Angular Momentum Properties Research Articles

Angular momentum properties of a circularly polarized vortex beam in the paraxial optical systems

The angular momentum (AM) properties of circularly polarized vortex beams (CPVBs) in two paraxial optical systems [free space and a gradient-index (GRIN) fiber] are demonstrated. The transverse light intensity, the longitudinal light intensity, the phase of the longitudinal electric field, the kinetic momentum, the total spin AM (SAM), the transverse-type SAM (t-SAM), the longitudinal-type SAM (l-SAM), and the orbital angular momentum (OAM) of CPVBs in the two paraxial optical systems are characterized. Spin-orbit coupling of CPVBs is studied during propagation in free space and in a GRIN fiber. When the OAM and the SAM of a CPVB have the same direction of rotation and when they have opposite directions of rotation, the spin-orbit coupling exhibits different characteristics in free space and in the GRIN fiber.

Circularly Polarized High-Harmonic Beams Carrying Self-Torque or Time-Dependent Orbital Angular Momentum.

In the rapidly evolving field of structured light, self-torque has been recently defined as an intrinsic property of light beams carrying time-dependent orbital angular momentum. In particular, extreme-ultraviolet (EUV) beams with self-torque, exhibiting a topological charge that continuously varies on the subfemtosecond time scale, are naturally produced in high-order harmonic generation (HHG) when driven by two time-delayed intense infrared vortex beams with different topological charges. Until now, the polarization state of such EUV beams carrying self-torque has been restricted to linear states due to the drastic reduction in the harmonic up-conversion efficiency with increasing the ellipticity of the driving field. In this work, we theoretically demonstrate how to control the polarization state of EUV beams carrying self-torque, from linear to circular. The extremely high sensitivity of HHG to the properties of the driving beam allows us to propose two different driving schemes to circumvent the current limitations to manipulate the polarization state of EUV beams with self-torque. Our advanced numerical simulations are complemented with the derivation of selection rules of angular momentum conservation, which enable precise tunability over the angular momentum properties of the harmonics with self-torque. The resulting high-order harmonic emission, carrying time-dependent orbital angular momentum with a custom polarization state, can expand the applications of ultrafast light-matter interactions, particularly in areas where dichroic or chiral properties are crucial, such as magnetic materials or chiral molecules.

Orbital angular momentum and rotational properties of off-axis vortex beams in two-dimensional media with anisotropic nonlocal nonlinearity.

By introducing anisotropy into nonlinear propagations, off-axis vortex beams exhibit significantly different characteristics compared to the isotropic case. The orbital angular momentum (OAM) is non-conservative and can periodically change between positive and negative values. Accordingly, the rotation of phase singularity can transit between clockwise and counterclockwise directions. Furthermore, the phase singularity can move to infinity when the OAM approaches zero. By using the Ehrenfest theorem, the motion of the beam center is obtained. Its trajectory can be circular and parabolic or follow other complex shapes, depending closely on the anisotropy of the nonlinearity. The rotational velocity of the beam center can be modulated by the nonlinearity anisotropy and can far exceed the initial value during its propagation. These results may find potential applications in beam shaping and optical manipulation.

Plasmonic vortex beam emitter

Terahertz vortices prompt numerous advanced applications spanning classical and quantum communications, sensing, and chirality-based detection, owing to the inherent physical properties of terahertz waves and orbital angular momentum (OAM). Nonetheless, existing methodologies for generating terahertz vortices face challenges such as unalterable topological charges and intricate feed networks. To address these limitations, we propose a novel approach to generate multi-mode and tunable vortex beams based on chiral plasmons. Through eigenmode analysis, the uniform helical gratings are demonstrated to support chiral plasmons carrying OAM. By leveraging their vortex characteristics and introducing modulation into the periodic system, these chiral plasmons are alternatively diffracted into high-purity vortex radiations according to the Bragg law. To validate the theory, the vortex beam emitter is fabricated and measured in the microwave regime based on the modulated scheme. Experimental results confirm the emission of vortex beams with desirable phase distributions and radiation patterns. Our findings highlight the potential of chiral plasmons as seeds for tunable and compact vortex radiation, offering promising applications in tunable vortex sources.

