Orbital Angular Momentum Vortex Beams Research Articles

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.

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.

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.

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.

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.

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.

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.

Dynamically tunable multimodal vortex beam generator based on reflective metasurface

How efficiently generating a multi-mode vortex beam is the key to the new system of wireless communication technology based on orbital angular momentum. Due to the advantages of simple structure and flexible tunability of the metasurface, in recent years, people have been devoted to generating dynamic orbital angular momentum more simply and efficiently using programmable metasurfaces. To address the current problem of a single number of modes for generating orbital angular momentum vortex beams from metasurface, the programmable reflective metasurface structure has been designed to realize the function of dynamically tunable multi-mode of vortex beams. By loading two varactor diodes inside each metal patch to construct eight digital coding units, and using the programmable bias circuit to modulate the coding distribution and resonance characteristics on the metasurface in real-time, the metasurface obtains low reflection loss and stable reflection phase, and finally obtains vortex beams with mode numbers of ℓ = ±1, ℓ = ±2, and ℓ = ±3, respectively. The designed metasurface also has the advantages of a simple structure, small electrical size, and low profile, which has a large potential value in the communication of vortex beams and a perfect application in wireless transmission.

Mid-infrared reconfigurable all-dielectric metasurface based on Ge2Sb2Se4Te1 phase-change material.

In this paper, a reconfigurable all-dielectric metasurface based on Ge2Sb2Se4Te1 (GSST) phase-change material is proposed. By changing GSST from amorphous state to crystalline state, the metasurface can achieve high circular dichroism (CD) and wideband polarization conversion for circularly polarized waves in the mid-infrared (MIR) band. The maximum CD value reaches 0.95 at 74 THz, and circular polarization conversion efficiency is more than 90% in the wideband range of 41 THz - 48 THz. In addition, based on Pancharatnam-Berry phase, three kinds of wavefront manipulation of light have been realized: abnormal refraction, orbital angular momentum vortex beam and orbital angular momentum vortex beam splitting. This work has potential applications in the future MIR optical integrated system.

Terahertz quasi non-diffraction Bessel vortex beam generation using three lattice types reflective metasurface.

Bandwidth, orbital-angular momentum (OAM) divergence, and mode purity are the three critical issues for the practical terahertz orbital angular momentum manipulation, especially in the next sixth-generation (6G) communication system. Here we propose the broadband high-order Bessel vortex beam carrying multiple OAM modes reflective metasurface in the terahertz domain. The simulation results agree with the theoretical expectation, and the diffracting divergence of OAM vortex beam characteristics has been alleviated. The research on the relationship between the varieties of lattice type and mode purity is also relatively scarce. Henceforth, a comparison study has been conducted between three lattice types, i.e., square lattice, triangular lattice, and concentric ring lattice. And corresponding results of the relationship of mode purity with those lattice types show that the concentric ring lattice has the best performance.

Low Sidelobe Orbital Angular Momentum Vortex Beams Based on Modified Bayliss Synthesis Method for Circular Array

Electromagnetic orbital angular momentum (OAM) vortex beam patterns generated by conventional methods are always of high sidelobe level, which severely limits its application. Considering the sidelobe suppression abilities of Bayliss synthesis method for difference beams, we present a modified Bayliss synthesis procedure to obtain low sidelobe OAM vortex beams. The Bayliss synthesis method cannot suppress the sidelobe level of difference pattern to the desired value when the element number of linear array is relatively small. By modifying the original zero locations that need to be removed in Bayliss pattern, we can further suppress the sidelobe level of difference pattern. This can be called the modified Bayliss pattern. Finally, by expanding the proposed synthesis method from linear source to circular array, theoretical results show that sidelobe levels of OAM beams can be suppressed to less than −30 dB when OAM mode number varies from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = 1 to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = 10. The proposed low sidelobe OAM vortex beam can be applied in antijamming communications of near-field in the future.

