Orbital Angular Momentum Research Articles

Fractional-order vortex beam diffraction process recognition using machine learning

Fractional-order vortex beams possess Fractional Orbital Angular Momentum (FOAM) modes, which theoretically have the potential to increase transmission capacity infinitely. Therefore, they have significant application prospects in the field of measurement, optical communication and microparticle manipulation. However, when fractional-order vortex beams propagate in free space, the discontinuity of the helical phase makes them susceptible to diffraction in practical applications, thereby affecting the accuracy of OAM mode recognition and severely limiting the use of FOAM-based optical communication. The problem of achieving machine learning recognition of fractional-order vortex beams under diffraction conditions is currently an urgent and unreported issue. This paper proposes a deep learning (DL) method based on ResNet for accurate recognition of the propagation distance and topological charge of fractional-order vortex beam diffraction process. Utilizing both experimental measured and theoretically simulated intensity distributions, a dataset of vortex beam diffraction intensity patterns in atmospheric turbulence environments was created. An improved 101-layer ResNet structure based on transfer learning was employed to achieve accurate and efficient recognition of the FOAM model at different propagation distances. Experimental results show that the proposed method can accurately recognize FOAM modes with a propagation distance of 100 cm, an interval of 5 cm, and a mode spacing of 0.1 under turbulent conditions, with an accuracy of 99.69%. This method considers the effect of atmospheric turbulence during spatial transmission, allowing the recognition scheme to achieve high accuracy even in special environments. It has the distinguishing capability for ultra-fine FOAM modes and propagation distances that traditional methods cannot achieve. This technology can be applied to multidimensional encoding and sensing measurements based on FOAM beam.

Calculation of spectroscopic constants for O<sup>-</sup><sub>2</sub> containing spin-orbit coupling

A comprehensive theoretical study on the low-energy electronic states of the superoxide anion (O<sup>-</sup><sub>2</sub>) is presented, focusing on the effects of spin-orbit coupling (SOC) on these states. Utilizing the complete active space self-consistent field (CASSCF) method combined with the multireference configuration interaction method with Davidson correction (MRCI+Q), and employing the aug-cc-pV5Z-dk basis set that includes Douglas-Kroll relativistic corrections, electron correlation and relativistic effects are accurately considered. The research concentrates on the first and second dissociation limits of O<sup>-</sup><sub>2</sub>, calculating the potential energy curves (PECs) and spectroscopic constants of 42 Λ-S states. After introducing SOC, 84 Ω states are obtained through splitting, and their PECs and spectroscopic constants are calculated. Detailed data on the electronic states related to the second dissociation limit are provided for the first time. The results show excellent agreement with existing literature, validating the reliability of the methodology. For the first time, this study confirms through calculations with different basis sets that the double-well structure of the a<sup>4</sup>Σ<sup>-</sup><sub>u</sub> state originates from an avoided crossing with the 2<sup>4</sup>Σ<sup>-</sup><sub>u</sub> state, and finds that the size of the basis set significantly affects the depth of its potential wells. After considering SOC, the total energy of the system decreases, especially for states with high orbital angular momentum (such as the 1²Φ<sub>u</sub> and 1⁴Δ<sub>g</sub> states), leading to energy level splitting and energy reduction, while other spectroscopic constants remain essentially unchanged. These findings provide valuable theoretical insights into the electronic structure and spectroscopic properties of O<sup>-</sup><sub>2</sub>, offering important reference data for future research in fields such as atmospheric chemistry, plasma physics, and molecular spectroscopy. The datasets presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00213.00076.

Unambiguous Detection of High-Energy Vortex States via the Superkick Effect

Vortex states of photons, electrons, and other particles are freely propagating wave packets with helicoidal wave fronts winding around the axis of a phase vortex. A particle prepared in a vortex state carries a nonzero orbital angular momentum projection on the propagation direction, a quantum number that has never been exploited in experimental particle and nuclear physics. Low-energy vortex photons, electrons, neutrons, and helium atoms have been demonstrated in experiment and found numerous applications, and there exist proposals of boosting them to higher energies. However, verification that a high-energy particle is indeed in a vortex state will be a major challenge, since the low energy techniques become impractical at higher energies. Here, we propose a new diagnostic method based on the so-called superkick effect, which can unambiguously detect the presence of a phase vortex. A proof-of-principle experiment with vortex electrons can be done with existing technology, and its realization will also constitute the first observation of the superkick effect. Published by the American Physical Society 2024