Unveiling the Dynamics of Dense Cores in Cluster-forming Clumps: A 3D Magnetohydrodynamics Simulation Study of Angular Momentum and Magnetic Field Properties

We conducted isothermal magnetohydrodynamics simulations with self-gravity to investigate the properties of dense cores in cluster-forming clumps. Two different setups were explored: a single rotating clump and colliding clumps. We focused on determining the extent to which the rotation and magnetic field of the parental clump are inherited by the formed dense cores. Our statistical analysis revealed that the alignment between the angular momentum of dense cores, , and the rotational axis of the clump is influenced by the strength of turbulence and the simulation setup. In single rotating clumps, we found that tends to align with the clump’s rotational axis if the initial turbulence is weak. In colliding clumps, however, this alignment does not occur, regardless of the initial turbulence strength. This misalignment in colliding clumps is due to the induced turbulence from the collision and the isotropic gas inflow into dense cores. Our analysis of colliding clumps also revealed that the magnetic field globally bends along the shock-compressed layer, and the mean magnetic field of dense cores, , aligns with it. Both in single rotating clumps and colliding clumps, we found that the angle between and is generally random, regardless of the clump properties. We also analyzed the dynamical states of the formed cores and found a higher proportion of unbound cores in colliding clumps. In addition, the contribution of rotational energy was only approximately 5% of the gravitational energy, regardless of the model parameters for both single and colliding cases.

Exploring the angular momentum – atomic gas content connection with eagle and IllustrisTNG

ABSTRACT We use the Evolution and Assembly of GaLaxies and their Environments (eagle) and IllustrisTNG (The Next Generation) cosmological simulations to investigate the properties of the baryonic specific angular momentum (j), baryonic mass (M), and atomic gas fraction (fatm) plane for nearby galaxies. We find EAGLE and TNG to be in excellent agreement with each other. These simulations are also consistent with the results obtained with eXtended GALEX Arecibo SDSS Survey (xGASS) for gas fractions greater than 0.01. This implies that the disagreements previously identified between xGASS and predictions from simple analytical disc stability arguments also holds true for eagle and tng. For lower gas fraction (the regime currently unconstrained by observations), both simulations deviate from the plane but still maintain good agreement with each other. Despite the challenges posed by resolution limits at low gas fractions, our findings suggest a potential disconnect between angular momentum and gas fraction in the gas-poor regime, implying that not all gas-poor galaxies have low specific angular momentum.

Propagation behavior of orbital angular momentum in vector anomalous vortex beams under maritime atmospheric turbulence

This study explores the propagation properties of orbital angular momentum (OAM) carried by a vector anomalous vortex beam (VAVB) in maritime atmospheric turbulence, utilizing the Rytov approximation. A comparative analysis is conducted between the VAVB and Laguerre-Gaussian beam, revealing that the VAVB exhibits a higher detection probability under specific circumstances. This suggests that the VAVB is more suitable for scenarios where maximizing detection probability is critical. The detection probability of the signal OAM mode is affected by the characteristics of maritime atmospheric turbulence and propagation distance, but can be significantly improved by manipulating beam parameters such as wavelength, beam order, beam waist, and quantum number, while considering the characteristics of maritime atmospheric turbulence. Hence, the use of VAVB has the potential to facilitate reliable optical communication in challenging maritime environments.

OAM-resolved polarization in random light beams.

We establish the properties of the cross-spectral density orbital angular momentum (CSD-OAM) matrix of a stationary optical beam-like field and use them to introduce the OAM-resolved polarization properties. It is shown that sufficiently general random fields contain two types of polarization, one relating to a single OAM mode and the other to a pair of modes.

