Switchable frequency terahertz vortex beam generator

The vortex beam has attracted much attention due to its great potential applications in communication and medicine area. To compare with the regular light, the vortex beam carrying the orbital angular momentum can greatly increase information capacity in communication. In this paper, a new method is presented to generate terahertz vortex beam based on the complementary open-rings metamaterial. The structure is composed of a metasurface-dielectric-metasurface, which top and bottom layer are both complementary open rings, and the dielectric layer is polyimide. And the vortex beam generators with topological charge of 1 and 2 have been designed according to the phase principle of Pancharatnam-Berry (P-B), which based on the different orientation of open ring unit cells. The amplitude and phase of the cross-polarized transmitted beam were manipulated by rotating the orientation of the complementary open rings, while ensure the amplitude of the transmitted beam stays unchanged, and the phase of the cross-polarized transmitted beam cover the range of 1*2π (where l is the topological charge). The vortex beam characteristics has been studied when circularly polarized light incident on the measurface of vortex beam generators with the CST MICROWAVE STUDIO. The simulation results show that the transmitted beam are well meet the characteristic of vortex beam, which the transmittance of cross-polarized vortex beam is as high as 64% at 1.48 THz while the phase can cover the range of 1*2π. That is, the presented transmissive vortex beam generators can convert incident circularly polarized beam into a cross-polarized vortex beam carrying orbital angular momentum. The approach demonstrated in this paper which is high conversion efficiency, simple structure, quick response, and has potential applications in the field of terahertz communication.

Broadband transparent terahertz vortex beam generator based on thermally tunable geometric metasurface

The paper reports the intensity characteristics of fractional vortex beam, and controls the coupling between photon orbital angular momentum and spin angular momentum by coherently manipulating Laguerre–Gaussian vortex beam with linearly polarized plane light and its conjugate beam. By comparing theory with experiment, fractional vortex beams with different topological charges are generated by using pure phase liquid crystal spatial light modulator, and their intensity characteristics and interference characteristics are observed. The results show that the radial notch size of the spot is related to the fractional part of the topological charge value, and the closer the fractional part of the topological charge value is to the half integer, the more obvious the radial notch is. With the increase of integer part of topological charge, the spot radius also increases. In addition, interference with linearly polarized plane light reveals that the number of branches is equal to the number of topological charges. Interfere with its conjugate beam, and judge the topological charge number of vortex beam by the relationship between petal number and topological charge number. The petal number of integer vortex beam is twice the topological charge number, and fractional order vortex beam destroys the ring structure of Laguerre–Gaussian beam.

The generation and application of the vortex beams are part of the hot topics in the optical field. In this paper, the phase structure of the four-step phase plates, analyzed by Fourier series expansion method, is composed of a series of spiral phase plates. When the phase plate is directly irradiated by linearly polarized light, multi-order diffraction waves with different topological charge numbers are generated. Unlike vortex waves, the intensity distribution of the multi-order diffraction has a deviation from the axial symmetry due to the interference with each other. On this basis, a new scheme is proposed to generate vortex beams by the four-step phase plates. With the help of Mach-Zehnder interferometer, the diffraction waves generated by two pieces of the four-step phase plates overlap each other. By adjusting the phase difference of the Mach-Zehnder interferometer, some orders of diffraction waves generate destructive interference while the others generate constructive interference. Thus the linear polarized light can be converted into vortex beams. The diffraction intensity and angular momentum distributions of the four-step phase plates with different cycle numbers are numerically simulated and compared with the spiral phase plates, we can provethat the vortex beams can be obtained by simple four-step phase plates which are the same as those obtained by spiral phase plates. In addition, the four-step phase plates with a small cycle number can generate vortex beams with a large topological charge number and the fabrication difficulty of the phase plates is reduced.

The single-pass second-harmonic generation (SHG) of a vortex beam under low fundamental wave depletion is systematically studied. Vortex modes at 1064 nm with integer topological charges from ±1 to ±9 and fractional ones at ±0.75 are generated by modulating the fundamental Gaussian beam with different spiral phase plates. The frequency doubling of these fundamental vortex modes is realized via single-pass SHG through the KTP. A detailed theoretical model is set up in the single-pass SHG of the vortex beams. Theoretical analysis indicates that the higher the order of the vortex beams, the lower the SHG efficiency, when the beam waists and fundamental power are given. The experimentally measured SHG output characteristics verify those obtained via theoretical analysis. Conservation of the orbital angular momentum during the SHG process is also verified, regardless of the fractional or integer vortex beams. SH LG0,2l vortex beams with high mode purity are obtained. The beam waists of fundamental/SH in KTP measured using a 4f system demonstrate that the Rayleigh ranges of the fundamental wave and SH wave are the same. The paper comprehensively presents some basic laws in the single-pass SHG of a vortex beam. In addition, it also indicates that SHG is an effective method to improve the mode purity of vortex beam.

