Showing papers on "Diffraction published in 2018"

Macromolecular Imaging using X-ray free-electron lasers*

Abstract: X-ray crystallography, which is used for the determination of most biomolecular structures, has relied on Bragg diffraction from single crystals for more than a century. For many difficult to crystallize proteins, the growth of large well-ordered single crystals is a major challenge. Single molecule diffraction is a challenging but highly desired approach to structure determination, as it abolishes the need for crystallization and provides about four times higher information content than needed to solve a structure, unlike Bragg diffraction which is usually insufficient for direct phasing. Even using the powerful X-ray Free Electron Lasers, the challenges of this method have so far not been overcome to acquire atomic resolution structures. Recently, it was shown that the structure of a protein can be solved based on continuous diffraction from crystals with translational disorder.

Susceptibility Derivation and Experimental Demonstration of Refracting Metasurfaces Without Spurious Diffraction

Abstract: Refraction represents one of the most fundamental operations that may be performed by a metasurface. However, simple phase-gradient metasurface designs suffer from restricted angular deflection due to spurious diffraction orders. It has been recently shown, using a circuit-based approach, that refraction without spurious diffraction, or diffraction-free, can fortunately be achieved by a transverse (or in-plane polarizable) metasurface exhibiting either loss–gain, nonreciprocity, or bianisotropy. Here, we re-derive these conditions using a medium-based—and hence, more insightful—approach based on generalized sheet transition conditions and surface susceptibility tensors, and experimentally demonstrate for the first time, beyond any doubt, two diffraction-free refractive metasurfaces that are essentially lossless, passive, bianisotropic, and reciprocal.

Structural and electron diffraction scaling of twisted graphene bilayers

Abstract: Multiscale simulations are used to study the structural relaxation in twisted graphene bilayers and the associated electron diffraction patterns. The initial twist forms an incommensurate moire pattern that relaxes to a commensurate microstructure comprised of a repeating pattern of alternating low-energy AB and BA domains surrounding a high-energy AA domain. The simulations show that the relaxation mechanism involves a localized rotation and shrinking of the AA domains that scales in two regimes with the imposed twist. For small twisting angles, the localized rotation tends to a constant; for large twist, the rotation scales linearly with it. This behavior is tied to the inverse scaling of the moire pattern size with twist angle and is explained theoretically using a linear elasticity model. The results are validated experimentally through a simulated electron diffraction analysis of the relaxed structures. A complex electron diffraction pattern involving the appearance of weak satellite peaks is predicted for the small twist regime. This new diffraction pattern is explained using an analytical model in which the relaxation kinematics are described as an exponentially-decaying (Gaussian) rotation field centered on the AA domains. Both the angle-dependent scaling and diffraction patterns are in quantitative agreement with experimental observations. A Matlab program for extracting the Gaussian model parameters accompanies this paper.

Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum

Abstract: In magnetic materials, skyrmions are nanoscale regions where the orientation of electron spin changes in a vortex-type manner. Here we show that spin-orbit coupling in a focused vector beam results in a skyrmion-like photonic spin distribution of the excited waveguided fields. While diffraction limits the spatial size of intensity distributions, the direction of the field, defining photonic spin, is not subject to this limitation. We demonstrate that the skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of light wavelength, which corresponds to about 10 nanometre lengthscale. The application of photonic skyrmions may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the spin structure of the field, not its intensity, can be applied to achieve deep-subwavelength optical patterns.

An instrument for in situ time-resolved X-ray imaging and diffraction of laser powder bed fusion additive manufacturing processes

Abstract: In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement.

Photonic hook: a new curved light beam

Abstract: It is well known that electromagnetic radiation propagates along a straight line, but this common sense was broken by the artificial curved light—the Airy beam. In this Letter, we demonstrate a new type of curved light beam besides the Airy beam, the so-called “photonic hook.” This photonic hook is a curved high-intensity focus by a dielectric trapezoid particle illuminated by a plane wave. The difference between the phase velocity and the interference of the waves inside the particle causes the phenomenon of focus bending.

Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement

Abstract: Single-camera stereo-digital image correlation (stereo-DIC) techniques have gained increasing attentions and demonstrated excellent prospects in the experimental mechanics community owing to their prominent advantages of cost-effectiveness, compactness, and the avoidance of the complicated camera synchronization. Using additional optical devices, e.g. a diffraction grating, a bi-prism or a set of planar mirrors, pseudo stereo images of a test sample surface can be recorded with a single camera. By correlating these stereo images using DIC, full-field three-dimensional (3D) shape and deformation can be retrieved. This review comprehensively summarizes the historical development, methodologies, strengths and weaknesses of the diffraction grating-based, prism-based, four-mirror-adaptor-based single-camera stereo-DIC techniques, and the recently proposed novel full-frame single color camera-based stereo-DIC technique for full-field 3D shape and deformation measurement. The optical arrangements, principles and calibration procedures of these single-camera stereo-DIC techniques are described in detail. Since high-speed deformation measurement is efficiently achieved by combining the single-camera stereo-DIC with one high-speed camera, single-camera stereo-DIC techniques show great potential in impact engineering, vibration and other dynamic tests.

Electron diffraction data processing with DIALS

Abstract: Electron diffraction is a relatively novel alternative to X-ray crystallography for the structure determination of macromolecules from three-dimensional nanometre-sized crystals. The continuous-rotation method of data collection has been adapted for the electron microscope. However, there are important differences in geometry that must be considered for successful data integration. The wavelength of electrons in a TEM is typically around 40 times shorter than that of X-rays, implying a nearly flat Ewald sphere, and consequently low diffraction angles and a high effective sample-to-detector distance. Nevertheless, the DIALS software package can, with specific adaptations, successfully process continuous-rotation electron diffraction data. Pathologies encountered specifically in electron diffraction make data integration more challenging. Errors can arise from instrumentation, such as beam drift or distorted diffraction patterns from lens imperfections. The diffraction geometry brings additional challenges such as strong correlation between lattice parameters and detector distance. These issues are compounded if calibration is incomplete, leading to uncertainty in experimental geometry, such as the effective detector distance and the rotation rate or direction. Dynamic scattering, absorption, radiation damage and incomplete wedges of data are additional factors that complicate data processing. Here, recent features of DIALS as adapted to electron diffraction processing are shown, including diagnostics for problematic diffraction geometry refinement, refinement of a smoothly varying beam model and corrections for distorted diffraction images. These novel features, combined with the existing tools in DIALS, make data integration and refinement feasible for electron crystallography, even in difficult cases.

Airy Wave Packets Accelerating in Space-Time.

Abstract: Although diffractive spreading is an unavoidable feature of all wave phenomena, certain waveforms can attain propagation invariance. A lesser-explored strategy for achieving optical self-similar propagation exploits the modification of the spatiotemporal field structure when observed in reference frames moving at relativistic speeds. For such an observer, it is predicted that the associated Lorentz boost can bring to a halt the axial dynamics of a wave packet of an arbitrary profile. This phenomenon is particularly striking in the case of a self-accelerating beam-such as an Airy beam-whose peak normally undergoes a transverse displacement upon free propagation. Here we synthesize an acceleration-free Airy wave packet that travels in a straight line by deforming its spatiotemporal spectrum to reproduce the impact of a Lorentz boost. The roles of the axial spatial coordinate and time are swapped, leading to "time diffraction" manifested in self-acceleration observed in the propagating Airy wave-packet frame.

Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum

Abstract: Electrons are diffracted by a standing light wave of light, a phenomenon known as the Kapitza–Dirac effect. A generalization of this effect opens perspectives for the manipulation of ultrashort electron wavepackets by intense laser fields. In the early days of quantum mechanics Kapitza and Dirac predicted that matter waves would scatter off the optical intensity grating formed by two counter-propagating light waves1. This interaction, driven by the ponderomotive potential of the optical standing wave, was both studied theoretically and demonstrated experimentally for atoms2 and electrons3,4,5. In the original version of the experiment1,5, only the transverse momentum of particles was varied, but their energy and longitudinal momentum remained unchanged after the interaction. Here, we report on the generalization of the Kapitza–Dirac effect. We demonstrate that the energy of sub-relativistic electrons is strongly modulated on the few-femtosecond timescale via the interaction with a travelling wave created in vacuum by two colliding laser pulses at different frequencies. This effect extends the possibilities of temporal control of freely propagating particles with coherent light and can serve the attosecond ballistic bunching of electrons6, or for the acceleration of neutral atoms or molecules by light.

Dark State Optical Lattice with a Subwavelength Spatial Structure.

