Convergent Beam Research Articles

Potentials of individual atoms by convergent beam electron diffraction

In convergent beam electron diffraction (CBED) on two-dimensional (2D) materials, the intensity distributions within the individual CBED spots map the local atomic arrangements within the probed region. In this study we demonstrate that the average intensities within the CBED spots essentially depend on the scattering parameters by a single atom, thus, offering the possibility of the direct measurement of such parameters. Scattering potential of an individual atom can be approximated by the Gaussian function and its parameters, the standard deviation and the maximal phase shift, can be recovered from the ratio of the intensities in the zero- and higher orders spots in CBED pattern. In order to demonstrate this, we simulated CBED patterns and extracted the atomic scattering parameters from such patterns. In the simulated examples, no weak phase object approximation is applied and the proposed method provides accurate results even for materials with large phase shifts up to 1.5 rad, as for example tungsten. The scattering parameters recovered from the experimental CBED patterns of graphene and twisted graphene-hBN structure show good agreement with the theoretically obtained values.

Convergence of illuminating beams suffering from scattering during the individual measurement of suspended particles

The convergent illuminating beam is the key factor during the individual measurement of suspended particles in aquatic suspensions. When the illuminating beam propagates in the suspension, the particle scattering in the optical path may destroy the convergence of the illuminating beam, especially in suspensions with a high particle concentration. In this paper, using the Monte Carlo simulation, the convergence of the illuminating beam is investigated by changing the physical properties of particles, such as size and concentration, and the optical path length of the illuminating beam. A dimensionless quantity, as the product of the scattering coefficient of suspension and the optical path length, is found to determine the achievement of the convergent beam. Moreover, an individual measurement setup based on the convergence of the illuminating beam is used to measure polystyrene microspheres with different concentrations. The experiment results are consistent with those of the simulations. Furthermore, improvement strategies are proposed and proved to effectively keep the convergence of the illuminating beam in turbid water. The results in this work can provide clues for designing a similar optical apparatus used in aquatic environment monitoring.

Thickness profiling of electron transparent aluminium alloy foil using convergent beam electron diffraction.

Convergent beam electron diffraction (CBED) was used to profile the thickness of aluminium alloys foils prepared by using the twinjet electropolishing method. The two-beam CBED condition was obtained by exciting the and aluminium diffracted g-vector. The aluminium alloy foil thicknesses were calculated at different distances from the sample hole edge. In areas where only one Kossel-Möllenstedt (K-M) minima fringe was obtained, the thickness was determined by matching the experimental with simulated convergent beam diffraction patterns. In areas far away from the sample edge, the thickness of foils was high enough to generate at least two (K-M) minima fringes, required for linear regression fitting.

Spatially enhanced electric field induced second harmonic (SEEFISH) generation for measurements of electric field distributions in high-pressure plasmas

The spatial resolution of the ps electric field induced second harmonic (EFISH) generation has been enhanced by using non-collinear pump laser beam arrangements. The pump laser beam (1064 nm, nominal pulse duration 150 ps, pulse energy 20–40 mJ) is separated into two coaxial or crossing beams, overlapping only near the focal point. The spatially enhanced EFISH (SEEFISH) signal is generated over a shorter beam overlap region compared to the collinear beam arrangement. Blocking of either of the two beams results in a complete suppression of the signal. The signal is spatially isolated from the ‘conventional’ EFISH signal and measured by a photomultiplier detector. Measurements of a known Laplacian field generated between two parallel cylinder electrodes in ambient air shows that SEEFISH improves the spatial resolution of the measurements by up to a factor of 2, such that the measurement results agree with the Laplacian field distribution. The spatial resolution is improved further by reducing the focal distance of the lens. The magnitude of the SEEFISH signal is significantly lower compared to that of the single-beam EFISH and decreases rapidly as the beam crossing angle is increased, due to the phase mismatch. This approach has a significant potential for measurements of electric field distributions in high-pressure plasmas, with an additional benefit of removing the stray second harmonic signal from the optical access windows.

