Interaction Of Orbital Angular Momentum Research Articles

High-accuracy transverse translation velocimeter enabled by OAM-assisted dual-point transverse Doppler effect

Laser Doppler velocimetry has developed as a powerful enabling technology within a myriad of realistic scenarios. While existing toolkits that exploit optical vortex allow access to longitudinal linear velocimetry and/or transverse rotational velocimetry of a motion object, achieving high-accuracy transverse translation velocimetry is still an open challenge. Here, we overcome this restriction by breaking the prevailing single-point metrological paradigm, instead employing dual-point transverse Doppler effect of customized orbital-angular-momentum (OAM) light fields as our pathway. We show that the dual-point transverse Doppler shifts can be generated and detected in a single velocimetry geometry for the first time, thereby enabling the determination of arbitrary transverse translation velocity. In particular, we demonstrate that the limited frequency resolution causes the velocimetry deviation and increasing the sampling duration can thus effectively improve velocimetry accuracy. As a consequence, a high-accuracy transverse translation velocimeter with a relative error below 0.08% has been well-established. Our validated findings not only provide a profound insight into the interaction of OAM with macroscopic motion object but also may find potential applications in clinical medicine, astronomical observation, and industrial manufacturing.

Generating superposed terahertz perfect vortices via a spin-multiplexed all-dielectric metasurface

Perfect optical vortices (POVs), characterized as a ring radius independent of topological charge (TC), possess extensive application in particle manipulation and optical communication. At present, the complex and bulky optical device for generating POVs has been miniaturized by leveraging the metasurface, and either spin-dependent or spin-independent POV conversions have been further accomplished. Nevertheless, it is still challenging to generate superposed POVs for incidences with orthogonal circular polarization. Here, a spin-multiplexed all-dielectric metasurface method for generating superposed POVs in the terahertz frequency range is proposed and demonstrated. By using the multiple meta-atom comprised structure as the basic unit, the complex amplitude of two superposed POVs is modulated, decoupled, and subsequently encoded to left- and right-handed circular polarization incidences. Furthermore, two kinds of metasurfaces are fabricated and characterized to validate this controlling method. It is demonstrated that the measured intensity and phase distributions match well with the calculation of the Rayleigh–Sommerfeld diffraction integral, and the radius of superposed POVs is independent of TCs. This work provides promising opportunities for developing ultracompact terahertz functional devices applied to complex structured light generation and terahertz communication, and exploring sophisticated spin angular momentum and orbital angular momentum interactions like the photonic spin-Hall effect.

Phase-dependent light-induced torque

We study an optical torque emerging through the interaction of orbital angular momentum with atoms/molecules in quantum systems with closed-loop interactions. First, we show how atoms with a closed-loop scheme experience a well-controlled torque whose features depend on the relative phase of applied fields. Such a controllable torque, along with simplicity of tuning the relative phase, can simplify the implementation of current flows in atomic Bose–Einstein condensates. Moreover, we calculate the optical torque exerted on chiral molecules with cyclic-transition structures and find that the enantiomers can experience different torques for proper choice of the parameters. Such features may find applications in sorting and separation of enantiomers.

Spatially and spectrally resolved orbital angular momentum interactions in plasmonic vortex generators

Understanding the near-field electromagnetic interactions that produce optical orbital angular momentum (OAM) is crucial for integrating twisted light into nanotechnology. Here, we examine the cathodoluminescence (CL) of plasmonic vortices carrying OAM generated in spiral nanostructures. The nanospiral geometry defines a photonic local density of states that is sampled by the electron probe in a scanning transmission electron microscope (STEM), thus accessing the optical response of the plasmonic vortex with high spatial and spectral resolution. We map the full spectral dispersion of the plasmonic vortex in spiral structures designed to yield increasing topological charge. Additionally, we fabricate nested nanospirals and demonstrate that OAM from one nanospiral can be coupled to the nested nanospiral, resulting in enhanced luminescence in concentric spirals of like handedness with respect to concentric spirals of opposite handedness. The results illustrate the potential for generating and coupling plasmonic vortices in chiral nanostructures for sensitive detection and manipulation of optical OAM.

Elastic Scattering of Electron Vortex Beams in Magnetic Matter.

Elastic scattering of electron vortex beams on magnetic materials leads to a weak magnetic contrast due to Zeeman interaction of orbital angular momentum of the beam with magnetic fields in the sample. The magnetic signal manifests itself as a redistribution of intensity in diffraction patterns due to a change of sign of the orbital angular momentum of the electron vortex beam. While in the atomic resolution regime the magnetic signal is most likely under the detection limits of present transmission electron microscopes, for electron probes with high orbital angular momenta, and correspondingly larger spatial extent, its detection is predicted to be feasible.

