Laguerre-Gaussian Laser Beams Research Articles

Chirality-enabled optical dipole potential energy for two-level atoms

We consider the optical dipole potential energy, which arises from the interaction of a two-level atom with a circularly polarized Laguerre-Gaussian laser beam of small waist. The beam is characterized by the existence of a longitudinal electric field component which is responsible for the appearance of a chiral term in the optical dipole potential energy. This term reverses sign if either the winding number or the wave polarization of the beam reverses sign. We propose a scheme of a bi-chromatic vortex interaction with the two-level atom in which the resulting optical dipole potential is fully chiral.

Electron Bernstein wave excitation and heating by nonlinear interactions of Laguerre and Hermite Gaussian laser beams in a magnetized plasma

In this paper, we have studied the nonlinear interaction of Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) laser beams in magnetized plasma, and electron Bernstein wave (EBW) excitation and heating of electron is evaluated. The beat wave with frequency ω=(ω1-ω2) and wave vector k→=k→1+k→2, exert a pondermotive force on electrons. This force has potential to drive electron Bernstein wave in both homogeneous and inhomogeneous plasma. The convective loss of waves in inhomogeneous plasma is discussed. An analytical theory is also developed for the anomalous heating of electrons. The variation of normalized potential and power distribution with normalized frequency and beam direction is demonstrated to excite EBWs under resonance conditions. The laser parameters such as beam width, modes index, and frequency affect the EBWs excitation and rate of electron heating. In addition, the spatial tuned shape of beams and the optimum laser beams parameter is proposed for much more exaction and heating.

Ground-state phase diagram and excitation spectrum of a Bose-Einstein condensate with spin-orbital-angular-momentum coupling

We investigate the ground-state phase diagram and excitation spectrum of an interacting spinor Bose-Einstein condensate with spin-orbital-angular-momentum (SOAM) coupling realized in recent experiments by introducing atomic Raman transition with a pair of copropagating Laguerre-Gaussian laser beams that carry different orbital angular momenta (OAM) [Chen et al., Phys. Rev. Lett. 121, 113204 (2018) and Zhang et al., Phys. Rev. Lett. 122, 110402 (2019)]. Because of the ground-state degeneracy of the single-particle Hamiltonian at vanishing detuning, several angular-stripe phases, which are superposition of states with different angular quantum numbers, appear in the phase diagram. However, these phases normally exist at small detuning, which makes them hard to be probed in experiments. We show that for a large OAM difference of the laser beams, an asymmetric kind of angular-stripe phase can exist even at large detuning. The excitation spectra in different phases exhibit distinct features: In the angular-stripe phase there exist two gapless bands corresponding to the broken U(1) and rotational symmetries, while in the half-skyrmion phase the gapless band exhibits a roton-like structure. Our predictions of the angular-stripe phases and the low-energy excitations can be examined in recently realized BECs with SOAM coupling.

Generation and regulation of electron vortices in an underdense plasma by Laguerre-Gaussian laser pulses

An analytical model is presented to control the angular momentum of electrons by impinging Laguerre-Gaussian (LG) laser beam in an underdense warm plasma. During intense laser-plasma interaction, radiative effects of intense long laser pulses become important, which are analyzed under quasi-static approximation. It is shown that the spot size of the laser beam controls the depth of doughnut-shaped ion-channel created in the plasma. Due to inhomogeneity of plasma density, the nonlinear beating of two fast-time-scale laser fields results in the generation of slow-time-scale “dc” magnetic fields. These fields confine the fast electrons in relativistic laser channel and support their direct coupling with the lasers’ fields. Inside the doughnut-shaped channel, the electrons are found to execute transverse betatron oscillations. At resonance, the laser field efficiently transfers the energy and momentum to the plasma electrons which can be controlled by the lasers and plasma parameters. The angular momentum of the electrons is also found to be dependent on the magnitude of the quasi-static axial magnetic field. Moreover, regulation of the electron angular momentum can explore various interesting areas including coherent vortex beam generation via synchrotron radiation process.

Diffraction of (l,n)th mode Laguerre-Gaussian laser beam by a curved fork-shaped grating

The transformation of (l,n)th mode Laguerre-Gaussian (LG) laser beam, in the process of its diffraction by a curved fork-shaped grating with topological charge p, is theoretically studied. The analytical solutions for the diffracted wave field amplitudes are derived in the Fresnel regime and in the back focal plane of a convergent lens. The zeroth-diffraction order is found as (l,n)th mode LG beam. The higher, th diffraction-order beam is described in the radial direction through a product of the Gauss-doughnut function by the double sum of hypergeometric Kummer functions. Its topological charge can be increased or reduced compared to that of the incident beam, or it can be equal to zero. The radial intensity distributions are plotted and compared to the corresponding ones valid when a fork-shaped grating is used. The results are specialized for the cases of incident LG beams of modes (l, n = 0), (l = 0,n) and (l = 0,n = 0) i.e. Gaussian mode.

