Generation Of Vortex Beams Research Articles

Generation of acoustic vortices and acousto-optic interactions with acoustic vortex beams.

We demonstrate the generation of acoustic vortex beams with the aid of an acoustic spiral phase plate and an acoustic grating having a fork-like bifurcation of its slits. The acoustic vortex beam generated in this manner influences the particles floating on a water surface and rotates them. Our experiments show that the intensity of the acoustic vortex beam increases from zero in its center to a certain maximal value near the shell of the beam. The phase of the beam changes by the angle 2π whenever the azimuthal angle is changed by the same value. We also study acousto-optic diffraction of the optical beam by the acoustic beam bearing a vortex and demonstrate that each of the appropriate diffraction maxima are split into two maxima. The latter correspond to diffraction by the elementary acoustic rays which are close to the opposite edges of the acoustic vortex beam. A dark region between the maxima appears due to destructive interference of the elementary optical rays originated from the diffraction by the opposite sides of the acoustic vortex beam, which are phase-shifted by π.

Direct Emission of Two-Color Vortex Beams at 1.0 μm and 1.5 μm Wavebands From an All-Fiber Laser

In this letter, we demonstrated an all-fiber laser that can simultaneously radiate dual-color vortex beams by incorporating a dual-channel mode-selective coupler (DC-MSC) into the laser cavity. By optimizing the phase-matching condition between LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes, wavelength-dependent mode coupling could be observed. In this case, a broadband mode conversion with two isolated bands can be achieved, which totally covers the gain bands of ytterbium and erbium ion, enabling the simultaneous generation of vortex beams (VBs) at 1.06 μm and 1.55 μm wavebands. With intra-cavity mode conversion and polarization controlling, a coaxial two-color VB is transmitted directly from the laser cavity, providing a more compact dual-beam light source for subsequent vortex nonlinear frequency conversions, such as wavelength extension to the red light and the mid-infrared (3 μm) wavebands.

Arbitrary Multifunctional Vortex Beam Designed by Deep Neural Network

AbstractAs topological charge constitutes an infinite‐dimensional Hilbert space, vortex beam has numerous applications in optical communications and other fields where signal capacity is a vital requirement. Multifunctional vortex beams, showing up to different controllable responses subjected to separate combinations of polarization states, have significantly exhibited improved capacity of signal transport. Relying on prior physical knowledge, complex requirement brings tremendous challenge to the design of multifunctional vortex beams. Here, a deep‐learning‐based platform for designing metasurfaces is proposed, which can intelligently generate predesigned multifunctional vortex beams. Employing the proposed strategy, the demonstrations of bifunctional and trifunctional vortex beams are consistent with the design targets. Three samples are fabricated and measured by a Michelson interferometer. Clear observed interference patterns revealed the topological nature of the generated vortex beams, unambiguously justifying the design platform. This intelligent design strategy, which may inspire new ideas in other scientific fields, lays a solid foundation for the high‐performance application of multifunctional vortex beams. This work fully exploits the potential of vortex beams for large‐scale dense data communication and quantum optics with high quantum numbers, which may further promote the development of the integrated photonic chip.

Superimposition of topological charges between vortex beams and singular points of photonic crystal slabs

The bound state in the continuum (BIC) is a singular point of polarization in the momentum space of a periodic structure. It has been demonstrated that vortex beams can be generated by utilizing the polarization vortex around BIC based on the geometric phase in momentum space. In this letter, we propose a reflective photonic crystal (PhC) to generate a vortex beam and demonstrate the superimposition of topological charges in the momentum space between PhC and the vortex beams. In addition to BICs, we demonstrate that other singularities like degenerate points can also be used to generate vortex beams.

