Net Orbital Angular Momentum Research Articles

Observation of Anomalous Orbital Angular Momentum Transfer in Parametric Nonlinearity.

Orbital angular momentum (OAM) conservation plays an important role in shaping and controlling structured light with nonlinear optics. The OAM of a beam originating from three-wave mixing should be the sum or difference of the other two inputs because no light-matter OAM exchange occurs in parametric nonlinear interactions. Here, we report anomalous OAM transfer in parametric upconversion, in which a Hermite-Gauss mode signal interacts with a specially engineered pump capable of astigmatic transformation, resulting in Laguerre-Gaussian mode sum-frequency generation (SFG). The anomaly here refers to the fact that the pump and signal both carry no net OAM, while their SFG does. We reveal experimentally that there is also an OAM inflow to the residual pump, having the same amount of that to the SFG but with the opposite sign, and thus holds system OAM conservation. This unexpected OAM selection rule improves our understanding of OAM transfer among interacting waves and may inspire new ideas for controlling OAM states via nonlinear optics.

Reading the tea leaves in the Mbh–M*,sph and Mbh–Re,sph diagrams: dry and gaseous mergers with remnant angular momentum

ABSTRACT We recently revealed that bulges and elliptical galaxies broadly define distinct, superlinear relations in the (black hole mass, Mbh)–(spheroid stellar mass, M*,sph) diagram or Mbh−M*,sph diagram, with the order-of-magnitude lower Mbh/M*,sph ratios in the elliptical galaxies due to major (disc-destroying, elliptical-building) dry mergers. Here, we present a more nuanced picture. Galaxy mergers, in which the net orbital angular momentum does not cancel, can lead to systems with a rotating disc. This situation can occur with either wet (gas-rich) mergers involving one or two spiral galaxies, e.g. NGC 5128, or dry (relatively gas-poor) collisions involving one or two lenticular galaxies, e.g. NGC 5813. The spheroid and disc masses of the progenitor galaxies and merger remnant dictate the shift in the Mbh−M*,sph and Mbh−Re,sph diagrams. We show how this explains the (previously excluded merger remnant) Sérsic S0 galaxies near the bottom of the elliptical sequence and core-Sérsic S0 galaxies at the top of the bulge sequence, neither of which are faded spiral galaxies. Different evolutionary pathways in the scaling diagrams are discussed. We also introduce two ellicular (ES) galaxy types, explore the location of brightest cluster galaxies and stripped ‘compact elliptical’ galaxies in the Mbh−M*,sph diagram, and present a new merger-built Mbh−M*,sph relation which may prove helpful for studies of nanohertz gravitational waves. This work effectively brings into the fold many systems previously considered outliers with either overly massive or undermassive black holes relative to the near-linear Mbh−M*,sph ‘red sequence’ patched together with select bulges and elliptical galaxies.

Asymmetric dynamics of the orbital angular momentum of vector beam propagation in nonlocal nonlinear media with -symmetric azimuthal potentials

We study the dynamics of vector beams (VBs) propagating in nonlocal nonlinear media with -symmetric azimuthal potentials. It is found that there is an apparent clockwise/counterclockwise symmetry (similar to left/right symmetry) breaking for the sub-beam with the opposite value of orbital angular momentum (OAM) of the VB. This symmetry breaking leads to the obvious asymmetric evolution of OAM, which results in net OAM generation. We numerically confirm that this phenomenon is induced by the -symmetric azimuthal potential.

Coherence-induced depolarization effects in polarization singular beams.

We demonstrate theoretically and experimentally coherence-induced depolarization effects in generic and higher index polarization singular beams endowed with C-point (or V-point) polarization singularity. The irradiance profiles and degree of polarization (DoP) distributions are found to be governed by spatial coherence length, polarization singularity index, and orbital angular momentum (OAM) of the superposition states of the beams. On reducing the coherence length, the DoP distribution in the V-point deteriorates uniformly. In contrast, C-point beams resist depolarization exhibiting anti-depolarization around the central core of the beam due to the nonzero net OAM of the beam. Interestingly, the polarization vortex structure remains preserved on reducing the spatial coherence length.