Robust quantum metrology with random Majorana constellations

Abstract Even the most classical states are still governed by quantum theory. A number of physical systems can be described by their Majorana constellations of points on the surface of a sphere, where concentrated constellations and highly symmetric distributions correspond to the least and most quantum states, respectively. If these points are chosen randomly, how quantum will the resultant state be, on average? We explore this simple conceptual question in detail, investigating the quantum properties of the resulting random states. We find these states to be far from the norm, even in the large-number-of-particles limit, where classical intuition often replaces quantum properties, making random Majorana constellations peculiar and intriguing. Moreover, we study their usefulness in the context of rotation sensing and find numerical evidence of their robustness against dephasing and particle loss. We realize these states experimentally using light’s orbital angular momentum degree of freedom and implement arbitrary unitaries with a multiplane light conversion setup to demonstrate the rotation sensing. Our findings open up new possibilities for quantum-enhanced metrology.

Decoherence of high-dimensional orbital angular momentum entanglement in anisotropic turbulence

The decoherence of high-dimensional orbital angular momentum (OAM) entanglement in the weak scintillation regime has been investigated. In this study, we simulate atmospheric turbulence by utilizing a multiple-phase screen imprinted with anisotropic non-Kolmogorov turbulence. The entanglement negativity and fidelity are introduced to quantify the entanglement of a high-dimensional OAM state. The numerical evaluation results indicate that entanglement negativity and fidelity last longer for a high-dimensional OAM state when the azimuthal mode has a lower value. Additionally, the evolution of higher-dimensional OAM entanglement is significantly influenced by OAM beam parameters and turbulence parameters. Compared to isotropic atmospheric turbulence, anisotropic turbulence has a lesser influence on high-dimensional OAM entanglement.

Nearfield observation of spin-orbit interactions at nanoscale using photoinduced force microscopy.

Optical spin and orbital angular momenta are intrinsic characteristics of light determined by its polarization and spatial degrees of freedom, respectively. At the nanoscale, sharply focused structured light carries coupled spin-orbital angular momenta with complex 3D nearfield structures, crucial for manipulating multidimensional information of light in nanophotonics. However, characterizing these interactions faces challenges with conventional farfield-based methods, which typically lack the essential accuracy and resolution to interrogate the structured nearfield with high fidelity. To address this challenge, we experimentally observe spin-orbit interactions at the nanoscale using photoinduced force microscopy. Such interactions are enabled through sharply focused circularly polarized optical vortices, which are then mapped by nearfield optical forces in high resolution. Because the optical forces can reveal both longitudinal and transverse nearfield structures with high fidelity, the spin-orbit interactions are eventually evaluated quantitatively at the nearfield, as an important inspiration to use the coupled momenta in dense optical nano-device systems.

Underwater Laguerre-Gaussian beam propagation and phase compensation method

Laguerre-Gaussian (LG) beams experience phase twist after long-distance transmission, making the orbital angular momentum (OAM) indistinguishable. This phenomenon becomes more severe in water due to the higher refractive index. Based on the physical principles of LG beams, this paper derives the mathematical expression for LG beam transmission in water to address this issue. It organizes and analyses the physical significance of each term. The exponential term responsible for phase twist is separated and phase compensation is applied to the initial LG beam. Simulation results show that after transmission through water, traditional LG beams exhibit a clockwise twisted distribution of isophase lines. By applying the phase compensation method proposed in this paper, the phase of the initial LG beam is modulated and when the LG beam reaches the observation plane, the isophase lines become straight, verifying the effectiveness of the method. This compensation method holds significant value for LG beams in advanced physics research.

Mask-coding-assisted continuous-variable quantum direct communication with orbital angular momentum multiplexing

Abstract Quantum secure direct communication (QSDC) is an approach of communication to transmitting secret messages based on quantum mechanics. Different from the quantum key distribution, secret messages can be transmitted directly on quantum channel with QSDC. Higher channel capacity and noise suppression capabilities are key to achieving longdistance quantum communication. Here we report a continuous-variable QSDC scheme based on mask-coding and orbital angular momentum, in which the mask-coding is employed to protect the security of the transmitting messages and to suppression the influence of excess noise. And the combination of orbital angular momentum and the information block transmission can effectively improve the secrecy capacity. In the 800 information blocks × 1310 bits length 10-km experiment, the results show that the statistical average of bit error rate reached 0.38%, and the system excess noise is 0.0184 SNU, and the final secrecy capacity is 6.319×106 bps. Therefore, this scheme reduces error bits while increasing secrecy capacity, providing a solution for long-distance large-scale quantum communication, which has capacity of transmitting text, images, and other information of reasonable size.