Convergent beam X-ray diffraction using multilayer Laue lenses – an exploration following in John's footsteps

Convergent beam X-ray diffraction using multilayer Laue lenses – an exploration following in John's footsteps

Multi-scale bullseye antennas.

We design, simulate and experimentally characterize a multi-scale bullseye antenna for the broadband manipulation of microwaves. The device achieves far-field beam-forming via tailored diffraction at the interface between two concentric bullseye geometries, with near-field energy concentration resulting from the overlap of the diffracted beams. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'.

Investigation of ACS image stabilization of on-board optoelectronic guidance and tracking devices

Introduction. The movement of the carrier and external factors (the effects of the atmosphere, temperature and pressure) degrade significantly the image quality of the servo optoelectronic systems (OES) and the positioning accuracy of the emitting OES. The issues of image quality improvement and the probability of keeping the image of the observation object (OO) on the optical axis of the servo EOS are considered.Materials and Methods. The development of an automatic control system for an optoelectronic device (ACS OED) involved solving a multi-criteria optimization problem taking into account a number of conflicting technical-andeconomic (TE) requirements. The determination of tolerated dynamic errors (TDE) of image stabilization was a key issue in the development of on-board optoelectronic devices (OOED). Lagrange equations of the second kind and the mixed Gilbert method made it possible to obtain a mathematical model of the CO OED. Then, the decomposition of a two-link ACS with nonlinear cross-couplings in the CO was performed. A functional diagram of the image formation model of the OOED was presented. The parameters of the matrix photodetector and the requirements for the dynamic error of the ACS OED, taking into account the permissible MTF of the OED, were listed. The functions of transferring modulation, as well as linear, harmonic and vibrational shift of the image corresponding to the permissible and achieved TDE were visualized. Logarithmic frequency characteristics were created in the Mathcad environment. The two-link control system of the OED with the specified parameters of the CO for the considered movement was presented as two independent azimuth and elevation control channels.Results. The processes of control of the on-board optoelectronic system in the stabilization and tracking modes were described. To study the dynamics of spatial control of the OOEP in accordance with the ACS methodology, a computer simulation model (CSM) of the digital automatic control systems (DACS) of the OED was developed. It was implemented in the Matlab environment and consisted of CSM CO, drives, proportional-integral-derivative (PID) controllers taking into account non-linearities, a central computing device (CCD), a guidance software device, a CSMcarrier that implemented the equations of motion. Harmonic vibrations of the carrier were described. The errors of tracking and stabilization in the tracking mode with an additional control action introduced in the form of a constant speed were determined. The dynamics of spatial control of the OOED was investigated. A computer simulation model of a digital automatic control system of an optoelectronic device, the results of modeling the DACS OED without considering the board movement, and the processes of OED control subject to movement were visualized.Discussion and Conclusions. The stabilization accuracy was calculated for the studied cases. It was established that the stabilization tens of times exceeded the previously stated indicators, and it tens of times reduced the requirements for the convergence of the laser beam and the laser radiation power when developing the optical path of the product in question. The proposed CSM can be used in the development of the on-board optoelectronic systems. In this case, the application of the presented methodology and CSM will help to reduce labor costs and minimize errors.

Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe2 Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy.

PdSe2 has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe2 is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe2. Monoclinic PdSe2 is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. The structure and composition of the monoclinic PdSe2 crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe2 flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe2 exhibits n-type charge carrier conduction with electron mobilities up to ∼298 cm2 V-1 s-1 and a strong in-plane electron mobility anisotropy of ∼1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.

