General Momentum Research Articles

Statistics of tidal and deformation eigenvalue fields in the primordial Gaussian matter distribution: the two-dimensional case

ABSTRACT We study the statistical properties of the eigenvalues of the primordial tidal and deformation tensor for two-dimensional random Gaussian cosmic density fields. With the tidal and deformation tensors, Hessians of the gravitational and velocity potential, being Gaussian, the eigenvalue fields are distinctly non-Gaussian. We extend the Doroshkevich formula for the joined distribution of eigenvalues to two-dimensional fields and evaluate the two- and three-point correlation functions. In addition, we assess the number densities of singular points of the eigenvalue fields and find their corresponding two- and three-point correlation functions. The incentive for the present study is based on our interest in developing a fully analytical theory for the structure and dynamics of the cosmic web. The tidal forces and the resulting mass element deformation shape the prominent anisotropic wall-like and filamentary components of the cosmic web. Less well-known is that the web-like spatial pattern is already recognizable in the primordial tidal and deformation eigenvalue field. Against the full phase-space assessment of structure formation in the Universe, the caustic skeleton theory entails an analytical framework for the non-linear evolution of the cosmic web. It accomplishes this by describing the folding of the dark matter sheet and quantifying the emerging caustic singularities, which are fully specified by the spatial properties of the deformation eigenvalues and eigenvectors. Finally, the eigenvalues of the primordial tidal tensor are crucial in the generation of the angular momentum of galaxies. Understanding their spatial distribution is a critical element in predicting the resulting rotation amplitude and orientation.

Does a concurrent motor process influence representational momentum?

Whether the direction of a hand motion that is congruent or incongruent with a concurrent target motion can influence representational momentum for that target was examined. Participants viewed a leftward or rightward moving target while moving their hand rightward, leftward, or not moving their hand. Prior studies of mental rotation found that congruency or incongruency of the direction of mental rotation and the direction of a concurrent physical rotation of a stimulus influenced mental rotation. As mental rotation and representational momentum each involve extrapolation of target motion, it could be predicted that congruency of the direction of hand motion and the direction of target motion might influence representational momentum of the target. Robust representational momentum occurred in all conditions, but there was no effect of congruency of hand motion and target motion, nor of the presence or absence of hand motion, on representational momentum. The results are consistent with a hypothesis that the generation of representational momentum involves sensory processes rather than motor processes.

Isotropic Ge3Sb2Te6 metasurfaces for mid-infrared wavefront steering

Due to unprecedented electromagnetic properties, germanium antimony telluride (GST) plays a crucial role in the field of electromagnetic wavefront control. With the purpose of mid-infrared wavefront reconfiguration, eight meta-atoms are proposed. Meta-atoms appear in different phase states as state is switched. As crystalline is involved, eight meta-atoms reach full phase coverage with phase gradient of 45°. As is switched to amorphous state, phase difference between adjacent meta-atoms no longer varies uniformly by 45°, thus losing original properties. Based on eight meta-atoms, four metasurfaces are presented to verify characteristics, which are two metalenses and two orbital angular momentum (OAM) generators. As crystalline is switched to amorphous , dynamical manipulation between focusing and defocusing is realized. Besides, OAM mode of vortex beam is reconstructed from or 2 to . Our designs could find potential applications in focusing, switching, and encryption.

Parameters identification and forward rushing motion optimization for a big calibre soft recoil gun

