Generation Of Vortex Beams Research Articles

Generation of dual-polarization orbital angular momentum vortex beams with reflection-type metasurface

Orbital angular momentum (OAM) vortex beams can provide a new degree of freedom for information modulation at a physical level, which has great prospects to expand channel capacity in the fields of wireless microwave and optical communication. In this paper, a reflection-type anisotropic metasurface is proposed to transform the incident plane wave into reflected vortex beams by modulating the phase of wavefront. Single vortex beam, multi-beam vortex beams with different mode beams are demonstrated. Under the incidence of orthogonal linearly polarized waves, independent vortex beams can be generated at 15 GHz for polarization multiplexing. Besides, the OAM spectrum decomposition is performed to analyze the purity of the OAM modes based on a Fourier transformation. The results presented in the paper demonstrate the feasibility of generating vortex beams on metasurface within a subwavelength thickness, which may promote the development of electromagnetic/optical integration devices with polarization multiplexing techniques.

Propagation of asymmetric optical vortex beams through turbulence and evolution of their OAM spectra

In the realm of wave propagation through turbulent media, the spectrum of the orbital angular momentum of optical vortex beams is known to undergo symmetric broadening. However, the evolution of beams that are initially azimuthally asymmetric represents a distinct phenomenon. In this work, we have developed an analytical model describing the propagation of asymmetric OAM beams through the so-called Kolmogorov turbulence. Our results describe how the perturbation strength and the initial beam properties lead to a nonsymmetric spectrum of OAM modes. These findings lay the groundwork for further use of asymmetric fields that propagate in inhomogeneous media and their applications such as communications and sensing.

Reconfigurable perfect vortex beam generator based on a liquid crystal spiral phase plate.

A transmissive adaptable optical setup to generate a range of perfect vortex beams (PVBs) carrying different topological charges (TC) without using moving parts is presented. The setup is composed of an ad hoc transparent reconfigurable liquid crystal (LC) spiral phase plate (SPP), a refractive axicon and a convergent refractive lens. The LC SPP electrodes are manufactured ablating indium-tin oxide (ITO) glass substrates using direct laser writing (DLW) resulting in a very high fill factor device. In-house tailored electronics drive the 72 LC SPP electrodes giving rise to 72 different configurations with orbital angular momentum. In this work, the generation of PVBs with 36 positive or 36 negative TCs using this optical setup is accomplished.

Single-celled metasurface for labeled vortex beam generator

Vortex light is a unique beam characterized by a spiral phase as it propagates. A fundamental parameter of vortex light is the topological charge, which determines the amount of angular momentum and plays a crucial role in tailoring its behavior. However, conventional measurement methods for determining the topological charge, such as those based on interference and phase modulation, tend to be intricate and complex. In this regard, a labeled vortex beam generator is proposed, composed of a metasurface with a single-celled configuration. When the metasurface is illuminated by light of the designed wavelength, the outgoing light exhibits a vortex structure. Furthermore, the topological charge numbers can be directly observed with distinct labeled patterns when the metasurface is placed in an orthogonal-polarized optical path. With advantages such as ultra-compactness, high robustness, and exceptional precision, the proposed metasurface exhibits significant potential for applications in optical communication, light manipulation, optical sensing, etc.

A simple structured and homo-frequency point mode switchable THz full-space metasurface based on temperature-controlled vanadium dioxide

Terahertz (THz) full-space metasurfaces have attracted great attention in the field of modern communication systems due to their inherent advantages in electromagnetic (EM) wave manipulation. However, nearly all the reported full-space metasurfaces are realized by multilayer dielectric cascade structures. Furthermore, for mode switchable metasurfaces, reflection and transmission modes are always switched at different frequency points, which limits their application prospects. In this paper, we propose a full-space THz metasurface that can switch between reflection and transmission modes at the same frequency point only by controlling a single substrate layer of vanadium dioxide. The reflection and transmission properties can be independently controlled by rotating the optimized meta-atoms on the metasurface independently. To demonstrate its application potential for multifunctional devices, full-space focusing vortex beam generator and near-field imaging are practiced. A mode switchable focusing vortex beam generator using right circularly polarized (RCP) light at 1.312 THz is designed. The mode purity of the vortex beam under both the transmission and reflection directions is above 85 %. Then a full-space near-field imaging of letters “UPC” at 200 μm from the reference plane is achieved both in the reflection and transmission direction. The transmission and reflection modes can be switched simply by regulating the temperature of the vanadium dioxide substrate through a thermistor. Our suggested design concept of homo-frequency points and simple structure is more conducive to future practical engineering applications of metasurface and provides a worthwhile direction for future research of full-space functional devices.