Effect of lens-to-sample distance on spatial uniformity and emission spectrum of flat-top laser-induced plasma

The optimal spectral excitation and acquisition scheme is explored by studying the effect of the lens-to-sample distance (LTSD) on the spatial homogeneity and emission spectra of flat-top laser converging spot induced plasma. The energy distribution characteristics before and after the convergence of the laser beam with quasi flat-top intensity profile used in this study are theoretically simulated and experimentally measured. For an aspheric converging mirror with a focal length of 100 mm, the LTSD (106 mm ≥ LTSD ≥ 96 mm) was changed by raising the stainless-steel sample height. The plasma images acquired by ICCD show that there is air breakdown when the sample is below the focal point, and a ring-like plasma is produced when the sample is above the focal point. When the sample is located near the focal point, the plasma shape resembles a hemisphere. Since the spectral acquisition region is confined to the plasma core and the image contains all the optical information of the plasma, it has a lower relative standard deviation (RSD) than the spectral lines. When the sample surface is slightly higher than the focal plane of the lens, the converging spot has a quasi flat-top distribution, the spatial distribution of the plasma is more uniform, and the spectral signal is more stable. Simultaneously, there is little difference between the RSD of the plasma image and the laser energy. In order to further improve the stability of the spectral signal, it is necessary to expand the spectral acquisition area.

Correction to: Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg-Si alloy, using convergent beam electron diffraction

Correction to: Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg-Si alloy, using convergent beam electron diffraction

Numerical Study on the Laser Annealing of Silicon Used in Advanced V-NAND Device.

Laser melt annealing of amorphous silicon (a-Si) and subsequent recrystallization of a-Si are essential processes for successfully implementing vertical NAND (V-NAND) flash memory devices developed based on the cell-over-periphery (COP) structure. The aim of this study was to develop the numerical model for the laser melting process of a-Si used in V-NAND COP structure. In this study, the numerical simulation predicting the temperature distribution induced by multipath laser scanning and beam overlapping was conducted. In particular, the temperature uniformity and melt duration issues, which are critical in practical laser melt annealing applications in semiconductor fabrication, were discussed based on the simulated temperature distribution results. According to the simulation results, it was found that the annealed surface was subjected to rapid heating and cooling. The heating and cooling rates after temperature stabilization were 4.7 × 107 K/s and 2.04 × 107 K/s, respectively. The surface temperature increased with time and beam overlap ratio owing to the preheating effect and increasing heat accumulation per unit area. Under the process conditions used in the simulation, the temperature in a-Si was far above its melting point (1440 K), which numerically indicated full melting of the a-Si layer. Temperature uniformity within the annealed area was significantly improved when an overlap ratio of 50% was used. It was also found that using an overlap ratio of 50% increased the melt duration by 29.8% compared with an overlap ratio of 25%.

Enhancing forced-convection heat transfer of a channel surface with piezo-fans

The beam shape of a piezo-fan and the inlet turbulence intensity (TI) of channel flow are important parameters that affect the heat transfer enhancement of a heated surface in an airflow channel with a vibrating piezo-fan. In this study, 3D numerical channel models of different piezo-fans are considered when investigating the influence of beam shapes and inlet turbulence intensity on the heat transfer performance of channel flow, within a range of 4000–30,000 Reynolds numbers (ReCF) and 5% to 20% inlet turbulent intensity (TI). The influence of these parameters is analyzed in terms of the distributions of the flow field, time-averaged heat transfer coefficient, and overall thermal performance. The results show that the piezo-fans with the widest rectangular shape and a convergent shape behave better in terms of overall thermal performance in the channel. However, as the turbulence intensity increases to 20%, the difference in heat transfer caused by the beam shape becomes negligible. The maximum overall thermal performance parameter (η) = 1.47 is obtained at ReCF = 13,000 and TI=20% for the convergent beam shape (Beam-E).

Tailored nanophononic wavefield in a patterned bilayer system probed by ultrafast convergent beam electron diffraction

Optically excited nanostructures provide a versatile platform for the generation of confined nanophononic fields with potential (non-)linear interactions between different degrees of freedom. Control of resonance frequencies and the selective excitation of acoustic modes still remains challenging due to the interplay of nanoscale geometries and interfacial coupling mechanisms. Here, we demonstrate that a semiconductor membrane patterned with a platinum stripe acts as a tailored source for high-frequency strain waves generating a multi-modal distortion wave propagating through the membrane. To locally monitor the ultrafast structural dynamics at a specific distance from the deposited metal stripe, we employ ultrafast convergent beam electron diffraction in a laser-pump/electron-probe scheme. Experimentally observed acoustic deformations are reproduced by numerical simulations in a continuous medium model, revealing a spatiotemporal evolution of the lattice dynamics dominated by local rotations with minor strain and shear contributions.

Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg–Si alloy, using convergent beam electron diffraction

Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg–Si alloy, using convergent beam electron diffraction

The Role of the Directivity of Various THz Detectors in Multiplexing Systems

Many modern and future systems, based on the wireless communication at the THz frequencies, could benefit from multichannel transmission. One of the possible approaches is to (de)multiplex several separate signals to and from a single transmission channel using dedicated diffractive optical elements. Proper selection of receivers for such systems is crucial and strongly depends not only on the frequencies used but also on the geometry of the setup. In this article, we present a complex analysis of the applicability of various detectors for the characterization of highly convergent and off-axis beams. Three three-focal-spot diffractive lenses have been designed, optimized and manufactured to verify the influence of parameters such as focal length, focal position shift, deflection angle or radiation frequency on the proper detection and separation of focal spots using different receivers. The reliable characterization of multi-focal-point structures can be performed only with high-acceptance-angle detectors, such as, for example, field-effect transistors equipped with a patch antenna. On the other hand, for the detection of a single demultiplexed signal, a much more directive receiver can be applied, as long as it is placed at a proper angle.

Transceiver Decoupling of Multivariate Symmetric Hybrid Precoding Based on 5G

Mobile Internet will promote the continuous change of human interaction, leading to an increase in mobile traffic, so the demand for network bandwidth and data volume is rising rapidly, which is also one of the problems that 5G needs to solve. The mobile communication network of the railway system has the characteristics of high-speed user mobility, large-scale group mobility of users, high certainty of user mobile lines, and high QoS requirements for dispatching information. In order to improve the transmission reliability requirements of the railway system for wireless communication, a quick search method algorithm based on GMCS model to encode the number of each subinterval is proposed. Hybrid precoding is designed according to multivariate symmetry rules. The target beam is designed according to the GMCS model, and the hierarchical training beam is designed to minimize the mean square error between the training beam and the target beam as the objective function. Then, the fast search model based on beam overlap is extended to NLoS to solve the problem of misjudgment caused by multipath. In the simulation experiment, it proves that the search success rate of the research in this paper is 10% higher than that of the traditional algorithm. It improves the search speed and has obvious advantages in complexity. It can provide a dynamic reliable conversion mechanism for the railway communication environment, reduce the transmission power of the base station, and optimize the actual effect of uplink and downlink service requirements.

Structure Factors and Charge Density Description of Aluminum: A Quantum Crystallographic Study.

Static structure factors and charge density for metallic aluminum were investigated by periodic calculations using atom-centered Gaussian-type basis sets with the Perdew-Burke-Ernzerhof (PBE) functional implemented in the CRYSTAL14 package and X-ray constrained wave function (XCW) fitting. The effects of additional diffuse d and f basis functions on structure factors were compared with synchrotron powder X-ray diffraction and quantitative convergent electron beam diffraction data. Changes in structure factors from an independent atom model at 022, 113, and 222 reflections introduced d and f basis functions similar to those of the experimental data. The XCW fitting was applied to different sizes of aluminum clusters. The charge density features for a 50-atom cluster clearly demonstrated electron accumulation at tetrahedral sites and electron depletion at octahedral sites. The resolution dependence of the XCW study suggests that structure factors of the five lowest resolution reflections with 0.1% accuracy were indispensable for determining the detailed bonding description in the case of metallic aluminum.