Average power of Laguerre-Gaussian laser beams along the atmospheric path

Numerical simulation of various mode propagation of a Laguerre-Gaussian laser beam reflected by a flat mirror at different intensities of optical turbulence along the atmospheric path are carried out. It is found that the strength of optical turbulence increases, the average power of the received signal at the end of a location path exceeds ones at the end of a propagation path double distance in the forward direction. The greater the mode order of the Laguerre-Gaussian beam, the stronger the effect.

Vortex mode excitation in a multimode fiber by the Laguerre-Gaussian laser beams

The coupling of Laguerre-Gaussian (LG) vortex modes into the Bessel vortex modes in a multimode fiber was analyzed using the vector form of LG beams. A formula for estimating the transmission coefficients of the excited vortex modes was developed. Analytical calculation results show that the coupling coefficients are affected by the focus size. With proper beam waist radius most the power of incident vortex mode can couple into single vortex mode. The numerical simulation was also carried out, and the simulation results are in good agreement with those from the analytical model.

Fresnel and Fraunhofer diffraction of (l,n)th-mode Laguerre–Gaussian laser beam by a fork-shaped grating

ABSTRACTWe study theoretically the problem of diffraction of a Laguerre-Gaussian (LG) laser beam with radial mode number n and azimuthal mode number l, by a fork-shaped grating (FG) with integer topological charge (TC) p. The diffracted wave field amplitude and intensity are calculated at any distance behind the FG and in the back focal plane of a convergent lens. The zeroth diffraction order is obtained as an (l,n)th-mode LG beam. The higher, mth diffraction order beam is described in the radial direction through a product of the Gauss-doughnut function of order by the finite sum of hypergeometric Kummer functions. It can be a vortex beam with increased or reduced TC compared to that of the incident beam, or it can be a non-vortex beam. The obtained results are specialized for two particular cases: when the incident LG beam is with zeroth radial mode number and azimuthal mode number l, and when the incident beam is with zeroth azimuthal mode number and radial mode number n. The presented research results can find interest in optical trapping experiments, fibre-optic multiplexing and quantum information processing.

Probability density of field and intensity fluctuations of structured light in a turbulent atmosphere

Distributions of the probability density of fluctuations of the intensity and real and imaginary parts of the complex field amplitude of the Laguerre–Gaussian laser beam propagating through a turbulent atmosphere, provided that the beam scintillation index takes values larger than unity, are studied. The calculations are performed through numerical simulation and with application of an analytical approach based on the expansion of the probability distribution density of the wave field into the Edgeworth series. The comparison of the results of numerical and analytical calculations has demonstrated that the probability density distribution of intensity fluctuations of the laser beam is resistant to small variations in parameters of the probability density distribution of fluctuations of the real and imaginary parts of the wave field. In addition, the earlier formulated statement that the normalized probability density of intensity fluctuations of a laser beam at any point of the space occupied by this beam is determined by the value of the scintillation index and does not depend directly on the turbulent propagation conditions is confirmed. It is shown that the probability density of intensity fluctuations constructed with the Edgeworth approximation of the probability density of field fluctuations is in good agreement with the results of numerical simulation and the fractional exponential distribution for the values of the scintillation index ranging from 1.0 to 1.6.

Terahertz radiation generation through the nonlinear interaction of Hermite and Laguerre Gaussian laser beams with collisional plasma: Field profile optimization

The nonlinear interaction of Hermite-Gaussian and Laguerre-Gaussian (LG) laser beams with a collisional inhomogeneous plasma is studied, and the amplitude of the emitted terahertz (THz) electric field is evaluated. The effects of laser beams and plasma parameters, including the beams width, LG modes, the plasma collision frequency, and the amplitude of density ripple on the evolution of THz electric field amplitude, are examined. It is found that the shape of the generated THz radiation pattern can be tuned by the laser parameters. In addition, the optimum values of the effective parameters for achieving the maximum THz electric field amplitude are proposed. It is shown that a significant enhancement up to 4.5% can be obtained in our scheme, which is much greater than the maximum efficiency obtained for laser beams with the same profiles.