Semiconductor spin qubits

Spin qubits, employing the fundamental quantum property of intrinsic angular momentum of individual electrons and nuclei, continue to develop and evolve. Lithographically patterned semiconductor chips play host to spin-qubit systems in which long coherence times, high-fidelity quantum gates, and single-shot quantum measurement are now routine. This review summarizes progress in four current qubit types: single spin qubits, donor spin qubits, singlet triplet spin qubits, and exchange-only spin qubits. All have been boosted by advances in mesoscopic physics. On the other hand, all systems still need to address challenges to further progress, such as the mitigation of noise and the establishment of long-distance quantum entanglement.

Optical Encoding Model Based on Orbital Angular Momentum Powered by Machine Learning

Based on orbital angular momentum (OAM) properties of Laguerre-Gaussian beams LG(p,ℓ), a robust optical encoding model for efficient data transmission applications is designed. This paper presents an optical encoding model based on an intensity profile generated by a coherent superposition of two OAM-carrying Laguerre-Gaussian modes and a machine learning detection method. In the encoding process, the intensity profile for data encoding is generated based on the selection of p and ℓ indices, while the decoding process is performed using a support vector machine (SVM) algorithm. Two different decoding models based on an SVM algorithm are tested to verify the robustness of the optical encoding model, finding a BER =10-9 for 10.2 dB of signal-to-noise ratio in one of the SVM models.

Reflective Metasurface With Steered OAM Beams for THz Communications

The abundant spectrum resources and low beam divergence of the terahertz (THz) band, can be combined with the orthogonal propagation property of orbital angular momentum (OAM) beams to multi-fold the capacity of wireless communication systems. Here, a reflective metasurface (RMTS) is utilized to enhance the coverage of the high gain THz OAM beams by enabling the non-line-of-sight (NLoS) component by reshaping the planar wavefront of the incident wave into the helical wavefront, so that it is re-directed towards the direction of interest. This can contribute to alleviating the concern of the small aperture size, since improving the channel capacity can be achieved at the low spectrum blocks of the THz band (larger aperture size). For validation, three <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$90 \times 90$ </tex-math></inline-formula> mm RMTSs are simulated, fabricated, and tested in the frequency range 90–110 GHz, to re-direct single and dual OAM beams towards the desired location.

Arcs on speedskating straightaways: forces, energy and angular momentum

The 10 000 m speedskating event during the 2022 winter Olympics brought a new world record at 12 min and 30.74 s. In a document released after the competitions, the gold medalist Nils van der Poel describes his technique, including a focus on horizontal forces and a rapid sideways motion in connection with stride changes on the straight sections of the track (the ‘straightaways’). Based on a video of the race, together with his written description, we suggest that van der Poel makes use of the properties of angular momentum to increase his speed during a stride by a rapid inwards shift of the centre of mass.

Linear and angular momentum properties induced by radial- and azimuthal-variant polarized beams in a strongly focused optical system.

Optical linear and angular momenta have attracted tremendous research interest in recent years. In this paper we theoretically investigate the electromagnetic fields and linear and angular momentum properties of tightly focused radial- and azimuthal-variant vector input beams. Calculations show that a uniform 3D optical cage can be achieved when the optical degree of freedom of polarization in the radial direction is introduced. Furthermore, the distributions of linear and angular momenta in the focal volume are revealed. Moreover, we numerically investigate the gradient, scattering, and total forces as well as spin and orbital torques on a Rayleigh particle generated by the optical cage. It is found that there are two equilibrium positions before and after the focal plane, both of which can achieve stable 3D particles capture. Most importantly, the longitudinal spin and orbital torques show the same patterns but in opposite directions in the two equilibrium positions, thus, the unwinding of the double helix can be expected to be achieved by virtue of this special optical torque.

