Finite Orbital Angular Momentum Research Articles

Transporting the Optical Chirality through the Dynamical Barriers in Optical Microcavities

AbstractIn mixed phase space structures, the stable islands and chaotic sea are separated by dynamical barriers. Considering the small tunneling rate, intuitively, the properties of long‐lived resonances within stable islands are supposed to be less affected by the perturbations in chaotic sea. Here, the modifications on the chaotic sea which can be transported through dynamical barriers and strongly affect long‐lived resonances within stable islands are experimentally demonstrated. In waveguide‐connected quadruple microdisks (WCQMs), the finite orbital angular momentum and the propagation directions of long‐lived resonances within stable islands could be changed and controlled by the waveguides connecting to the chaotic sea. The numerical calculations and the corresponding 4 × 4 non‐Hermitian Hamiltonian theoretical model match the experimental results well and demonstrate the essential roles of asymmetrical scattering in the chaotic sea and chaotic‐to‐regular tunneling. This research will be interesting for fundamental studies on quantum chaos and practical applications in optical sensing.

Growth rate instabilities with finite OAM in non-Maxwellian plasmas: Saturn's magnetosphere

The electrostatic twisted waves with finite orbital angular momentum (OAM) states are studied in a doubly κ− distributed plasma, containing superthermal hot and cool electrons in addition to positive ions. In the framework of Vlasov-Poisson model, the expression for a generalized plasma response function is derived by considering the Laguerre-Gaussian (LG) solutions for the perturbed distribution function and electrostatic potential. Modified dispersion relations and instability growth rates of the twisted electron-acoustic (TEA) and twisted ion-acoustic (TIA) waves are derived in an unmagnetized double κ−distributed superthermal plasma. For parametric analysis, the effects of spectral indices (κh and κc), twisted parameter (η) and hot-to-cool electron density ratio (f) are numerically examined on the real and imaginary frequencies of TEA and TIA waves. The threshold conditions for TEA and TIA waves are also analyzed to investigate their instability growth rates both analytically and numerically. The relevance of the present results to Saturn's outer magnetosphere is discussed, where two groups of electrons with different finite temperatures are modeled by the double κ−distributions.

Kinetic study of twisted electron plasma waves in q-nonextensive plasmas

Relying on the kinetic description, the electron-plasma (EP) waves carrying distinct orbital angular momentum (OAM) states are investigated in q-nonextensive plasma consisting of inertial electrons with static background of positive ions. Deriving an expression for the dielectric response function, the dispersion relation and Landau damping frequencies are analyzed both analytically and numerically. It is found that weakly damped regions of the EP waves strongly depend on the non-extensivity (q) and twist (η) parameters. The presence of finite OAM results in the longer range of the values in terms of normalized wavenumber for which the EP waves are weakly damped. However, maximum range of weakly damped wavenumbers is obtained when the longitudinal and azimuthal wavenumbers are equal, viz. twist parameter becomes unity. For very large twist parameter, the results of the twisted wave model coincide with the planer wave model.

Twisted waves and instabilities in a permeating dusty plasma

New features of the twisted dusty plasma modes and associated instabilities are investigated in permeating plasmas. Using the Vlasov–Poisson model equations, a generalized dispersion relation is obtained for a Maxwellian distributed plasma to analyse the dust-acoustic and dust-ion-acoustic waves with finite orbital angular momentum (OAM) states. Existence conditions for damping/growth rates are discussed and showed significant modifications in twisted dusty modes as compared to straight propagating dusty modes. Numerically, the instability growth rate, which depends on particle streaming and twist effects in the wave potential, is significantly modified due to the Laguerre–Gaussian profiles. Relevance of the study to wave excitations due to penetration of solar wind into cometary clouds or interstellar dusty plasmas is discussed.

Twisted ion waves carrying orbital angular momentum in a dense pair-ion plasma

Electrostatic ion oscillations carrying finite orbital angular momentum are investigated in an unmagnetized dense quantum degenerate plasma containing electrons, positrons, and ions. Instead of conventional plane wave solutions, Laguerre-Gaussian (LG) type density and potential profiles are considered to solve the wave equation in paraxial approximation. The solutions exhibit finite orbital angular momentum states associated with the ion waves. The electric field structure, energy density, and orbital angular momentum density are theoretically examined. In addition, the numerical analysis is carried out to study the effects of positron concentration on the waves for various radial and angular mode numbers and azimuthal angles. The variation of LG potential and azimuthal field amplitudes with positrons is elaborated. The study has relevance with the possible phonon modes and twisted excitation in dense plasmas in the environment of radiation beams associated with optical vortices or stimulated Brillouin scattering.

