Asymmetric Beam Research Articles

Design and simulation of asymmetric Y-junction beam splitter with controllable splitting based on adjusted air-hole defect

Design and simulation of asymmetric Y-junction beam splitter with controllable splitting based on adjusted air-hole defect

Single-Cavity Gyromultipliers With Asymmetric Electron Beams

A new approach to realization of a high cyclotron harmonic gyro-oscillator based on the frequency multiplication effect is proposed and numerically studied. This approach utilizes axially asymmetric electron beam optics, which nevertheless may be as simple as the one of the conventional gyrotrons. Numerical simulations show that the proposed method can provide a watt-level output power in THz and sub-THz frequency ranges using existing electron guns.

Asymmetric rotating array beams with free movement and revolution

We introduce a new class of partially coherent asymmetric array beams. When the beam propagates, the spectral density of each lobe and the corresponding degree of coherence have rotating behavior. Especially, not only can array-like lattices revolve arbitrarily, but also they can move freely by controlling transverse plane shifts. Furthermore, we have generated this kind of beam experimentally, and the experimental phenomena are consistent with the numerical simulation results. Such a rotating beam with free movement and revolution may broaden the way for optical applications. More importantly, it inspires further studies in the field of asymmetric coherence gratings and lattices.

Influence of the test methodology on the fatigue behaviour of a composite subjected to a mixed fracture mode I/II

Composite materials manufactured by means of the superposition of layers and some specific morphologies, are susceptible to losing part of their resistant capacity due to the presence of delaminations between their  layers. The objective of this work is to evaluate experimentally the generation and propagation of these interlaminar cracks in a carbon-epoxy composite subjected to dynamic stress under mixed fracture mode I/II, using for this purpose two different test methodologies, on the one hand the standardized MMB (Mixed-Mode Bending) and on the other hand the ADCB (Asymmetric Double Cantilever Beam), with the intention of exploring the viability of the ADCB test as a simpler alternative of execution, especially in fatigue tests, to the MMB test. For this purpose and after a previous static characterization of the material in which the critical values of the energy release rate were determined in both test methods, the energy release rate levels to be applied in the fatigue tests were defined for two GII/Gc mixed mode grades of 0.2 and 0.4(0,33 ADCB)   and a ratio of stress levels to fatigue of R= Gmin/Gmax=0.1.DG-N fatigue initiation curves were obtained from these experimental data. The most relevant result of the work is that, in general, the MMB mixed mode provides different fatigue limits depending on the degree of mixed mode, whereas in the ADCB type test the fatigue limits tend to have the same value.

The analysis of mode decomposition in interlaminar fracture tests of asymmetric specimens

Interlaminar failure tests are usually carried out in specimens where crack arms are symmetric. Asymmetric Double Cantiliver Beam (ADCB) is an exception where both crack arms have different thickness in order to get mixed mode I/II. The asymmetry in ADCB specimens could be applied also to te case of End Notched Flexure Tests, obtaining also mixed mode I/II. The decomposition proposed by Williams is not valid in the case of ADCB tests. On the other hand, articles related to asymmetric ENF tests including the mode decompositiion have not been found. The present study deals with the analsys of the mode decomposition in the case of specimens with asymmetric arms. The asymmetry may be related to the different thickness of the crack arms, to different materials or a combination of both factors. The results obtained will be compared with the results of mode decompsition that have been published in the case of ADCB tests.

Nonlinear Deformation Analysis of Fg-Gpls Reinforced Asymmetric Sandwich Beams with Auxetic Core on Nonlinear Elastic Foundation Under Hygrothermal Conditions

Nonlinear Deformation Analysis of Fg-Gpls Reinforced Asymmetric Sandwich Beams with Auxetic Core on Nonlinear Elastic Foundation Under Hygrothermal Conditions

Analysis on fracture behaviour of stitched foam sandwich composites using interlaminar tension test

Sandwich composites are susceptible to interfacial delamination, owing to the mismatches in the material properties between the face sheets and core. Previous studies have shown that stitching can improve the performance of sandwich composites. In this study, an analysis approach was developed to investigate the fracture behaviour of stitched foam sandwich composites. The stitched foam sandwich composites were manufactured by a vacuum-assisted resin transfer moulding process. Interlaminar tension tests revealed the effects of the linear thread density on the failure mechanisms of the stitched foam sandwich composites. Asymmetric double cantilever beam tests were performed to investigate the influences of the stitch thread reinforcement on the fracture behaviour. An analytical approach combining extended finite element method and nonlinear spring elements was proposed to predict the failure behaviour of the stitched sandwich composites. Experiment and simulation approaches were employed to investigate the influences of the stitch parameters (stitch pitch and linear thread density) on the ultimate load and energy absorption. The results show that stitched method can significantly enhance the mechanical properties of sandwich composites. The energy absorption and ultimate load values of the specimens tend to increase with an increase in the linear thread density or a decrease in the stitch pitch.