Abstract: We report on the experimental realization of a conservative optical lattice for cold atoms with a subwavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a one-dimensional array of ultranarrow barriers with widths less than 10 nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice and find good agreement with theoretical predictions. Even on resonance, the observed lifetimes of atoms trapped in the lattice are as long as 44 ms, nearly 10^{5} times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.

Deep-subwavelength control of acoustic waves in an ultra-compact metasurface lens.

Abstract: Space-coiling acoustic metasurfaces have been largely exploited and shown their outstanding wave manipulation capacity. However, they are complex in realization and cannot directly manipulate acoustic near-fields by controlling the effective path length. Here, we propose a comprehensive paradigm for acoustic metasurfaces to extend the wave manipulations to both far- and near-fields and markedly reduce the implementation complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate. A semi-analytical approach for such a design is established using a microscopic coupled-wave model, which reveals that the acoustic diffractive pattern at every slit exit is the sum of the initial transmission and the secondary scatterings of the coupled fields from other slits. For proof-of-concept, we examine two metasurface lenses for sound focusing within and beyond the diffraction limit. This work provides a feasible strategy for creating ultra-compact acoustic components with versatile potentials. Here, the authors propose an acoustic metasurface design to extend the wave manipulations to both far- and near-fields while reducing the complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate.

Structure of Carbon Materials Explored by Local Transmission Electron Microscopy and Global Powder Diffraction Probes

Abstract: Transmission electron microscopy and neutron or X-ray diffraction are powerful techniques available today for characterization of the structure of various carbon materials at nano and atomic levels. They provide complementary information but each one has advantages and limitations. Powder X-ray or neutron diffraction measurements provide structural information representative for the whole volume of a material under probe but features of singular nano-objects cannot be identified. Transmission electron microscopy, in turn, is able to probe single nanoscale objects. In this review, it is demonstrated how transmission electron microscopy and powder X-ray and neutron diffraction methods complement each other by providing consistent structural models for different types of carbons such as carbon blacks, glass-like carbons, graphene, nanotubes, nanodiamonds, and nanoonions.

Diffraction by a Wide Slit

Abstract: The problem of diffraction of scalar waves by an infinite conducting plane with a slit is investigated. Approximate expressions for the near and far fields, taking into account the interaction between the edges, are derived in terms of the well‐known solutions for the field produced when an isolated conducting half‐plane is excited by (a) a plane wave, and (b) a line source. Results of numerical calculation are given for the case of a plane wave normally incident on the slit. Twelve values of slit width ranging from 0.96 to 2.5 wavelengths are considered. A comparison of transmission coefficients is given. It is found that the new approximate solution agrees well with the exact solution, and provides a significant correction to the noninteraction solution. The accuracy increases with the slit width, so that the result is useful in the range where interaction cannot well be neglected but where the exact solution converges so slowly that computation is impracticable.

Measuring Three-Dimensional Strain and Structural Defects in a Single InGaAs Nanowire Using Coherent X-ray Multiangle Bragg Projection Ptychography

Abstract: III–As nanowires are candidates for near-infrared light emitters and detectors that can be directly integrated onto silicon. However, nanoscale to microscale variations in structure, composition, and strain within a given nanowire, as well as variations between nanowires, pose challenges to correlating microstructure with device performance. In this work, we utilize coherent nanofocused X-rays to characterize stacking defects and strain in a single InGaAs nanowire supported on Si. By reconstructing diffraction patterns from the 2110 Bragg peak, we show that the lattice orientation varies along the length of the wire, while the strain field along the cross-section is largely unaffected, leaving the band structure unperturbed. Diffraction patterns from the 0110 Bragg peak are reproducibly reconstructed to create three-dimensional images of stacking defects and associated lattice strains, revealing sharp planar boundaries between different crystal phases of wurtzite (WZ) structure that contribute to charg...

Polarization-Dependent Second Harmonic Diffraction from Resonant GaAs Metasurfaces

Abstract: Resonant semiconductor metasurfaces are an emerging versatile platform for nonlinear photonics. In this work, we investigate second-harmonic generation from metasurfaces consisting of two-dimensional square arrays of gallium arsenide nanocylinders as a function of the polarization of the fundamental wave. To this end, we perform nonlinear second harmonic microscopy, where the pump wavelength is tuned to the resonances of the metasurfaces. Furthermore, imaging the generated nonlinear signal in Fourier space allows us to analyze the spatial properties of the generated second harmonic. Our experiments reveal that the second harmonic is predominantly emitted into the first diffraction orders of the periodic arrangements, and that its intensity varies with the polarization angle of the fundamental wave. While this can be expected from the structure of the GaAs nonlinear tensor, the characteristics of this variation itself are found to depend on the pump wavelength. Interestingly, we show that the metasurface c...

