crystallography, the same tool would be desirable. However, the standard programs for least-squares structure refinement used in X-ray diffraction may give wrong results using electron diffraction data because the kinematically calculated diffracted intensity is not valid for the interaction of electrons and crystals thicker than about 20 A for strong scatterers. In this paper, a new approach is presented that overcomes this problem and in addition takes into account all the advantages contained in dynamic scattering. The multislice method, well known in high-resolution electron microscopy (HREM), was combined with a least-squares algorithm, resulting in the multislice least-squares (MSLS) procedure. Experiments show that the atomic positions obtained by the new procedure are of the same accuracy as those obtained from single-crystal X-ray diffraction. However, the size of the single crystals used is much smaller (diameters down to ±100 A). Also, light-atom positions can be determined with high precision by using data sets from crystal areas with different thicknesses. The multislice refinement gave good results up to 150 to 400 A depending on the composition of the crystal, with R values based on the intensities of less than 5%. An additional advantage of the approach is that some extra quantities (e.g. crystal thickness, crystal orientation) can be refined at the same time.

Optical gradient forces of strongly localized fields

Abstract: We present a new approach for determining optical gradient forces applied by strongly focused laser beams on dielectric particles. We show that when the electromagnetic field is focused to a diffraction limited spot a dipole approximation is valid for any particle size. We derive intuitive predictions for force-displacement curves, maximal trapping forces, and force constants. The theory fits well with recent measurements of particles trapped by laser tweezers. We also discuss effects of radiation pressure and gravity. [S0031-9007(98)06883-5]

Analyses of the Diffraction Efficiencies, Birefringence, and Surface Relief Gratings on Azobenzene-Containing Polymer Films

Abstract: Dynamical experimental studies on the diffraction efficiencies and the formation of birefringence grating and surface relief grating on doped and/or covalently bonded azobenzene derivatives containing polymer films were carried out using laser beams with different polarizations. From polarization analyses of the first-order (±1) diffracted beams, the contributions to the diffraction efficiency are separated into an anisotropic (or birefringence) part and a surface relief part. During the growth of the gratings the dynamical responses of both contributions appear to be quite distinct, and estimates of the time variations of the anisotropic phase shift, Δφ, due to the induced birefringence and of the surface relief height, 2Δd, due to the polymer mass transport are obtained. Calculations and simulations of the theoretical expressions allow us to confirm the experimental findings and to reproduce all the observed polarized first-order diffraction curves with good agreement, even when the surface relief is im...

Hard?X-ray diffraction study of charge stripe order in La1.48Nd0.4Sr0.12CuO4

Abstract: High-energy photon diffraction is used to investigate the charge ordering previously studied by neutron diffraction in La148Nd04Sr012CuO4 Besides confirming the existence of superlattice peaks due to charge order, the temperature dependence of the peak intensity, width, and position has been determined with improved precision Furthermore, we show that the scattered intensity has a sinusoidial modulation along c*, consistent with long-range Coulomb interactions between ordered charges within the CuO2 planes

Modeling of pulsed finite-amplitude focused sound beams in time domain.

Abstract: A time-domain numerical model is presented for simulating the finite-amplitude focused acoustic pulse propagation in a dissipative and nonlinear medium with a symmetrical source geometry. In this method, the main effects responsible in finite-amplitude wave propagation, i.e., diffraction, nonlinearity, and absorption, are taken into account. These effects are treated independently using the method of fractional steps with a second-order operator-splitting algorithm. In this method, the acoustic beam propagates, plane-by-plane, from the surface of a highly focused radiator up to its focus. The results of calculations in an ideal (linear and nondissipative) medium show the validity of the model for simulating the effect of diffraction in highly focused pulse propagation. For real media, very good agreement was obtained in the shape of the theoretical and experimental pressure-time waveforms. A discrepancy in the amplitudes was observed with a maximum of around 20%, which can be explained by existing sources of error in our measurements and on the assumptions inherent in our theoretical model. The model has certain advantages over other time-domain methods previously reported in that it: (1) allows for arbitrary absorption and dispersion, and (2) makes use of full diffraction formulation. The latter point is particularly important for studying intense sources with high focusing gains.

Magnesium dichloride-alcohol adducts, process for their preparation and catalyst components obtained therefrom

Abstract: The present invention relates to MgCl2.mROH.nH2O adducts, where R is a C1-C10 alkyl, 2≤m≤4.2, 0≤n≤0.7, characterized by an X-ray diffraction spectrum in which, in the range of 2υ diffraction angles between 5° and 15°, the three main diffraction lines are present at diffraction angles 2υ of 8.8 ± 0.2°, 9.4 ± 0.2° and 9.8 ± 0.2°, the most intense diffraction lines being the one at 2υ=8.8 ± 0.2°, the intensity of the other two diffraction lines being at least 0.2 times the intensity of the most intense diffraction line. Catalyst components obtained from the adducts of the present invention are capable to give catalysts for the polymerization of olefins characterized by enhanced activity and stereospecificity with respect to the catalysts prepared from the adducts of the prior art.

Atomic-scale structure of amorphous TiO2 by electron, X-ray diffraction and reverse Monte Carlo simulations

Abstract: Amorphous TiO2 materials, layers and powders, have been produced by different preparation techniques. The layers have been subjected to electron diffraction and the bulk powders to X-ray diffraction experiments. Three-dimensional structure models consistent with the experimental structure functions have been constructed by reverse Monte Carlo simulations. Partial distribution functions, interatomic distances and coordination numbers have been extracted from the model atomic configurations. The atomic arrangement in all amorphous TiO2 materials investigated has been found to resemble that occurring in brookite, a crystalline modification of TiO2, and to be well described as an assembly of short, staggered chains of Ti–O octahedra. The crystallization behaviour of amorphous TiO2 materials has also been discussed.

Computation of the near-field pattern with the coupled-wave method for transverse magnetic polarization

Abstract: In this paper, an accurate method for computing the elecromagnetic field distribution in lamellar gratings is proposed. The method that relies on the rigorous coupled-wave analysis provides accurate numerical results and avoids possible sources of artefacts due to permittivity discontinuities. Its performance is analysed through various lamellar grating cases, including dielectric and metallic materials, the visible and near-infrared (1–10 μm) regions of the spectrum. Special attention is placed on field singularities which are in general present at the grating wedges for transverse magnetic polarization.