Orbital Angular Momentum Beams Research Articles

Orbital and spin angular momentum Raman scattering of methanol, benzene, hexane, and carbon tetrachloride liquids

Spontaneous Raman spectra using Orbital angular momentum (OAM) and Spin angular momentum (SAM) from organic liquids with circularly polarized Laguerre-Gaussian (L= 1 and 2) were experimentally observed. Certain Raman peaks from vibrational bonds under circularly polarized OAM L= 1 and L= 2 mainly for CH3, and C-C arising from pi and sigma bonds were enhanced or decreased relative to the Raman of a Gaussian beam with L= 0. For example, methanol shows changes for circularly polarized OAM Raman ranging from 4.5% to 66.8% for different vibrational modes. The relevant enhancement and decrease in the Raman intensity from the various studied liquids are attributed to the matching up of the multipoles from the vibrational orbital structure from electron clouds symmetry with the multipoles of the OAM light. For example, Methanol and Hexane are more dipole-like (L=1); while Benzene is more quadrupole-like (L=2), and Carbon Tetrachloride is more monopole-like (L=0) in the interaction with L for OAM beams. In summary, we show that the use of OAM and SAM offers a new approach for larger changes of the Raman effect in contrast to circular dichroism and Raman optical activity methods, which have changes of less than 10−4.

Studying superposition of multiple orbital angular momentum modes for beam concentration using circular arrays for long‐range communication

The most important limitation in using orbital angular momentum (OAM) multiplexing in communication is the range, so that because of OAM beam propagation, it cannot be used in the long range. It was found that when several original OAM modes were superimposed together, the concentrated EM wave in a certain angular range, with its effective phase diagram, could constitute a new quasi-OAM (QOAM) mode. In this article, the superposition of multiple OAM modes for beam concentration using uniform circular array is presented to in order to realize long-range communication. The simulation results show that the orthogonality characteristic of the two QOAMs produced by the proposed method is more than 10 decibels.

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a 0, which scales as . A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10−14 kg m2 s−1 is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 1024 photons s−1 mm−2 mrad−2 per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.

Time diffraction-free transverse orbital angular momentum beams

The discovery of optical transverse orbital angular momentum (OAM) has broadened our understanding of light and is expected to promote optics and other physics. However, some fundamental questions concerning the nature of such OAM remain, particularly whether they can survive from observed mode degradation and hold OAM values higher than 1. Here, we show that the strong degradation actually origins from inappropriate time-delayed kx–ω modulation, instead, for transverse OAM having inherent space-time coupling, immediate modulation is necessary. Thus, using immediate x–ω modulation, we demonstrate theoretically and experimentally degradation-free spatiotemporal Bessel (STB) vortices with transverse OAM even beyond 102. Remarkably, we observe a time-symmetrical evolution, verifying pure time diffraction on transverse OAM beams. More importantly, we quantify such nontrivial evolution as an intrinsic dispersion factor, opening the door towards time diffraction-free STB vortices via dispersion engineering. Our results may find analogues in other physical systems, such as surface plasmon-polaritons, superfluids, and Bose-Einstein condensates.

Polarized deep diffractive neural network for sorting, generation, multiplexing, and de-multiplexing of orbital angular momentum modes.

The multiplexing and de-multiplexing of orbital angular momentum (OAM) beams are critical issues in optical communication. Optical diffractive neural networks have been introduced to perform sorting, generation, multiplexing, and de-multiplexing of OAM beams. However, conventional diffractive neural networks cannot handle OAM modes with a varying spatial distribution of polarization directions. Herein, we propose a polarized optical deep diffractive neural network that is designed based on the concept of dielectric rectangular micro-structure meta-material. Our proposed polarized optical diffractive neural network is optimized to sort, generate, multiplex, and de-multiplex polarized OAM beams. The simulation results show that our network framework can successfully sort 14 kinds of orthogonally polarized vortex beams and de-multiplex the hybrid OAM beams into Gauss beams at two, three, and four spatial positions, respectively. Six polarized OAM beams with identical total intensity and eight cylinder vector beams with different topology charges have also been sorted effectively. Additionally, results reveal that the network can generate hybrid OAM beams with high quality and multiplex two polarized linear beams into eight kinds of cylinder vector beams.