The Dose Distribution from Iridium-192 Source on Cervical Cancer Brachytherapy by Manchester System Using Monte Carlo Simulation

One treatment for cervical cancer is to use radioactive sources that directly target the cancer cell called brachytherapy. This study is aimed to determine dose distribution at phantom pelvis using the DOSXYZnrc Monte Carlo code. The phantom was derived from a CT scan image of the DICOM-type pelvis with a size of 50 × 50 × 28.8 cm obtained from Santosa Kopo Hospital. The source used was Ir-192, which makes an asymmetrical beam with a size of 0.45 × 0.09 × 0.09 cm. Monte Carlo simulation was performed to determine the dose distribution of the Ir-192 source on cervical cancer CT images based on the Manchester system. The Monte Carlo simulation was divided into two models with distance variations on the applicator. Model A used TPS data with a distance between sources of 0.9 cm, while model B had a distance between sources of 0.5 cm. The distribution of dose resulting from the Monte Carlo simulation was analyzed and compared with TPS data. The results showed that at the range of 50 %, dose distribution in model A reaches the end of 3.9 cm. When compared to the range of 50 % dose distribution at the TPS results that reaches the point of 4 cm, it produces a deviation value of 2.5 %, which is still within the tolerance range. Model A and Model B provide different dose distribution. In model B, it reaches 3.86 cm, resulting in a deviation of 1.02 %, which is still within the tolerance range. The resulting γ-index value for the 50 % dose distribution was 2.26, while the whole area's GPR value was 94.13 %. This indicates a difference in dose distribution between the two models. Therefore, the smaller the distance between the sources, the shorter the dose distribution range with relatively more uniform dose distribution.

Spatially displaced and superposed Bessel beams for transparent material laser microprocessing

Invariant Bessel–Gauss beams have attracted great interest for transparent material microprocessing applications. The small central peak and elongated focal region of the zeroth-order Bessel–Gauss beam provide advantages in laser micromachining. However, many transparent material processing applications need asymmetric beams to induce directional cracking. Therefore, in this work, we investigate analytically, numerically, and experimentally a beam that was generated by spatially displaced axicon halves, which induces noncylindrical symmetry in the beam phase profile. Analytical solutions were found and verified experimentally. High power and quality beams were generated by writing a needed phase alteration of the beam with a geometric phase element. These beams were able to demonstrate applicability to micromachine processing of transparent material.

Synthesis of sparse linear arrays using reweighted gridless compressed sensing

Pattern synthesis of the sparse linear array (SLA) has played an important role when the antenna size is extremely limited. Although grid-based compressed sensing (CS) algorithms have been widely utilised to synthesise the SLA, the performance is greatly affected by the grid mismatch problem. To solve the problem, a reweighted gridless CS (RGCS) algorithm based on the reweighted atomic norm minimisation and the rotational invariance propagator method is introduced. In the RGCS algorithm, the number of antenna elements can be efficiently reduced through the reweighted gridless convex optimisation, which utilises a reweighted strategy to break the limit of the atomic norm and improves the performance of the SLA. More importantly, in addition to the focussed-beam pattern, the proposed algorithm can also synthesise the SLA with the asymmetric beam pattern. Numerical experiments show that the RGCS algorithm can save about 18.75%–46% array elements for the uniform linear array.

Two-dimensional asymmetric Laguerre-Gaussian diffraction-free beams

We find asymmetric diffraction-free Laguerre-Gaussian solutions of the two-dimensional paraxial wave equation in free space. They are constructed by the Laguerre polynomials and trigonometric functions, and described by the radial and angular mode numbers, and modulation depth. Different optical field distributions of the asymmetric diffraction-free Laguerre-Gaussian beams are presented and analyzed. By selecting appropriate angular mode numbers, we identify novel types of asymmetric diffraction-free beams, obtained by the linear superposition of solutions, such as asymmetric vortex and asymmetric necklace-ring beams. Intensity distribution of the asymmetric diffraction-free beams at four times Rayleigh range with different mode parameters is presented analytically and simulated numerically. This research provides theoretical basis for possible experimental verification and utilization of asymmetric diffraction-free beams in free space or uniform transparent dielectrics.

