Dual-band vortex beam generation with different OAM modes using single-layer metasurface

Wideband reflective metasurface for independent control of mode and circular polarization of orbital angular momentum

Pancharatnam-Berry (PB) phase, usually utilized for phase manipulation of circularly polarized (CP) waves, has inherent symmetrical response on left-handed polarized (LCP) and right-handed polarized (RCP) for orbital angular momentum (OAM), which severely hinders its application. By modulating both propagation and PB phase allows independent control of LCP and RCP of OAM, but increases the design difficulty. Here, we propose a phase compensation scheme to independent control the CP states of OAM only utilizing PB phase, where arbitrary topological charges and deflection directions of LCP and RCP beams can be realized. Two wideband metasurfaces are designed to independent control the mode, circular polarization and beam directions of OAM at the frequency range of 10-20 GHz. This work significantly motivates the development of polarization division multiplexing in wireless communication system.

Dual-mode vortex beam transmission metasurface antenna based on linear-to-circular polarization converter

The generation of multi-mode vortex beams at the same aperture is currently emerging as a research hotspot. In this paper, a method based on a linearly polarized-circularly polarized translational transmission metasurface (TM) is proposed to enable a dual-circularly polarized dual-mode vortex beam generation. Through the judicious implementation of an additional rotational phase and the combination of the initial transmission phase, the phases of the left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves can be manipulated arbitrarily and independently. Meanwhile, the design of the array phase is utilized for the dual-mode dual-circularly polarized beam generation. Simulation and sample measurements provide validation data for the feasibility of this method, in which the measurement results are in excellent consistency with the simulation ones. This proposed method paves the way toward the enhancement of the channel capacity of mobile communication.

Dual-band metasurface generating multiple OAM beams independently in full polarizations

In this paper, a dual-band metasurface (MS) generating multiple orbital angular momentum (OAM) beams independently in full polarizations is proposed. First, the design principle of controlling full polarizations independently is analyzed. Second, the frequency selective surface is introduced to the meta-atom design that ensures the meta-atom operates at Ku- and Ka-band independently, while, at each band, sixteen optimized meta-atoms realize the high reflection amplitude and enough phase coverage. Next, the optimized dual-band meta-atom controlling full polarizations independently is utilized to design the MS, which could generate eight independent OAM beams including the x-polarized, y-polarized, left hand circularly polarized, and right hand circularly polarized OAM beams at dual-band. Finally, the MS is designed, fabricated, and measured. Both simulated and measured results verify that the proposed MS could generate multiple OAM beams in full polarizations at dual-band, showing the perspective in the OAM-based area, such as the wireless communication, target detection, and security encryption.

Highly-twisted states of light from a high quality factor photonic crystal ring

Twisted light with orbital angular momentum (OAM) has been extensively studied for applications in quantum and classical communications, microscopy, and optical micromanipulation. Ejecting high angular momentum states of a whispering gallery mode (WGM) microresonator through a grating-assisted mechanism provides a scalable, chip-integrated solution for OAM generation. However, demonstrated OAM microresonators have exhibited a much lower quality factor (Q) than conventional WGM resonators (by >100×), and an understanding of the limits on Q has been lacking. This is crucial given the importance of Q in enhancing light-matter interactions. Moreover, though high-OAM states are often desirable, the limits on what is achievable in a microresonator are not well understood. Here, we provide insight on these two questions, through understanding OAM from the perspective of mode coupling in a photonic crystal ring and linking it to coherent backscattering between counter-propagating WGMs. In addition to demonstrating high-Q (10⁵ to 10⁶), a high estimated upper bound on OAM ejection efficiency (up to 90%), and high-OAM number (up to l = 60), our empirical model is supported by experiments and provides a quantitative explanation for the behavior of Q and the upper bound of OAM ejection efficiency with l. The state-of-the-art performance and understanding of microresonator OAM generation opens opportunities for OAM applications using chip-integrated technologies.

Generating multiple vortex beams simultaneously and independently in different directions by elaborately splicing multiple transmissive metasurfaces featuring polarization isolation

In this paper, by elaborately splicing multiple transmissive metasurfaces (MSs) featuring polarization isolation, multiple linear polarized (LP) vortex beams are generated simultaneously and independently in different directions. Specifically, by carefully optimizing the radius of the array and the distance between the MS and array, each MS generates a well-performed deflection vortex beam with a low side-lobe level and little diffraction, resulting in a minor effect on other deflection vortex beams. Subsequently, four transmissive MSs are elaborately spliced, showing the polarization isolation characteristic between the adjacent MS, and thereby each MS is only illuminated by the respective antenna array. In addition, each MS only generates the desired LP vortex beam, and the corresponding cross-polarization is suppressed. Finally, the simulation and measurement results show that multiple LP vortex beams carrying different orbital angular momentum (OAM) modes are generated simultaneously and independently in different directions, verifying the effectiveness of the proposed method.

