Showing papers on "Diffraction published in 2021"

Generating Dual-Polarized Vortex Beam by Detour Phase: From Phase Gradient Metasurfaces to Metagratings

Abstract: Traditional methods of generating vortex beams based on metasurfaces consist mainly in modulating propagation phase or geometric phase. Here, by introducing detour phase, we propose the construction of dual-polarized vortex beam generators in the form of metasurface and metagrating (MG). The phase is modulated through moving the position of meta-atoms instead of varying the geometrical parameters or rotating the unit cells. To use detour phase, two kinds of unit cells are designed to achieve specific diffraction order. Each unit can arbitrarily and independently adjust the operation frequency and diffraction angle of transverse electric (TE) and transverse magnetic (TM) polarizations. Two vortex beam generators are designed and fabricated with different topological charges carried by orthogonal polarizations. To demonstrate the ability to independently manipulate, two polarizations of the generator based on MG are designed in different frequency bands. Both the simulation and experimental results validate the proposed method, showing great potential for polarization division multiplexing in orbital angular momentum (OAM) communication systems.

RETICOLO software for grating analysis

Abstract: RETICOLO implements the rigorous coupled wave analysis (RCWA) for 1D (classical and conical diffraction) and 2D crossed gratings. It operates under a MATLAB environment and incorporates an efficient and accurate toolbox for computing Bloch modes and visualizing the electromagnetic field in the grating region. As a spin-off, the Version V9 launched in 2021 includes a toolbox for the analysis of stacks of arbitrarily anisotropic multilayered thin-films.

Existence, symmetry breaking bifurcation and stability of two-dimensional optical solitons supported by fractional diffraction

Abstract: We study existence, bifurcation and stability of two-dimensional optical solitons in the framework of fractional nonlinear Schrodinger equation, characterized by its Levy index, with self-focusing and self-defocusing saturable nonlinearities. We demonstrate that the fractional diffraction system with different Levy indexes, combined with saturable nonlinearity, supports two-dimensional symmetric, antisymmetric and asymmetric solitons, where the asymmetric solitons emerge by way of symmetry breaking bifurcation. Different scenarios of bifurcations emerge with the change of stability: the branches of asymmetric solitons split off the branches of unstable symmetric solitons with the increase of soliton power and form a supercritical type bifurcation for self-focusing saturable nonlinearity; the branches of asymmetric solitons bifurcates from the branches of unstable antisymmetric solitons for self-defocusing saturable nonlinearity, featuring a convex shape of the bifurcation loops: an antisymmetric soliton loses its stability via a supercritical bifurcation, which is followed by a reverse bifurcation that restores the stability of the symmetric soliton. Furthermore, we found a scheme of restoration or destruction the symmetry of the antisymmetric solitons by controlling the fractional diffraction in the case of self-defocusing saturable nonlinearity.

Mesoporous WO3-TiO2 heterojunction for a hydrogen gas sensor

Abstract: In this study, the WO3-TiO2 composite was fabricated as a hydrogen gas sensing element, and the heterojunction effect produced by the combination of WO3 and TiO2 was applied to hydrogen sensing under room temperature. X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive spectrometry, multipoint Brunner-Emmet-Teller model and transmission electron microscopy were used to examine the structure and morphology of the samples. The fabricated 4.0 wt% WO3-TiO2 possessing mesoporous structure was obtained with a high surface area of 109.8 m2/g. 4.0 wt% WO3-TiO2 -based sensor exhibited a high sense response of 5.26–10,000 ppm H2 with a short recovery time (5 s). The sensing time required for a complete single cycle was 182 s, a drop of 90.8 % from that for the TiO2-based sensor (1974s). Additionally, good stability, repeatability, and selectivity of the sensing material were observed. These experimental results reveal that the 4.0 wt% WO3-TiO2 heterojunction with mesoporous structure exhibits considerable potential as a hydrogen sensing element. A possible sensing mechanism was proposed.