Three-point bending of physically asymmetric metal sandwich beams with aluminum foam core: Failure behavior and optimal design

The failure behavior of physically asymmetric metal foam core sandwich beams under three-point bending is studied experimentally and analytically. Five failure modes including indentation, face yield, core shear (Type A), core shear (Type AB) and core shear (Type A-AB) were observed in the three-point bending tests. Analytical formulae for the critical loads of different failure modes are presented, and failure mechanism maps are constructed based on the formulae. There is a reasonable agreement between the analytical predictions and the experimental results. The effects of face material strain hardening and exchanging two faces, on the failure behavior of physically asymmetric metal sandwich beams are studied. Optimal designs are performed for the load-carrying capacity of physically asymmetric sandwich beams under three-point bending. Based on the optimization results, a method to compare the performances of physically asymmetric sandwich beams with various material combinations is proposed.

Improvement in interfacial fracture toughness of multi-material additively manufactured composites through thermal annealing

An experimental investigation is performed to investigate the effect of post-processing heat treatment on the interfacial fracture toughness of bi-material additively manufactured semi-crystalline polymer composite. An asymmetric double cantilever beam (ADCB) and single-leg bending (SLB) specimens made of polylactic acid (PLA) and Nylon are considered for the mode-I and mixed-mode fracture characterization, respectively. Specimens are isothermally heated in a forced convection oven for a wide range of temperatures and durations. Fracture toughness decreases significantly for both mode-I and mixed-mode conditions when specimens are annealed below the melting temperature of PLA (150 °C). An increase of the crystallinity at the high-temperature annealing prevents the polymer chain mobility, hinders the neck growth, and provides poor intermolecular diffusion resulting in decrease of fracture toughness by 88% as compared to the unannealed specimen. Annealing at 160 °C improves the bi-material interfacial fracture toughness by a maximum of 1225% via sufficient interfacial wetting, higher molecular diffusion, and a longer polymer chain entanglement process. Material transfer, void collapse, and filament impression on the fracture surface of high temperature annealed specimen indicate a better molecular diffusion and strong interlaminar bond at the interface.

Optical beam cooperation‐based secrecy capacity enhancement for visible light communications

To a large extent, the physical layer security (PLS) performance of a visible light communication (VLC) system is affected by light source radiation characteristics. Previous works are limited to well-known Lambertian beam configurations, which fail to support flexible coordinated coverage. Unlike existing works, this article proposes one secrecy capacity (SC) enhancement scheme based on asymmetric non-Lambertian optical beam cooperation. Without loss of generality, this article presents a numerical analysis of coordinated and non-coordinated PLS performance in a typical indoor application scenario. Compared with a non-coordinated benchmark, the numerical results illustrate that the proposed cooperation-based scheme could induce about 0.88 bps/Hz average SC gain without a transmission power increase. Therefore, non-Lambertian beam cooperation could be viewed as one promising secure VLC performance enhancement technology.

High Efficiency Uniform Wakefield Acceleration of a Positron Beam Using Stable Asymmetric Mode in a Hollow Channel Plasma.

Plasma wakefield acceleration in the blowout regime is particularly promising for high-energy acceleration of electron beams because of its potential to simultaneously provide large acceleration gradients and high energy transfer efficiency while maintaining excellent beam quality. However, no equivalent regime for positron acceleration in plasma wakes has been discovered to date. We show that after a short propagation distance, an asymmetric electron beam drives a stable wakefield in a hollow plasma channel that can be both accelerating and focusing for a positron beam. A high charge positron bunch placed at a suitable distance behind the drive bunch can beam-load or flatten the longitudinal wakefield and enhance the transverse focusing force, leading to high efficiency and narrow energy spread acceleration of the positrons. Three-dimensional quasistatic particle-in-cell simulations show that an over 30% energy extraction efficiency from the wake to the positrons and a 1% level energy spread can be simultaneously obtained. Further optimization is feasible.

