Wave Angular Momentum Research Articles

The Design of a Multifunctional Coding Transmitarray with Independent Manipulation of the Polarization States.

Manipulating orthogonally polarized waves independently in a single metasurface is pivotal. However, independently controlling the phase shifts of orthogonally polarized waves is difficult, especially in the same frequency bands. Here, we propose a receiver-phase shift-transmitter transmitarray with independent control of arbitrary polarization states in the same frequency bands, in which transmission rates reach more than 90% in the frequency bands 4.2~4.9 GHz and 5.3~5.5 GHz. By introducing a phase-regulation structure to each element, phases covering 360° for different polarized incident waves can be independently controlled by different geometric parameters, and two-bit coding phases can be obtained. The design principle based on the two-port network's scattering matrix has been analyzed. To verify the independent tuning abilities of the proposed transmitarray for different polarization incidences in the same frequency bands, a multifunctional receive-phase shift-radiation coding transmitarray (RPRCT), which is composed of 16×16 elements, with functions of anomalous refraction (for example, orbital angular momentum wave) and focusing transmission for different polarized incident waves was simulated and measured. The measured results agree reasonably well with the simulated ones. Our findings provide a simple method for obtaining a multifunctional metasurface with orthogonal polarization in the same frequency bands, which greatly improves the capacity and spectral efficiency of communication channels.

Space‐Time Coding Metasurface For Multifunctional Holographic Imaging

AbstractHolographic imaging as one of promising image techniques reconstructs object image using the amplitude and phase information scattered from objects. Programmable metasurface composed of digitally coding unit cells can arbitrarily manipulate amplitude and phase of the scattering wave by tailoring operation states of the unit cells, and thus achieves high‐resolution imaging reconstruction. However, the conventional metasurface can only achieves a desired holographic image on an imaging plane. This paper presented a time‐space coding 2‐bit metasurface to overcome this challenge. By rapidly switching PIN diodes loaded in metasurface unit cell according to the customized time sequences, the magnitude and phase of the ±1st order harmonic fields can be fully controlled, achieving the hologram imaging at the higher order harmonics. The proof‐of‐concept measurements are given to demonstrate the versatile holographic imaging functions at the ±1st order harmonics including two different holographic images on an imaging plane, holographic imaging and beam scanning, and holographic imaging and orbital angular momentum (OAM) wave generation, which undoubtedly enriches the applications of the programmable metasurfaces in the field of the holographic imaging.

Investigation of the OAM EM wave tissue irradiation at millimeter-wave frequencies

Recently, there has been an increase of interest in the use of electromagnetic (EM) waves with helical wavefronts, known as the orbital angular momentum (OAM) waves. Applications in the field of biomedicine have been foreseen, such as medical imaging and diagnosis, deep-tissue imaging, biosensing, and communication with medical implants. Other possible applications include various localized tissue treatments or tissue ablation. The available references mainly study the interaction of OAM light with biological structures, offering some insights into the biophotonics effects, but without the investigation of how to plan tissue exposures or how to estimate the EM field parameters in a particular case of application. We use the previously developed short dipole modeling of OAM EM fields to study the above problems by altering the OAM beam parameters and the distance from the target tissue. The results could guide the design of components and devices based on OAM EM waves.

High resolution imaging of acoustic orbital angular momentum wave based on orthogonal matching tracking

The orbital angular momentum (OAM) wave has shown great potential for improving radar imaging and underwater communication performance due to its helical wavefront phase and infinite orthogonal modes. However, there are currently no known applications of this technology in underwater imaging. In this paper, we employed acoustic OAM wave for underwater imaging and established transceiver signal models using the uniform circular array. We concurrently achieved two-dimensional imaging of azimuth and elevation angles, which differs from radar imaging. We proposed a matching process for the echo signal in the modal domain, the OAM wave beam image's sidelobe decreased by 7.9 dB in the elevation direction and 6.1 dB in the azimuth direction compared to the plane wave, with the mainlobe decreased by 0.2° in the elevation direction and 0.4° in the azimuth direction. Furthermore, this paper introduced OAM wave high-resolution image reconstruction based on the orthogonal matching pursuit (OMP) algorithm. Finally, we implemented broadband acoustic OAM wave for underwater imaging and introduced an image reconstruction method based on the modal domain OMP algorithm. Simulation results demonstrate that the use of OAM wave in underwater imaging is feasible, and the proposed scheme can achieve high-resolution imaging.