Spectroscopic, Structural, Thermal, and Mechanical Properties of B2O3-CeO2-PbO2 Glasses

Abstract: The melt-quenching method has been used to fabricate a PbO2–B2O3–CeO2 glass system. The XRD diffractometer procedure was used to check the status of these samples. It can be concluded, from the X-ray diffraction curves, that the tested samples have high levels of glassiness. As the CeO2 concentration increased most of the [BO4] are converted into [BO3] structural units with the formation of non-bridging oxygen ions in the borate matrix. It can be cross verified with the decrease of the N4 fraction from 0.654 to 0.239. This decrease may be attributed to the formation of [CeO7] structural units that needs more oxygen atoms. The ultrasonic velocities of the prepared glasses are decreased with the increase of CeO2 content. Different types of elastic modules were evaluated (experimental and theoretical) for the prepared glasses are increases with the increase of CeO2 content. Glass transition temperature (Tg), crystallization temperature (Tc), the peak of crystallization temperature (Tp) and thermal stability values decreases with the increase of CeO2 content. The refractive index of these samples is increasing with the increase in the reflection and the density.

Polarization-sensitive optical diffraction tomography

Abstract: Polarization of light has been widely used as a contrast mechanism in two-dimensional (2D) microscopy and also in some three-dimensional (3D) imaging modalities In this paper, we report the 3D tomographic reconstruction of the refractive index (RI) tensor using 2D scattered fields measured for different illumination angles and polarizations Conventional optical diffraction tomography (ODT) has been used as a quantitative, label-free 3D imaging method It is based on the scalar formalism, which limits its application to isotropic samples We achieve imaging of the birefringence of 3D objects through a reformulation of ODT based on vector diffraction theory The off-diagonal components of the RI tensor reconstruction convey additional information that is not available in either conventional scalar ODT or 2D polarization microscopy Finally, we show experimental reconstructions of 3D objects with a polarization-sensitive contrast metric quantitatively displaying the true birefringence of the samples

End-to-end nanophotonic inverse design for imaging and polarimetry

Abstract: By co-designing a meta-optical front end in conjunction with an image-processing back end, we demonstrate noise sensitivity and compactness substantially superior to either an optics-only or a computation-only approach, illustrated by two examples: subwavelength imaging and reconstruction of the full polarization coherence matrices of multiple light sources. Our end-to-end inverse designs couple the solution of the full Maxwell equations---exploiting all aspects of wave physics arising in subwavelength scatterers---with inverse-scattering algorithms in a single large-scale optimization involving $\gtrsim 10^4$ degrees of freedom. The resulting structures scatter light in a way that is radically different from either a conventional lens or a random microstructure, and suppress the noise sensitivity of the inverse-scattering computation by several orders of magnitude. Incorporating the full wave physics is especially crucial for detecting spectral and polarization information that is discarded by geometric optics and scalar diffraction theory.

Conceptual-based design of an ultrabroadband microwave metamaterial absorber

Abstract: By introducing metallic ring structural dipole resonances in the microwave regime, we have designed and realized a metamaterial absorber with hierarchical structures that can display an averaged -19.4 dB reflection loss (∼99% absorption) from 3 to 40 GHz. The measured performance is independent of the polarizations of the incident wave at normal incidence, while absorption at oblique incidence remains considerably effective up to 45°. We provide a conceptual basis for our absorber design based on the capacitive-coupled electrical dipole resonances in the lateral plane, coupled to the standing wave along the incident wave direction. To realize broadband impedance matching, resistive dissipation of the metallic ring is optimally tuned by using the approach of dispersion engineering. To further extend the absorption spectrum to an ultrabroadband range, we employ a double-layer self-similar structure in conjunction with the absorption of the diffracted waves at the higher end of the frequency spectrum. The overall thickness of the final sample is 14.2 mm, only 5% over the theoretical minimum thickness dictated by the causality limit.

Azimuthal modulation of electromagnetically induced grating using structured light.