Ultrafast beam pattern modulation by superposition of chirped optical vortex pulses

As an extension of pulse shaping techniques using the space–time coupling of ultrashort pulses or chirped pulses, we demonstrated the ultrafast beam pattern modulation by the superposition of chirped optical vortex pulses with orthogonal spatial modes. The stable and robust modulations with a modulation frequency of sub-THz were carried out by using the precise phase control technique of the constituent pulses in both the spatial and time/frequency domains. The performed modulations were ultrafast ring-shaped optical lattice modulation with 2, 4 and 6 petals, and beam pattern modulations in the radial direction. The simple linear fringe modulation was also demonstrated with chirped spatially Gaussian pulses. While the input pulse energy of the pulses to be modulated was 360 upmu J, the output pulse energy of the modulated pulses was 115 upmu J with the conversion efficiency of sim 32%. Demonstrating the superposition of orthogonal spatial modes in several ways, this ultrafast beam pattern modulation technique with high intensity can be applicable to the spatially coherent excitation of quasi-particles or collective excitation of charge and spin with dynamic degrees of freedom. Furthermore, we analyzed the Poynting vector and OAM of the composed chirped OV pulses. Although the ring-shaped optical lattice composed of OV pulse with topological charges of pm , ell is rotated in a sub-THz frequency, the net orbital angular momentum (OAM) averaged over one optical period is found to be negligible. Hence, it is necessary to require careful attention to the application of the OAM transfer interaction with matter by employing such rotating ring-shaped optical lattices.

Vortex Laser Based on a Plasmonic Ring Cavity

The orbital angular momentum (OAM) of the structure light is viewed as a candidate for enhancing the capacity of information processing. Microring has advantages in realizing the compact lasers required for on-chip applications. However, as the clockwise and counterclockwise whispering gallery modes (WGM) appear simultaneously, the emitted light from the normal microring does not possess net OAM. Here, we propose an OAM laser based on the standing-wave WGMs containing clockwise and counterclockwise WGM components. Due to the inhomogeneous intensity distribution of the standing-wave WGM, the single-mode lasing for the OAM light can be realized. Besides, the OAM of the emitted light can be designed on demand. The principle and properties of the proposed laser are demonstrated by numerical simulations. This work paves the way for exploring a single-mode OAM laser based on the plasmonic standing-wave WGMs at the microscale, which can be served as a basic building block for on-chip optical devices.

Nonlinear Landau damping of plasma waves with orbital angular momentum.

We present, using three-dimensional particle-in-cell simulations, an observation that orbital angular momentum (OAM) is transferred to resonant electrons proportionally to longitudinal momentum when Laguerre-Gaussian plasma waves are subjected to Landau damping. A higher azimuthal mode number leads to a larger net orbital angular momentum transfer to particles traveling close to the phase velocity of the plasma wave, implying a population of electrons that are orbiting the same center of rotation as the plasma wave. This observation has implications on magnetic field excitation as a result of the formation and damping of OAM plasma waves. The energy distributions of electrons in damping Laguerre-Gaussian plasma waves are significantly changed as a function of azimuthal mode number. This leads to larger numbers of lower energy particles tending towards a significant narrowing of the energy distribution of accelerated particles.

