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

Vortex beams carrying orbital angular momentum are extensively studied owing to its potential to expand channel capacity of microwave and optical communication. By utilizing the Pancharatnam-Berry phase concept, an ultra-wideband 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 . 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 respectively. 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.

Ultra-wideband Reflection-type Metasurface for Generating Integer and Fractional Orbital Angular Momentum

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

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

Based on the significant applications of phase gradient metasurface in the control of electromagnetic wavefronts, a double-layer broadband reflecting element is used to design a reflecting broadband vortex beam metasurface, and the vortex beam generator is fabricated and simulated. The simulated and experimental results verify that the vortex waves can be flexibly generated by using subwavelength reflective metasurfaces in 8.S-11.SGHz.

Tong Cai

Papers

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

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

Design of Broadband Vortex Generator