Abstract: We propose a theoretical scheme for creating a two-dimensional Electromagnetically Induced Grating in a three-level $$\Lambda $$ -type atomic system interacting with a weak probe field and two simultaneous position-dependent coupling fields—a two dimensional standing wave and an optical vortex beam. Upon derivation of the Maxwell wave equation, describing the dynamic response of the probe light in the atomic medium, we perform numerical calculations of the amplitude, phase modulations and Fraunhofer diffraction pattern of the probe field under different system parameters. We show that due to the azimuthal modulation of the Laguerre–Gaussian field, a two-dimensional asymmetric grating is observed, giving an increase of the zeroth and high orders of diffraction, thus transferring the probe energy to the high orders of direction. The asymmetry is especially seen in the case of combining a resonant probe with an off-resonant standing wave coupling and optical vortex fields. Unlike in previously reported asymmetric diffraction gratings for PT symmetric structures, the parity time symmetric structure is not necessary for the asymmetric diffraction grating presented here. The asymmetry is due to the constructive and destructive interference between the amplitude and phase modulations of the grating system, resulting in complete blocking of the diffracted photons at negative or positive angles, due to the coupling of the vortex beam. A detailed analysis of the probe field energy transfer to different orders of diffraction in the case of off-resonant standing wave coupling field proves the possibility of direct control over the performance of the grating.

Data-driven discovery of a universal indicator for metallic glass forming ability

Abstract: Despite the importance of glass forming ability as a major alloy characteristic, it is poorly understood and its quantification has been experimentally laborious and computationally challenging. Here, we uncover that the glass forming ability of an alloy is represented in its amorphous structure far away from equilibrium, which can be exposed by conventional X-ray diffraction. Specifically, we fabricated roughly 5,700 alloys from 12 alloy systems and characterized the full-width at half-maximum, Δq, of the first diffraction peak in the X-ray diffraction pattern. A strong correlation between high glass forming ability and a large Δq was found. This correlation indicates that a large dispersion of structural units comprising the amorphous structure is the universal indicator for high metallic glass formation. When paired with combinatorial synthesis, the correlation enhances throughput by up to 100 times compared to today’s state-of-the-art combinatorial methods and will facilitate the discovery of bulk metallic glasses. The glass forming ability of alloys is found to be strongly correlated with the full-width at half-maximum of the first diffraction peak in the X-ray diffraction pattern, which facilitates the discovery of bulk metallic glass compositions.

4D-STEM of Beam-Sensitive Materials.

Abstract: ConspectusScanning electron nanobeam diffraction, or 4D-STEM (four-dimensional scanning transmission electron microscopy), is a flexible and powerful approach to elucidate structure from "soft" materials that are challenging to image in the transmission electron microscope because their structure is easily damaged by the electron beam. In a 4D-STEM experiment, a converged electron beam is scanned across the sample, and a pixelated camera records a diffraction pattern at each scan position. This four-dimensional data set can be mined for various analyses, producing maps of local crystal orientation, structural distortions, crystallinity, or different structural classes. Holding the sample at cryogenic temperatures minimizes diffusion of radicals and the resulting damage and disorder caused by the electron beam. The total fluence of incident electrons can easily be controlled during 4D-STEM experiments by careful use of the beam blanker, steering of the localized electron dose, and by minimizing the fluence in the convergent beam thus minimizing beam damage. This technique can be applied to both organic and inorganic materials that are known to be beam-sensitive; they can be highly crystalline, semicrystalline, mixed phase, or amorphous.One common example is the case for many organic materials that have a π-π stacking of polymer chains or rings on the order of 3.4-4.2 A separation. If these chains or rings are aligned in some regions, they will produce distinct diffraction spots (as would other crystalline spacings in this range), though they may be weak or diffuse for disordered or weakly scattering materials. We can reconstruct the orientation of the π-π stacking, the degree of π-π stacking in the sample, and the domain size of the aligned regions. This Account summarizes illumination conditions and experimental parameters for 4D-STEM experiments with the goal of producing images of structural features for materials that are beam-sensitive. We will discuss experimental parameters including sample cooling, probe size and shape, fluence, and cameras. 4D-STEM has been applied to a variety of materials, not only as an advanced technique for model systems, but as a technique for the beginning microscopist to answer materials science questions. It is noteworthy that the experimental data acquisition does not require an aberration-corrected TEM but can be produced on a variety of instruments with the right attention to experimental parameters.

