Orbital Angular Momentum Vortex Research Articles

Polarization-insensitive, wideband reflective metasurface generating orbital angular momentum vortex beam

In this paper, a low-profile polarization-insensitive reflective metasurface is proposed to generate high gain and wideband orbital angular momentum (OAM) in X-band (in the frequency range of 8−12GHz). By adjusting a subwavelength pseudo-screw-shaped meta-atom’s size the required phase compensation has been achieved for designing a reflectarray antenna at 10GHz. The theoretical analyses and simulation results confirm that the proposed structure can effectively generate the first-, second-, third-, and fourth-order OAM vortex beams at the operating frequency. To verify the bandwidth of the suggested metasurface, the structure is implemented for first-order OAM mode generation at various frequencies. The outcomes of investigations show the OAM operational bandwidth and 1.5-dB gain bandwidth are attained to be 40 %, and 27.5 %, respectively. Meanwhile, approximately 14.35 % reflection efficiency was procured for the overall design at the operating frequency of 10GHz. The numerical aperture of the reflectarray metasurface is also conceptually defined and calculated for varying positions of the center-feed antenna. The admitted performance and substantial characteristics of the proposed OAM reflectarray antenna may make it a useful candidate for radar imaging and detection applications as well as broadband microwave communications.

Generating terahertz multiple vortex beams using graphene metasurfaces

This paper investigates the generation of orbital angular momentum vortex beams using a graphene metasurface in the terahertz frequency band. The proposed design consists of 20 × 20 unit-cell elements to operate in 1.2 THz applications. Each element is a graphene ring patch printed on a silicon dioxide substrate backed with a polysilicon ground plane of size 75 × 75 × 25 µm3. The graphene reconfigurable surface conductivity is used to control the beam shape, direction, and directivity radiated from the metasurface, through the application of DC biasing voltages. A parametric study on the effect of graphene chemical potential, relaxation time and temperature on the unit-cell reflection properties is introduced. The reflection magnitude varies from − 2.1 dB to -0.8 dB with a 350-degree phase variation for µc ranging from 0.25 eV to 1.6 eV at \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au$$\\end{document} =5 ps, and T = 300 K. The effect of graphene relaxation time from 0.3 ps to 10 ps on the reflection coefficient at µc = 0.7 eV, and T = 300 K is investigated. The metasurface radiation characteristics are investigated under the illumination of two types of incidence sources, plane-wave, and focused-waves. A depiction of a single vortex beam in various orientations θ = 0, 30o, 50o, and 70o, φ = 90o for l = 1 is presented. The purity of the OAM single beam shows that 94% of the power is concentrated in the designed mode. A graphene metasurface can to convert linearly polarized input into multiple beams exhibiting orthogonal modes. Two/four vortex beams in different directions are demonstrated. The capacity for wireless communication in the terahertz band can be enhanced by utilizing a graphene metasurface.

Realization of Flat-Topped Electromagnetic Orbital Angular Momentum Vortex Wave With a Square Array

Realization of Flat-Topped Electromagnetic Orbital Angular Momentum Vortex Wave With a Square Array

Design of an ultra wideband THz reflectarray antenna based on orbital angular momentum

A Reflectarray antenna (RA) that can generate orbital angular momentum vortex waves in the THz band was proposed. First of all, a reflective phase-shifting element composed of a notched double-shaped ring and a single Matal structure was designed to form a radiation patch. The reflective phase-shifting element has the advantages of good linearity, high smoothness, and a large phase shift range, which is suitable for the subsequent production of reflective array antennas. Then, the phase distribution of the reflected wave of the OAM vortex wave carrying mode l=+1、+2、+3、+4 is given, and the size of each element is determined based on the reflected phase of each element. Using the proposed element, an OAM reflectarray antenna operating within the waveguide standard WR-5 (120GHz–220GHz) is designed. The optimal frequency band for antenna operation in mode l=+1 is 150GHz–190 GHz, and the maximum gain of the reflector array antenna is 19.4 dBi, and the minimum divergence angle of the antenna array is 5.2°, which has good directivity and beam radiation ability. The spiral structure of the OAM beam space generated by the RA in this paper is clearly visible, proving that the implementation of OAM vortex waves in the THz frequency band is feasible. This design also provides a new method for generating OAM vortex waves in wireless and microwave wireless communications. The proposed RA is suitable for 6G wireless communication systems.