Longitudinal-transverse coupled vibrations of variable-height asymmetric beams: Modeling, analysis, and case study

In general, the cross-sectional centroids of the variable-height asymmetric beam change in the height direction. The Timoshenko beam theory does not consider the influence of the varying cross-sectional centroids and cannot accurately reveal the vibration of variable-height asymmetric beams. Considering that the cross-sectional centroids are dislocated, we propose a novel dynamic model for the variable-height asymmetric beam element. The proposed dynamic model is based on the rigid-plane assumption and can reflect its main mechanical characteristics. The non-collinear axial forces produce the complementary moment. The rotational radiuses of the two ends of longitudinal fibers are not equal. After retaining the main mechanical characteristics, the variable-height element is roughly treated as a constant-height element to simplify the governing differential equations. A bonded-type piezoelectric transducer with variable-height asymmetric beams is taken as an example to verify the developed dynamic model. The theoretical and experimental resonant frequencies are 39338 Hz and 38704 Hz, respectively, with a difference of 634 Hz and a relative error of 1.638%. The calculated displacement vibration shapes are consistent with the experimental results. Finally, the modeling differences between variable-height symmetric and asymmetric beams are clarified from three aspects.

Delay-differential-equation model for mode-locked lasers based on nonlinear optical and amplifying loop mirrors

Delay differential equation model of a nonlinear optical--nonlinear amplifying loop mirror mode-locked laser is developed that takes into account the finite relaxation rate of the gain medium and asymmetric beam splitting at the entrance of the nonlinear mirror loop. Asymptotic linear stability analysis of the continuous wave solutions performed in the limit of large delay indicates that in a class-B laser flip instability is preceded by the modulational instability and therefore cannot give rise to stable square wave patterns. Numerically it is shown that the model can demonstrate large windows of regular fundamental and harmonic mode-locked regimes with single and multiple pulses per cavity round trip time separated by domains of irregular pulsing.

Scan-path dependence of surface ripples on Si thin films induced by laser irradiation with an asymmetric Gaussian beam

A Nd:YAG linearly polarized nanosecond laser emitting at a wavelength of 355 nm was used to irradiate amorphous-Si thin film (50 nm thickness) on a glass substrate at a repetition rate of 14 kHz and a pulse duration of 5.5 ns. By scanning the laser beam with an asymmetric Gaussian shape in the lateral and vertical directions with various scan pitches at maximum fluences of 55 mJ/cm2 and 90 mJ/cm2, uniform surface textures were obtained when the laser was scanned in the vertical direction at a scan pitch of ∼35–40 μm along the long beam axis. The surface textures were composed of nanoparticle arrays embedded in the Si layer, whose height gradually increased up to 100–150 nm as the number of pulses increased. However, when over-irradiated, the nanoparticles with a rounded shape were transformed to an irregular shape, and the surface roughness also increased. These results would enable application of a 355-nm nanosecond laser to achieve uniform surfaces as a result of the laser crystallization process for organic light-emitting diode displays or photovoltaic devices.

Design and validation of simple bend beam specimen for covering the full range of I+II fracture modes

Using short beam specimen containing an edge crack (perpendicular to the beam axis) and subjected to asymmetric three-point bend loading a new test configuration (called ASBB) was suggested for investigating the mixed mode I/II fracture. Despite the conventional beam specimen with high aspect ratio of length to width that is not able to produce mode II or dominantly mode II conditions, complete mode mixities were obtained using the proposed three-point bend asymmetric short beam specimen. The fracture parameters including modes I and II stress intensity factors and T-stress were determined numerically for this test specimen with different crack lengths and bottom rollers distances relative to the crack. The practical ability of the ASBB specimen was examined experimentally by conducting several mixed-mode I/II fracture tests on PMMA material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using MTS and GMTS criteria. The fracture toughness of PMMA tested with the ASBB specimen was increased by increasing the contribution of mode II component. The effects of beam length aspect ratio, crack type and loading type on the mixed mode fracture of cracked beam samples subjected to three-point bend loading were also examined.

Optical tunnels: long-range optical trapping and manipulation in aqueous media

In this study, we demonstrate an asymmetric counter-propagating beam system with engineered optical forces allowing for long-range particle trapping and manipulation. We achieved this by breaking the symmetry of the well-known counter-propagating optical trapping beams. By doing so, we extend the range of optical forces for particle confinement and transportation to significantly larger foci separations, creating an optical tunnel. These tunnels are capable of moving matter forward and back with controllable speeds for more than a millimeter length with the ability to bring them to a full stop at any point, creating a stable 3D trap. Our trap stiffness measurements for the asymmetric trapping system demonstrate at least one order of magnitude larger values with respect to the symmetric counter-propagating beams so far reported. Our system is quite versatile as it allows for single or multi trapping with flexible positioning of any size particle ranging from tens of nanometers to tens of microns with powers as low as a few milliwatts.