Effects of exchange symmetry and quantum diffraction on amplitude-modulated electrostatic waves in quantum magnetoplasma

Abstract: In this paper we have made use of reductive perturbation technique (RPT) and carried out homotopy analysis method (HAM) to investigate the effect of exchange correlation and quantum diffraction on the electrostatic waves in quantum magnetoplasma. We have derived a nonlinear Schrodinger equation (NLSE) by using RPT that describes the spatiotemporal evolution of an initial waveform. Apart from this technique, we have made use of HAM to second our initial findings. It has been shown that both quantum diffraction H and parameter streaming velocity $$u_0$$ have significant effects in determining the stability criteria and the growth or decay of any instability created therein. The stable parametric regimes are crucial from the experimental point of view as well.

Experimental method to quantify the ring size distribution in silicate glasses and simulation validation thereof.

Abstract: Silicate glasses have no long-range order and exhibit a short-range order that is often fairly similar to that of their crystalline counterparts. Hence, the out-of-equilibrium nature of glasses is largely encoded in their medium-range order. However, the ring size distribution-the key feature of silicate glasses' medium-range structure-remains invisible to conventional experiments and, hence, is largely unknown. Here, by combining neutron diffraction experiments and force-enhanced atomic refinement simulations for two archetypical silicate glasses, we show that rings of different sizes exhibit a distinct contribution to the first sharp diffraction peak in the structure factor. On the basis of these results, we demonstrate that the ring size distribution of silicate glasses can be determined solely from neutron diffraction patterns, by analyzing the shape of the first sharp diffraction peak. This method makes it possible to uncover the nature of silicate glasses' medium-range order.

Linear and nonlinear optical properties of potassium dichromate in solution and solid polymer film

Abstract: A film with polyvinyl alcohol and aqueous solution of potassium dichromate are prepared where various linear optical constants and nonlinear properties are determined via the measurements of the film optical absorbance and transmittance spectra in the wavelength range 312–900 nm and via the diffraction ring patterns by transmitting a visible, 473 nm, continuous wave with single fundamental transverse mode, low power laser beam. As high as 1010 sec−1 optical conductivity and nonlinear refractive index of 10−13 cm2/W of the film are obtained due to the linear parameters and 10−7 cm2/W of the aqueous solution due to the diffraction ring patterns. Fraunhofer approximations of the Fresnel-Kirchhoff theory are used to simulate the diffraction ring patterns.

Can 3D electron diffraction provide accurate atomic structures of metal–organic frameworks?

Abstract: Many framework materials such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are synthesized as polycrystalline powders, which are too small for structure determination by single crystal X-ray diffraction (SCXRD). Here, we show that a three-dimensional (3D) electron diffraction method, namely continuous rotation electron diffraction (cRED), can be used for ab initio structure determination of such materials. As an example, we present the complete structural analysis of a biocomposite, denoted BSA@ZIF-CO3-1, in which Bovine Serum Albumin (BSA) was encapsulated in a zeolitic imidazolate framework (ZIF). Low electron dose was combined with ultrafast cRED data collection to minimize electron beam damage to the sample. We demonstrate that the atomic structure obtained by cRED is as reliable and accurate as that obtained by single crystal X-ray diffraction. The high accuracy and fast data collection open new opportunities for investigation of cooperative phenomena in framework structures at the atomic level.