Nonlinear relativistic electron Thomson Scattering for laser radiation with orbital angular momentum

The classical nonlinear incoherent Thomson Scattering (TS) power spectrum from free relativistic electrons moving in a laser beam with orbital angular momentum (OAM) is investigated. The main focus in this paper is on the TS process as a diagnostic technique for this type of beams. Linearly polarized incoming radiation and electrons of very low initial kinetic energy are considered. Averaged spectra from electrons randomly covering the transverse laser pattern have different shape in the case of a beam with OAM as compared with the TEM00 case (with vanishing net OAM). Hence, spectrally resolved measurements are needed to discriminate between both cases. If electrons are distributed over the laser spot as thin stripes at a given angle with respect to the polarization direction, computations show non-trivial angular dependencies of integrated power of a laser with OAM as compared with the TEM00 mode. An experimental test of the OAM state of a laser beam is proposed based on these results. The numerical code developed is general enough to deal with more complex polarization states of the laser beam and/or electrons having arbitrary initial kinetic energies.

Orbital angular momentum bistability in a microlaser.

Light's orbital angular momentum (OAM) is an unbounded degree of freedom emerging in helical beams that appears very advantageous technologically. Using chiral microlasers, i.e., integrated devices that allow generating an emission carrying a net OAM, we demonstrate a regime of bistability involving two modes presenting distinct OAM (ℓ=0 and ℓ=2). Furthermore, thanks to an engineered spin-orbit coupling of light in these devices, these modes also exhibit distinct polarization patterns, i.e., circular and azimuthal polarizations. Using a dynamical model of rate equations, we show that this bistability arises from polarization-dependent saturation of the gain medium. Such a bistable regime appears very promising for implementing ultrafast optical switches based on the OAM of light. As well, it paves the way for the exploration of dynamical processes involving phase and polarization vortices.

Radiation emission in laser-wakefields driven by structured laser pulses with orbital angular momentum

High-intensity X-ray sources are invaluable tools, enabling experiments at the forefront of our understanding of materials science, chemistry, biology, and physics. Laser-plasma electron accelerators are sources of high-intensity X-rays, as electrons accelerated in wakefields emit short-wavelength radiation due to betatron oscillations. While applications such as phasecontrast imaging with these betatron sources have already been demonstrated, others would require higher photon number and would benefit from increased tunability. In this paper we demonstrate, through detailed 3D simulations, a novel configuration for a laser-wakefield betatron source that increases the energy of the X-ray emission and also provides increased flexibility in the tuning of the X-ray photon energy. This is made by combining two Laguerre-Gaussian pulses with non-zero net orbital angular momentum, leading to a rotation of the intensity pattern, and hence, of the driven wakefields. The helical motion driven by the laser rotation is found to dominate the radiation emission, rather than the betatron oscillations. Moreover, the radius of this helical motion can be controlled through the laser spot size and orbital angular momentum indexes, meaning that the radiation can be tuned fully independently of the plasma parameters.

Tunable Orbital Angular Momentum Radiation from Angular-Momentum-Biased Microcavities.

Lasers and light emitters do not typically radiate fields with orbital angular momentum (OAM). Here we show that a suitable scheme of spatiotemporal modulation of a microring cavity laser can impart a synthetic angular momentum, resulting in beams with well-defined OAM. The phenomenon relies on a traveling wave modulation of the refractive index of the microring, which breaks the degeneracy of oppositely oriented whispering gallery modes. In parallel, a static structural grating on the periphery of the microring enables efficient vertical radiation. The proposed structure is inherently tunable and can also emit fields with zero net OAM while retaining toroidal energy distributions similar to the effect of an axicon lens.

Magetostatic amplifier with tunable maximum by twisted-light plasma interactions

Laser beams with Laguerre–Gaussian (LG) mode carry orbital angular momentum (OAM); however, when interacting with plasmas, the net angular momentum acquired by plasmas is basically zero after interaction. Here, we find when there exists a small magetostatic seed along the laser propagation direction, the barrier would be broken, giving rise to dramatic angular momentum transfer from LG-lasers to plasmas. Hence, the net OAM remaining in the plasmas system would continuously enhance the magetostatic field, until the corresponding Larmor frequency of electrons is comparable to the laser frequency in vacuum. Three-dimensional particle-in-cell simulations are performed to confirm our theory, producing spatial-uniform, temporal-stable and extremely-intense magetostatic fields.

