High Orbital Angular Momentum Research Articles

Generation of arbitrary vector Bessel beams on higher-order Poincaré spheres with an all-dielectric metasurface

Vector Bessel beams may have a spatially varying polarization state combined with a Bessel-type transversal intensity profile. Such beams can carry both spin angular momentum and orbital angular momentum (OAM) and be characterized by points on higher-order Poincar\'e (HOP) spheres. We report a method to produce vector Bessel beams on any point of a HOP sphere of any topological charge by utilizing only a single all-dielectric planar metasurface. Specifically, by a proper choice of the input polarization and distance from the metasurface, the entire HOP sphere surface can be covered. The proposed method is simple and efficient and applies to any $n\mathrm{th}$-order HOP sphere. Furthermore, it yields an extremely high OAM mode purity. Our paper demonstrates a robust approach for generating arbitrary high-quality vector Bessel beams and will likely inspire future applications ranging from optical manipulation to laser beam engineering.

Mm-wave Rydberg-Rydberg transitions gauge intermolecular coupling in a molecular ultracold plasma.

Out-of-equilibrium, strong correlation in a many-body system can trigger emergent properties that act to constrain the natural dissipation of energy and matter. Signs of such self-organization appear in the avalanche, bifurcation, and quench of a state-selected Rydberg gas of nitric oxide to form an ultracold, strongly correlated ultracold plasma. Work reported here focuses on the initial stages of avalanche and quench and uses the mm-wave spectroscopy of an embedded quantum probe to characterize the intermolecular interaction dynamics associated with the evolution to plasma. Double-resonance excitation prepares a Rydberg gas of nitric oxide composed of a single selected state of principal quantum number, n0. Penning ionization, followed by an avalanche of electron-Rydberg collisions, forms a plasma of NO+ ions and weakly bound electrons, in which a residual population of n0 Rydberg molecules evolves to a state of high orbital angular momentum, ℓ. Predissociation depletes the plasma of low-ℓ molecules. Relaxation ceases and n0ℓ(2) molecules with ℓ ≥ 4 persist for very long times. At short times, varying excitation spectra of mm-wave Rydberg-Rydberg transitions mark the rate of electron-collisional ℓ-mixing. Deep depletion resonances that persist for long times signal energy redistribution in the basis of central-field Rydberg states. The widths and asymmetries of Fano line shapes witness the degree to which coupling in the arrested bath (i) broadens the allowed transition and (ii) mixes the local network of levels in the ensemble.

Ultra-intense vortex laser generation from a seed laser illuminated axial line-focused spiral zone plate.

Relativistic vortex laser has drawn increasing attention in the laser-plasma community owing to its potential applications in various domains, e.g., generation of energetic charged particles with orbital angular momentum (OAM), high OAM X/γ-ray emission, high harmonics generation, and strong axial magnetic-field production. However, the generation of such relativistic vortex laser is still a challenge to the current laser technology. Using micro-structure targets named axial line-focused spiral zone plate (ALFSZP), we propose a novel scheme for ultra-intense vortex laser generation. In the scheme, a relativistic Gaussian laser pulse irradiates an ALFSZP, and diffracts as it passes through the ALFSZP. Due to the focusing and radial Hilbert transform capabilities of the ALFSZP, the seed laser is converted efficiently to a vortex one which is then well focused in a tunable focal volume. Three-dimensional particle-in-cell simulations indicate that using a seed laser pulse with intensity of 1.3 × 1020 W/cm2, the vortex laser intensity achieved is as high as 1.3 × 1021 W/cm2 with the averaged angular momentum per photon up to 0.73ℏ, promising diverse applications in various fields aforementioned.