Generating Different Polarized Multiple Vortex Beams at Different Frequencies from Laminated Meta-Surface Lenses.

We demonstrate the generation of multiple orbital angular momentum (OAM) vortex beams with different radiating states at different frequencies through a laminated meta-surface lens consisting of a dual polarized meta-array interconnected with a frequency selective meta-array. The co-linearly polarized (LP) waves from the source can directly penetrate the meta-surface lens to form multiple OAM vortex beams at one frequency. On the other hand, the meta-surface lens will be capable of releasing the cross-LP counterparts at another frequency with high-efficient polarization conversions to have multiple OAM vortex radiations with different radiating directions and vortex modes. Our design, using laminated meta-surface lens to synthesize multiple OAM vortex beams with orthogonal polarizations at different frequencies, should pave the way for building up more advanced vortex beam communication system with expanded diversity of the meta-device.

Polarization-independent quadri-channel vortex beam generator based on transmissive coding metasurface

In the 1990s, it was recognized that light beams carrying orbital angular momentum (OAM) have benefited applications ranging from optical manipulation to quantum information processing. In recent years, attention has been directed towards the opportunities for communication systems due to the inspiring application potential in both the optical and microwave fields. In this paper, a polarization-independent quadri-channel vortex beam generator based on transmissive metasurface is proposed that can achieve selectivity of polarization, 2-bit OAM modes and spatial distribution in the quadri-channel simultaneously. The transmissive metasurface consists of four metallic layers and three dielectric layers and is designed, fabricated, and experimentally demonstrated to generate multi-mode and dual-polarization OAM vortex beams at 10.0 GHz. Orthogonal polarization and 2-bit information are carried by OAM modes +1, −1 + 2 and −2 and a different phase gradient is superimposed at each channel to realize beam steering, ensuring the accuracy and integrity of the information. The simulation and experimental results verify that the vortex beams with different OAM modes in dual polarizations can be flexibly generated by using transmissive metasurfaces. The proposed method and metasurface pave a way to add extra channels to create an additional set of data carriers for space-division multiplexing (SDM).

Switchable transverse mode operation of a fiber laser with an external feedback cavity

In this paper, we demonstrate a switchable transverse mode operation fiber laser. Lasing in a specific transverse mode is enabled by employing a Fabry–Pérot (F-P) cavity forming an external feedback cavity architecture as an effective transverse mode selector. High spatial quality tunable output modes, including vector vortex beam, orbital angular momentum (OAM) vortex beam and flat-topped beam, could be achieved by optimizing the cavity length and appropriate polarization control. The purity of the vector vortex beam is more than 95%. The topological charge of the OAM mode can be tuned from −1 to +1. The purity of the ±1 OAM vortex beams are all more than 95%. The normalized root mean square of the flat-topped beam is 0.035 and the steep degree is 0.815. Furthermore, this approach has great scalability and can be applied to other modes and spectral regimes.

Vortex Beam Optimization Design of Concentric Uniform Circular Array Antenna with Improved Array Factor

In this paper, an improved array factor of the concentric uniform circular array (CUCA) antenna is proposed for the orbital angular momentum (OAM) vortex beam optimization design. From the perspective of the radiation pattern’s power conservation principle, a correction factor is introduced to the conventional array factor of CUCA. Then, based on the improved array factor, by adjusting the rings’ radii parameters of the CUCA, we optimize the vortex beam’s sidelobe level through the generic algorithm (GA). Two different CUCA antenna model are calculated as examples to further illustrate the effectiveness of the improved array factor. Subsequently, an electromagnetic simulation model of two rings CUCA antenna is built at C band for generating low sidelobe vortex beam carrying OAM mode. The electromagnetic simulation model of the designed CUCA antenna is also fabricated and measured. The corresponding antenna far-field vortex beam radiation pattern and near-field vortex wave electric field distributions are measured. The simulation results and the measurement results are in good agreement. The proposed designs of antenna and OAM vortex beam regulation are expected to be used for 5G and 6G communications applications