Ultrathin and Ultra‐Broadband Terahertz Single‐Layer Metasurface Based on Double‐Arrow‐Shaped Resonator Structure for Full‐Space Wavefront Manipulation

AbstractIn this paper, a simple design of an ultrathin and ultra‐broadband single‐layer MS based on double‐arrow‐shaped resonator (DASR) structure for both transmission and reflection modes in terahertz (THz) region is presented. The single‐layer MS is composed of a periodic array of the combination of the metal DASR and complementary circular patch (CCP) adhered on an ultrathin dielectric substrate. Numerical results show that the MS structure can convert CP wave to its orthogonal component after reflection and transmission simultaneously, and the corresponding amplitudes are ≈0.45 on average from 0.45 to 1.75 THz with a relative bandwidth of 118.2%, in which the CP conversion efficiency is closing to theoretical limit (25%) of the single‐layer structure. Furthermore, the 0–2π phase shifts for the reflected and transmitted orthogonal CP wave can be achieved simultaneously by adjusting the oriented angle (α) of the DASR structure along the wave propagation direction. For the concept proofs, wave beams deflection, vortex beam generation, and focusing effect for both reflection and transmission modes are demonstrated numerically. The design can provide new possibilities and potential to develop multifunctional full‐space devices.

Highly Efficient 3D Nonlinear Photonic Crystals in Ferroelectrics

AbstractNonlinear photonic crystals (NPCs) with spatially modulated second‐order nonlinear coefficients are indispensable in nonlinear optics and modern photonics. To access full degrees of freedom in the generation and control of coherent light at new frequencies, three dimensional (3D) NPCs have been recently fabricated employing the femtosecond laser writing technique in ferroelectric crystals. However, the nonlinear interaction efficiencies in 3D NPCs so far are rather low, which has been a major barrier to further development. Here, fabrication challenges of large‐scale 3D NPCs have been overcome to realize more than four orders of magnitude efficiency improvement over previously reported crystals. With the generation of second harmonic vortex beam as an example, the measured conversion efficiencies as high as 2.9 × 10−6 W−1 (femtosecond pulse‐pumped) and 1.2 × 10−5 W−1 (continuous‐wave‐pumped) are comparable with those in 1D or 2D NPCs. A general approach of structural simplification to relax fabrication challenge of truly complex 3D structures without sacrificing the predesigned functions is also presented. This realization of efficient nonlinear interactions in 3D NPCs paves the way for their novel applications in nonlinear photonics and quantum optics including nonlinear orbital angular momentum multiplexing and high‐dimensional entangled source.

Optically polarized selective transmission of a fractional vector vortex beam by the polarized atoms with external magnetic fields.

We investigate the role of external magnetic fields and linearly polarized pump light, especially when their directions are parallel or vertical, on the propagation of the fractional vector vortex beams (FVVBs) through a polarized atomic system. Herein, the different configurations of external magnetic fields lead to various optically polarized selective transmissions of FVVBs with different fractional topological charge α caused by the polarized atoms, which is theoretically demonstrated by the atomic density matrix visualization analysis and experimentally explored by Cesium atom vapor. Meanwhile, we find that the FVVBs-atom interaction is a vectorial process due to the different optical vector polarized states. In this interaction process, the atomic optically polarized selection property provides potential for the realization of the magnetic compass based on warm atoms. For the FVVBs, due to the rotational asymmetry of the intensity distribution, we can observe some transmitted light spots with unequal energy. Compared with the integer vector vortex beam, it is possible to obtain a more precise magnetic field direction by fitting the different "petal" spots of the FVVBs.

Tunable Polarization‐Preserving Vortex Beam Generator Based on Diagonal Cross‐Shaped Graphene Structures at Terahertz Frequency

AbstractThe vortex beams carrying orbital angular momentum (OAM) are of extreme importance for applications in communication, nanoscience, and optical micro‐manipulation. The development of dynamically tunable vortex beam generators with compact size is a critical step in the realization of vortex beam modulation and application. Here, a series of ultrathin OAM‐tunable polarization‐preserving vortex beam generators based on graphene metasurfaces are proposed, which offer a new scheme to generating vortex beams with variable topological charges at terahertz frequency. By introducing the diagonal cross‐shaped graphene structures, the abrupt phase shift range reaches 2π for linearly polarized incident light while maintaining the original polarization state. Planar spiral phase generators are theoretically constructed to convert beams with plane wavefront into beams with spiral phase. More importantly, the topological charges of the generated vortex beams can be dynamically tuned by the graphene Fermi energy in a broadband frequency range from 4.5 to 5.5 THz. This work provides a simple technology for the generation of vortex beams and the flexible modulation of the topological charge, which would inspire the application potential of the vortex beam.