Aiming to deal with the problem of the forward rushing velocity reduction caused by the increase of the firing angle of the soft recoil gun, a dynamical model of the soft recoil gun during forward rushing motion is established, and an improved immune clone selection algorithm is proposed. The vaccination strategy is introduced into the immune clone selection algorithm to improve the convergence speed. The random crossover strategy and elite learning mechanism enhances the ability to escape from the local extremes and avoids premature algorithmic maturity. Based on the introduction of the above three strategies, the convergence speed and calculation accuracy of the algorithm are improved, and the convergence time is greatly shortened which saves a lot of time for subsequent parameter identification and optimization calculations. By using the improved immune clone selection algorithm, the unknown parameters in the dynamical model can be identified accurately and quickly. Then, the parameters are used to correct the dynamical model which increase the accuracy of the optimization results. Finally, the algorithm is used to optimize the initial pressure of the gas spring in the forward momentum generator, so as to ensure that different firing angles have the same maximum forward rushing velocity, which improves the stability of the forward rushing motion and the reliability of the firing. Through the optimization calculation, the pressure variation curve of the gas spring with the change of the firing angle is finally determined. The forward rushing velocity is stable at 5.65m/s with different firing angles.

Generation of OAM Beam with High Azimuthal Symmentry through Planar UCA for Vehicular Communication

In this paper, a uniform circular array (UCA) with circularly polarized (CP) square patches is presented for the generation of orbital angular momentum (OAM) beam with high azimuthal symmetry. The proposed CP UCA is a compact structure with a simple feed network generating OAM beams. The design consists of eight circularly polarized square patch antennas which are geometrically rotated to obtain the required phase distribution. The left hand circularly polarized square patch used as a radiating element in UCA exhibits l = + 1 OAM mode, while the right hand circularly polarized square patch exhibits l = - 1 OAM mode. In addition, the antenna exhibits a single-layer structure, which facilitated the fabrication of the design and reduced the cost as well. The simulated and measured results are reported showing that the antenna exhibits an OAM beam of l = + 1 and l = - 1 modes at 5.85 GHz with high azimuthal symmetry. The mode purity estimation is also reported for the OAM l = + 1 and l = - 1 modes. The gain of the conical shaped OAM beam is almost 11 dBi which makes it quite viable for applications in wireless and vehicular communications.

Boosting the intrinsic anomalous Hall effect through a topological phase transition in the magnetic Weyl semimetal Co3Sn2S2

The anomalous Hall Effect (AHE) is one of the most fundamental phenomena related to band topology and holds great potential for technological applications. A promising approach to search for large AHE is to begin with a magnetic Weyl semimetal (WSM) and manipulate the Weyl nodes in the momentum-energy $(k\ensuremath{-}E)$ space. However, controlling the Weyl points is generally difficult because Weyl nodes can robustly exist at a generic momentum. Here we realize this approach in an experimentally confirmed ferromagnetic WSM ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ via tension. Our calculations systematically demonstrate that the tension applied to ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ can drive the movement and annihilation of Weyl nodes in the $k\ensuremath{-}E$ space, triggering a high-Chern-number three-dimensional quantum AHE topological phase transition and thus greatly boosting the AHE. We also found a rare way of annihilation of Weyl nodes occurring near the Brillouin zone boundary. Our work offers a promising way to boost the AHE and sheds light on the search for three-dimensional quantum anomalous Hall insulators.

Gait-phase specific transverse-plane momenta generation during pre-planned and late-cued 90 degree turns while walking

Turning while walking is ubiquitous and requires linear and angular momenta generation to redirect the body’s trajectory and rotate towards the new direction of travel. This study examined strategies that healthy young adults used during each gait phase to generate transverse-plane momenta during pre-planned and late-cued 90° turns. During leftward turns, we expected that momenta would be generated most during the gait phases known to generate leftward linear and angular momenta during straight line gait. We found distinct roles of gait phases towards generating momenta during turns that partially supported our hypotheses. Supporting one hypothesis, the change in transverse-plane angular momentum and average moment were greater during double support with the left foot in front vs. other gait phases. Also, the change in leftward linear momentum and average leftward force were greater during right single support vs. other gait phases during straight-line gait and late-cued turns. However, during pre-planned turns, the average leftward force was not significantly greater during right single support vs. other gait phases. Overall, transverse-plane angular momentum generation during turns is similar to its generation during straight-line gait, suggesting that healthy young adults can leverage momenta control strategies used during straight-line gait during turns.