Quantitative Microwave Imaging of High-contrast Targets with the Incidence of Orbital Angular Momentum Wave

Quantitative Microwave Imaging of High-contrast Targets with the Incidence of Orbital Angular Momentum Wave

Predicting angular-momentum waves based on Yang–Mills equations

As one of the most elegant theories in physics, Yang–Mills (YM) theory not only incorporates Maxwell’s equations unifying electromagnetism, but also underpins the standard model explaining the electroweak and strong interactions in a succinct way. Whereas the highly nonlinear terms in YM equations involving the interactions between potentials and fields retard the resolution for them. In the U(1) case, the solutions of Maxwell’s equations are the electromagnetic waves, which have been applied extensively in the modern communication networks all over the world. Likewise the operator solutions of the YM equations under the assumptions of weak-coupling and zero-coupling predict the SU(2) angular-momentum waves, which is the staple of this work. Such angular-momentum waves are hopefully realized in the experiments through the oscillation of spin angular momentum, such as the “spin Zitterbewegung” of Dirac’s electron.

Wire antenna with skew planar wheel structure for generating high‐purity OAM beam

AbstractThis paper presents the generation of an orbital angular momentum (OAM) wave using a wire antenna with a skew planar wheel structure at 5.8 GHz. The wire antenna consists of four arms, each of which behaves like a left‐handed circularly polarized antenna. The orientation and rotation of each arm are adjusted to obtain the desired phase shift, resulting in the generation of an OAM wave. The proposed design is fabricated and tested, and the results demonstrate that the OAM beam produced has high mode purity, with a peak gain of 2.8 dBi, divergence angle of , and high azimuthal symmetry. Both simulated and measured results confirm the effectiveness of the proposed wire antenna in generating an OAM wave. The proposed wire antenna has a wide range of potential applications, including communication systems, remote sensing, and radar imaging. In particular, the azimuthal symmetry of the OAM beam makes it particularly suitable for unmanned aerial vehicle applications.

Generation of Terahertz OAM Waves with Six Modes Based on Three-Layer Z-Shaped Reflective Metasurface

In this paper, a broadband and efficient three-layer Z-shaped reflective metasurface for linear polarization conversion is designed and six different modes of orbital angular momentum (OAM) waves are generated in the terahertz band. The designed metasurface consisted of several units, and it is divided into twelve regions. The phase difference is achieved by changing the structural parameters of the units, and then different modes of OAM waves are generated. The terahertz OAM waves with the modes of ±1, ±2, and ±3 are generated by metasurface with high efficiency and wide bandwidth. The results show that the designed metasurface could produce high purity terahertz OAM waves with six different modes, and the reflection amplitude of the metasurface unit is more than 0.9 in the frequency range of 1.0 THz to 1.8 THz. The generated OAM waves with the modes of ±1 and ±2 have a mode purity more than 90%. The designed metasurface has good wavefront control ability, which provides an effective method to generate multimode OAM waves.

Digital coding metasurface for Multi-Beam and Multi-Mode OAM in Full-Space

A digital coding metasurface is proposed to generate multi-beam and multi-mode orbital angular momentum (OAM) wave in the full space. The 3-bit coding elements are designed for multifunctional integrated digital coding metasurface (MIDCMS) based on the propagation phase theory. The variable-sized phase-shift scheme is applied on metal structures to realize different functionalities in linearly polarized (LP) incidence. Furthermore, the phase distribution of MIDCMS is determined by the pattern convolution theorem. The transmitted and reflected wavefronts are manipulated simultaneously through MIDCMS. As a proof of concept, a metasurface device was fabricated and measured for the four-beam OAM with +1 mode in transmission space and the four-beam OAM with −1 mode in reflection space. Both experimental results and simulated values verify that the proposed MIDCMS can effectively realize the full-space multi-beam multi-mode OAM generation and improve the capacity of information channels, which has potential applications for comprehensive implementations in the future wireless communication systems.