Wavefront twisting by rotating black holes: Orbital angular momentum generation and phase coherent detection

In this paper we study wave propagation and scattering near a rotating black hole. In particular, we assume a coherent emission source near the black hole and investigate the wavefront distortion as seen by a distant observer. Near the observer, the propagating wave can be decomposed using the Laguerre-Gaussian mode basis and its wavefront distortion can be characterized by the decomposition coefficient. We find that this decomposition spectrum is symmetric for wave sources located near a Schwarzschild black hole, but is generically asymmetric if the host black hole is rotating. The spectrum asymmetry, or the net orbital angular momentum carried by the wave, is intimately related to the black hole spin and mass, the wave frequency and the locations of the source and the observer. We present semi-analytical expressions and numerical results for these parameter-dependences. If the radiation is temporally coherent, our results show that the secondary images of the source can be almost as bright as its primary image. In the case of temporally-incoherent radiation, we show that the non-fundamental spectrum components could be resolved by spatially-separated telescopes, although that would be degenerate with the telescope direction. Finally, our results suggest that the black-hole-induced spectrum asymmetry is generally too weak to be observed in radio astronomy, even if the observer were located near a caustic point.

Inverse Faraday Effect with Linearly Polarized Laser Pulses

The inverse Faraday effect is usually associated with circularly polarized radiation; here, we show that it can also occur for linearly polarized radiation. The quasistatic axial magnetic field generated by a laser propagating in plasma can be calculated by considering both the spin and the orbital angular momenta of the laser pulse. A net spin is present when the radiation is circularly polarized and a net orbital angular momentum is present if there is any deviation from perfect rotational symmetry. The orbital angular momentum gives an additional contribution to the axial magnetic field that can enhance or reduce the effect usually attributed to circular polarization and strongly depends on the intensity profile of the Laguerre-Gaussian modes involving the azimuthal and radial mode numbers.

Phenomenological analysis of the nucleon spin contents and their scale dependence

In the past few years, a great deal of evidence has accumulated which indicates that the gluon polarization inside the nucleon is likely to be small at least at the low renormalization scales. On the other hand, the recent lattice QCD analyses suggest that the net orbital angular momentum carried by the quarks is nearly zero. There is also some indication, noted by Brodsky and Gardner based on the COMPASS observation of small single-spin asymmetry on the isoscalar deuteron target, that the gluon orbital angular momentum inside the nucleon is likely to be small. Naively combining all these observations, we are led to a rather embarrassing conclusion that the nucleon constituents altogether do not carry an adequate amount of angular momentum saturating the total nucleon spin. We show that this somewhat confused state of affairs can be cleared up only by paying careful attention to the scale dependencies of the nucleon spin decomposition.

Generalized Bardeen-Cooper-Schrieffer States and the Proposed Low-Temperature Phase of LiquidHe3

Particle interactions in a Fermi gas may be such as to attract pairs near the Fermi surface more strongly in $l=1, 2, 3$ or higher states than in the simple spherically symmetrical $s$ state. In that case the Bardeen-Cooper-Schrieffer condensed state must be generalized, and the resulting state is an anisotropic superfluid. We have studied the properties of this type of state in considerable detail, especially for $l=1 \mathrm{and} 2$. We have derived expressions for the energy, the moment of inertia, the magnetic susceptibility and the specific heat. We also derive the density correlation function and the density-current density correlation; in some cases the latter implies that the liquid has net surface currents and a net orbital angular momentum. The ground state for $l=2$ is different from those previously considered, and has cubic symmetry and no net angular momentum. A general method for replacing the possibly rather complicated potential by a simple scattering matrix is given. A brief discussion of possible collective effects is included. We apply our results to liquid ${\mathrm{He}}^{3}$; after correction for scattering by a method due to Suhl, it is found that the predicted transition should take place below 0.02\ifmmode^\circ\else\textdegree\fi{}K. Other possible applications are suggested.