Investigation of kaonic atom optical potential by the high precision data of kaonic 3He and 4He atoms

Abstract We investigate kaonic atom optical potentials of 3He and 4He phenomenologically based on the latest high precision data of the 2p states of the kaonic atoms in helium (J-PARC E62). We consider a simple phenomenological form for the optical potential proportional to the nuclear density distributions and clarify the meanings and implications of the data by determining the potential parameters by the latest data. We find two sets of potential parameters for each of K− − 3He and K− − 4He that reproduce the data. We then extract the potential parameters consistent with the data of 3He and 4He atoms simultaneously. We report the possible strong isospin dependence of the potential, and the different strength of the data of K− − 3He and K− − 4He in the restrictions on the potential parameters. We mention that the study of the 1s states of the kaonic helium atom is very interesting as a next step. Observing these states could provide valuable information on potential parameters and kaonic nuclear s states because of the mutual influence between the atomic and the nuclear states of kaon with the same orbital angular momentum.

Physics-driven untrained neural network for vortex beam compensation in adaptive optics aided underwater wireless optical communications

Orbital angular momentum (OAM) multiplexing is emerging as a critical technique for achieving high data capacity in underwater wireless optical communications (UWOC). Nonetheless, wavefront distortions induced by underwater turbulence compromise the orthogonality of OAM modes. In this paper, we introduce a physics-driven untrained learning approach for adaptive optics that operates independently of extensive amplitude datasets. Without iterative processing and pre-trained datasets, the underwater turbulence characteristics can be retrieved accurately by only relying on a one-shot distorted probe beam and a priori known amplitude of the probe beam. By leveraging a single distorted diffraction pattern and a priori known amplitude of the probe beam, the characteristics of underwater turbulence can be accurately retrieved without iterative processing or pre-trained datasets. Furthermore, by implementing a hybrid input/output alternating projection algorithm with a square constraint area, the retrieved underwater turbulence phase screen beyond the [0, 2π] range aligns with the target pattern. This consistency indicates that the proposed wavefront recovery technology is validated across a broad range of turbulence strengths. As a demonstration of feasibility, numerical simulations, and optical experiments were conducted to validate the compensation of OAM beams. Furthermore, the theoretical bit error rate (BER) and channel capacity were inferred based on the purity of OAM modes and the level of crosstalk.

Enhanced empirical formulas for α-decay for heavy and superheavy nuclei: Incorporating deformation effects of daughter nuclei

Abstract The latest experimental data of $\alpha$-decay half-lives for 573 nuclei within the range of $52 \leq Z \leq 118$ have been utilized to enhance empirical formulas with updated coefficients. These formulas were enhanced by analyzing the contributions of orbital angular momentum and isospin asymmetry. The effect of deformation of daughter nuclei on the $\alpha$-decay half-life is modeled by incorporating two additional terms, dependent on the quadrupole and hexadecapole deformation parameters, into the empirical formulas for $\alpha$-decay half-lives. Incorporating these deformation-dependent terms, along with angular momentum and isospin asymmetry, improved the standard deviation by approximately $17\%$. The revised empirical formulas for $\alpha$-decay half-lives demonstrated better agreement with experimental data when deformation factors were included. Validation of the modified formulas was conducted through comparisons with recent experimental results and further theoretical predictions. This study presents and compares $\alpha$-decay half-life predictions for several isotopes of superheavy nuclei with $Z = 120-126$, which are yet to be experimentally synthesized. For various isotopes of each element, the variation of $\log_{10} T_{\alpha}$ with changes in neutron number is also explored.