Optical spin–orbit interaction in spontaneous parametric downconversion

Optical spin–orbit interaction (SOI), which can be used to simultaneously control the spin and orbital angular momentum of light, is important for both classical and quantum information applications. In linear and nonlinear optics, the SOI of light has been extensively explored in both artificial structures and conventional optical crystals. However, optical SOI in quantum nonlinear optical processes, such as spontaneous parametric downconversion (SPDC), has not been studied before. Here, we experimentally demonstrate that optical SOI in the SPDC process can be realized through a nonlinear crystal with threefold rotational symmetry. Two-photon quantum states with controlled angular momentum can be generated through the symmetry selection rules in nonlinear optics and the SOI of the pump wave. The proposed methodology may facilitate the generation and control of spin and orbital angular momentum of entangled photons.

Existence of weak solutions for a nonhomogeneous incompressible cell–fluid Navier–Stokes model with chemotaxis

This work is concerned with the mathematical analysis of a general cell–fluid Navier–Stokes model with the inclusion of chemotaxis. This general model relays on a mixture theory multiphase formulation. It consists of two mass balance equations and two general momentum balance equations, respectively, for the cell and fluid phase, combined with a convection–diffusion–reaction equation for oxygen. We investigate the existence of weak solutions in a two‐ or three‐dimensional bounded domain when the fluids are assumed to be incompressible with constant volume fraction.

A Low-Profile Dielectric Resonator Antenna Array for OAM Waves Generation at 5G NR Bands.

This paper presents the generation of orbital angular momentum (OAM) vortex waves with mode +1 using dielectric resonator antenna (DRA) array. The proposed antenna was designed and fabricated using FR-4 substrate to generate OAM mode +1 at 3.56 GHz (5G new radio band). The proposed antenna consists of 2 × 2 rectangular DRA array, a feeding network, and four cross slots etched on the ground plane. The proposed antenna succeeded in generating OAM waves; this was confirmed by the measured radiation pattern (2D polar form), simulated phase distribution, and intensity distribution. Moreover, mode purity analysis was carried out to verify the generation of OAM mode +1, and the purity obtained was 53.87%. The antenna operates from 3.2 to 3.66 GHz with a maximum gain of 7.3 dBi. Compared with previous designs, this proposed antenna is low-profile and easy to fabricate. In addition, the proposed antenna has a compact structure, wide bandwidth, high gain, and low losses, thus meeting the requirements of 5G NR applications.

Mode Conversion Between Beams Carrying Orbital Angular Momentum With Opposite Topological Charges Using Two-Dimensional Multimode Interference Waveguides

Recently, the implementation of rectangular waveguides for generation and manipulation of orbital angular momentum (OAM) modes, has been developed to exploit the outstanding features of these modes in more complex integrated devices. Multimode interference (MMI) structures have been widely used in both one and two dimensions as the basic elements in many integrated optical devices like optical beam splitters, mode converters, couplers, wavelength-division (de)multiplexers, and switches. According to the various applications of OAM modes in quantum and classical communications, the study of their propagation properties in MMI waveguides is useful in order to facilitate the way into higher security and capacity systems. This paper presents the results of numerical investigation of a proposed integrated optical OAM mode converter which is performed based on the mode decomposition properties of propagating OAM modes in 2D MMI waveguides, as well as the fact that any order of OAM mode can be represented as superposition of an odd mode and a quarter-wave shifted even mode. The proposed device, which consists of two MMI waveguides connected by phase shifters and linear waveguides, provides mode conversion between beams carrying OAM with odd opposite topological charges. The design procedure is performed for OAM modes with topological charge values of l = ±1 and l = ±3 using beam propagation method. The proposed device is passive with reciprocal behavior and has the output mode purity of 94% and 82% for beams carrying l = ±1 and l = ±3, respectively.