Separation of coherent and incoherent light by using optical vortex via spatial mode projection

We present a spatial light filter method which can distinguish light by its coherency. The unique orbital angular momentum and spiral phase front properties of the vortex beam generate a null intensity center area that can serve as a spatial window to filter out incoherent light. A spatial light modulator can be used to modulate the wavefront of the incident light beam. Thanks to diffraction, the spectra from the coherent and incoherent part are spatially separated at the backfocal plane of a lens. The coherent part forms a ring, and the incoherent part forms a spot in the null intensity center of the ring. This method offers a new perspective to enhance the signal-to-noise ratio in the field of applied optics, such as optical communication, and active remote sensing, such as light detection and ranging.

Momentum, angular momentum, and spin of waves in an isotropic collisionless plasma

We examine the momentum and angular momentum (including spin) properties of linear waves, both longitudinal (Langmuir) and transverse (electromagnetic), in an isotropic nonrelativistic collisionless electron plasma. We focus on conserved quantities associated with the translational and rotational invariance of the wave fields with respect to the homogeneous medium; these are sometimes called pseudomomenta. There are two types of the momentum and angular momentum densities: (i) the kinetic ones associated with the energy flux density and the symmetrized (Belinfante) energy-momentum tensor and (ii) the canonical ones associated with the conserved Noether currents and canonical energy-momentum tensor. We find that the canonical momentum and spin densities of Langmuir waves are similar to those of sound waves in fluids or gases; they are naturally expressed via the electron velocity field. In turn, the momentum and spin densities of electromagnetic waves can be written either in the forms known for free-space electromagnetic fields, involving only the electric field, or in the dual-symmetric forms involving both electric and magnetic fields, as well as the effective permittivity of plasma. We derive these properties both within the phenomenological macroscopic approach and microscopic Lagrangian field theory for the coupled electromagnetic fields and electrons. Finally, we explore implications of the canonical momentum and spin densities in transport and electrodynamic phenomena: the Stokes drift, the wave-induced magnetization (inverse Faraday effect), etc.

Directional dependence of the event-by-event neutron- γ multiplicity correlations in Cf252(sf)

We differentiate the event-by-event n-$\gamma$ multiplicity data from \ce{^{252}Cf}(sf) with respect to the energies of the emitted particles as well as their relative angles of emission. We determine that neutron emission enhances $\gamma$-ray emission around $0.7$ and $1.2$ MeV, but the only directional alignment was observed for $E_\gamma \leq 0.7$ MeV and tended to be parallel and antiparallel to neutrons emitted in the same event. The emission of $\gamma$ rays at other energies was determined to be nearly isotropic. The presence of the emission and alignment enhancements is explained by positive correlations between neutron emission and quadrupole $\gamma$-ray emission along rotational bands in the de-exciting fragments. This observation corroborates the hypothesis of positive correlations between the angular momentum of a fragment and its intrinsic excitation energy. The results of this work are especially relevant in view of the recent theoretical and experimental interest in the generation of angular momentum in fission. Specifically, we have determined an alignment of the fragments angular momenta in a direction perpendicular to the direction of motion. We interpret the lack of $n$-$\gamma$ angular correlations for fission fragments near closed shells as a weakening of the alignment process for spherical nuclei. Lastly, we have observed that statistical $\gamma$ rays are emitted isotropically, indicating that the average angular momentum removed by this radiation is small. These results, and the analysis tools presented in this work, represent a stepping stone for future analysis of $n$-$\gamma$ emission correlations and their connection to angular momentum properties.