Active modulation of terahertz vortex beams by Dirac semimetals-based space–time-coding​ metasurface

Coding metasurfaces have developed rapidly in the last few years because of the digital thought. Recently, spatiotemporal metasurface have attracted considerable attention for the additional degrees of freedom they provide when manipulating electromagnetic waves in space and time domains. This study proposed Dirac semimetal based space–time-coding metasurfaces in the terahertz band. The conversion from a low- to high-bit programmable metasurface was achieved using a spatiotemporal metasurface, that is, 2-bit coding was equated to 3-bit coding. Space–time-coding metasurfaces facilitated the realization of the function of wide-band terahertz vortex beam generator switching according to the vortex phase arrangement, and were combined with the convolution theory for the successful realization of the modulation of terahertz vortex deflection and beam splitting under space–time coded modulation. The modulation of the equivalent 3-bit vortex beam produced under space–time coded modulation considerably reduced the gain of the side lobes and achieved precise modulation of the terahertz vortex beam. Moreover, the principle and method proposed in this study are also applicable to microwave and visible wavelengths. The results of this study are expected to result in promising applications in future terahertz high-data-capacity communication systems and multi-target precision tracking and guidance.

Binary encoding-inspired generation of vector vortex beams

Binary encoding-inspired generation of vector vortex beams

Guided-Wave Inspired Metasurfaces for Multifunctional Vortex Beam Generation and Manipulation

Vortex beam, manifesting the characteristic helical phase front and amplitude singularity, carries inherent orbital angular momentum (OAM) and has received intensive research from its generation, detection to various promising applications. To flexibly impart OAM to the electromagnetic (EM) waves and further control the topological charge, propagation, polarization, bandwidth and efficiency of the vortex beams, various approaches have been proposed by the science community. In this review, we briefly introduce the established methods for the generation and manipulation of vortex beams, with the focus on the fast-growing guided-wave metasurface based approach. Guided-wave metasurface contains sub-wavelength guided-wave subsystems that are capable of versatile control of the EM wave and we review some recent advances that enabled multifunctional vortex beam generation and manipulation, with an outlook towards the future development in this field. OAM topological charges switching, polarization control, divergence control, full-space vortex beam generation, broadband vortex beam generation and vortex multibeam are covered in this paper aiming to accommodate the ever-increasing demands from the vortex beam applications.

Frequency-Doubled High-Power Optical Vortex Beam With Sub 500 fs Pulse Duration

We report on the generation of high-power ultrashort-pulsed optical vortex beams (VBs) with orbital angular momentum (OAM) of +1 in the infrared (IR) and +2 in the visible (VIS) spectral regions. The IR vortex beam was created from a Gaussian beam by employing a nanograting (S-waveplate) mode converter. With this approach we obtained an average power of up to 802 W at a wavelength of λ = 1030 nm in the IR-VB with a pulse energy of 802 µJ, a pulse duration of 460 fs, and a mode-conversion efficiency of 90.1%. By subsequent second harmonic generation, the IR-VB was frequency-doubled inside a lithium triborate (LiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> ) crystal. An average power of up to 320 W at λ = 515 nm was generated, with 320 µJ of pulse energy, 382 fs of pulse duration, and an IR-VB to SH-VB conversion efficiency of 39.9%. The generated IR and VIS VBs were characterized by a Mach-Zehnder type interferometric measurement, confirming the helical wavefronts at these high average powers. These results show that it is possible to efficiently generate high-quality VBs at multi hundreds of watts and confirm the OAM scaling law in harmonic generation processes.