A novel approach for the generation of OAM beam with high mode purity using the reflectarray

In this paper, a reflectarray composed of 14 × 14 unit elements is proposed for generating orbital angular momentum (OAM) beam (l = +1) at 5.75 GHz with reduced phase errors and side lobe levels. Unlike traditional designs, the proposed reflective surface is made up of two different types of unit cell structures. Together, the two-unit cells can offer a complete 360°phase range with linear slope as a function of the phasing element size. The proposed reflectarray prototype is 350 mm×350 mm in size, it is fabricated on commercial FR4 laminates, and experimentally tested. Additionally, antipodal vivaldi (AV) antenna is considered as the feed for the proposed reflective surface. The experimental results confirm that the OAM vortex beam with l=+1 is successfully generated at 5.75GHz. The generated OAM beam has a high mode purity, with a gain of 15dBi and divergence angle of ± 8°.

Dual-mode switchable metasurface for multi-type OAM vortex beam generation and dual-band perfect absorption in terahertz band

As metasurface technology is developing rapidly in the past decades, multi-operating mode and tunability are evolving into hot spots in its development. In this paper, we present a dual-operating mode metasurface consisting of vanadium dioxide (VO2). At room temperature (25 °C), it operates as a reflection mode. Eight metasurface unit cells with different reflection phases are designed, which can achieve 2π phase coverage in the frequency band of 0.4 THz-0.5 THz. Furthermore, by bringing encoded convolution and superposition theorems into the design of metasurface arrays, vertically incident circularly polarized (CP) waves can be transformed into single-beam, multi-beam, deflected and superimposed orbital angular momentum (OAM) vortex beams, respectively. On the other hand, at high temperature (68 °C), it operates as a dual-band terahertz absorber. It achieves near-perfect absorption at 1.71 THz and 1.87 THz with 99.9% and 98.9%, and also has polarization insensitivity. Therefore, the metasurface designed in this paper has promising applications in future terahertz communications, high-resolution imaging, and electromagnetic stealth.

Four‐Beams‐Reconfigurable Circular‐Ring Array Antennas for Monopulse Radar Applications

AbstractWe propose a new approach to the mask‐constrained power synthesis of reconfigurable circular‐ring array antennas for monopulse radar applications. By resorting to Orbital Angular Momentum vortex beams, and without increasing either the complexity of the beam forming network or the signal processing times with respect to the state‐of‐art solutions, the proposed method allows improving the precision pointing with respect to usual techniques. The overall synthesis is cast as a Convex Programming optimization problem aimed at reducing as much as possible the number of control points of the array, and it is tested in relevant application scenarios.

Demodulation method of orbital angular momentum vortex wave by few fields sampled on an extremely small partial aperture in radiation region

In this paper, a demodulation method of electromagnetic wave carrying orbital angular momentum (OAM) has been developed by using few electric fields sampled on an extremely small portion of the whole circumference in the radiation region. Considering that the field of the OAM wave is naturally of the form of exp (ilφ) in the far-field region, the matrix pencil method is used to extract the magnitude terms and the exponential terms according to the real and imaginary parts of the fields sampled on a small portion of the whole circumference, and further, the mode purities corresponding to the desirable OAM modes can be easily solved. The proposed demodulation method only uses four sampling electric fields with the sampling angle ranges of 3.3° and 9° to well demodulate the mode purities of the OAM waves with the single mode and the mixed dual-mode, respectively. The prototype of a dual uniform circular array with the equal divergent angle for the OAM waves of the modes l = 1 and l = 2 is fabricated and measured. Good agreement between the simulation and the measurement is observed to verify the proposed demodulation method.