Small mode volume topological photonic states in one-dimensional lattices with dipole-quadrupole interactions

We study the topological photonic states in one-dimensional lattices analog to the Su-Schrieffer-Heeger model beyond the dipole approximation. The electromagnetic resonances of the lattices supported by near-field interactions between the plasmonic nanoparticles are studied analytically with coupled dipole-quadrupole method. The topological phase transition in the bipartite lattices is determined by the change of Zak phase. Our results reveal the contribution of quadrupole moments to the near-field interactions and the band topology. It is found that the topological edge states in nontrivial lattices have both dipolar and quadrupolar nature. The quadrupolar edge states are not only orthogonal to the dipolar edge states, but also spatially localized at different sublattices. Furthermore, the quadrupolar topological edge states, which coexist at the same energy with the quadrupolar flat band have shorter localization length and hence smaller mode volume than the conventional dipolar edge states. The findings deepen our understanding in topological systems that involve higher-order multipoles, or in analogy to the wave functions in quantum systems with higher-orbital angular momentum, and may be useful in designing topological systems for confining light robustly and enhancing light-matter interactions.

Application of the Lagrange-mesh method in continuum-discretized coupled-channel calculations

We apply the Lagrange-mesh method to discretize continuum states of weakly bound nuclei for continuum-discretized coupled-channel (CDCC) calculations of three-body breakup reactions. This discretization method is compared with the bin method, which is regarded as the standard continuum discretization method, for the d and 6Li induced reactions. An improved Numerov algorithm is used to solve the coupled channels equations, which permits a fast integration of equations and a convenient treatment of the closed channels. A new CDCC model code named CDCC-R is developed. In all cases, the combination of the Lagrange-mesh method and the Numerov algorithm shows high efficiency and accuracy for the CDCC calculations of the elastic scattering and breakup reactions. Especially, various numerical and physical aspects are discussed for 6Li induced reactions. The coupling effects of the continuum states with high orbital angular momentum and closed channels are discussed. Moderate effects are found in the calculations for elastic scattering and breakup reaction when F- and G-wave continuum states are included in CDCC model space for 6Li induced reactions at the incident energies well above the Coulomb barrier. The closed channel effect is found to be indispensable for 6Li breakup reaction calculation when the incident energy is around the Coulomb barrier.

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.

Combination regulation of terahertz OAM vortex beams on multi-bit metasurface

High efficiency terahertz orbital angular momentum (OAM) vortex beam manipulation is of crucial importance for ultra large capacity wireless communication in terahertz frequencies region. We present a silicon-based multi-bit encoded metasurface structure based on convolution and addition theorem to generate flexible regulation of OAM multiple combinations such as vortex beam and beam splitting combination, vortex beam splitting, deflected beam and vortex beam combination. The proposed metasurfaces is composed of periodic array of silicon pillar with quadrangular ridge on top of a metal plate substrate. The advantage of this scheme in producing vortex beam is its simple structure, flexible design and controls to arbitrary presented directions easily. This work provides a new approach to realize powerful and flexible adjustment of terahertz wave.

Macroscopic phase-matching mechanism for orbital angular momentum spectra of high-order harmonics by mixing two Laguerre-Gaussian vortex modes

The high-order harmonic generation (HHG) by the interaction of an intense vortex infrared laser and a gas medium has been employed to produce the extreme ultraviolet (XUV) light beam carrying a high value orbital angular momentum (OAM). Here we extend this approach to generate the XUV light with superimposed OAM modes by mixing two different Laguerre-Gaussian (LG) modes as the driving beam. By solving the three-dimensional Maxwell's wave equations of the high-harmonic field and calculating the coherence lengths of HHG, we reveal the complex nature of phase-matching conditions in the gas medium, especially their dependence on the azimuthal angle. We demonstrate that the OAM spectra of the HHG can be tailored by varying the harmonic order, the position of gas medium, and the energy ratio of two LG beams. We also uncover that only including short-trajectory contribution in the single-atom response could broaden the OAM spectrum of the macroscopic HHG because of the featured phase-matching characteristics caused by the atomic intrinsic phase. We anticipate this work to stimulate some interests in generating an XUV vortex beam with the tunable OAM spectrum through the gaseous HHG process as well as in separating different OAM modes of an XUV vortex beam by advanced techniques.

Suspended Metasurface for Broadband High-Efficiency Vortex Beam Generation

Electromagnetic (EM) waves carrying orbital angular momentum (OAM) exhibit phase vortex and amplitude singularity. Broadband OAM generation with high efficiency is highly desired with suggested applications such as broadband imaging and communications. In this paper, suspended metasurface structure achieving low-Q factor is proposed to realize broadband phase control and excellent reflection efficiency. Broadband vortex beam generation with OAM order of 1 and 2 are realized using the proposed suspended structure. Furthermore, by analyzing different metasurface aperture phase distribution schemes, the efficiency of the OAM generator is maximally achieved. The designs are validated by simulation and measurement. The proposed OAM generators work across 4–10 GHz with efficiency higher than 82%. This design provides a route to broadband metasurface realization and high efficiency OAM generation.