Vortex Dynamics and Structured Darkness of Laguerre-Gaussian Beams Transmitted Through Q-Plates Under Weak Axial-Asymmetric Incidence

We describe the rich vortex dynamics at normal and near-normal or off-centered Laguerre-Gaussian beam transmission through the q-plate – a waveplate capable of generating vortex beams from circularly polarized incident beams. First, we explore the basics of the spin-to-orbit angular momentum conversion and show the vortex constellations under normal incidence for the case of quarter-wave retardation. Second, we analyze the optical Gaussian and vortex beam profile transformations accompanying the giant beam shifts under axial-asymmetric incidence. Namely, we consider the near-normal (for the Fourier-space q-plate) and off-centered (for the real-space q-plate) incidence, when the incident angle becomes comparable with beam angular divergence and off-center distance becomes comparable with beam width, respectively. In this regime, one can control the positions of the single-charge vortices with the specified polarization components of the beam, including those residing in its structured darkness. All mentioned effects arise due to the extreme spin-orbit coupling. The results obtained form a novel platform for a plethora of optical vortex transformation and structured light phenomena.

40.1: Liquid Crystal Polymer Phase Plates Fabricated by Direct Laser Writing

Optical vortex is a type of structured beam that carry orbital‐angular momentum (OAM) with different topological charge numbers. In this paper, we fabricate liquid crystal polymer phase plates to generate vortex beams by femtosecond 3D direct laser writing. Instead of using traditional photoresists, we compound liquid crystal monomers to create different structures. As a two‐photon direct laser writing technique, Nanoscribe is of high resolution of 100 nm, which implements high accuracy of our structures. We design spiral phase plate (SPP) with diameter of 25µm and a total height difference h of 1.25um to generate vortex beams with different topological charge numbers. We also set up the interference system to characterize the vortex beam. With the interference beam coupled, the presence of the vortex is experimentally indicated by the fringe “dislocation defects” at the center of the vortex in visible region. It presents a new way to fabricate liquid crystal polymer structures with unique birefringent properties, indicating new possibilities in photonics application.

Generation, Topological Charge, and Orbital Angular Momentum of Off-Axis Double Vortex Beams

Compared with the on-axis vortex beam and the off-axis single vortex beam, the off-axis double vortex beam has more control degrees of freedom and brings rich physical properties. In this work, we investigate theoretically and experimentally the generation, topological charge (TC), and orbital angular momentum (OAM) of off-axis double vortex beams. It is demonstrated that the tilted lens method can detect not only the magnitudes and signs of two TCs of the off-axis double vortex beam but also the spatial distribution of the TCs. Moreover, the average OAM value of the off-axis double vortex beam decreases nonlinearly as the off-axis distance increases, although its TC is independent of the off-axis distance of phase singularities. The results indicate that the average OAM of the off-axis double vortex beam can be easily controlled by changing the relative position of two-phase singularities, thereby realizing the applications of multi-degrees of freedom particle manipulation, optical communication, and material processing.

Generation of perfect vectorial vortex beams by employing coherent beam combining.

Based on coherent beam combining, we propose a method for generating the perfect vectorial vortex beams (VVBs) with a specially designed radial phase-locked Gaussian laser array, which is composed of two discrete vortex arrays with right-handed (RH) and left-handed (LH) circularly polarized states and in turn adjacent to each other. The simulation results demonstrate that the VVBs with correct polarization order and topological Pancharatnam charge are successfully generated. The diameter and thickness of generated VVBs independent of the polarization orders and topological Pancharatnam charges further prove that the generated VVBs are perfect. Propagating in free space, the generated perfect VVBs can be stable for a certain distance, even with half-integer orbital angular momentum. In addition, constant phases φ0 between the RH and LH circularly polarized laser arrays has no effect on polarization order and topological Pancharatnam charge but makes polarization orientation to rotate φ0/2. Moreover, perfect VVBs with elliptically polarized states can be flexibly generated only by adjusting the intensity ratio between the RH and LH circularly polarized laser array, and such perfect VVBs are also stable on beam propagation. The proposed method could provide a valuable guidance for high power perfect VVBs in future applications.