A Transmissive Metasurface Generating Wideband OAM Vortex Beam in the Ka-Band

In this paper a wideband transmitting metasurface is proposed to generate the orbital angular momentum (OAM) vortex beam in the Ka-band. The proposed unit cell is a novel combination of dual-square loops and a cross dipole element printed on four dielectric layers separated by an airgap. This combination of two types of elements helps to attain a linear transmission phase response with more than a 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> phase compensation range with a transmission magnitude equal to or better than 1.1 dB. The proposed structure of the unit cell is also suitable for dual polarized applications due to its symmetrical configuration. The proposed metasurface is also fabricated and measured to verify its simulated performance. The overall measured outcome shows that the proposed structure has successfully achieved wideband performance for the OAM vortex beam with +1 mode. The overall mode purity is greater than 72% from 28.0 to 38.0 GHz, which is about 33.3% OAM bandwidth. Moreover, the peak gain of 23.8 dBi is achieved with 22.1% aperture efficiency.

Optically transparent anisotropic holographic impedance metasurface for launching orbital angular momentum vortex wave

The visualization of electronic products is essential for the digitization and visualization of human-computer exchange in the future smart world. However, due to its large non-translucent aperture, the conventional hologram metasurface urgently needs transparency for wide scene usage. In this paper, an optically transparent holographic impedance metasurface with a low profile is investigated for generating orbital angular momentum (OAM) vortex waves in the radio-frequency domain for the first time. Indium tin oxide is applied to perform the metal functionality, which is featured by high optical transparency, and polymethyl methacrylate is proposed as the substrate material in the meta-atom design. By virtue of classic optical metasurface theory and leaky-wave principle, holographic impedance mapping is achieved, so the surface wave generated by the monopole port is effectively converted into a radiating vortex wave at 15 GHz. The prototype has been manufactured and fabricated, and the experiment results exhibit good agreements with theory and simulation analyses, showing a good prospect for OAM vortex wave launch in visualized applications.

Polarization insensitive reflectarray for OAM beam generation over octave bandwidth for 5G applications

In this paper, a wide band, polarization-insensitive reflectarray composed of 14 × 14 unit elements is proposed to generate vortex beams of different orbital angular momentum (OAM) modes with higher mode purity. The multiple resonance architecture of the unit element is designed and achieved 750°of the reflection phase by varying geometrical dimensions, which contributes to the remarkable increase in linear phase bandwidth. The unit element provides stable responses for different incident angles of electromagnetic wave. The prototype of proposed reflectarray with the size of 350 mm × 350 mm is designed, fabricated using printed board technology, and experimentally tested. The measured results confirms that a OAM vortex beam l = +1 is successfully generated at 5.75 GHz with a high gain of 21.3 dBi and a low divergence angle of 12°.

Helical Circular Array Configurations for Generation of Orbital Angular Momentum Beams

The vortex electromagnetic waves carrying the orbital angular momentum (OAM) are an increasing subject of study, and the design of a system capable of generating such waves is also of crucial importance. In this letter, a helical circular array (HCA) has been proposed to generate OAM radio beams. The proposed HCA has a more straightforward and more feasible scheme when compared to uniform circular arrays (UCAs) that require additional phase-shifting devices to form inter-element phases. The heights of the proposed HCA elements along the z-axis are exploited to generate the different OAM modes. For the high-order OAM modes, the height of the HCA is made more compact with the help of the proposed helical circular subarray (HCSA) configurations with spiral phase positions providing a linear phase delay with the azimuth angle. In addition, at high mode values, the HCSA structure creates a smoother OAM vortex beam than HCA. The full-wave simulation results show that the proposed HCA scheme can be used to generate vortex electromagnetic waves with a spiral phase wavefront structure.