Short-range transmission using OAM-carrying waves generated by uniform circular arrays

The orbital angular momentum (OAM or vortex) waves are expected to provide ten-fold and larger increases in wireless data rates, required for short-range communications within the beyond 5G and 6G concept of all-connected life and industry. We address the specifics of short-range communications employing OAM-carrying waves generated by small uniform circular arrays (UCAs) at lower, i.e., 10 GHz, transmission frequencies. Comparing the link budgets obtained using (i) asymptotic analytical formulas, (ii) numerical electromagnetic simulations, and (iii) measurements on two pairs of manufactured prototypes comprising 8 microstrip-patch-element UCAs, we point out the limitations of simplified models which do not account for various effects, such as coupling, parasitic radiation, and insertion loss. The observed effects are expected to be relevant at millimeter-wave frequencies as well.

OAM mode purity improvement based on antenna array

Orbital Angular Momentum (OAM) waves are characterized by helical wave fronts and orthogonality between different modes. Therefore, OAM waves have huge potential in improving wireless communications' channel capacity and radar imaging's resolution. Consequently, the generation and application of OAM waves have attracted a lot of attention. And many methods are proposed to generate OAM waves. Although antenna array is the most popular method of generating OAM waves, OAM waves generated by antenna array have redundant modes. However, all advantages of OAM waves are closely related to infinite OAM modes. Thus, to better apply OAM waves to wireless communications and radar, it is very important to reduce unnecessary OAM modes and improve the OAM mode purity. In order to improve the OAM mode purity, two combined antenna arrays composed of X direction antenna and Y direction antenna array are proposed in this paper. The X direction antenna array and the Y direction antenna array are supplied by the excitations with the same amplitude and fixed phase shift. The overall phase shift of the X direction antenna array is π/2 more or less than that of the Y direction antenna array. The results of formulas and antenna models in CST show that the combined antenna arrays can generate OAM waves with less redundant modes in x component, y component and z component. Besides, the z component carries pure OAM modes.

Eccentric binary black holes: Comparing numerical relativity and small mass-ratio perturbation theory

The modeling of unequal mass binary black hole systems is of high importance to detect and estimate parameters from these systems. Numerical relativity (NR) is well suited to study systems with comparable component masses, ${m}_{1}\ensuremath{\sim}{m}_{2}$, whereas small mass ratio (SMR) perturbation theory applies to binaries where $q={m}_{2}/{m}_{1}\ensuremath{\ll}1$. This work investigates the applicability for NR and SMR as a function of mass ratio for eccentric nonspinning binary black holes. We produce 52 NR simulations with mass ratios between $1:10$ and $1:1$ and initial eccentricities up to 0.8. From these we extract quantities like gravitational wave energy and angular momentum fluxes and periastron advance, and assess their accuracy. To facilitate comparison, we develop tools to map between NR and SMR inspiral evolutions of eccentric binary black holes. We derive post-Newtonian accurate relations between different definitions of eccentricity. Based on these analyses, we introduce a new definition of eccentricity based on the (2,2)-mode of the gravitational radiation, which reduces to the Newtonian definition of eccentricity in the Newtonian limit. From the comparison between NR simulations and SMR results, we quantify the unknown next-to-leading order SMR contributions to the gravitational energy and angular momentum fluxes, and periastron advance. We show that in the comparable mass regime these contributions are subdominant and higher order SMR contributions are negligible.