Vortex beam carrying orbital angular momentum (OAM) reveals essential application potential in regimes ranging from communication to high-resolution imaging due to the capability to expand channel capacity. However, many current methods for generating vortex beams suffer from narrow bandwidth, low efficiency, and high profile, which undoubtedly will limit their practical application. In this paper, a microwave ultra-wideband single-layer Pancharatnam–Berry metasurface is proposed to convert incident plane waves into vortex waves. The proposed novel stacked-arc-shaped meta-atoms of the metasurface can realize high-performance spin angular momentum-OAM conversion with the efficiency of more than 83% within a considerable bandwidth from 5.85 to 19.56 GHz (fractal bandwidth of 108%), which achieves high-efficiency operation within a wider bandwidth compared to some other wideband metasurface-based vortex beam generator. Three metasurface prototypes for generating vortex beams with the topological charges of l = ±2, −1, −3 respectively are designed. In addition, a metasurface for generating a vortex beam with the topological charge of l = ±2 is fabricated as an example to demonstrate the feasibility and high performance of our vortex beam generator. The generated vortex beams with the mode order of ±2, −1, −3 both in the near-field and the far-field zone are simulated theoretically and the generated vortex beam with the mode order of l = −2 is measured experimentally. The results of the simulation and experiment coincide with each other, which verified the proposed vortex beam generation method successfully. The proposed vortex beam generator is valuable and user-friendly in practical application because of the advantages of low profile, ultra-wide bandwidth, high efficiency, and easy fabrication.

Optical vortex beams, carrying orbital angular momentum (OAM), are of great interest for applications in optical communications, particle manipulation, and quantum information processing. However, achieving high‐efficiency generation and manipulation of vortex beams with a single compact element, particularly in the visible range, remains challenging. Here, we demonstrate laser‐written geometric phase fork gratings in silica glass that enable simultaneous generation and propagation control of vortex beams with an efficiency up to 98.4% at 515 nm. By tailoring the phase‐gradient period and topological charge ( l = 1–100), we achieve tunable control over the vortex beam's topological charge, diameter, and diffraction position. The fabricated gratings maintain high efficiency across a broad spectral range from 450 to 690 nm. Furthermore, we demonstrate spin–orbit coupling between incident vortices and fork gratings, verifying that the output OAM values follow the expected addition rule l out = l in ± l fork . In addition, the fork gratings exhibit excellent thermal stability (up to 800°C) and optical damage threshold (close to pristine silica glass). These results establish nanopore‐based geometric‐phase fork gratings as a scalable, broadband, high‐performance platform with outstanding thermal and optical tolerance for next‐generation OAM photonics.

In this paper, a reflective terahertz reconfigurable multifunctional coding metasurface based on topology optimization using liquid crystal (LC) is presented. First, the LC metasurface unit is topologically optimized using the NSGA-II multi-objective optimization algorithm. By applying the bias voltage to dynamically adjust the dielectric constant of the LC, the metasurface unit is capable of 2-bit coding at the frequency of 0.67 THz. Then, based on the designed metasurface unit, the array arrangements of the coding metasurface are reversely designed, which makes the metasurface realize flexible control and dynamic switching between beam assignment and vortex beam generation. The results show that for the beam assignment, the single-beam and multi-beam control can be realized with the reflected electromagnetic waves at continuous arbitrary angles in the range of elevation angle of 40° and azimuth angle of 360°. Moreover, the elevation angle and azimuth angle of each beam in the multi-beam can be controlled independently. Using the Fourier convolution addition operation, the control range of the single-beam elevation angle can be expanded. For the vortex beam, the single vortex beam and the multi-vortex beam can be generated in the range of elevation angle of 40° and azimuth angle of 360° with topological charges l=±1, ±2, ±3 at arbitrary angles. This provides flexibility and diversity in the generation of vortex beams. Therefore, the proposed terahertz LC metasurface can realize flexible control of reconfigurable functions and has certain application prospects in terahertz communication, phased array radar, and vortex radar.

Direct generation of optical vortex beams with tunable topological charges up to 18th using an axicon

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch’s theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid‑infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources.