Broadband Achromatic Metalens for Tunable Focused Vortex Beam Generation in the Near-Infrared Range.

Vortex beams accompanied with orbital angular momentum have attracted significant attention in research fields due to their formidable capabilities in various crucial applications. However, conventional devices for generating vortex beams still suffer from bulky sizes, high cost, and confined performances. Metalens, as an advanced platform to arbitrarily control the optical waves, has promising prospects to address the predicament for conventional devices. Although great progress has been demonstrated in the applications of vortex beams, they are still confronted with fixed functionality after fabrication that severely hinders their application range. In this work, the phase-change material of Ge2Sb2Te5 (GST) is employed to design the meta-atoms to realize tunable optical responses. Moreover, the focused vortex beam can be accomplished by superimposing a helical phase and hyperbolic phase, and the chromatic aberrations in near-infrared (NIR) range can be corrected by introducing an additional phase compensation. And the design strategy is validated by two different metalenses (BAMTF-1 and BAMTF-2). The numerical results indicate that the chromatic aberrations for two metalens can be corrected in 1.33-1.60 μm covering the telecom range. Moreover, the average focusing efficiency of BAMTF-1 is 51.4%, and that of BAMTF-2 is 39.9%, indicating the favorable performances of designed BAMTF. More importantly, their average focal lengths have a relative tuning range of 38.82% and 33.17% by altering the crystallization ratio of GST, respectively. This work may provide a significant scheme for on-chip and tunable devices for NIR imaging and communication systems.

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.

High-power vortex beams generated from a Yb:YAG thin-disk laser with spot-defect mirrors

We report the generation of high-power and high-order vortex beams based on a Yb:YAG thin-disk laser oscillator with a spot-defect mirror. High-quality vortex beams were directly emitted by the oscillator with an output power of 80 W, which is the highest output power of any vortex laser oscillator. Tunable high-order Hermite-Gaussian modes from HG1,1 to HG5,1 were also generated, and subsequently converted to Laguerre-Gaussian beams by a cylindrical-lens mode converter. The vortex characteristics of the output were confirmed with a Mach-Zehnder interferometer. This work demonstrates an effective way to generate high-quality, high-power and high-order optical vortex beams.

Comparison of double-phase hologram and binary amplitude encoding: holographic projection andvortex beam generation.

Utilizing computer-generated holograms is a promising technique because these holograms can theoretically generate arbitrary waves with high light efficiency. In phase-only spatial light modulators, encoding complex amplitudes into phase-only holograms is a significant issue, and double-phase holograms have been a popular encoding technique. However, they reduce the light efficiency. In this study, our complex amplitude encoding, called binary amplitude encoding (BAE), and conventional methods including double-phase hologram, iterative algorithm, and error diffusion methods were compared in terms of the fidelity of reproduced light waves and light efficiency, considering the applications of lensless zoomable holographic projection and vortex beam generation. This study also proposes a noise reduction method for BAE holograms that is effective when the holograms have different aspect ratios. BAE is a non-iterative method, which allows holograms to be obtained more than 2 orders of magnitude faster than iterative holograms; BAE has about 3 times higher light efficiency with comparable image quality compared to double-phase holograms.

Multiplexed vortex beam-based optical tweezers generated with spiral phase mask

The design and implementation of a multiplexed spiral phase mask in an experimental optical tweezers setup are presented. This diffractive optical element allows the generation of multiple concentric vortex beams with independent topological charges and without amplitude modulation. The generalization of the phase mask for multiple concentric vortices is also shown. The design for a phase mask of two multiplexed vortices with different topological charges is developed. We experimentally show the transfer of angular momentum to the optically trapped microparticles by enabling nearly independent orbiting dynamics around the optical axis within each vortex. The angular velocity of the confined particles versus the optical power in the focal region is also discussed for different combinations of topological charges.

Generating the 1.5 kW mode-tunable fractional vortex beam by a coherent beam combining system.