Orbital Angular Momentum Wave Generation and Multiplexing: Experiments and Analysis Using Classical and Quantum Optics

Optical fiber communication is known as the backbone of communication today for voice, video, and data transmission. Recent advancements in optical technologies are leading us to many new forms of generation of orbital angular momentum (OAM) waves and its transmission technologies. Among them are hypergeometric waveform (HG), Bessel’s waveforms, Laguerre Gaussian waveforms, and other forms of OAMs. This mode of communication is by the twisted form of electromagnetic wave which can accommodate a larger number of channels making communication simpler for wired and wireless communication. In this paper, various methods of OAM waveform generation are presented and are compared in details discussing the various needs in communication. OAM multiplexing is done by spatial division multiplexing (SDM) and wavelength division multiplexing (WDM). Methods of OAM communication, generation techniques, power analysis, and bit error analysis prove efficient transmission capability of OAM waves using low power consumption and low bit error rate in optical communication networks. OAM from fiber can be directly communicated to atmosphere which is found very useful for short distance wireless communication. Experimental validation is discussed for HG, Bessel’s waveforms, and LG waveforms. Pure Optical Vertices (OV) are generated by VCSELs (vertical cavity surface emitting lasers) and transmitted by (MMF) multimode fibers. Various atmospheric effects and challenges are also elaborated in this paper. RSoft OptSim software is used for simulation.

A reflective metasurface for broadband OAM vortex wave generation

In this paper, we proposed a reflective metasurface which controls the phase delay of the electromagnetic (EM) wave through geometric phase manipulation. By coding the metasurface unit cells in different orientation angles, an orbital angular momentum (OAM) wave can be obtained through the metasurface reflection. A broadband reflective OAM wave with mode l = −1 was demonstrated which was experimentally observed from 8.3 GHz to 11.6 GHz. The mode spectrum of OAM wave was analyzed and the spectrum weight was about 0.82 through numerical calculation and about 0.47 through experimental measurement for the mode l = −1. The realization of this broadband OAM wave has the potential application to improve the OAM wave based communication capacity.

How convolutional-neural-network detects optical vortex scattering fields

Light scattering through disordered media is a critical topic in optical engineering as its ubiquity in natural and artificial systems. Recent progress has shown that deep learning is capable to recognize topological charge values carried by orbital angular momentum (OAM) waves with ultra-fine resolution under scattering environment. However, the physical mechanism of how a deep learning convolutional neural network (CNN) fulfills such tasks remains unclear. In this paper, in perspective of optical vortex scattering field detection, we studied the basic physical mechanism of the CNN on recognizing scattered vortex beams carrying OAMs. It has been demonstrated that a CNN uses statistical invariance of both spatial phase front of an incident OAM wave and intrinsic features of a specific disordered medium across large-scale datasets to identify the OAM topological charge values from speckles. This work can provide insightful reference for CNN-assisted OAM-based scattering detection.

Research of Crack Defect Detection in Metal Pipes Based on Microwave Antenna Array

This study evaluates the feasibility of using antenna array to detect crack defects in metal pipes. Antenna arrays are set at the port of a metal pipe in radial direction, and the microwave signal can be effectively coupled into the metal pipe. It can produce orbital angular momentum (OAM) waves. The cracks in the pipe will change the microwave transmission characteristics, and they can be inspected by time domain reflectometry (TDR) of vector network analyzer (VNA). By studying the transmission characteristics of current on the inner surface of the metal pipe, the feasibility of linearly polarized TE11 mode microwaves for crack inspection was preliminarily verified. Three galvanized metal pipes with an inner diameter of 8 cm and a total length of about 270 cm were used in the experiment. Axial slits and circumferential slits were used to simulate the crack, respectively. The slits penetrated the pipe wall with a breach of approximately 10 cm × 0.7 cm . The experimental results demonstrated that the radial direction antenna array can detect both the axial slit and circumferential slit through TDR and standing wave ratio (SWR) at the port, especially effective for the axial slit. It provides a new inspection method for metal pipe defect evaluation.

