We report the first experimental generation of high-order Mathieu beams and confirm their propagation invariance over a limited range. In our experiment we use a computer-generated phase hologram. The peculiar behaviour of the vortices in Mathieu beams gives rise to questions about their orbital-angular-momentum content, which we calculate by performing a decomposition in terms of Bessel beams.

Holographic generation and orbital angular momentum of high-order Mathieu beams : Special issue on atoms and angular momentum of light

In this paper, we review the generation of vortex beams carrying orbital angular momentum in the microwave domain. We firstly present the theory of Laguerre–Gaussian beams where it is demonstrated that they carry such type of momentum. We further provide an overview of the classical methods used to generate orbital angular momentum vortex beams, which rely on two main methods; plane wave to vortex wave conversion and direct generation using radiating antennas. Then, we present recent progress in the physics of metasurfaces devoted to the generation of vortex beams with a discussion about reflective and transmissive metasurfaces for plane wave to vortex wave conversion as well as methods to reduce the intrinsic divergence characteristics of vortex beams. Finally, we conclude on this rapidly developing research field.

A Review of Orbital Angular Momentum Vortex Beams Generation: From Traditional Methods to Metasurfaces

Vortex beams carrying orbital angular momentum (OAM) are extensively studied owing to its potential to expand channel capacity of microwave and optical communication. By utilizing the Pancharatnam–Berry phase concept, an ultrawideband single-layer metasurface is proposed to realize the conversion from incident plane waves to reflected vortex beams covering a considerable bandwidth from 6.75 to 21.85 GHz (>105%). An equivalent circuit model combined with broadband phase shift network is developed to effectively design the meta-atoms in metasurface. It is the first time to design wideband metasurfaces with the phase-based characteristics. To verify the proposed model, some deformed square loop meta-atoms are proposed to construct the metasurfaces with broadband OAM characteristic. Moreover, the vortex beams with the integer ( ${l} = -3$ ), fractional ( ${l} = -1.5$ ), and high-order ( ${l} = -10$ ) OAM mode are generated. Based on an OAM spectral analysis, the mode purity of the generated vortex waves is discussed in detail. The experimental results achieve a good agreement with those obtained from the simulation, thus proving the effectiveness and practicability of the proposed method.

Ultrawideband Reflection-Type Metasurface for Generating Integer and Fractional Orbital Angular Momentum

Vortex beam has attracted growing attention in recent years due to its remarkable abilities in the communication system since it is believed to be an effective way to improve the channel capacity efficiency. However, available vortex beam generators suffer from the issues of complex configurations, low efficiency as well as narrow bandwidths, especially for the transmissive case. Here, we proposed a broadband transmissive metasurface to generate vortex beam with l=1 in a broad band ranging from 8GHz to 13 GHz. We enhance the working bandwidth by carefully designing the meta-atoms which provide high transmittances along with similar slopes of the phase responses within a large frequency interval. More importantly, the designed vortex beam generator exhibits very high working efficiencies (more than 83% within the whole working band). Our finding opens a door for the design of high-efficiency broadband transmissive vortex beam generators and operating at other frequency domains.

https://findresearcher.sdu.dk:8443/ws/files/146402496/oe_27_4_4281.pdf

High-efficiency broadband vortex beam generator based on transmissive metasurface

Quad-Band Transmissive Metasurface with Linear to Dual-Circular Polarization Conversion Simultaneously

High-performance broadband vortex beam generator using reflective Pancharatnam-Berry metasurface

Ultra-wideband linear-to-circular polarization converter with ellipse-shaped metasurfaces

Vortex beam generators, particularly those operating in transmission mode, are extremely important in modern communication systems because they are believed to be an effective way to extend the capacity of a communication channel. However, current approaches have suffered from complex configurations, fixed operation modes, and low efficiency, particularly in a transmissive case. In this study, we propose a new strategy for improving the transmissive amplitude and bandwidth of metasurfaces by optimizing microstructures with a non-uniform thickness based on the transfer matrix method (TMM). As a result, our designed meta-atom can achieve a high transmission of greater than 0.9 within a wide frequency interval of 12.6-16.2 GHz. As a proof of concept, we designed a vortex beam generator with topological charge l = 1 using the designed meta-atom. We conducted near- and far-field experiments to characterize its performance, indicating that our meta-device can realize a pure vortex beam with an efficiency of higher than 82.1% at a central frequency of 15 GHz. Our findings will lay a theoretical foundation for studies on metasurfaces with a non-uniform thickness and provide a way to design high-performance meta-devices with broad bandwidths.

/pdf/high-performance-transmissive-broadband-vortex-beam-53iuqnvgs3.pdf

High-Performance Transmissive Broadband Vortex Beam Generator Based on Pancharatnam–Berry Metasurface

Wavefront manipulation is of great importance in the development of science and technology, and the generation of vortex beam is one of the most effective methods to improve the channel capacity and communication accuracy. However, current available vortex beam generators suffer from complex structure, large size, especially narrow bandwidth. To solve these issues, we propose a general strategy to enhance the bandwidth of the vortex beam generator by using double-layered reflective metasurface. The well-optimized double-layered reflective element possesses dual resonance, which realizes complete phase coverage (larger than 360 degrees) and a flat slope of the reflection phase, resulting in a broad bandwidth (8.5-11.5GHz). A compact and low-profile vortex-beam generator is proposed by combining the parabolic phase and the spiral phase plate phase. Both near field and far field experiments are performed to demonstrate the predesigned effects. A pure vortex beam is observed clearly with a topological charge of m=1 in a wide frequency window of 3 GHz (8.5-11.5GHz). More importantly, the working efficiency of the vortex beam is better than 90% at center frequency 10 GHz. The findings in this paper motivate the realization of high-performance reflective metasurfaces and other functional metadevices.

High-performance broadband vortex beam generator based on double-layered reflective metasurface

In this work, an ultra-wideband microwave metasurface absorber with optically transparent and flexible properties is proposed. The metasurface is composed of a reflective backplane and a microwave absorption layer sandwiched between two dielectric substrates. The impedance matching curves of the microwave absorption layer are deduced based on the impedance matching theory, which is quite helpful and useful to improve the accuracy and efficiency of the broadband optimization design. Simulated results show that absorption higher than 90% can be achieved in the frequency band ranging from 5.8 GHz to 27.3 GHz, which covers the radar wavebands of C, X, Ku and K. The relative bandwidth reaches up to 130%, thus realizing ultra-wideband absorption while the thickness of the metasurface is only 0.12 times the upper-cutoff wavelength. For the TE (transverse electric) wave incidence, the metasurface maintains good performance when incident angle θ ≤ 50°, while for the TM (transverse magnetic) wave incidence, the absorption higher than 90% can be still achieved in a broad frequency band when θ ≤ 60°. It can be seen that using double-layer dielectric substrate in the metasurface not only greatly expands the microwave absorption bandwidth, but also improves the oblique incident properties. In addition, the metasurface is insensitive to polarization since its unit cell is symmetrical. Moreover, by rationally designing materials, the metasurface in this work is optically transparent and flexible, thus quite suitable for window radar stealth and equipment conformal stealth.

Yuzhou Ran

Papers

High-performance broadband vortex beam generator using reflective Pancharatnam-Berry metasurface

Ultra-wideband linear-to-circular polarization converter with ellipse-shaped metasurfaces

High-Performance Transmissive Broadband Vortex Beam Generator Based on Pancharatnam–Berry Metasurface

High-performance broadband vortex beam generator based on double-layered reflective metasurface

Ultra-wideband, optically transparent, and flexible microwave metasurface absorber