The generation and manipulation of optical vortex beams are essential to optical sensing, communication, and processing applications. The integrated on‐chip generation of vortex beams by the optical phased array (OPA) paves the way of compact, adjustable, and robust optical systems. To address the limited field‐of‐view challenge in 2D architecture OPAs, this work proposes an innovative far‐field envelope reconstruction strategy and implements a grating‐lobe‐free 2D OPA with a bicyclic configuration on a silicon integrated photonic platform. The fabricated device realizes a side‐lobe suppression ratio (SLSR) of ≈11.2 dB, a main lobe full width at half‐maximum (FWHM) of roughly 0.16°, and maintains intensity fluctuations of under 3 dB across even an 80° × 60° steering range, achieving unprecedented ≈187 500 far‐field resolvable points. The high‐performance OPA chip is demonstrated to rapidly generate and steer free‐space vortex beams carrying tunable optical orbital angular momentum (OAM) by accurately calibrating the phase shift and configuring the computed voltage, which provides broad prospects for applications such as optical coherence tomography and high‐capacity optical communications.

Terahertz vortex beams, carrying orbital angular momentum (OAM), are quite desirable to enhance the data transmission capability in telecommunication. However, it faces fundamental and technical challenge in a single metasurface to simultaneously generate orthogonal basis vortices with linear polarization (x- and y-polarity) and circular polarization (left- and right-handed polarity). Here, we proposed a chip-integrated all-dielectric metasurface in terahertz regime, to demonstrate the simultaneous generation of four-channel orthogonal polarized vortex beams at various topological charges. The versatile metasurface was designed only with propagation phase strategy, consisting of polarization-maintaining and polarization-conversion meta-atoms. Simultaneous control of polarization and topological charges in vortex beams was realized by properly arranging birefringent meta-atom arrays to induce additional phases of x- and y-polarization as customized, showing more degrees of freedom for carrying information. The experimental results are in good agreement with the simulations. Such metasurface approach provides complete polarization bases for further synthesis of diverse polarization vortices required for huge-capacity communication.

Terahertz vortex beams, carrying orbital angular momentum (OAM), are quite desirable to enhance the data transmission capability in telecommunication. However, it faces fundamental and technical challenge in a single metasurface to simultaneously generate orthogonal basis vortices with linear polarization (x- and y-polarity) and circular polarization (left- and right-handed polarity). Here, we proposed a chip-integrated all-dielectric metasurface in terahertz regime, to demonstrate the simultaneous generation of four-channel orthogonal polarized vortex beams at various topological charges. The versatile metasurface was designed only with propagation phase strategy, consisting of polarization-maintaining and polarization-conversion meta-atoms. Simultaneous control of polarization and topological charges in vortex beams was realized by properly arranging birefringent meta-atom arrays to induce additional phases of x- and y-polarization as customized, showing more degrees of freedom for carrying information. The experimental results are in good agreement with the simulations. Such metasurface approach provides complete polarization bases for further synthesis of diverse polarization vortices required for huge-capacity communication.

The acoustic vortex beam carrying orbital angular momentum (OAM) provides a new degree of freedom for sound manipulation, with promising applications in diverse fields ranging from acoustic communication to particle manipulation. However, their propagation in free space is subject to the severe divergence effect, resulting in limited operating distance and reduced signal intensity. Here, we propose a mechanism for generating a divergence-suppressed acoustic vortex beam through subwavelength dipole metamaterials in a broadband, flexible, and compact way. We theoretically reveal that the enhanced directivity of the man-made dipole can effectively reduce the high-spatial-frequency wave-vector component of the generated vortex beam in the radial direction and thereby suppress the beam's divergence. As a practical implementation, a blocklike acoustic metamaterial is systematically designed for converting the monopole emission into a dipole in a broad bandwidth, whose acoustic properties are also analytically deduced. The effectiveness of a dipole metamaterial-empowered phased array is demonstrated both numerically and experimentally, showcasing that the divergence angle and the main lobe width of a particular first-order vortex beam are reduced by 45% and 23% on average, respectively, within the range of 1000--1600 Hz. Thanks to the compatibility with the phased array, both the topological charge and the deflection angle of the emitted vortex beam can be flexibly modulated without altering metamaterials. We anticipate that our proposed mechanism will promote the application of acoustic vortex beams in various fields, such as long-distance OAM communication and beyond.

The results of an experiment on formation of a scalar vortex beam in the system of coherent combining of fiber laser beams are reported. Stabilization of phase relations between optical beams is achieved by placing a helicoid phase forming diffraction optical element (DOE) made as a phase plate with helical phase profile in the active feedback loop. Time-stable vortex beams with the topological charge p = ±1 are obtained. The vortex character of the obtained beams is confirmed by the results of interferometric measurements.