108.96 nm Broadband Generation of Second-Order OAM Beam Using an Angular Modulated Cascading LPFG

Broadband generation of orbital angular momentum (OAM) beam in fiber has a lot of applications in optical communications and fiber sensing. However, the efficient broadband generation of second-order OAM beam using long-period fiber grating (LPFG) is still a great challenge due to the low intermodal overlap. In this paper, we achieved a 108.96 nm efficient broadband generation of second-order OAM beams with a conversion efficiency over 95% in experiment using an angular modulated cascading LPFG with low insertion loss. By changing the refractive index modulation area in certain angle, not only the coupling between fundamental mode and second-order mode in fiber can be greatly enhanced, but also second-order even and odd modes are generated with approximately equal magnitudes, which is very beneficial to high-purity OAM beams generation. Finally, by choosing proper cascading parameters and an adaptive refractive index modulation fabrication strategy, we achieved a LPFG mode converter with a conversion efficiency over 95% in a 108.96 nm from 1520.96 nm to 1629.92 nm. The insertion loss of the LPFG is measured only ∼0.48 dB.

Integrated-resonant metadevices: a review

Integrated-resonant units (IRUs), associating various meta-atoms, resonant modes, and functionalities into one supercell, have been promising candidates for tailoring composite and multifunctional electromagnetic responses with additional degrees of freedom. Integrated-resonant metadevices can overcome many bottlenecks in conventional optical devices, such as broadband achromatism, efficiency enhancement, response selectivity, and continuous tunability, offering great potential for performant and versatile application scenarios. We focus on the recent progress of integrated-resonant metadevices. Starting from the design principle of IRUs, a variety of IRU-based characteristics and subsequent practical applications, including achromatic imaging, light-field sensing, polarization detection, orbital angular momentum generation, metaholography, nanoprinting, color routing, and nonlinear generation, are introduced. Existing challenges in this field and opinions on future research directions are also provided.

Generating reconfigurable acoustic orbital angular momentum with double-layer acoustic metasurface

In this paper, a double-layer acoustic metasurface (DAM) composed of a fixed lower acoustic metasurface (LAM) and a rotatable upper acoustic metasurface (UAM) is proposed for the generation of mode-reconfigurable acoustic orbital angular momentum (OAM). The UAM and LAM are divided into multiple sections, in which the hybrid structures combining cascaded Helmholtz resonators and a straight pipe are adopted to achieve specific phase compensation. By rotating the UAM, the incident acoustic plane wave can be efficiently converted into the vortex acoustic waves of reconfigurable topological charges ranging from −5 to +5 with distinguishable purity. Furthermore, the influences of the parameters on the purity of the generated topological charges have been investigated and discussed, such as the distance between LAM and UAM, rotatable angle error, and operating frequency. With the capability of reconfigurable OAM modes, the proposed DAM can be used to efficiently increase capacity or to conveniently switch between different channels in underwater vortex acoustic communications.

Rotational dynamics of indirect optical bound particle assembly under a single tightly focused laser.

The optical binding of many particles has the potential to achieve the wide-area formation of a "crystal" of small materials. Unlike conventional optical binding, where the entire assembly of targeted particles is directly irradiated with light, if remote particles can be indirectly manipulated using a single trapped particle through optical binding, the degrees of freedom to create ordered structures can be enhanced. In this study, we theoretically investigate the dynamics of the assembly of gold nanoparticles that are manipulated using a single trapped particle by a focused laser. We demonstrate the rotational motion of particles through an indirect optical force and analyze it in terms of spin-orbit coupling and the angular momentum generation of light. The rotational direction of bound particles can be switched by the numerical aperture. These results pave the way for creating and manipulating ordered structures with a wide area and controlling local properties using scanning laser beams.