Compensating the distorted OAM beams with near zero time delay

Vortex beams, carrying orbital angular momentum (OAM), have great potential to increase the information capacity of optical communication systems for the orthogonality and infinite mode number. For OAM beams propagating in free space, however, the atmospheric turbulence may cause mode distortions and hinder their utilization in practice. In this work, we propose a kind of diffractive deep neural network (D2NN) to compensate the distorted OAM beams. Different from those D2NNs reported before, the network reported here is dissipative, rather than unitary. In our system, the common features of various wavefront distortions are extracted and compensated, while the random distortions are filtered out by a diaphragm, which is achieved by constructing an improved loss function. The results show that multiple OAM beams with different degrees of distortions can be compensated simultaneously, and good agreement between simulations and experiments is obtained. The D2NN based OAM beam compensating reported here will greatly improve the robustness and efficiency of free space optical communication.

Hard X-ray helical dichroism of disordered molecular media

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life sciences to materials science. Chirality-sensitive spectroscopic methods, such as circular dichroism, exhibit weak signal contributions on an achiral background. Helical dichroism, which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an ångström-size orbital. Here we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the helical dichroism spectra of a disordered powder sample of enantiopure salts of the molecular complex of [Fe(4,4′-diMebpy)3]2+ at the iron K edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the helical dichroism spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the OAM of light.

Metasurface-based THz reflectarray antenna with vortex multiplexing and beam-steering capabilities for future wireless communications

SummaryThis article investigates the unexplored potentials of vortices or orbital angular momentum (OAM) beams using the low-cost and high-gain dielectric reflectarray antennas (RAs) at the terahertz (THz) band. It proposes a paradigm to enable 3D beam-steering or OAM multiplexing by a single structure via tilted OAM beams. That, in turn, requires reaching the maximal attainable angles either to send multiple beams to different receivers or to focus the OAM beams of different modes in a desired direction. For this reason, two concepts are addressed in this work: (i). generating a single 3D steered OAM beam and (ii) producing multiple off-centered OAM beams with different modes. A volumetric unit cell is adopted to be accurately tuned through the aperture to steer the generated beams towards the desired direction(s). The proposed paradigm can be utilized to produce RAs with beam-steering or OAM multiplexing capabilities as candidates for THz indoor communications.

Deep Learning of Coherent Laser Arrays in Angular Domain for Orbital Angular Momentum Beams Customization

Artificial intelligence methods for the phase control of laser arrays have been extensively developed in the recent years. To tailor high-power and fast switchable orbital angular momentum (OAM) beams by the coherent combination of laser arrays, complex phase control difficulties arise, and the introduction of deep-learning (DL) methods to learn the mapping from the intensity information to the relative phases of array elements offers a promising solution. In this work, we theoretically propose a DL-based phase control scheme that can customize OAM beams with desirable topological charges from laser array systems, which relies on the acquisition of the optical field information in the view of angular domain. When the system in closed loops, the optical field and phase errors information of laser array are extracted and decoupled in the angular domain perspective by using an OAM demultiplexer. The processed optical field of the coherently combined beam serves the DL-based phase control servo to realize the efficient phase stabilization, thus ensuring the controllable customization of OAM beams. This work could provide a useful reference on the intelligent control of laser array systems for structured light beams customization.

Orbital angular momentum comb generation from azimuthal binary phases

Since Allen et al. demonstrated 30 years ago that beams with helical wavefronts carry orbital angular momentum (OAM), the OAM of beams has attracted extensive attention and stimulated lots of applications in both classical and quantum physics. Akin to an optical frequency comb, a beam can carry multiple various OAM components simultaneously. A series of discrete, equally spaced, and equally weighted OAM modes comprise an OAM comb. Inspired by the spatially extended laser lattice, we demonstrate both theoretically and experimentally an approach to producing OAM combs through azimuthal binary phases. Our study shows that transition points in the azimuth determine the OAM distributions of diffracted beams. Multiple azimuthal transition points lead to a wide OAM spectrum. Moreover, an OAM comb with any mode spacing is achievable through reasonably setting the position and number of azimuthal transition points. The experimental results fit well with theory. This work presents a simple approach that opens new prospects for OAM spectrum manipulation and paves the way for many applications including OAM-based high-security encryption and optical data transmission, and other advanced applications.

Utilizing multiplexing of structured THz beams carrying orbital-angular-momentum for high-capacity communications.