Twisted electrostatic waves in a self‐gravitating dusty plasma

A generalized response (dielectric) function for twisted electrostatic waves is derived for an un‐magnetized self‐gravitating thermal dusty plasma, whose constituents are the Boltzmann‐distributed electrons and positive ions in the presence of negatively charged micrometre‐sized massive dust particulates. For this purpose, a set of Vlasov–Poisson coupled equations is solved along with the perturbed Laguerre–Gauss distribution function, as well as the electrostatic and gravitational potentials in the limit of paraxial approximation. For plane wave solution, the wavefronts of the dust‐acoustic (DA) wave are assumed to have a constant phase with electric and gravitational field lines propagating straight along the propagation axis. On the other hand, non‐planar wave solutions show helical (twisted) wavefronts, in which field lines spiral around the propagation axis owing to the azimuthal velocity component to account for the finite orbital angular momentum (OAM) states. The dispersion relation and damping rate for twisted DA waves are studied both analytically and numerically. It is shown that finite OAM states, the dust to electron temperature ratio, and dust self‐gravitation effects significantly affect the linear dispersion and Landau damping frequencies. In particular, the phase speed of twisted DA waves is reduced with the variation of the twist parameter η (= k/lqϕ), dust concentration δ (= nd0/ni0), and dust self‐gravitation α (= ωJd/ωpd). The relevance of our findings to interstellar dust clouds is also discussed for micrometre‐sized massive dust grains.

Focusing light with orbital angular momentum by circular array antenna.

We experimentally demonstrated focusing of light with orbital angular momentum (OAM) using an 8-element circular array of linear antennas. A spiral phase plate was used to generate a vortex beam with an OAM of ħ in the terahertz (THz) frequency region. We used THz near-field microscope to directly measure the phase vortex. A beam profile with a center dark spot and 2π phase rotation was observed in the small center gap region of the circular array antenna after the vortex beam excitation. The beam size is reduced by a factor of 3.4 ± 0.2. Half-wave resonance of the antenna element is responsible for the focusing function, indicating the scalability of this method to other frequency regions. This method will enable deep subwavelength focusing of light with OAM and eliminate the obstacle for the observation of the dipole forbidden transition with finite OAM of the vortex beam.

Vortex type oscillations in a multi-component plasma

Abstract Owing to the fact that the helical wave structures with azimuthal phase dependence can exist and propagate longitudinally in a plasma, a kind of linearized ion-acoustic vortices or phonon modes are studied with orbital angular momentum in an unmagnetized uniform multi-component plasma. Laguerre-Gaussian (LG) type density and potential perturbations are considered to solve the wave equation in paraxial approximation. The solutions determine finite orbital angular momentum states associated with the ion motion giving rise to phonon modes exhibited as vortex-like (twisted) ion-acoustic waves. The electric field energy and angular momentum density are calculated and the effects of the twist due to azimuthal phase components of density and potential profiles are elaborated. The role of density balance and other parameters like radial and angular mode numbers and phase angle are highlighted in the numerical analysis. The relevance of the work with plasma motion in the environment of twisting fields is discussed.

Controlling multiple filaments by relativistic optical vortex beams in plasmas.

Filamentation dynamics of relativistic optical vortex beams (OVBs) propagating in underdense plasma is investigated. It is shown that OVBs with finite orbital angular momentum (OAM) exhibit much more robust propagation behavior than the standard Gaussian beam. In fact, the growth rate of the azimuthal modulational instability decreases rapidly with increase of the OVB topological charge. Thus, relativistic OVBs can maintain their profiles for significantly longer distances in an underdense plasma before filamentation occurs. It is also found that an OVB would then break up into regular filament patterns due to conservation of the OAM, in contrast to a Gaussian laser beam, which in general experiences random filamentation.