Quantum model of dynamic Stark‐induced degenerate vibronic state by two nonresonant shift‐up lasers for π‐electron angular momentum generation

AbstractA theoretical scenario for the generation of coherent π‐electron angular momentum in a low‐symmetry aromatic molecule using two nonresonant shift‐up lasers is presented. The effects of vibrational modes on coherent π‐electron angular momentum are considered in the adiabatic approximation. In this scenario, two electronic excited states are irradiated with two linearly polarized nonresonant lasers. This causes a consistent up‐shift in the vibronic energy levels of each electronically excited state toward higher energy as a result of the dynamic Stark effect. For the weak coupling case of the electronic‐vibrational interactions, the lowest vibronic states in the two electronic excited states are designed to be degenerate by varying the parameters (frequency and intensity) of each laser. The resultant degenerate state, called the dynamic Stark‐induced degenerate vibronic state (DSIDVS), is the origin of the coherent π‐electron angular momentum generation. An optimal laser parameter set is determined under conditions in which the lowest two vibronic states form the DSIDVS. The DSIDVS was formulated by the method of Laplace transformations. An analytical expression for the coherent π‐electron angular momentum was derived in the displaced harmonic oscillator model. We examined the dependence of the time‐averaged z‐component of the coherent angular momentum on the ratio of the two detunings and potential displacements of toluene, a typical low‐symmetry aromatic ring molecule, which belongs to the weak coupling case and is adopted to demonstrate the theoretical scenario. The inclusion of potential displacements in the angular momentum generation was found to reduce the value of by half, compared with that in the frozen nuclear approximation.

Computational Fluid Dynamics Analyses of a Wing with Distributed Electric Propulsion

The efficiency increase that distributed propulsion could deliver for future hybrid-electric aircraft is in line with the urgent demand for higher aerodynamic performances and a lower environmental impact. Several consolidated proprietary tools (not always available) are developed worldwide for distributed propulsion simulation. Therefore, prediction and comparisons of propeller performances, with computational fluid dynamic codes featuring different implementation of solvers, numerical schemes, and turbulence models, is of interest to a wider audience of research end-users. In this framework, the paper presents a cross-comparison study among different CFD solvers, the SU2 Multiphysics Simulation and Design Software, the CIRA proprietary flow solver UZEN, and the commercial ANSYS-FLUENT code, for the simulation of a wing section with a tractor propeller at different flow attitudes. The propeller is modelled as an actuator disk according to the general momentum theory and is accounted for in the flow solvers as a boundary condition, for the momentum and energy equations. In this study, a propeller with a fixed advance ratio J=0.63 is considered, while propeller performances are assumed variable along with the radius. To perform the comparisons among the solvers, an in-house procedure, which provides the input thrust and torque distributions in a unified format among the three solvers, is developed. Steady RANS simulations are performed at Re∞=1.7×106 and M∞=0.11, for the flowfield of an isolated propeller. Successively, a wing section with a fixed forward-mounted propeller configuration with no nacelle, is studied at α=0∘,4∘, and 8∘ angles of attack. The comparisons in terms of the lift coefficient show a good agreement among the three flow solvers both in power-off and power-on conditions. Simulations also evidenced the strong stability preserving property of upwind schemes, applied to propeller simulation at low-Mach number. Some discrepancies in the drag coefficient are observed and related to different levels of numerical diffusion between the three codes, which affects the downstream wake. Differences in flow properties in near disk region are observed and explained considering the different hub implementations.

Helically twisted double-cladding-three-core photonic crystal fiber for generation of orbital angular momentum

Aiming at the shortcomings of helically twisted single-cladding-few-core photonic crystal fibers in generating orbital angular momentum (OAM), the double-cladding and three-core structures with non-uniform inner and outer air holes are introduced into a photonic crystal fiber for the first time and the generation of high-order OAM modes through helical twisting is realized. The fiber is expected to reduce the losses of the generated OAM modes by introducing a specially designed double-cladding structure, while the three cores distributed in a regular triangle around the center are expected to increase the number of generated OAM modes. On the basis of optical transformation theory, the optical fiber is systematically analyzed by the finite element method. It is found that with the twist rate <i>α</i> = 7853.98 rad/m, the generated OAM modes include “OAM<sub>–4,1</sub>, OAM<sub>+9,1</sub>, OAM<sub>+10,1</sub>, OAM<sub>+11,1</sub>, OAM<sub>+13,1</sub>”, where +13 is the highest order in the OAM modes currently generated by using helically twisted fibers. And the losses of OAM modes are all less than 1.64×10<sup>–3</sup> dB/m, which is at least two orders of magnitude lower than the lowest OAM mode loss reported in the existing references (Napiorkowski M, Urbanczyk W S 2018 <i>Opt. Express</i> <b>26</b> 12131), and their purity is greater than 93%. Further studies show that the generation of orbital angular momentum depends on the resonant coupling between the core supermode and the ring-core mode, and the parity of the order of the generated OAM modes is related to the polarization direction of the fiber core supermode and the ring-core mode.