Structured electromagnetic (EM) waves have been explored in various frequency regimes to enhance the capacity of communication systems by multiplexing multiple co-propagating beams with mutually orthogonal spatial modal structures (i.e., mode-division multiplexing). Such structured EM waves include beams carrying orbital angular momentum (OAM). An area of increased recent interest is the use of terahertz (THz) beams for free-space communications, which tends to have: (a) larger bandwidth and lower beam divergence than millimeter-waves, and (b) lower interaction with matter conditions than optical waves. Here, we explore the multiplexing of THz OAM beams for high-capacity communications. Specifically, we experimentally demonstrate communication systems with two multiplexed THz OAM beams at a carrier frequency of 0.3 THz. We achieve a 60-Gbit/s quadrature-phase-shift-keying (QPSK) and a 24-Gbit/s 16 quadrature amplitude modulation (16-QAM) data transmission with bit-error rates below 3.8 × 10-3. In addition, to show the compatibility of different multiplexing approaches (e.g., polarization-, frequency-, and mode-division multiplexing), we demonstrate an 80-Gbit/s QPSK THz communication link by multiplexing 8 data channels at 2 polarizations, 2 frequencies, and 2 OAM modes.

Triple Coupled Ring‐Core Fiber with Dual Highly Dispersive Windows for Orbital Angular Momentum Mode

Orbital angular momentum (OAM) beams, featured by the helical phase front and doughnut intensity profile, have been demonstrating great potential for various applications. Ring‐core optical fiber is also experimentally proven to be an effective waveguide for OAM mode. In general, managing the chromatic dispersion in the optical fiber is important for a wide range of applications. In this article, a highly dispersive germanium‐doped ring‐core fiber with dual selectable operating windows for OAM mode is proposed. The proposed fiber consists of three concentric high‐refractive‐index germanium‐doped rings with different mole fractions. The chromatic dispersion of the OAM1,1 mode in the ring fiber can be as low as −129 964 ps/(nm·km) at 1644.4 nm as the light launches into the fiber from the innermost ring and −270 017 ps/(nm·km) at 1651 nm as the light enters from the outermost ring. Furthermore, the effect of the geometrical and material parameters on chromatic dispersion for OAM1,1 mode is investigated. When the parameters of the innermost and outermost rings are varied separately, the corresponding operation windows are changed independently. An extremely negative dispersion for the OAM1,1 mode of −817 576 ps/(nm·km) can be achieved. This design can serve as a dispersive element in various OAM‐based optical fiber systems.

Design of Dual-Functional Metaoptics for the Spin-Controlled Generation of Orbital Angular Momentum Beams

The capability of multiple orbital angular momentum (OAM) modes generation with high resolution and diversified functionalities in the visible and near-infrared regime is challenging for flat and integrated optical devices. Additionally, having a static tiny optical device capable of generating multiple structured spots in space reduces the complexity of optical paths that typically use dynamic optical components and/or many standard elements, leading to unprecedented miniaturization and compactness of optical systems. In this regard, we propose dual-functional transmission dielectric metalenses based on a set of Pancharatnam-Berry phase meta-atoms with different cross-sections, for the combined manipulation of the dynamic and geometric phases. In particular, we present and describe the numerical algorithms for the computation of dual-functional metaoptics and we apply those techniques to the design of optical elements which are able to generate and focus different OAM modes at distinct points in space. In the specific, the designed elements enable the independent or simultaneous manipulation of right-handed and left-handed circularly polarized waves, by acting on the helicity of the input beam to enable or disable a specific optical operation. The theoretical proof-of-concept results highlight the capability of the designed metalenses to generate multiple high-resolution focused OAM modes at different points in space by exploiting the polarization of the incident beam as a degree of freedom, thus providing new integrated optics for applications in the fields of high-resolution microscopy, optical manipulation, and optical communications, both in the classical and single-photon regimes.

Real-time measurement of dynamic micro-displacement and direction using light's orbital angular momentum

In this Letter, we propose an orbital angular momentum (OAM) sensor to simultaneously measure the dynamic micro-displacement and the direction of a moving object in real time. The micro-displacement of the moving object can be detected by the calculation of the petals' rotation angle caused by the coaxial interference between the measured OAM beam and its reference OAM beam, and the direction (forward or backward) of the moving object can be achieved by the clockwise or anticlockwise of the petals' rotation. We also develop an algorithm to monitor the petals' rotation angle and the rotation direction. The experimental results demonstrate that the proposed sensor can achieve high precision (±16.5995 nm) and a longer measuring range (0–1100 cm). Additionally, the OAM sensor is sensitive to the topological charge in the OAM mode, the initial distance, and the velocity of the moving object. The sensor can perform the non-contact measurement, so it will be a promising method in micro-vibration sensing, surface unevenness sensing, and microbial movement sensing.