Twisted Landau damping rates in multi-component dusty plasmas

Keeping in view the kinetic treatment for plasma particles, the electrostatic twisted dust-acoustic (DA) and dust-ion-acoustic (DIA) waves are investigated in a collisionless unmagnetized multi-component dusty plasma, whose constituents are the electrons, singly ionized positive ions, and negatively charged massive dust particulates. With this background, the Vlasov–Poisson equations are coupled together to derive a generalized dielectric constant by utilizing the Laguerre-Gaussian perturbed distribution function and electrostatic potential in the paraxial limit. The dispersion and damping rates of twisted DA and DIA waves are analyzed with finite orbital angular momentum states in a multi-component dusty plasma. Significant modifications concerning the real wave frequencies and damping rates appeared with varying twisted dimensionless parameter and dust concentration. In particular, it is shown that dust concentration enhances the phase speed of the DIA waves in contrary to DA waves, whereas the impact of twisted parameter reduces the frequencies of both DA and DIA waves. The results should be useful for the understanding of particle transport and trapping phenomena caused by wave excitation in laboratory dusty plasmas.

Electrostatic twisted modes in multi-component dusty plasmas

Various electrostatic twisted modes are re-investigated with finite orbital angular momentum in an unmagnetized collisionless multi-component dusty plasma, consisting of positive/negative charged dust particles, ions, and electrons. For this purpose, hydrodynamical equations are employed to obtain paraxial equations in terms of density perturbations, while assuming the Gaussian and Laguerre-Gaussian (LG) beam solutions. Specifically, approximated solutions for potential problem are studied by using the paraxial approximation and expressed the electric field components in terms of LG functions. The energy fluxes associated with these modes are computed and corresponding expressions for orbital angular momenta are derived. Numerical analyses reveal that radial/angular mode numbers as well as dust number density and dust charging states strongly modify the LG potential profiles attributed to different electrostatic modes. Our results are important for understanding particle transport and energy transfer due to wave excitations in multi-component dusty plasmas.

Spatio-temporal coherence mapping of few-cycle vortex pulses.

Light carrying an orbital angular momentum (OAM) displays an optical phase front rotating in space and time and a vanishing intensity, a so-called vortex, in the center. Beyond continuous-wave vortex beams, optical pulses with a finite OAM are important for many areas of science and technology, ranging from the selective manipulation and excitation of matter to telecommunications. Generation of vortex pulses with a duration of few optical cycles requires new methods for characterising their coherence properties in space and time. Here we report a novel approach for flexibly shaping and characterising few-cycle vortex pulses of tunable topological charge with two sequentially arranged spatial light modulators. The reconfigurable optical arrangement combines interferometry, wavefront sensing, time-of-flight and nonlinear correlation techniques in a very compact setup, providing complete spatio-temporal coherence maps at minimum pulse distortions. Sub-7 fs pulses carrying different optical angular momenta are generated in single and multichannel geometries and characterised in comparison to zero-order Laguerre-Gaussian beams. To the best of our knowledge, this represents the shortest pulse durations reported for direct vortex shaping and detection with spatial light modulators. This access to space-time coupling effects with sub-femtosecond time resolution opens new prospects for tailored twisted light transients of extremely short duration.

Kinetic study of ion-acoustic plasma vortices

The kinetic theory of electron plasma waves with finite orbital angular momentum has recently been introduced by Mendonca. This model shows possibility of new kind of plasma waves and instabilities. We have extended the theory to ion-acoustic plasma vortices carrying orbital angular momentum. The dispersion equation is derived under paraxial approximation which exhibits a kind of linear vortices and their Landau damping. The numerical solutions are obtained and compared with analytical results which are in good agreement. The physical interpretation of the ion-acoustic plasma vortices and their Landau resonance conditions are given for typical case of Maxwellian plasmas.

Finite orbital angular momentum states and Laguerre–Gaussian potential in two-temperature electron plasmas

Electron-acoustic waves are studied with orbital angular momentum (OAM) in an unmagnetized collisionless uniform plasma, whose constituents are the Boltzmann hot electrons, inertial cold electrons and stationary ions. For this purpose, we employ the fluid equations to obtain a paraxial equation in terms of cold electron density perturbations, which admits both the Gaussian and Laguerre–Gaussian (LG) beam solutions. Furthermore, an approximate solution for the electrostatic potential problem is found, which also allows us to express the components of the electric field in terms of LG potential perturbations. Calculating the energy flux of the electron-acoustic waves, an OAM density for these waves is obtained. Numerically, it is found that the parameters, such as, azimuthal angle, radial and angular mode numbers, and the beam waist strongly modify the LG potential profiles associated with electron-acoustic waves. The present results should be helpful to study the trapping and transportation of plasma particles and energy as well as to understand the electron-acoustic mode excitations produced by the Raman backscattering of laser beams in a uniform plasma.