Perfect Vortex Modes for Nondestructive Characterization of Mode Dependent Loss in Ring Core Fibers

Ring core fibers (RCF) enable high-performance modal multiplexing with low crosstalk and can support orbital angular momentum (OAM) modes. RCFs are challenging to characterize due to the lack of commercial multiplexers, especially for high OAM orders. For fibers supporting large numbers of modes, typical cutback techniques for characterization are extremely wasteful of fiber, especially as one cutback is required for each mode. We show the differential modal loss across modes 3 to 10 was significantly underestimated using an OTDR when exciting modes individually or when exciting all modes indiscriminately. We exploit perfect vortex beams to achieve reliable and nondestructive characterization of mode-dependent loss (MDL) for OAM modes. Perfect vortex beams allow us to maximize the coupling efficiency at each mode launch, increasing the accuracy of MDL estimate. We fabricated fiber with a refractive index difference between the ring core and the cladding of a <inline-formula><tex-math notation="LaTeX">$5.1\times 10^{-2}$</tex-math></inline-formula>. For this fiber, mode orders 3 to 10 are the most suitable for data transmission and were the focus of our work (the fiber support up to OAM order 13). Such a high index difference can lead to MDL. We demonstrate that the modal loss spans from 2.14 to 4.38&#x00A0;dB/km for orders 3 to 10.

Long-range model of vibrational autoionization in core-nonpenetrating Rydberg states of NO.

In high orbital angular momentum (ℓ ≥ 3) Rydberg states, the centrifugal barrier hinders the close approach of the Rydberg electron to the ion-core. As a result, these core-nonpenetrating Rydberg states can be well described by a simplified model in which the Rydberg electron is only weakly perturbed by the long-range electric properties (i.e., multipole moments and polarizabilities) of the ion-core. We have used a long-range model to describe the vibrational autoionization dynamics of high-ℓ Rydberg states of nitric oxide (NO). In particular, our model explains the extensive angular momentum exchange between the ion-core and the Rydberg electron that had been previously observed in vibrational autoionization of f (ℓ = 3) Rydberg states. These results shed light on a long-standing mechanistic question around these previous observations and support a direct, vibrational mechanism of autoionization over an indirect, predissociation-mediated mechanism. In addition, our model correctly predicts newly measured total decay rates of g (ℓ = 4) Rydberg states because for ℓ ≥ 4, the non-radiative decay is dominated by autoionization rather than predissociation. We examine the predicted NO+ ion rotational state distributions generated by vibrational autoionization of g states and discuss applications of our model to achieve quantum state selection in the production of molecular ions.

Kilo-Tesla axial magnetic field generation with high intensity spin and orbital angular momentum beams

Absorption of angular momentum from a high intensity laser pulse can lead to the generation of strong axial magnetic fields in plasma. The effect, known as the inverse Faraday effect, can generate kilo-Tesla strength, multipicosecond, axial magnetic fields extending over hundreds of microns in underdense plasma. In this paper we explore the effect with ultrahigh intensity circularly polarized Gaussian beams and linearly polarized orbital angular momentum beams comparing analytic expressions with three-dimensional particle-in-cell simulations. We develop a model for the transverse magnetic field profiles, introduce a model for the temporal decay, and show that while the magnetic field strength is independent of plasma density, it has a strong dependence on the laser beam waist.

Excitation of high order orbital angular momentum modes in ultra-short chiral long period fiber gratings.

A class of ultra-short chiral long period fiber gratings (CLPFGs) are prepared by writing a spiral curve on the surface of a six-mode fiber. The CLPFGs are applied to excite ±2nd- and ±3rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the CLPFG in these modes can be as high as 99%, when the length is only 0.5cm. The polarization characteristic of the excited higher-order OAM modes in CLPFGs was theoretically analyzed and experimentally investigated. Results show that the obtained ±2nd- and ±3rd-order OAM modes are polarization independent, as expected.