Thermalization of Orbital Angular Momentum Beams in Multimode Optical Fibers.

We report on the thermalization of light carrying orbital angular momentum in multimode optical fibers, induced by nonlinear intermodal interactions. A generalized Rayleigh-Jeans distribution of asymptotic mode composition is obtained, based on the conservation of the angular momentum. We confirm our predictions by numerical simulations and experiments based on holographic mode decomposition of multimode beams. Our work establishes new constraints for the achievement of spatial beam self-cleaning, giving previously unforeseen insights into the underlying physical mechanisms.

Interactions between Plasmonic Nanoantennas and Vortex Beams.

The orbital angular momentum (OAM) of light offers a new degree of freedom for light-matter interactions, yet how to control such interactions with this physical dimension remains open. Here, by developing a numerical method enabling optical OAM simulations, we provide insights into complex plasmon behaviors with the physical dimension of OAM, and we prove in theory that plasmonic nanostructures can function as efficient antennas to intercept and directionally reradiate the power of OAM beams. The interplay between optical OAM and spin angular momentum (SAM) generates novel particle polarizations and radiations, which were inaccessible before. For arrayed nanoparticles, coherent surface plasmons with specific phase retardations determined by OAM of the beams enable directional power radiations, making a phased array antenna. These findings expand our knowledge of nanoplasmonics in the OAM area and are promising for quantum information processing and dynamic sensing of ultraweak biosignals.

Amplitude Non-Uniformity of Millimeter-Wave Vortex Beams Generated by Transmissive Structures

We investigate the origin of non-uniformity in an orbital angular momentum (OAM) beam generated by a transmissive structure in the millimeter-wave frequency range. This work includes the study of amplitude non-uniformity including modal composition analysis using the angular spectrum method (ASM) for OAM beams generated by spiral phase plates (SPPs) and flat-surface metamaterial structures (MSs). The dependency of the axial dimension, over which the OAM beam transforms from a Gaussian beam, on the amplitude non-uniformity of both the structures has been rigorously analyzed. For experimental verification, we designed a perforated planar dielectric slab to convert the Gaussian beam into an OAM beam of order 2 with improved amplitude uniformity. Within the manufacturing limitations, the design of the transmissive MS was fabricated and tested to obtain optimum amplitude uniformity in the generation of OAM beams and to compare with simulation results.

Modeling of high intensity orbital angular momentum beams for laser–plasma interactions

In this work, we explore the field of high intensity orbital angular momentum (OAM) beams, their generation with spiral phase mirrors, and the theory behind modeling both ideal and realistic beams. We explore OAM beam asymmetries introduced by aberrations in the beam, manufacturing defects, and bandwidth. A full three-dimensional description of the paraxial Laguerre–Gaussian (LG) modes suitable for modeling beams down to f/2 focal geometries is derived. A perturbative approach to modeling asymmetric OAM beams is introduced showing that only three LG modes are sufficient to model a wide variety of OAM asymmetries. The models are compared with experimental results followed by a discussion on the future of high intensity OAM beams in plasma physics.

3D Engineering of Orbital Angular Momentum Beams via Liquid‐Crystal Geometric Phase (Laser Photonics Rev. 16(6)/2022)

Orbital Angular Momentum Beams In article number 2200118, Peng Chen, Yan-Qing Lu, and colleagues report a robust method for orbital angular momentum (OAM) beam tailoring in 3D space via preprogrammed liquid crystals. 3D OAM beam lattices with volumetric identical or space-variant OAM modes are generated efficiently in a spin-controllable and electrically-tunable manner. This opens a promising avenue for OAM harnessing with both large capacity and good flexibility.

Simultaneous and independent regulation of circularly polarized terahertz wave based on metasurface.

A single metasurface-based device possessing multiple functionalities is highly desirable for terahertz technology system. In this paper, we design a reflective metasurface to generate switchable vortex beams carrying orbital angular momentum (OAM), focusing beams, focusing beams with arbitrary positions, and vortex beams with arbitrary topological charges in the terahertz region. By combining the spin decoupling principle and the phase addition theorem, the superposition states of OAM and focusing beams with arbitrary positions can be independent manipulated under right-handed and left-handed circularly polarized (LCP/RCP) waves illumination. Such a diversified functionalities device provides a promising application in the field of terahertz communication and terahertz super-resolution imaging.