Compact cascaded meta-surface system for controlling the spin and orbital angular momentum of electromagnetic fields simultaneously

We propose a compact cascaded meta-surface system (CCMS) to produce well converged orbital angular momentum (OAM) vortex waves with tailored spin angular momentum (SAM) by integrating a meta-surface lens (ML) with an assistant meta-mirror (AM). Specifically, the co-linearly polarized (LP) waves from the feed would be reflected by the ML firstly and then twisted into the cross-LP counterparts by the AM to penetrate the ML for the perfect synthesis of the OAM vortex beams while performing the linear-to-circular polarization conversion. Especially, the CCMS can pack the ML and the AM closely together with a quarter of the ML focal length when we apply proper phase distributions on the AM. In addition, the proposed CCMS can readily be extended to the generation of multiple circularly polarized OAM vortex waves with different modes. Our design should thus pave the way for building up more efficient wireless communication systems with expanded channel capacity.

Formation of Millimeter Waves with Electrically Tunable Orbital Angular Momentum

A method for forming electromagnetic waves with a tunable nonzero orbital angular momentum (OAM) is proposed. The approach is based on transforming an incident plane wave into a helical one using an electrically tunable ferroelectric lens. It uses high-resistive thin/thick film electrodes with a special discrete topology. The correlation between film electrodes topology and the highest order of OAM modes that the lens can form is described. A lens prototype based on Ba0.55Sr0.45TiO3 ferroelectric material and operating at a frequency of 60 GHz was designed, manufactured, and tested. The amplitude and phase distribution of the OAM wave with l = +1 formed by prototype were measured to confirm the effectiveness of the proposed method. The proposed lens has a combination of advantages such as low dimensions, electrical control over the OAM modes, and the possibility to operate in the millimeter wavelength range.

Focusing and imaging of a polarization-controlled bifocal metalens

Metalenses are a kind of flat optical device, which consist of an array of nanoantennas with subwavelength thickness that manipulates the incoming light wavefront in a precisely tailorable manner. In this work, we proposed a bifocal metalens that can realize switchable multiplane imaging, controlled by changing the polarization state of an incident light. The polarization-dependent metalens was designed and fabricated by arranging polysilicon nanobeam unit elements. We simulated and experimentally characterized the focus performance of the bifocal metalens. Under the light incidence with left-handed circular polarization, the focal length is 250 µm. By changing the polarization state to right-handed circular polarization, the focal length is tuned to 200 µm. Experimental results and numerical simulations are in good agreement. Moreover, when a linear polarization light is used, two focal spots will appear at the same time. Such a bifocal metalens is suitable for multiplane imaging applications.

Dual-band multi-bit programmable reflective metasurface unit cell: design and experiment

Programmable reflective metasurfaces that combine the features of reconfigurable phased array antennas and reflectors are an effective solution for radar and modern communication systems. However, most of the demonstrated active metasurfaces support tunable responses for a specific frequency band. Thus, we propose a programmable metasurface that combines the advantages of multi-bit phase quantization and dual-band operations. To actively control the diverse functions, two PIN diodes are integrated on the radiating element, and these diodes are controlled by the biasing voltage. The unit cell is fabricated, and experimental characterization is performed in the waveguide measurement setup. The proposed design can be applied for imaging and high-capacity wireless communications.

Terahertz smart dynamic and active functional electromagnetic metasurfaces and their applications

The demand for smart and multi-functional applications in the terahertz (THz) frequency band, such as for communication, imaging, spectroscopy, sensing and THz integrated circuits, motivates the development of novel active, controllable and informational devices for manipulating and controlling THz waves. Metasurfaces are planar artificial structures composed of thousands of unit cells or metallic structures, whose size is either comparable to or smaller than the wavelength of the illuminated wave, which can efficiently interact with the THz wave and exhibit additional degrees of freedom to modulate the THz wave. In the past decades, active metasurfaces have been developed by combining diode arrays, two-dimensional active materials, two-dimensional electron gases, phase transition material films and other such elements, which can overcome the limitations of conventional bulk materials and structures for applications in compact THz multi-functional antennas, diffractive devices, high-speed data transmission and high-resolution imaging. In this paper, we provide a brief overview of the development of dynamic and active functional electromagnetic metasurfaces and their applications in the THz band in recent years. Different kinds of active metasurfaces are cited and introduced. We believe that, in the future, active metasurfaces will be combined with digitalization and coding to yield more intelligent metasurfaces, which can be used to realize smart THz wave beam scanning, automatic target recognition imaging, self-adaptive directional high-speed data transmission network, biological intelligent detection and other such applications. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.