Experimental generation and orbital angular momentum properties of focused field for vector vortex beams on Hybrid-order Poincaré sphere

Due to the anisotropic spatial polarization state and spiral phase distribution, the orbital angular momentum (OAM) of vector vortex beams (VVBs) has attracted increasingly attention in recent years. In this paper, the Hybrid-order Poincaré sphere VVBs are constructed by orthogonal left-handed (L-H) and right-handed (R-H) circularly polarized vortexes. The vector vortex light fields of different orders at the focal plane are collected and analyzed experimentally. Based on the vector diffraction integral theory, the effects of the topological charge and polarization order for VVBs, as well as the initial phase difference for L-H and R-H components on the OAM properties of focused field for the longitudinal component are investigated numerically. It is found that the OAM density distribution of the longitudinal component shows inner and outer two layers on the whole, and the layers mainly concentrate the corresponding L-H and R-H components respectively. Meanwhile, the signs of the topological charges for L-H and R-H components determine the positive and negative distributions of the inner and outer OAM density respectively. When the initial phase difference of L-H and R-H components ϕ0 changes in the range of 0–2π, the OAM density rotates the angle ϕ0/2 along the clockwise. However, the OAM of the longitudinal component does not change with ϕ0, and increases as the polarization order m increases when the topological charge of VVBs l>2. The results could be helpful to further explore the applications of the OAM for VVBs in the fields of optical detection and communication.

Novel ultrafast structured EUV/x-ray sources from nonlinear optics

Coherent extreme-ultraviolet (EUV)/x-ray laser sources, structured in their temporal/spectral, spatial and angular momentum properties are emerging as unique tools to probe the nanoworld. One of the key ingredients for the emergence of such sources is the extraordinary coherence in the up-conversion of infrared laser sources through the highly nonlinear process of high-order harmonic generation. In this contribution we will review the advances during the last decade that led to the generation of structured EUV/xray sources, such as circularly polarized attosecond pulses, harmonic vortices with time-varying orbital angular momentum, ultrafast vector and vector/vortex beams, tunable high-order harmonic combs or attosecond pulse trains with time-dependent polarization states. The use of such sources is being already applied to the investigation of chiral matter or magnetic materials. In the latter case, structured ultrafast sources are very promising to achieve a complete understanding of the electronic and spin interactions that govern sub-femtosecond magnetization dynamics.

Orbit of Galactic Globular Cluster of NGC 7089, NGC 7099, NGC 1851, Pal 1, and Pal 2 Around the Milky Way Galaxy Constructed from the Gaia EDR3

We report that we estimated the mean proper motion, the full space velocity of five Globular clusters, Pal 1, Pal2, NGC 1815, NGC 7089, and NGC 7099, using Gaia Early Data Release 3 (Gaia EDR3). We focus on these five clusters to measure proper motion, internal dispersion, and dynamical properties. Also, we have integrated the orbits using the potential of MWPotential2014 for five backyards Gyr. We have derived energy, angular momentum, and orbital properties for each cluster. In addition, we find that there are four metal-poor GCs with metallicity less than -1.1 dex and only one metal-rich GCs with metallicity greater than -1.0 dex. We estimate the distance modulus of these five globular clusters and examine isochrone fitting to the color-magnitude diagram from the PAdova and TRieste Stellar Evolution Code (PARSEC). Among five globular clusters, we have found that the NGC 7099 is the oldest globular cluster age of ∼ 13 Gyr with metallicity ([Fe/H]) of 2.19, whereas the youngest globular cluster we find in our sample is Pal 1 with metallicity ([Fe/H]) of -0.65 and age of ∼ 8 Gyr.

Probability property of orbital angular momentum distortion in turbulence

The probability property of the orbital angular momentum (OAM) distortion of the Bessel Gaussian beam propagating through the turbulence is investigated in this study. The mean and variance of the beam harmonic intensity are derived from the Rytov theory with a bias of less than 6% when compared with the data calculated by the phase-screen method. Based on these statistics, the probability density function (PDF) of the harmonic intensity fluctuation is proposed to characterize the randomness property of the beam OAM distortion, which agrees well with the result obtained from the phase-screen method. The PDF of the intensity difference between the fundamental and its adjacent crosstalk modes is derived. Furthermore, the probability of the OAM decoding error is also provided. This study not only facilitates beam OAM crosstalk characterization, but also provides the applicable condition of beam multiplexing for the beam parameter selection and the communication link design.