Anisotropic plasmon-coupling dimerization of a pair of spherical electron gases

We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled non-concentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. We demonstrate the anisotropic inter-sphere Coulomb coupling in space and present semi-analytic results in the random-phase approximation both perpendicular and parallel to the axis of quantization. For the incidence of light with a finite orbital or spin angular momentum, the magnetic field generated from an induced oscillating electric dipole on one sphere can couple to an induced magnetic dipole on another sphere in a way that is dependent on whether the direction is parallel or perpendicular to the probe E field. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart.

Inverse Faraday effect with plasmon beams

The angular momentum conservation equation is considered for an electron gas, in the presence of Laguerre–Gaussian (LG) plasmons propagating along the z-axis. The LG plasmons carry a finite orbital angular momentum despite longitudinal nature, which can be partly transfered to the electrons. For short timescales, such that ion motion can be neglected, plasmons primarily interact with the electrons, creating an azimuthal electric field and generating an axial magnetic field. This effect can be called an inverse Faraday effect due to plasmons. Numerically, it is found that the magnitude of the magnetic field enhances with the plasmon density or with the energy of the electron plasma waves. A comparison of the magnitudes of the axial magnetic field is made for the inverse Faraday effect excited by both plasmons and transverse photons.

Bistable and Addressable Localized Vortices in Semiconductor Lasers

We demonstrate experimentally that localized emission states in coupled broad-area semiconductor lasers can carry a finite orbital angular momentum. The resulting structures therefore possess the chirality of optical vortices together with the properties of localized structures in dissipative systems, namely, the coexistence with a low intensity homogeneous emission and the mutual independence. These results open the way to the realization of arrays of optically addressable and bistable chiral laser pixels.

Landé-like formula for thegfactors of hole-nanowire subband edges

We have analyzed theoretically the Zeeman splitting of hole-quantum-wire subband edges. As is typical for any bound state, their $g$ factor depends on both an intrinsic $g$ factor of the material and an additional contribution arising from a finite bound-state orbital angular momentum. We discuss the quantum-confinement-induced interplay between bulk-material and orbital effects, which is nontrivial due to the presence of strong spin-orbit coupling. A compact analytical formula is provided that elucidates this interplay and can be useful for predicting Zeeman splitting in generic hole-wire geometries.

Global quark polarization in noncentralA+Acollisions

Partons produced in the early stage of noncentral heavy-ion collisions can develop a longitudinal fluid shear because of the unequal local number densities of participant target and projectile nucleons. Under such fluid shear, local parton pairs with nonvanishing impact parameters have finite local relative orbital angular momentum along the direction opposite to the reaction plane. Such a finite relative orbital angular momentum among locally interacting quark pairs can lead to global quark polarization along the same direction because of spin-orbital coupling. Local longitudinal fluid shear is estimated within both the Landau fireball and the Bjorken scaling model of initial parton production. Quark polarization through quark-quark scatterings with the exchange of a thermal gluon is calculated beyond the small-angle scattering approximation in a quark-gluon plasma. The polarization is shown to have a nonmonotonic dependence on the local relative orbital angular momentum dictated by the interplay between electric and magnetic interactions. It peaks at a value of relative orbital angular momentum which scales with the magnetic mass of the exchanged gluons. With the estimated small longitudinal fluid shear in semiperipheral Au+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider (RHIC), the final quark polarization is found to be small $|{P}_{q}|<0.04$ in the weak coupling limit. Possible behavior of the quark polarization in the strong coupling limit and implications on the experimental detection of such global quark polarization at RHIC and CERN Large Hadron Collider (LHC) are also discussed.

Generation of Spatially Broadband Twin Beams for Quantum Imaging

We generate spatially multimode twin beams using 4-wave mixing in a hot atomic vapor in a phase-insensitive traveling-wave amplifier configuration. The far-field coherence area measured at 3.5 MHz is shown to be much smaller than the angular bandwidth of the process and bright twin images with independently quantum-correlated subareas can be generated with little distortion. The available transverse degrees of freedom form a high-dimensional Hilbert space that we use to produce quantum-correlated twin beams with finite orbital angular momentum.