Metasurface Spiral Focusing Generators with Tunable Orbital Angular Momentum Based on Slab Silicon Nitride Waveguide and Vanadium Dioxide (VO2)

The metasurface spiral focusing (MSF) generator has gained attention in high-speed optical communications due to its spatial orthogonality. However, previous MSF generators only can generate a single orbital angular momentum (OAM) mode for one polarized light. Here, a MSF generator with tunable OAM is proposed and it has the ability to transform linearly polarized light (LPL), circularly polarized light or Gaussian beams into vortex beams which can carry tunable OAM at near-infrared wavelength by controlling the phase transition of vanadium dioxide (VO₂). Utilizing this MSF generator, the beams can be focused on several wavelength-sized rings with efficiency as high as 76%, 32% when VO₂ are in the insulating phase and in the metallic phase, respectively. Moreover, we reveal the relationship between the reflective focal length and transmissive focal length, and the latter is 2.3 times of the former. We further demonstrate the impact of Gaussian beams with different waist sizes on MSF generators: the increase in waist size produces the enhancement in spiral focusing efficiency and the decrease in size of focal ring. The MSF generator we proposed will be applicable to a variety of integrated compact optical systems, such as optical communication systems and optical trapping systems.

Orbital-angular-momentum-amplifying helical vector modes in Yb3+-doped three-core twisted microstructure fiber

A helical Yb³⁺-doped three-core microstructure fiber (YTMF) amplifier is proposed in this paper, so as to solve the problem of generation and transmission of the orbital angular momentum (OAM) beams. The fiber is composed of three Yb³⁺-doped cores with a regular triangle shape and a longitudinal helical structure. The experimental results show that the 1064nm laser can be amplified due to the fluorescence amplification characteristics of the doped material Yb³⁺. Furthermore, theoretical analysis indicates the modes in YTMF at 1064nm, which is located in the amplified wavelength, can support nine modes carrying OAM. Therefore, the related experiments were performed and verified that the transmission modes can respectively carry 1, 2, and 3-order OAM at 1064nm in different coupling cases. These excellent properties indicate that the combination of doped materials and helical fiber provide favorable conditions for the generation and amplification of OAM, which provides a basis for the further development of OAM beams in the field of quantum communication and dense space division multiplexing.

Full control of dual-band vortex beams using a high-efficiency single-layer bi-spectral 2-bit coding metasurface

Vortex beams (VBs) carrying orbital angular moment (OAM) modes have been proven to be promising resources for increasing communication capacity. Although considerable attention has been paid on metasurface-based VB generators due to the unprecedented advantages of metasurface, most applications are usually limited at a single band with a fixed OAM mode. In this work, an emerging dual-band reflection-type coding metasurface is proposed to mitigate these issues by newly engineered meta-atoms, which could achieve independent 2-bit phase modulations at two frequency bands. The proposed coding metasurface could efficiently realize and fully control dual-band VBs carrying frequency selective OAM modes under the linearly polarized incidence. As the first illustrative example, a dual-band VB generator with normal beam direction is fabricated and characterized at two widely used communication bands (Ku and Ka bands). Moreover, by encoding proper coding sequences, versatile beams carrying frequency selective OAM modes can be achieved. Therefore, by adding a gradient phase sequence to the first VB generator, the second one is designed to steer the generated beams to a preset direction, which could enable diverse scenarios. The measurement results of both VB generators agree very well with the numerical ones, validating the full control capability of the proposed approach.

Generation of wideband vortex beam with different OAM modes using third-order meta-frequency selective surface

Orbital angular momentum (OAM) beam generators have attracted tremendous interests recently due to their excellent performance and potential applications in wireless communication. However, the existing transmissive OAM generators suffer from several limitations, such as narrow bandwidth, high profile and low efficiency. In this study, a new wideband third-order meta-frequency selective surface (meta-FSS) for generating focusing vortex beam is developed. The proposed meta-FSS element is designed at X- band with a third-order band-pass filter property, which exhibits the merits of low profile, high transmissivity, and large angular stability. By employing the proposed meta-FSS element, prototypes of OAM generators for + 1 and -2 modes are designed, fabricated, and measured. Experimental results verify the ability of the proposed design to convert an incident left/right-handed circularly polarized (L/RHCP) spherical wave into a transmitted R/LHCP vortex carrying OAM wave from 9.0 GHz to 11.0 GHz with high mode purity. A good agreement is achieved between the experimental and numerical results, which demonstrates that the proposed structure paves the wave for generating desired OAM modes, and provides new possibility for designing novel low-profile wireless communication devices.

Broadband low-scattering metasurface using a combination of phase cancellation and absorption mechanisms

In this paper, a broadband low-scattering metasurface is proposed by using a combination of phase cancellation and absorption mechanisms. The metasurface is composed of two structural layers. One layer adopts the geometric phase cell that can obtain a different reflection phase by changing its orientation. Through the random phase distribution design, electromagnetic diffusion can be realized to reduce the backward scattering energy. The other layer is made of a resistive frequency selective surface (RFSS) that can absorb the incident wave by converting it into Ohmic loss. The above two physical mechanisms respectively play the great roles at two distinct frequency bands, and finally make our metasurface achieve the RCS reduction over a wide frequency band ranging from 13 to 31.5 GHz. Both simulation and experimental results are in good agreement, which fully demonstrates our design method. The analysis of the scattering patterns, electric-field distribution and power loss density are given to explain the hybrid RCS-reduction mechanism of our metasurface.