As an essential component of the vortex beam, the fractional vortex beam has significantly advanced various applications, such as optical imaging, optical communication, and particle manipulation. However, practical applications face a significant challenge as generating high average power fractional vortex beams remains difficult. Here, we proposed and experimentally demonstrated a high average power mode-tunable fractional vortex beam generator based on an internally sensed coherent beam combining (CBC) system. We presented the first, to the best of our knowledge, successful generation of a 1.5 kW continuous wave fractional vortex beam. Moreover, real-time tuning of the topological charge (TC) from -2/3 to +2/3 was easily achieved using the programmable liquid crystals (LCs). More importantly, the fractional vortex beam copier was presented as well, and the generated fractional vortex beam could be easily transformed into a fractional vortex beam array by changing the fill factor of the laser array. This work can pave the path for the practical implementation of high average power structured light beams.

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.

Elliptic perfect vortex beam generation with selective modes based on single layer metasurface in broadband near-infrared range

Asymmetric and multiple modes of perfect vortex beams (PVBs) are of great significance in the manipulation of nanoscale biological cells. As a powerful artificial subwavelength platform, miscellaneous metasurfaces have been reported to generate PVBs. However, these reported PVBs are always symmetric, that is, only one simple annular shape in the observation plane, which cannot provide multiple modes. Here, using multi-coordinate transformation techniques (MCTT), we established the phase profile of mode-selective elliptic perfect vortex beams (MS-EPVBs) in elliptical coordinate and generated multiple modes of EPVBs with arbitrary eccentricity and orientation in broadband near-infrared range based on all-dielectric metasurface, greatly enriching the modes of PVBs. Composed of optimized half-waveplate-like silicon cylinders, the designed metasurface can freely transform the incident light into EPVBs with arbitrary eccentricity and orientation with high efficiency. Furthermore, during the mode transformation, the properties of the “perfect” and broadband were preserved. This work will find potential applications in advanced fields like biophotonics and genetic engineering.

3-bit reconfigurable THz metasurface based on structured light illumination for vortex beams and holographic imaging

Metasurfaces have attracted widespread attention due to their excellent control ability of terahertz wavefronts. Terahertz metasurfaces have made rapid progress in areas such as biological detection, high-speed communication, and imaging. However, terahertz metasurfaces currently have a single function, and the primary goal of this research is to create a reconfigurable and multifunctional terahertz metasurface. In this work, we designed a novel eight-gaps-splits-ring resonator (EGSRR). The EGSRR is filled with photosensitive semiconductor material, and the metasurface unit's amplitude-phase characteristics are controlled by different encodings of structured light. Finally, under the incidence of right-handed circularly polarized (RCP) waves, 3-bit phase encoding of its reflected wave with cross-polarization is achieved. To verify the ability of metasurface to control terahertz wavefronts, the 3-bit metasurface units are assembled into an array to achieve different-order vortex beams and reconfigurable holographic images. The novel optically controlled reconfigurable metasurface not only has promising applications in generating vortex beams and holographic imaging, but it also represents a novel approach to designing multifunctional and programmable terahertz metamaterials.

Spatially structured optical effects in semiconductor quantum dots via biexciton coherence

In this paper, we study the spatially structured optical effects that occur when weak laser lights interact with coherently prepared semiconductor quantum dots (SQDs). Initially, the SQD is prepared in a coherent superposition of the lower exciton states. By utilizing two weak optical vortex fields that couple to a biexciton state, we observe spatially dependent effects of the absorption of probe fields. Using the well-established Maxwell–Bloch equations, we analyze the generation of composite optical vortex beams within this system. Our investigation revolves around the formation of different types of spatially dependent beams, exploring their properties and characteristics. Additionally, the transfer of optical vortices through the parametric generation process is examined, for the case where only one vortex beam is present at the beginning of the medium. This study provides insights into the spatially structured optical phenomena in coherently prepared SQDs and contributes to the understanding of light–matter interactions in such systems.

Generation of integer and fractional vortex beams based on liquid crystal electronically reconfigurable spiral phase plates.

The manufacturing and characterization of a large-size 72-electrode liquid crystal-based reconfigurable spiral phase plate (SPP) is presented. The SPP is addressed by a custom-made driver with 72 independent channels, which allows for the generation of any arbitrary integer or fractional optical vortex beam with topological charges ranging from -24 to +24. The 25 mm diameter device is fabricated using direct laser writing, leading to a fill factor over 99%. The device performance and flexibility exceed previous transparent reconfigurable SPP in terms of size, tuning range, and fill factor. The device and the light path have been simulated using the angular spectrum propagation method, showing excellent correspondence.