Circular anti-resonance fibre supporting orbital angular momentum modes with flat dispersion, high purity and low confinement loss

A novel ring anti-resonance fibre (ARF) is designed and demonstrated for the transmission of orbital angular momentum (OAM) modes. The fibre consists of two layers of negative curvature tubes arranged orderly inside and outside the ring region and OAM modes can be transmitted in the ring region. The properties of the fibre are investigated by the finite element method based on the COMSOL multiphysics software and a manufacturing method is proposed. The fibre shows stable transmission of 30 OAMs in the 1.5–1.6 μm band, low and flat dispersion (5.98 ps/(km nm)), extremely small nonlinear coefficient (0.539 km−1 W−1 at 1.55 μm) and confinement loss (10−9–10−11 dB/m), as well as high OAM purity (larger than 98.295%). Therefore, the structure has immense potential in optical communication.

CONNECTING THE FORMATION OF STARS AND PLANETS. II: COUPLING THE ANGULAR MOMENTUM OF STARS WITH THE ANGULAR MOMENTUM OF PLANETS

A sample of 46 stars, host of exoplanets, is used to search for a connection between their formation process and the formation of the planets rotating around them. Separating our sample into two, stars hosting high-mass exoplanets (HMEs) and low-mass exoplanets (LMEs), we found the former to be more massive and to rotate faster than the latter. We also found the HMEs to have higher orbital angular momentum than the LMEs and to have lost more angular momentum through migration. These results are consistent with the view that the more massive the star and the higher its rotation, the more massive was its protoplanetarys disk and rotation, and the more efficient was the extraction of angular momentum from the planets.

Numerical Study of Photonic Crystal Fiber Supporting 180 Orbital Angular Momentum Modes With High Mode Quality and Flat Dispersion

We propose a photonic crystal fiber with a high refractive index ring, which supports up to 180 orbital angular momentum (OAM) modes without any high order radial modes from 1.5 um to 1.7 um. The finite element method is employed to numerically study the properties of the designed fiber. Further research shows that the refractive index differences of all the eigenmodes are above 2 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 3</sup> , which indicates that the corresponding eigenmodes can be well-separated and ensures the stable transmission of OAM modes. In addition, it is quite interesting that the high order radial modes are significantly suppressed via increasing the radius of the cladding in the design, while most of the OAM modes are without phase distortion, which is beneficial for the (de)multiplexing of the OAM modes. Particularly, the mode quality of all the eigenmodes is higher than 94.9% and the chromatic dispersion is flat with minimum value of the dispersion variation is 0.36 ps/(km·nm). Moreover, the designed fiber possesses various advantages such as low nonlinear coefficient (<; 0.7 /W/km) and confinement loss (on the order between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 7</sup> dB/m). The proposed fiber has shown great potential for high capacity OAM mode division multiplexing communications.

Vortex laser beam generation from laser interaction with azimuthal plasma phase slab at relativistic intensities.

It is shown theoretically and by simulation that a Gaussian laser beam of relativistic intensity interacting with a uniform-thickness plasma slab of azimuthally varying density can acquire orbital angular momentum (OAM). During the interaction, the laser ponderomotive force and the charge-separation force impose a torque on the plasma particles. The affected laser light and plasma ions gain oppositely directed axial OAM, but the plasma electrons remain almost OAM free. High OAM conversion efficiency is achieved due to the strong azimuthal electromagnetic energy flow during the laser phase modulation. The present scheme should provide useful reference for applications requiring relativistic-intense vortex light.