Generation of all-fiber third-order orbital angular momentum modes based on femtosecond laser processing of long-period grating

The generation of orbital angular momentum (OAM) modes is very important, for they have a variety of applications such as in optical tweezers, quantum optics, and optical communication systems. Particularly, how can high-order OAM modes be generated efficiently in fibers with the advantage of low cost and compatible with fiber system? The Traditional method for first order to third order OAM is based on long period fiber grating (LPFG) fabricated by carbon dioxide laser. However, high power and large focused spot of carbon dioxide laser are unfavorable for stable and repeatable generation of higher-order OAM, which needs the LPFG with small grating pitch. In order to solve this problem, a third-order OAM mode converter based on femtosecond microfabrication is proposed and fabricated for the first time. With the advantage of 4.4 μm focused spot size near the core, lower power and lower heat absorption efficiency, this method can be more stable and promising. Therefore, we first carry out the mode filed analysis and simulate the intensity and phase profiles of the superposed mode field in LP odd-even mode on different scales and phases patterns to obtain OAM mode. Second, we use the coupled-mode theory to analyze and simulate the transmission spectrum of LPFG, which guides the setting of the grating parameters such as the grating pitch, the depth of modulation and the length of the grating. By experimental verification, an asymmetric modulated long-period fiber grating with a pitch setting to 194 μm is fabricated on a six-mode fiber. The fundamental mode can be converted into the third-order angular linear polarization mode LP<sub>31</sub> mode with 98% mode conversion efficiency near 1550 nm, and further converted into the OAM<sub>±3</sub> modes by superposition of the odd and even LP<sub>31</sub> mode with ±π/2 phase difference. At the same time, this fiber grating can also generate LP<sub>12</sub> mode with 90% mode conversion efficiency near 1325 nm. Then we can take the same approach to transform LP<sub>12</sub> mode into OAM modes with angular first-order as well as radial second-order. The experimental result is consistent with the simulation result. Thus, this scheme provides an idea for generating the high-order OAM modes in all-fiber systems by using only one grating with high repeatability.

Launching directional hypersonic surface waves in monolithic gallium phosphide nanodisks: two holes are better than one.

The emergence and rapid progress of all-dielectric nanoantennas have provided unprecedented platforms for applications in sensing, optical control of light, opto-mechanics and metrology at the nanoscale. We present a general figure-of-merit (FOM) considering both optical and vibrational responses. Detectable mechanical vibrations ranging from gigahertz to terahertz in gallium phosphide (GaP) structures on sub-wavelength scales are found to surpass their metallic counterparts in a 400-800 nm pump-probe configuration. Then, we tailored low-aspect ratio GaP disks being probed near their optical anapole resonance. We further broke the isotropy of the nanodisks and achieved pronounced directional propagation for launching surface acoustic waves (SAWs) with a double-hole structure rather than with a one-hole configuration, which could be attributed to the constructive superposition of vibration induced by the two holes in the appropriate direction. Finally, we demonstrated that the orbital angular momentum of SAWs could be generated with a spiral distribution of the two-hole nanodisks. Our work paves a new way to monolithic GaP nanoantennas towards photoacoustic applications such as hypersound routers, stirring up inverse designs of individual antennas for phononic metasurfaces, topological phononics as well as quantum phononics.