Dual-comb spectroscopy for in-plane angle measurement using OAM vortex light.

By combining dual-comb spectroscopy and optical vortices, which have the orbital angular momentum (OAM) of light, we propose a novel measurement concept, "dual-vortex-comb spectroscopy (DVCS)." We extend the dual-comb spectroscopy to angle dimensions by utilizing the characteristic helical phase structure of optical vortices. We demonstrate a proof-of-principle experiment on DVCS that realizes in-plane azimuth-angle measurements with an accuracy of ∼0.1 mrad after cyclic error correction, whose origin is verified by a simulation. We also demonstrate that the measurable angle range is tailored by the topological number of the optical vortices. This is the first demonstration of dimensional conversion between the in-plane angle and dual-comb interferometric phase. This successful result can expand the applicability of optical frequency comb metrology to new dimensions.

Laser cavity creation of one or dual orbital angular momentum vortex beams by edge diffraction regulation

In the past three decades, a unique beam called vortex beam with orbital angular momentum has become the research focus of researchers. However, to date, almost all the methods for generating vortex beams are to adjust the phase distribution of the central part of the beam cross section. Is there a method to generate high-purity one or dual orbital angular momentum vortex beams based on the edge diffraction regulation? Specifically, it has been shown that a variety of complex laser modes can be generated in a special laser cavity, and these modes are difficult to control. How to create vortex beams from these complex patterns is an important scientific problem. This paper solves this scientific problem based on edge diffraction regulation in laser cavities. Only three variable aperture diaphragms are needed to realize the generation of one or dual orbital angular momentum vortex beams. Moreover, the topological charge of the beam is proved by interference experiments. This work can not only break the understanding of traditional vortex beam generation method and deepen the understanding of laser cavity regulation technology but also bring opportunities for optics, electromagnetics, and other related fields.

Multiple Orbital Angular Momentum Beams with High‐Purity of Transmission‐Coding Metasurface

AbstractVortex electromagnetic wave carrying orbital angular momentum (OAM) is expected to break through the shortage of spectrum resource because of its multiple independent modes. Nowadays, there are various methods to generate OAM vortex beam. Based on the concepts of digital metasurface and propagation phase, a broadband 3‐bit coding transmission metasurface is designed and illustrated by simulation and measurement. The elements exhibit the different phase by changing the size of metal patch, and the focusing phase is introduced to reduce the divergence of OAM beams. Based on the principle of phase superposition, the wide‐band, high‐purity, and multi‐beam OAM vortex waves can be generated by the transmission‐coding metasurface (TCMS). The experimental results verify that TCMS realizes the multiple OAM beams along different directions simultaneously from 9 to 10.5 GHz, which are consistent with the theoretical results. The high purity more than 90% of OAM mode is demonstrated by both the simulated and measured results. The TCMS provides a potential and effective scheme for OAM multiplexing and multi‐target positioning in wireless systems.

Synthesis of Orbital Angular Momentum Antennas for Target Localization

AbstractThe exploitation of Orbital Angular Momentum (OAM) is proposed to design circular‐support continuous aperture distributions and concentric‐ring arrays for target‐localization applications. The proposed approach allows generating a single isolated null of the field in the target direction, which is of interest with respect to common techniques leading to a line (or perpendicular lines) of power‐pattern zeroes in the spectral plane. In the presented design procedure, the optimal continuous aperture source is first identified by resorting to the most suitable OAM vortex and effective tools such as the Singular Value Decomposition of the involved operator and a smart Convex Programming optimization. Then, by exploiting fast deterministic algorithms, this continuous source is discretized into circular‐ring arrays emulating it at best. Relevant applications including equispaced arrays, sum‐difference reconfigurable arrays, and isophoric sparse arrays are presented. The proposed approach is also compared with the state‐of‐the‐art techniques and assessed through full‐wave electromagnetic simulations.