Nitrogen-rich salts based on the combination of 1,2,4-triazole and 1,2,3-triazole rings: a facile strategy for fine tuning energetic properties

Abstract: The synthesis of four neutral 4-nitro-5-(1,2,4-triazol-3-yl)-2H-1,2,3-triazole compounds in combination with different energetic moieties like amino, nitrimino, nitro, and azo groups is presented. Furthermore, a novel family of energetic salts based on the 4-nitro-5-(5-R-1,2,4-triazol-3-yl)-2H-1,2,3-triazole (2: R = nitrimino; 3: R = nitro) monoanion and dianion were controllably synthesized. All new compounds were fully characterized by IR and multinuclear NMR spectroscopy, elemental analysis and DSC measurements. The molecular structures of 2, 3, 8 and 17 were determined by single-crystal X-ray diffraction. In addition, the heats of formation and detonation performance of all the energetic compounds were calculated using Gaussian 09 and EXPLO5 v6.01 programs, respectively. Both experimental and theoretical evaluations show promising properties for these energetic compounds, such as high density, positive heats of formation, high thermal stabilities, good sensitivities and excellent detonation performances. Among them, 6, 7, 14 to 16, 24 and 25 exhibit favorable overall properties as energetic materials.

Sharp acoustic vortex focusing by Fresnel-spiral zone plates

Abstract: We report the optimal focusing of acoustic vortex beams by using flat lenses based on a Fresnel-spiral diffraction grating. The flat lenses are designed by spiral-shaped Fresnel zone plates composed of one or several arms. The constructive and destructive interferences of the diffracted waves by the spiral grating result in sharp acoustic vortex beams, following the focal laws obtained in analogy with the Fresnel zone plate lenses. In addition, we show that the number of arms determines the topological charge of the vortex, allowing the precise manipulation of the acoustic wave field by flat lenses. The experimental results in the ultrasonic regime show excellent agreement with the theory and full-wave numerical simulations. A comparison with beam focusing by Archimedean spirals also showing vortex focusing is given. The results of this work may have potential applications for particle trapping, ultrasound therapy, imaging, or underwater acoustic transmitters.

Controlling Diffraction Patterns with Metagratings

Abstract: Although various wavefront-manipulation capabilities have been demonstrated with metasurfaces, both fundamental and practical difficulties remain. This study elaborates on a synergistic approach that combines metamaterials and gratings to achieve complete control of diffraction patterns. Unlike in a metasurface, in a metagrating the number of scatterers is significantly reduced, relaxing fabrication tolerance. Strong control of diffraction with simple excitation, ultrawide bandwidth, and significantly fewer scatterers is particularly interesting at optical and infrared frequencies, for $e.g.$ efficient, reconfigurable antennas in microwave communication systems.

Anomalous Bessel vortex beam: modulating orbital angular momentum with propagation

Abstract: Abstract Zero-order and higher-order Bessel beams are well-known nondiffracting beams. Namely, they propagate with invariant profile (intensity) and carry a fixed orbital angular momentum. Here, we propose and experimentally study an anomalous Bessel vortex beam. Unlike the traditional Bessel beams, the anomalous Bessel vortex beam carries decreasing orbital angular momentum along the propagation axis in free space. In other words, the local topological charge is inversely proportional to the propagation distance. Both the intensity and phase patterns of the generated beams are measured experimentally, and the experimental results agree well with the simulations. We demonstrate an easy way to modulate the beam’s topological charge to be an arbitrary value, both integer and fractional, within a continuous range. The simplicity of this geometry encourages its applications in optical trapping and quantum information, and the like.

Total Structure Determination of Au16(S-Adm)12 and Cd1Au14(StBu)12 and Implications for the Structure of Au15(SR)13

Abstract: Ultrasmall nanoclusters (e.g., Au15(SR)13) are crucial in not only real applications such as bioapplication but also in understanding the structure transition from gold complexes to gold nanoclusters. However, the determination of these transition-sized gold nanoclusters has long been a major challenge. In this work, two new nanoclusters in the transition regime, including the thus far smallest thiolated alloy nanocluster Cd1Au14(StBu)12 and the homogold nanocluster Au16(S-Adm)12, are obtained and their atomic structures are fully determined by single crystal X-ray diffraction. Moreover, based on the structures of Cd1Au14(SR)12 and Au16(SR)12, we perform DFT calculations to predict the structure of the “transformation” nanocluster, Au15 (Au15(SR)12– and Au15(SR)13). Overall, this work bridges the gaps between gold complexes and nanoclusters.