Verification of selective laser melting heat source models with operando X-ray diffraction data

Abstract: The output obtained from operando X-ray diffraction experiments on Ti-6Al-4V is used to verify the accuracy of four FEM models in predicting the temperature evolution of the solidified domain, the cooling rates of the α and β phases, and the influence of the scanning vector length on the duration of the β phase. Three different laser heat sources are considered: a simple ellipsoid, a double ellipsoid, and a cylindrical source with a parabolic penetration curve. The comparison between simulated and experimental results allows to verify the role of radiation loss and symmetric/asymmetric enhanced thermal conductivity on the cooling evolution. Furthermore, it is shown that the evolution of the lattice strains evidence the formation of residual stresses and can be used for the further development of phase field and thermomechanical inspired FEM-based models.

Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals.

Abstract: Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called "atomically precise". That is unsurprising, given how challenging it is to quantify the degree of structural order in these materials. However, once that order crosses a certain threshold, the constructive interference of X-rays diffracted by the nanocrystals dominates the diffraction pattern, offering a wealth of structural information. By treating nanocrystals as scattering sources forming a self-probing interferometer, we developed a multilayer diffraction method that enabled the accurate determination of the nanocrystal size, interparticle spacing, and their fluctuations for samples of self-assembled CsPbBr3 and PbS nanomaterials. The multilayer diffraction method requires only a laboratory-grade diffractometer and an open-source fitting algorithm for data analysis. The average nanocrystal displacement of 0.33 to 1.43 A in the studied superlattices provides a figure of merit for their structural perfection and approaches the atomic displacement parameters found in traditional crystals.

Experimental realization of a pillared metasurface for flexural wave focusing

Abstract: A metasurface is an array of subwavelength units with modulated wave responses that show great potential for the control of refractive/reflective properties in compact functional devices. In this work, we propose an elastic metasurface consisting of a line of pillars with gradient heights, erected on a homogeneous plate. The change in the resonant frequencies associated with the height gradient allows us to achieve transmitted phase response covering a range of 2π, while the amplitude response remains at a relatively high level. We employ the pillared units to design a focusing metasurface and compare the properties of the focal spots through simulation and experiment. The subwavelength transverse and lateral full width at half maximum of the focusing intensity profiles are observed in both simulation and experiment, with the underlying mechanism being the interference and diffraction of the scattered waves from the resonant pillars as well as the boundaries (especially for experiment). The good correspondence between the experimental and simulated relative focal lengths shows the robustness of the focusing pillared metasurfaces with respect to fabrication imperfections. This proposed compact, simple, and robust metasurface with unaffected mechanical properties provides a new platform for elastic wave manipulation for energy harvesting, wave communication, sensing, and non-destructive testing among others.

Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction

Abstract: X-ray diffraction is routinely used for structure determination of stationary molecular samples. Modern X-ray photon sources, e.g., from free-electron lasers, enable us to add temporal resolution to these scattering events, thereby providing a movie of atomic motions. We simulate and decipher the various contributions to the X-ray diffraction pattern for the femtosecond isomerization of azobenzene, a textbook photochemical process. A wealth of information is encoded besides real-time monitoring of the molecular charge density for the cis to trans isomerization. In particular, vibronic coherences emerge at the conical intersection, contributing to the total diffraction signal by mixed elastic and inelastic photon scattering. They cause distinct phase modulations in momentum space, which directly reflect the real-space phase modulation of the electronic transition density during the nonadiabatic passage. To overcome the masking by the intense elastic scattering contributions from the electronic populations in the total diffraction signal, we discuss how this information can be retrieved, e.g., by employing very hard X-rays to record large scattering momentum transfers.

Noncontact super-resolution guided wave array imaging of subwavelength defects using a multiscale deep learning approach:

Abstract: Subwavelength defect imaging using guided waves has been known to be a difficult task mainly due to the diffraction limit and dispersion of guided waves. In this article, we present a noncontact su...