Multipolar invariants and the eccentricity enhancement function parametrization of gravitational radiation

Gravitational radiation can be decomposed as an infinite sum of radiative multipole moments, which parametrize the waveform at infinity. The multipolar-post-Minkowskian formalism provides a connection between these multipoles and the source multipole moments, known as explicit integrals over the matter source. The gravitational wave energy, angular momentum and linear momentum fluxes are then expressed as multipolar expansions containing certain combinations of the source moments. We compute several gauge-invariant quantities as "building blocks" entering the multipolar expansion of both radiated energy and angular momentum at the 2.5 post-Newtonian (PN) level of accuracy in the case of hyperboliclike motion, by completing previous studies through the calculation of tail effects up to the fractional 1PN order. We express such multipolar invariants in terms of certain eccentricity enhancement factor functions, which are the counterpart of the well known enhancement functions already introduced in the literature for ellipticlike motion. Finally, we use the complete 2.5PN-accurate averaged energy and angular momentum fluxes to study the associated adiabatic evolution of orbital elements under gravitational radiation reaction.

Shielding of Orbital Angular Momentum Waves by a Cavity With Apertures

In this article, for the first time, the shielding of orbital angular momentum (OAM) waves by a cavity with apertures is evaluated. This is particularly interesting because the distinguishing feature between the OAM modes is their phase distribution in space, and, hence, shielding effectiveness is a mode-dependent phenomenon. A perfectly electrically conducting cavity with multiple apertures and two OAM antennas, inside and outside the cavity, is investigated using a method of moments based tool. The behavior of OAM waves is analyzed by comparing the transmission between the OAM arrays for different cases, including their field patterns in front of and behind the apertures. The effects of the orientation and position of the transmitting antenna, the rotation, size, and form of the apertures and different cavities are explored. Additionally, the effect on the distance and the orientation angle of the outer antenna are studied. It is shown that the propagation of OAM waves through apertures is possible and that the resulting signal loss is similar to the loss of a plane wave or the field of a monopole. It is also shown that certain OAM waves penetrate the apertures better than others.

Integrated Physical Optics for Calculating Electric-Large Metallic Sphere Scattering Irradiated by Vortex Wave in Microwave Frequency Band

In this letter, a novel integrated physical optics (PO) method for calculating high-frequency scattering of nonplane wave is proposed, and the orbital angular momentum (OAM) wave scattering upon an electric-large metallic sphere is illustrated. A high-order Bessel wave expanded by a series of plane wave angular spectra is selected to irradiate a perfect electric conductor sphere. Combining the PO approximation, the scattered field from the facet element is calculated, and the total scattered effect is obtained by vector superposition of the scattering from all facets. The calculated results are verified by the EM simulated software FEKO and scattering measurement experiment. Numerical results show that the scattered field of the vortex wave passing through the standard target still has the OAM characteristics, retaining the same topological charge. It can be seen that the proposed combined PO method is feasible at high frequency and can be extended to solve the scattering of other beams and objects, which has a broad range of applications.

Mode‐switchable orbit angular momentum waves by mechanically rotating 2 bit phase quantization transmitarray

In this article, mode-reconfigurable orbit angular momentum (OAM) waves is developed by mechanically rotating a 2 bit phase quantization (0°, 90°, 180°, and 270°) transmitarray (TA) with size 6.2 λ 0 $$ {\lambda}_0 $$ × 6.2 λ 0 $$ {\lambda}_0 $$ × 0.98 λ 0 $$ {\lambda}_0 $$ ( λ 0 $$ {\lambda}_0 $$ wavelength in free space). The TA consists of three-layer metasurfaces (MSs), which are in the top, middle, and bottom layers, respectively. The TA is excited by a horn. The top layer MS converts the spherical wave from the horn into plane wave. The middle and bottom layers are same, and they together convert the plane wave into OAM wave with mode l, and then reconfigurable mode can be achieved by rotating the bottom layer MS. To the knowledge of the authors, it is the first time to design mechanical OAM mode reconfigurability by phase quantization TA. As example, two TAs are designed for reconfigurable modes at 15.5 GHz: (1) Among l = 1, 0, −1. (2) Among l = 2, 0, −2. The TAs have the following advantages: high gain (15.1–21.5 dBi), high aperture efficiency (the highest up to 29.2%), high purity (92.67%–98.93%), and small size (6.2 λ 0 $$ {\lambda}_0 $$ × 6.2 λ 0 $$ {\lambda}_0 $$ ). The measured and simulated results agree well, and validate the design method. The developed OAM mode reconfigurable TAs have potential application for future high capacity communication.