Liquid-crystal-based polarization volume grating applied for full-color waveguide displays.

Abstract: In this Letter, we demonstrate polarization volume grating (PVG)-based couplers for a double-layer waveguide display to realize a full-color near-eye display. The polarized interference exposure with photo-alignment methods was employed to generate a birefringent spiral configuration with two-dimensional periodicity in a chiral-dopant reactive mesogen material. Such a structure presents a unique highly efficient single-order Bragg diffraction with polarized selectivity. The prepared PVG couplers exhibited over 80% diffraction efficiency with large diffraction angles at spectra of blue (457 nm), green (532 nm), and red (630 nm). The demonstrated waveguide prototype showed a full-color display with a diagonal field of view of around 35°. The overall optical efficiency was measured as high as 118.3 cd/m2 per lumen with a transparency of 72% for ambient light.

Breaking the Diffraction Limit with Radially Polarized Light Based on Dielectric Metalenses

Abstract: DOI: 10.1002/adom.201800795 which suggested potential intriguing possibilities of metalenses in commercial imaging systems. Achromatic metalenses have also been reported with multiwavelengths[23,24] and continuous bandwidth.[25] A 3D Luneburg lens at optical frequencies was demonstrated with an ideal focusing ability.[26] Recently, a wide-angle metalens that can perform Fourier transform was realized, which proved the capabilities of mathematical operations with arrays of nanoparticles.[27] However, the resolution of such metalenses is still limited by theoretical Abbe limit since their design principles are based on scalar diffractive optics. Light beam with spatially inhomogeneous distributed polarization states, also known as vector beam, plays a critical role in many optical systems. Radially polarized light (RPL) has been investigated for many years in both conventional optics[28,29] and metasurfaces[30,31] due to its great potential in optical trapping, data storage, quantum dots, and so on. Since the diffraction of vector beams includes different electric components related to the propagation direction, the focal points of vector beams are not limited to the scalar diffraction–based Abbe limit. Several methods have been proposed to break the diffraction limit based on vector beams especially RPL. For example, an annular aperture was utilized to get tight RPL focusing with long depth of focus (DOF).[32,33] A parabolic mirror with high NA has also been demonstrated, which can focus RPL to a diffraction-limited dot at the visible.[34,35] In addition, by using an additional phase modulation, such as two discrete phase 0 and π, the fullwidth at half maximum (FWHM) of focal point can be reduced according to theoretical calculation.[36] The abovementioned methods are based on traditional optical media, which suffers from bulky size, limited capability of phase and polarization modulation, and weak strength of light-matter interaction. In recent years, some metasurfaces have been designed to transform the linearly polarized light or circularly polarized light to RPL,[29,37,38] and vice versa. Although phase and polarization of light beams can both be modulated through metasurfaces,[12] super-resolution with RPL based on a single-layer metalens has not been realized due to the absence of studying simultaneous control of RPL generation and focusing. In this study, we demonstrate a single-layer metalens to generate and focus RPL simultaneously based on elliptic silicon post arrays. We realize super-resolution with two different methods by applying a circular aperture (CA) and additional phase distribution to the metalens, respectively. The CA is Dielectric metalenses with high efficiency and compact size have been widely investigated recently, but still suffer from Abbe diffraction limit. Herein, with linear polarization incidence, a dielectric metalens is demonstrated to efficiently generate and focus radially polarized light simultaneously. Two novel methods are proposed to achieve super-resolution. First, a circular high-pass aperture is utilized to enhance the longitudinal field component in the vicinity of focus with the focal spot size of 0.138λ2, much smaller than the theoretical limit of 0.212λ2. The key parameters that impact the focusing size are explored in detail, such as radius of the circular aperture and numerical aperture of the metalens. Second, an extra phase distribution is added on the metalens to filter the transversely polarized component, which leads to a focal spot size of 0.144λ2. The approach provides a wide platform for sub-resolution focusing and imaging, which offers the capability of subdiffraction techniques for microscopy systems and information processing with extensive channels.