Enhanced proton acceleration using spiral-phase plasma mirrors

Abstract We present a method for generating intense vortex laser beams using spiral-phase plasma mirrors. These single-use micro-metric formations imprint a set amount of orbital angular momentum (OAM) to an arbitrary high power focusing beam. Using these beams, we irradiated ultrathin foils and measured the emission of ions from their back side. Utilizing a PW class laser, 5 J@45fs@2w resulting in an Intensity of 2×1020 W cm − 2 at spot focus, we observed an increase in the energy and numbers of these ions, compared to shots taken with reflection off flat plasma mirrors. A 45% increase in the proton numbers and a 35% increase in cutoff energy was observed. This is a major advance since the area of interaction and intensity are reduced due to the planar shape of the generated OAM laser. We review the available data-sets on laser ion acceleration using vortex beams, and conclude with a comparison to our findings.

Single-pixel orbital angular momentum detection in the temporal domain

Abstract Orbital angular momentum (OAM) beam’s spatial information captured using a camera, employs millions of single-pixel detectors arranged in a two-dimensional matrix. Poses challenges in OAM detection due to significant storage requirements, reduced speed due to low frame rate, and increased post-processing time. To address this bottleneck, a novel OAM detection technique utilizing only a diffuser and a single-pixel fast photodiode (FPD) is reported. The sixteen OAM beams, l = [ − 8 + 8 ] interact with the rotating diffuser resulting in a temporally varying speckle field. The 1D temporal speckle information (TSI) has been segregated from the temporally varying 2D speckle pattern images for each beam. The segregated 1D TSI has been classified by employing a machine learning model and achieved a classification accuracy of 97.1% and 97.3% on simulated and experimental data respectively. To further validate the proposed single-pixel OAM detection method, the 1D TSI is captured directly using a FPD. The experimentally captured 1D TSI captured via photodiode has been classified by a lightweight custom-designed 1D convolutional neural network and achieved an increased classification accuracy of 96%. This approach redefines OAM detection, operating in the temporal domain with a single-pixel FPD, thereby significantly reducing storage and computational costs while promising high-speed OAM detection.

Incoherent scattering ion line spectra of vortex beams by magnetized ionospheric plasma with bi-Maxwellian distribution

Abstract Incoherent scattering theory is one of the important means of plasma parameter diagnosis. The incoherent scattering of vortex beams carrying orbital angular momentum can bring new incremental information to plasma diagnosis. Based on the angular spectrum theory of Bessel vortex beams and the theory of incoherent scattering, this paper studies the incoherent scattering of the ionospheric magnetized plasma to vortex electromagnetic waves under the bi-Maxwellian distribution along the magnetic field drift. The numerical results show that when the angle between the scattering wave vector and the magnetic field direction is small, the ion spectral lines have an obvious frequency shift. When the angle is close to 90°, the ion line spectra produce harmonic oscillation, and the drift speed restrains this oscillation phenomenon. The linear Doppler shift will shift the spectral line as a whole, while the rotating Doppler shift of the vortex beam will weaken the spectral line oscillation. This work may provide a possibility for further research on the scattering of vortex beams by magnetized plasma and the development of ionospheric plasma irregularities parameter inversion.

Fiber Bragg Grating-Based Tunable Wavelength Orbital Angular Momentum Beam Generation with

This paper introduces and theoretically analyze the generation of orbital angular momentum (OAM) modes with different polarization in a few-mode optical fiber using short-period gratings (Bragg gratings) at telecommunication wavelengths. The modal characteristics of the supported modes have been studied using a full-vector true-mode analysis. Additionally, a appropriately designed raised-cosine apodized grating is employed to couple the HE11 core mode to the counter-propagating cladding mode. In contrast to the long-period grating based OAM mode generation, the present scheme is free from periodic back-coupling of optical power into the fundamental core mode which drastically enhances the OAM mode purity. The ±1-order OAM beam has been generated by combining the first higher order core modes with a π/2 phase shift between them. This paper also shows that by combining different higher order modes, it is possible to obtain a linearly polarised OAM mode or a circularly polarised OAM mode. Finally, I also report that in fibers with larger core radius, the higher order OAM mode of topological charge ±2 or higher can also be generated using similar approach of using short-period gratings. The topological charge of the OAM mode is verified by observing both the coaxial and off axial interference fringes.

Experimental demonstration of the equivalence of entropic uncertainty with wave-particle duality.

Wave-particle duality is one of the most notable and counterintuitive features of quantum mechanics, illustrating that two incompatible observables cannot be measured simultaneously with arbitrary precision. In this work, we experimentally demonstrate the equivalence of wave-particle duality and entropic uncertainty relations using orbital angular momentum (OAM) states of light. Our experiment uses an innovative and reconfigurable platform composed of few-mode optical fibers and photonic lanterns, showcasing the versatility of this technology for quantum information processing. Our results provide fundamental insights into the complementarity principle from an informational perspective, with implications for the broader field of quantum technologies.