Numerical investigation of the energy distribution of Low-intensity transcranial focused ultrasound neuromodulation for hippocampus

ObjectiveUltrasonic neuromodulation as a safe and non-invasive brain stimulation method that delivers a low-intensity, focused ultrasound to nervous system tissue in a targeted area of the brain. The objective of this study is to numerically investigate the ultrasound wave propagation and the energy distribution within the brain tissues using customized single element focused ultrasound transducers (SEFT), targeting the hippocampus. MethodsA high resolution detailed human head model with seven tissue types was constructed from magnetic resonance imaging (MRI). A full-wave finite-difference time-domain simulation platform, Sim4life, was then used to simulate a 3D non-linear ultrasound wave equation to the specific region of interest, the hippocampus. Three customized SEFT were used to test the effect of transducer positions, and another customized transducer was used to compare the sensitivity effect on heterogeneous and homogeneous brain models. Finally, the sensitivity and performance of low intensity focusing ultrasound stimulation were evaluated. ResultsAn optimized application of SEFT was customized to deliver 100 W/m2 intensity of energy deposition at the hippocampus region. About 85.65% of the generated volume beam was delivered to the targeted hippocampus region and the beam overlap parameter was affected by different transducer positions. Deflection angle changes of SEFT at the range of ± 5% did not have a significant effect on energy delivery and position displacement. Only 0.5% of peak pressure change was observed between heterogeneous and homogeneous brain models. The sensitivity analysis also showed that the sound speed is the most influential acoustic parameter. SignificanceThis study demonstrated that ultrasound neuromodulation targeting the depth brain tissue of the hippocampus could be a potential and promising alternative method to some non-acoustic brain stimulation modalities. In the numerical study of ultrasound brain stimulations, ultrasound parameters and the brain model need to be properly determined to simulate the ultrasonic neuromodulations.

Nanosecond laser pulse modulation using seed electrons from cascade ionization induced by inverse-Bremsstrahlung photon absorption.

Nanosecond (ns) laser pulses are modulated by seeding electrons on the laser beam path. The seed-electrons are from auxiliary ns-laser-induced breakdown (ALIB), and the ALIB is induced by a focused 1064-nm pulse, which is split after the frequency-doubling that generates the 532-nm pulse; therefore, the 532-nm and 1064-nm pulses are synchronized. The slowly converging (focal length = 500 mm) 532-nm pulse is re-directed to transmit through the region in where the ALIB-generated electrons reside. The seed-electrons from the ALIB then absorb the 532-nm photons via the inverse-Bremsstrahlung photon absorption (IBPA) process. The number density of the seed-electrons on the 532-nm beam path (ne,ALIB) is controlled by varying 1) the 532-nm pulse arrival time at the ALIB region (ΔPAT) after the 1064-nm pulse triggers the ALIB and 2) the location of the 532-nm beam relative to the core of the ALIB; the electron number density in ALIB is highly non-uniform and evolves in time. Electron-seeded laser-induced breakdown (ESLIB) occurs when ne,ALIB is sufficiently high. The 532-nm beam convergence (controlled by the focusing lens) is adjusted so that the breakdown does not occur without the electron seeding. The ESLIB immediately stops the transmission of the trailing edge of the laser pulse acting as a fast shutter, and ne,ALIB above a threshold can cut the pulse leading edge to modulate the 532-nm laser pulse.

Effect of the γ’volume fraction on the creep strength of Ni-base superalloys

AbstractThree single-crystalline Ni-base superalloys with varying content of the γ′ phase are investigated with respect to their microstructure and creep behaviour at 1000 °C. It is shown by convergent beam electron diffraction, X-ray diffraction and energy-dispersive X-ray spectroscopy that the composition of the matrix and precipitates as well as the lattice misfit between both phases are similar for all three model alloys despite a variation of the γ′ content from about 50 to about 65 %. This allows to study the influence of the precipitate volume fraction on high-temperature behaviour in separation from other microstructural parameters. Highest creep strength is found for the alloy with intermediate particle content (about 58 %), which is significantly below the particle volume fraction of contemporary single-crystal superalloys such as CMSX-4.