Wideband and high efficiency orbital angular momentum generator based on bi-layer metasurface

A broadband and high-efficieny bi-layer metasurface is proposed in this paper. The unit cell of the metasurface is formed by symmetrically etching two cross-type metal patches on both sides of a dielectric plate. Furthermore, the two metal patches have a displacement of half a period along the &lt;i&gt;y&lt;/i&gt;-axis. By employing the displacement, the transmission bandwidth of the bi-layer metasurface is significantly expanded. In order to obtain a physical insight into bandwidth broadening, a π-type equivalent circuit that presents the electromagnetic coupling between within the bi-layer metasurfaces is successfully extracted to investigate the influence of electromagnetic coupling on transmission performance. The results show that by shifting the metal patches along the &lt;i&gt;y&lt;/i&gt;-axis by half a period, the coupling impedance (&lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;12&lt;/sub&gt; or &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;21&lt;/sub&gt;) of bi-layer metasurface can be significantly modified, which further changes the electromagnetic coupling of the bi-layer metasurface. Correspondingly, the impedances &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;p&lt;/sub&gt; and &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;s&lt;/sub&gt; in the π-type circuit are changed to approximately meet the resonant condition of circuit in broadband, resulting in the bandwidth expansion of the proposed device. By using Pancharatnam-Berry phase theory, we redesign the proposed metasurface unit cell into a broadband orbital angular momentum generator. The simulation and measurement results verify that the bi-layer metasurface can convert a left-hand circularly polarized wave into a right-hand circularly polarized wave carrying orbital angular momentum in a frequency range between 11 GHz and 12.8 GHz, demonstrating the performance of device.

Design, Analysis, and Experiment on High-Performance Orbital Angular Momentum Beam Based on 1-Bit Programmable Metasurface

In this study, a high-performance orbital angular momentum (OAM) beam is designed, analyzed and experimented using a 1-bit programmable metasurface. OAM as a novel technology is investigated systematically in this paper. Moreover, this study is motivated by the application requirements of real-time controllable communication. The proposed programmable metasurface comprises 1-bit phase modulation units with only two phase states, which is evolved from an electromagnetic surface composed of 360° continuous phase units based on the principle of 1-bit phase quantization. First, the necessity and feasibility of developing the OAM beam based on 1-bit programmable metasurface are studied. Then, qualitative analysis of OAM beams is conducted on different OAM modes and phase modulation elements. Next, the quantitative indexes of OAM beams such as the peak gain in far-field, the divergence angle of main lobe, and circumferential symmetry in azimuth planes are systematically analyzed through a survey of key factors. It is noteworthy that this analysis provides a powerful research basis to achieve an excellent OAM beam with the adjustable function. Thereafter, an active reflective programmable metasurface with $48\times48$ elements is fabricated to verify the feasibility of developing an OAM beam using a 1-bit element. The experimental and simulation results are in good agreement. Furthermore, a high gain OAM beam with a narrow divergence angle is realized by using the large-scale 1-bit programmable metasurface. Notably, a high-gain beam and an OAM beam can be both generated and can converted into each other, which lays the foundation to achieve the OAM beam with the function of real time dynamic control in future.

General laws of the propagation of few-cycle optical pulses with orbital angular momentum in free space

We conduct a theoretical study of the propagation of few-cycle, ultrafast vortices (UFVs) carrying orbital angular momentum (OAM) in free space. Our analysis reveals much more complex temporal dynamics than that of few-cycle fundamental Gaussian-like beams, particularly when approaching the single-cycle regime and the magnitude of the topological charge $l$ is high. The recently described lower bound $\sqrt{|l|}$ to the number of oscillations of UFVs with propagation-invariant temporal shape (isodiffracting UFVs) is found to hold on average also for UFVs of general type, with variations along the propagation direction above and below that bound, even vanishing locally. These variations are determined by the so-called Porras factor or $g_0$-factor characterizing the dependence of the Rayleigh distance of the spectral constituents with frequency. With a given available bandwidth, UFVs must widen temporally with increasing magnitude of the topological charge, and must widen or may shrink temporally during propagation as a result of the axially varying, $g_0$-dependent lower bound. Under very restrictive conditions in their generation, an UFV can be shrunk below the lower bound $\sqrt{|l|}$ at a focus into a kind of locally compressed state of OAM, but it broadens well-above $\sqrt{|l|}$ and distorts in a tiny fraction of the depth of focus because of the dispersions introduced by Gouy's phase and wave front mismatch. These propagation phenomena have implications and should be taken into account in experiments and applications of UFVs, such as the generation of high-harmonics and attosecond pulses with high OAM, or in OAM-based ultrafast communications systems, as well as in other areas of physics such as acoustics or electron waves.