A broadband orbital angular momentum generator utilizing polarization conversion metasurface at microwave frequencies

In this paper, we achieved first- and second-order orbital angular momentum (OAM) vortex beams by a single-layer polarization conversion metasurface (PCM). Firstly, we present the basic PCM unit, composed of a short line and two pairs of oval split rings along the diagonal direction attached to a substrate layer. The agreement of numerical simulations and experimental measurements proves that the employed structure’s polarization conversion ratio (PCR) is more than 90% from 8.12 to 22.81 GHz under normal incidence (corresponding to the fractional bandwidth of 94.9%). We reveal the physical mechanism of the proposed PCM by an equivalent circuit method in broadband. Furthermore, to construct the reflective PCM array, the reflection phase of 2π can be obtained by adjusting the geometric parameters of the units. The OAM vortex beams with different topological charges (l= −1, l= −2) can be achieved by specific compensation phases. Simultaneously, we calculate the purity of different OAM modes from 10 to 15 GHz, demonstrating that the main-mode dominates broadband. Finally, the proposed reflective array realizes a stable response to the variations in the incidence angle from 0° to 45°. The proposed method provides an opportunity to realize different topological charges of OAM waves by PCM units in broadband.

Bidirectional Terahertz Vortex Beam Regulator

Most of the reported vortex beam generators with orbital angular momentum (OAM) in the terahertz region only operate in either the reflection mode or the transmission mode, which greatly limits the integration and application in terahertz technology systems. Herein, we propose a full-space vortex beam regulator at two different frequencies. By changing the VO2 phase transition state, the transmission and reflection mode OAM beams can be flexibly controlled by a single metasurface. For the transmission mode, the proposed structure realizes an OAM beam at the topological charges of l = 1 and 2 at 0.6 THz and 1.4 THz. For the reflection mode, our structure generates an OAM beam at the topological charges of l = 1 and 2 at 0.9 THz and 1.5 THz. Based on the superposition theorem and convolution operation principle, the regulation of an OAM vortex beam with a specific deflection angle and a symmetrical deflection OAM vortex beam are realized. The designed metasurface integrates multiple transmitted and reflected vortex beam functions in full space and has potential application in different terahertz systems.

A broadband vortex beam generator using graphene metasurface reflectarray in terahertz region

In this paper, a broadband vortex beam generator based on the graphene metasurface reflectarray is demonstrated in terahertz region. The designed graphene metasurface reflectarray structure realizes the generation of orbital angular momentum vortex wave with topological charges of l = ±1, l = ±2 and l = ±3 ranging from 2 to 2.4 THz by modulating the chemical potential and the radius of the graphene unit. Moreover, all of the vortex beam with topological charges of l = +1, l = +2 and l = +3 have a high purity in the range from 2 to 2.4 THz. Our finding shows that the graphene metasurface reflectarray structure might find applications in the fields of broad band vortex wave devices in THz domains and also provide new insights into innovative low-profile devices in the wireless communication.

Independent control of both amplitude and phase for orthogonal circularly polarized electromagnetic waves through polarization conversions

Spin-selective multi-functional integrated metasurfaces have attracted much research attention due to its promising application prospects. However, the goal of independent and arbitrary control of both amplitude and phase for orthogonal circularly polarized (CP) waves still has not been achieved fully. This is because it requires mirror asymmetric structure to achieve such a goal but there is no apparent physical relation between the unit cell structure and the spin-selective properties. A simple method with clear physical pictures is proposed in this paper to achieve the goal of arbitrary spin-selective manipulations. The idea is to convert the incident orthogonal CP waves to orthogonal linearly polarized (LP) waves first, then manipulate the LP waves with matured techniques, and finally convert the LP waves back to the corresponding CP waves. Simulations and experiments have been carried out to validate the method. Using this idea, spin-selective focusing, spin-selective deflection and spin-selective orbital angular momentum vortex beam have been demonstrated.