Single-Layered Phase-Change Metasurfaces Achieving Polarization- and Crystallinity-Dependent Wavefront Manipulation

As a promising platform for versatile electromagnetic (EM) manipulations, metasurfaces have drawn wide interest in recent years due to their unique EM properties and small footprints. However, although great efforts have been made to achieve multifunctionalities, the design of tunable metasurfaces with high compactness is still challenging. Here, a simple yet powerful design methodology for single-layered reconfigurable metasurfaces composed of nonvolatile phase-change material Ge2Sb2Se4Te1 (GSST) is proposed with average working amplitudes of 72.6% and 53% at different crystallization levels. The proposed metasurfaces could not only enable independent phase control at different crystallization levels but also introduced another polarization degree of freedom. As a proof of concept, we numerically demonstrate three kinds of metadevices in the infrared region achieving a multi-focus metalens with tunable foci, multistate vortex beam generator with adjustable topological charges and multi-channel meta-hologram with three independent information channels. It is believed that these multifunctional metasurfaces with both tunability and compactness are promising for various applications including information encryption, chiroptical spectroscopy, chiral imaging and wireless communication.

Numerical Simulation of Integrated Generation and Shaping of Airy and Bessel Vortex Beams Based on All-Dielectric Metasurface

Integrating multiple independent functions into a single optical component is one of the most important topics in research on photoelectric systems. In this paper, we propose a multifunctional all-dielectric metasurface that can achieve a variety of non-diffractive beams depending on the polarization state of the incident light. Using the anisotropic TiO2 rectangular column as the unit structure, the three functions of generating polygonal Bessel vortex beams under left-handed circularly polarized incidence, Airy vortex beams under right-handed circularly polarized incidence and polygonal Airy vortex-like beams under linearly polarized incidence are realized. In addition, the number of polygonal beam sides and the position of focal plane can be adjusted. The device could facilitate further developments in scaling complex integrated optical systems and fabricating efficient multifunctional components.

High-efficiency broadband polarisation converter via patterned multilayer twisted liquid crystal polymer

ABSTRACT We propose an ultra-compact polarisation converter based on liquid crystal (LC) material for effectively converting the obliquely incident unpolarised light into horizontal linearly polarised beam within the visible frequency range. Multilayer twisted configuration is introduced here to tackle the inherent dispersion issue from the LC itself and extended Jones matrix is employed to analyse the propagation of the electric field inside the LC medium under possible oblique illumination. Spatial angular separation of orthogonal polarised lights and polarisation conversion into the same polarisation direction are realised through specific patterned structure design, which effectively reduce the thickness of the device without sacrificing performance. Experimental results demonstrated that our fabricated LC-based device can deliver >80% polarisation conversion efficiency at normal illumination and 59%–80.5% efficiency for ±10° oblique incidence over the visible spectrum. Therefore, we envision that our presented device might find numerous potential applications associated with polarisation regulation such as vortex beam generator, augmented reality and optical computing.

Generation of vortex beams with nonuniform phase jumps in azimuthal locations

The phase angle of the vortex beam along a closed loop centered on the optical singularity changes by 2πℓ, where ℓ is the number of phase jumps (PJs) from 0 to 2π and indicates the topological charge of the vortex beam. In this paper, the generation and specification of a new type of vortex beam, in which their PJs are asymmetrically embedded in the phase pattern, are reported. In contrast to Laguerre–Gaussian vortex beams, where PJs are equally spaced azimuthally around the optical singularity, the presented vortex beams have PJs embedded at arbitrary azimuthal angles. By designing a particular forked grating and displaying it on a spatial light modulator, this type of vortex beam is experimentally generated. As with conventional forked grating, the designed grating produces vortex beams with the opposite orbital angular momentum (OAM) sign in the first diffraction order. By measuring the relative orientation of the intensity profile of these OAM beams in the first diffraction order, the position of the PJs on the wavefront of a vortex beam with ℓ = 2 can be determined. This type of vortex beam could have potential applications in various fields of photonics, especially in optical communications based on optical vortices.