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu2+, Mn2+ and Fe3+ in ZnWO4 are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d9 and 3d5 complexes. The impurity centres on Zn2+ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C2 axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu2+, Mn2+ and Fe3+, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn2+ site with obvious axial elongation (∼0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ≪ 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.

INVESTIGATIONS OF THE EPR PARAMETERS FORDy3+CENTER INZrSiO4CRYSTAL

The electronic paramagnetic resonance g factors g‖, g⊥of Dy3+centers and the hyperfine structure constants A‖and A⊥of161Dy3+and163Dy3+isotopes in ZrSiO4crystal are theoretically studied from the perturbation formulas of the spin Hamiltonian parameters for a 4f9ion in tetragonal symmetry. In these formulas, the contributions to g factors due to the J-mixing among the ground6H15/2, the first excited6H13/2and second excited6H11/2states via crystal-field interaction, the admixtures among the states with the same J value via spin-orbit coupling interaction and the interactions between the lowest Kramers doublet Γγ and other 20 Kramers doublets ΓXwithin the states6HJ(J=15/2, 13/2 and 11/2) via crystal-field and orbital angular momentum interactions are considered. The calculated EPR parameters show reasonable agreement with the observed values. The results are discussed.

The g Factors and Defect Structure of Orthorhombic Dy3+ Ions in CdF2 Crystals

The electronic paramagnetic resonance g factors gx, gy and gz of an orthorhombic Dy3+ center in CdF2 are studied by the perturbation formulae of the g factors for a 4f9 ion in orthorhombic symmetry. In these formulae, the contributions to g factors due to J-mixing among the ground 6H15/2, the first excited 6H13/2 and the second excited 6H11/2 states via crystal-field interaction, lead to admixtures among the states with the same J-value via spin-orbit coupling. In addition, the admixtures between the lowest Kramers doublet Γγ and other 20 Kramers doublets Γ x within the states 6HJ (J = 15/2, 13/2 and 11/2) via crystal-field and orbital angular momentum interactions are considered. In the above orthorhombic center, the impurity Dy3+ is associated with co-doped crystals with alkali metal ions in the [110] axis. By analyzing the g factors, we find that the impurity Dy3+ ion does not reside in the ideal Cd2+ site but moves towards the co-doped alkali metal ion along the [110] direction by a displacement Δ Z ≈ 0.272 Å.

CALCULATION OF EPR PARAMETERSgFACTORS FORCe3+ION INYBa2Cu3O6+xSUPERCONDUCTOR

The perturbation formulae of EPR parameters g factors gx, gyand gzfor the lowest Kramers doublet of 4f1ion in orthorhombic symmetry are established. In these formulae, the contributions of the covalency effects, the admixture between J=7/2 and J=5/2 states as well as the second-order perturbation (which is not considered in the previous works) via crystal-field and orbital angular momentum interactions are all included. According to these formulae, and by using the superposition model, the g-factors gx, gyand gzfor Ce3+center in YBa2Cu3O6+ xsuperconductor are calculated. The results are discussed.

Study of the anisotropic g-factors for Dy 3+ ions in DyBa 2Cu 3O 6+ x superconductor

The perturbation formulae of EPR g-factors g x , g y and g z for the lowest Kramers doublet of 4f 9 ion in rhombic symmetry are established. In these formulae, the contributions to g-factors are considered due to J-mixing among the ground 6H 15/2, the first excited 6H 13/2 and second excited 6H 11/2 states via crystal-field interaction, the admixtures among the states with the same J value via spin–orbit coupling interaction and those between the lowest Kramers doublet Γr and other 20 Kramers doublets Γ X within the states 6H J ( J=15/2, 13/2 and 11/2) via crystal-field and orbital angular momentum interactions. According to these formulae, the g-factors g x , g y and g z for Dy 3+ ions in DyBa 2Cu 3O 6+ x superconductor are calculated. The results are in reasonable agreement with the observed values.

EPR g factors of trigonal Dy3+ center in ThO2 crystal

The perturbation formulas of EPR g factors g‖ and g⊥ for the lowest Kramers doublet of 6H15/2 of a 4f9 ion in trigonal symmetry are established in this paper. In these formulas, besides the contribution due to the interaction within the lowest 6H15/2 manifolds considered in the previous papers, the contributions due to the J-mixing among the 6H15/2, 6H13/2 (first excited state) and 6H11/2 (second excited state) via crystal-field interaction, the admixtures among the states with the same J value via spin-orbit coupling interaction and those between the lowest Kramers doublet Γγ and other Kramers doublets ΓX within the states 6HJ (J=15/2,13/2,11/2) via crystal-field and orbital angular momentum interactions are included. From these formulas, the g factors g‖ and g⊥ for the trigonal Dy3+ center in ThO2 crystal are calculated. The results are discussed.