Vortex beam generator based on exceptional topological phase in terahertz all-dielectric metasurface

In this study, an all-dielectric metasurface with an F-shaped chiral structure in the terahertz region has been proposed. The coupling between the electric and magnetic dipole modes supported by the metasurface can be adjusted by varying the structural parameters. In the parameter space composed of l l and s, any closed path around exceptional point in the circular polarization conversion channel t rl generates a topologically protected exceptional topological phase. A vortex beam generator with a unit topological charge was proposed to provide a novel concept for designing terahertz metasurface wavefront modulation devices.

Transmissive Pancharatnam-Berry metasurfaces with stable amplitude and precise phase modulations using dartboard discretization configuration.

Metasurfaces are ultra-thin artificial structures capable of flexibly manipulating electromagnetic (EM) waves. Among various applications, phase modulation of electromagnetic (EM) waves using metasurfaces holds great significance. The Pancharatnam-Berry (P-B) metasurfaces provides a complete 2π phase modulation by simply rotating the meta-atom. However, the fixed lattice in rotation employed by traditional P-B metasurfaces often results in unstable amplitude and imprecise P-B phase, leading to performance degradation. In this work, we demonstrate transmissive P-B metasurfaces with stable amplitude and precise phase modulation. To ensure stable amplitude and precise P-B phase, we adopt a dartboard discretization configuration with a hexagonal lattice for the meta-atom design. By applying topology optimization to the encoding sequence formed by surface pixels and dimensions, we significantly enhancing the high transmissive bandwidth of the optimized meta-atom. Furthermore, the optimized meta-atom exhibits a stable amplitude and precise P-B phase for each rotation angle. As proof-of-concept demonstrations, two metasurfaces for single and multiplexed vortex beams generating are designed utilizing the optimized meta-atom. Both the simulated and measured results indicate high mode purity of generated vortex beams. The design method can also be readily extended to other high performance metasurfaces with stable amplitude and precise phase manipulations, which can enhance the efficiency and capacity of metasurface-assisted holographic imaging and 6 G wireless communication systems.

Amplitude gradient-based metasurfaces for off-chip terahertz wavefront shaping

Metasurfaces provide an effective technology platform for manipulating electromagnetic waves, and the existing design methods all highlight the importance of creating a gradient in the output phase across light scattering units. However, in the emerging research subfield of meta-waveguides where a metasurface is driven by guided modes, this phase gradient-oriented approach can only provide a very limited emission aperture, significantly affecting the application potential of such meta-waveguides. In this work, we propose a new design approach that exploits the difference between meta-atoms in their light scattering amplitude. By balancing this amplitude gradient in the meta-atoms against the intensity decay in the energy-feeding waveguide, a large effective aperture can be obtained. Based on this new design approach, three different wavefront shaping functionalities are numerically demonstrated here on multiple devices in the terahertz regime. They include beam expanders that radiate a plane wave, where the beam width can increase by more than 900 times as compared to the guided wave. They also include a metalens that generates a Bessel-beam focus with a width 0.59 times the wavelength, and vortex beam generators that emit light with a tunable topological charge that can reach −30. This amplitude gradient design approach could benefit a variety of off-chip light shaping applications such as remote sensing and 6G wireless communications.

Nonreciprocal terahertz wavefront manipulation based on a magneto-optical metasurface with an orthogonal meta-atom pair

Active wavefront manipulation and nonreciprocal transmission give key performances in beam steering, unidirectional isolation, and multichannel multiplexing. In this work, we propose a magneto-optical (MO) Pancharatnam–Berry (PB) metasurface, which consists of a pair of orthogonal anisotropic Si and InSb columns. This orthogonal meta-atom pair (OMP) structure leads to destructive interference in the geometric phase so that the wavefront manipulation is in the OFF state without a biased magnetic field (BMF). When the BMF is applied, the nonreciprocal transmission effect for different spin states in InSb breaks the original destructive interference, so the devices provide different geometric phase distribution for different magnetic field directions and spin states, achieving the ON state of the nonreciprocal wavefront manipulation, where cross-polarization circular dichroism is higher than 0.98 and the polarization conversion rate is over 97%. Furthermore, based on this mechanism, a beam deflector and a vortex beam generator are designed by arranging the OMP with different azimuthal angles. Compared with ordinary reciprocal wavefront manipulation, this work shows that this MO PB metasurface with OMP structure has irreplaceable functions in the isolation, selection, and conversion of the two conjugate spin beams, providing a unique path for active THz wavefront manipulation.