Aperture partitioning holographic metasurface for the generation of a directional vortex beam

In this paper, what we believe to be a new aperture partitioning holographic metasurface is designed to effectively generate a directional vortex beam with mixed orbital angular momentum (OAM) modes. The proposed holographic metasurface consists of four adjacent doughnut-shaped regions, and a monopole probe is set in the center for feeding. These four well-designed regions generate four different OAM mode numbers of -1, -2, -3, and -4 respectively, and thus realize a directional vortex beam with mixed OAM modes. The proposed holographic metasurface operating at 10 GHz is simulated, fabricated, and measured. The measurement results agree well with the simulation results. The realized directional vortex beam contains both beam directionality and rich vortex phase vibrations property, which makes it useful in improving vortex beam transmission distance and applications in Multiple-Input Multiple-Output (MIMO) communication systems.

Chiral Steric Effect for Improved Phase Transition and Exciton Dynamics in Efficient and Stable All‐Polymer Solar Cells

AbstractChiral materials exhibit distinctive properties that improve the performance of organic solar cells (OSCs). In this study, chiral PM6 is integrated into PM6:PY‐IT blends to fabricate chiral quasi‐binary OSCs (cOSCs). The produced devices displayed an increased short‐circuit current and fill factor, achieving a peak efficiency of 16.60% compared to 15.96% for the standard PM6:PY‐IT cell. Additionally, the incorporation of chiral PM6 significantly enhanced the thermal stability, with the cOSCs retaining over 80% of their initial efficiency after 1000 h of operation. The steric effect of chiral PM6 altered the molecular packing, thereby improving the overall crystallinity. The optimized nanoscale domain morphology resulted in a more effective blend film, leading to superior device performance. Moreover, the chirality‐induced orbital angular momentum increased the ratio of triplet states, which is a key factor for increasing the photocurrent in OSCs. The delocalization of electron wavefunctions is driven by chiral steric effects, and it diminished the Coulomb attraction between the interfacial electron–hole pairs, which enhanced the efficient charge generation. This study provides valuable insights into the role of chirality in optimizing both the performance and stability of OSCs.

Is it Possible to Detect a Rotating Spherical Colloidal Particle?

A single micrometer‐size spherical colloid has been set in rotation by transfer of light orbital angular momentum. This particle is floating at an air–water interface. Steady‐state rotational frequencies of the order of one hertz have been observed, depending on the topological charge of the beam and on its power, in agreement with expected values. The detection is performed using the rotational Doppler shift of the diffused light. Two time constants have been evidenced in the rotational velocity dynamics. The first one is related to the friction of the colloid with the fluid (air and water), whereas the other one is principally associated with the wall friction of the air–liquid interface with the container. This measurement technique makes it possible to identify dynamic parameters of the rotational movement of any spherical object, which is usually impossible to detect.

Quasi‐Bound States in the Continuum on Dislocated Bilayer Metal Gratings for Spatiotemporal Vortex Pulse Generation

AbstractSpatiotemporal vortex pulses (STVPs) with transverse orbital angular momentum (OAM) have recently stimulated great interest, influencing wide disciplines from classical to quantum optics. However, generating sophisticated STVPs in high‐efficiency and compact systems remains a challenge. This work outlines an ultra‐compact metasurface approach to efficiently generate STVPs through the coupling of free‐space plane waves with a quasi‐bound state in the continuum (quasi‐BIC). The approach leverages external excitation of the quasi‐BIC at the Γ‐point by slightly dislocating two sub‐layer gratings to break the mirror symmetries. This operation converts the incident wave into a unidirectional surface mode and induces a Fano resonance, resulting in STVPs in the frequency‐momentum domain. The method is demonstrated experimentally with a meta‐grating to produce an electromagnetic (EM) STVP with a topological charge of l = −1 with high fidelity and use the set‐up to unravel the hitherto unexplored diffraction and dispersive properties of near‐field STVPs. The present work can be extended from the microwave to the visible light regime by simply scaling the metasurface, and allowing high‐order OAM generation through cascaded elements, thereby paving a robust way to build up ultra‐compact STVP multiplexing devices for future applications.