The space-time Talbot effect

Abstract: The Talbot effect, epitomized by periodic revivals of a freely evolving periodic field structure, has been observed with waves of diverse physical nature in space and separately in time, whereby diffraction underlies the former and dispersion underlies the latter. To date, a combined spatiotemporal Talbot effect has not been realized in any wave field because diffraction and dispersion are independent physical phenomena, typically unfolding at incommensurable length scales. Here, we report the observation of an optical “space–time” Talbot effect, whereby a spatiotemporal optical lattice structure undergoes periodic revivals after suffering the impact of both diffraction and dispersion. The discovered space–time revivals are governed by a single self-imaging length scale, which encompasses both spatial and temporal degrees of freedom. Key to this effect is the identification of a unique pulsed optical field structure, which we refer to as a V-wave, that is endowed with intrinsically equal diffraction and dispersion lengths in free space, thereby enabling self-imaging to proceed in lockstep in space and time.

Structural characterization of polycrystalline thin films by X-ray diffraction techniques

Abstract: X-ray diffraction (XRD) techniques are powerful, non-destructive characterization tool with minimal sample preparation. XRD provides the first information about the materials phases, crystalline structure, average crystallite size, micro and macro strain, orientation parameter, texture coefficient, degree of crystallinity, crystal defects etc. XRD analysis provides information about the bulk, polycrystalline thin films, and multilayer structures, which is very important in various scientific and material engineering fields. This review discusses the diffraction related phenomena/principles such as powder X-ray diffraction, and thin-film/grazing incidence X-ray diffraction (GIXRD) comprehensively for thin film samples which are used frequently in various branches of science and technology. The review also covers few case studies on polycrystalline thin-film samples related to phase analysis, preferred orientation parameter (texture coefficient) analysis, stress evaluation in thin films and multilayer, multiphase content identification, bifurcation of multiphase on multilayer samples, depth profiling in thin-film/ multilayer structures, the impact of doping effect on structural properties of thin films etc., comprehensively using GIXRD/XRD.

Properties of the generation and propagation of spatiotemporal optical vortices

Abstract: Spatiotemporal optical vortex (STOV) light is a new type of vortex light with transverse orbital angular momentum (OAM) which is different from conventional spatial vortex light. Understanding the properties of STOV are meaningful before STOV are applied. We present a theoretical study on the generation and propagation of spatiotemporal vortices step by step based on diffraction theory. The properties of the output pulses with different topological charges generated using 4 f pulse shaper in both the near-field and the far-field are analyzed. Using spiral phase mask, the intensity profiles of the output pulses immediately after the 4 f pulse shaper are of multi-lobe structures. With energies circulating around the phase singularity in the space-time plane, energy coupling occurs between the spatial and temporal domains in the wave packets during propagation, then the intensity profiles evolve into multi-hole shapes, and the holes tend to be merged for higher order STOV. The conservation of OAM in the space-time domain is shown clearly. The profiles of the output pulses in the near-field form donut rectangle shapes using π-step mask, and in the far-field, they split into a multi-lobe structure. The rules of the generation and evolution of STOV are revealed. The results demonstrate the physical properties of the STOV and the generation and propagation processes directly and clearly. It provides a guidance on the application of STOV.

Space-time wave packets localized in all dimensions

Abstract: Optical wave packets that are localized in space and time, but nevertheless overcome diffraction and travel rigidly in free space, are a long sought-after field structure with applications ranging from microscopy and remote sensing, to nonlinear and quantum optics. However, synthesizing such wave packets requires introducing non-differentiable angular dispersion with high spectral precision in two transverse dimensions, a capability that has eluded optics to date. Here, we describe an experimental strategy capable of sculpting the spatio-temporal spectrum of a generic pulsed beam by introducing arbitrary radial chirp via two-dimensional conformal coordinate transformations of the spectrally resolved field. This procedure yields propagation-invariant `space-time' wave packets localized in all dimensions, with tunable group velocity in the range from $0.7c$ to $1.8c$ in free space, and endowed with prescribed orbital angular momentum. By providing unprecedented flexibility in sculpting the three-dimensional structure of pulsed optical fields, our experimental strategy promises to be a versatile platform for the emerging enterprise of space-time optics.