Photon polarization tensor in pulsed Hermite- and Laguerre-Gaussian beams

In this article, we provide analytical expressions for the photon polarization tensor in pulsed Hermite- and Laguerre-Gaussian laser beams. Our results are based on a locally constant field approximation of the one-loop Heisenberg-Euler effective Lagrangian for quantum electrodynamics. Hence, by construction they are limited to slowly varying electromagnetic fields, varying on spatial and temporal scales significantly larger than the Compton wavelength/time of the electron. The latter criterion is fulfilled by all laser beams currently available in the laboratory. Our findings will, e.g., be relevant for the study of vacuum birefringence experienced by probe photons brought into collision with a high-intensity laser pulse which can be represented as a superposition of either Hermite- or Laguerre-Gaussian modes.

Investigation of vortex laser beam injection into an optical fiber

We investigate Laguerre-Gaussian vortex laser beam injection into an optical fiber. Modelling of radiation entering an optical fiber with plane (cylinder) and axicon (cone with diffrent apex angle) micro-relief is numerically investigated by the finite difference time domain (FDTD) method.

Ponderomotive beatwave ion acceleration using twisted light

An all-optical mechanism of ion acceleration in vacuum with two counter-propagating plane waves has been proposed by F. Peano et al., IEEE Trans Plasma Sci. 36, 1857 (2008). A suitable frequency chirping of lasers drives a beat wave with variable phase velocity that traps particles and accelerates them longitudinally. In this paper, direct ion acceleration by two counter-propagating focused laser beams with variable frequencies is considered, and the multi-dimensional effects associated with the finite transverse dimension of lasers are investigated. It is shown that the Gaussian laser beams provide a defocusing transverse force that stops the acceleration process as ions propagate towards regions of smaller laser fields. On the other hand, the Laguerre-Gaussian laser beams with identical orbital angular momentum can confine the off-axis ions radially as they accelerate to high energies. It is shown that the orbital angular momentum of the Laguerre-Gaussian lasers can be used to control the angular momentum of the accelerated ion beam.

Effective factors on twisted terahertz radiation generation in a rippled plasma

Based on the beating of two Laguerre–Gaussian laser beams in a rippled plasma medium, the effective factors such as plasma density, Gouy phase and orbital angular momentums of input lasers on the output twisted terahertz radiation are investigated. As a result, the amplitude of the generated vortex terahertz radiation is an increasing function of plasma density. The vortex terahertz intensity is strongly dependent on the orbital angular momentum of the input lasers. The terahertz output amplitude increases by decreasing the orbital angular momentum of the Laguerre–Gaussian input lasers. Here, by employing a suitably ripple wavenumber, the destroyer effect of the relative Gouy phase of the input lasers is removed, and perfect phase matching is satisfied.

Terahertz twisted beams generation in plasma

The resonant vortex terahertz beam generation by the cross-focusing of two twisted coaxial laser beams is investigated. For the resonant excitation of terahertz radiation, the rippled density in plasma and the ripple wave number is suitably chosen to satisfy the phase matching condition. The nonlinear current density at terahertz frequency arises due to the spatial variation of two Laguerre-Gaussian coupled field. The terahertz intensity scales as the ponderomotive force of laser beams which imparts an oscillatory velocity to the electrons and, in fact, input Laguerre-Gaussian laser beams properties such as vortex charge number and beam waist. Various laser and plasma parameters are employed to yield vortex terahertz radiation with higher efficiency. Also, it is shown that when the beating frequency approaches plasma frequency, the amplitude of THz radiation increases.

Asymmetric Laguerre-Gaussian beams

We introduce a family of asymmetric Laguerre-Gaussian (aLG) laser beams. The beams have been derived via a complex-valued shift of conventional LG beams in the Cartesian plane. While propagating in a uniform medium, the first bright ring of the aLG beam becomes less asymmetric and the energy is redistributed toward peripheral diffraction rings. The projection of the orbital angular momentum (OAM) onto the optical axis is calculated. The OAM is shown to grow quadratically with increasing asymmetry parameter of the aLG beam, which equals the ratio of the shift to the waist radius. Conditions for the OAM becoming equal to the topological charge have been derived. For aLG beams with zero radial index, we have deduced an expression to define the intensity maximum coordinates and shown the crescent-shaped intensity pattern to rotate during propagation. Results of the experimental generation and rotation of aLG beams agree well with theoretical predictions.

Technique to separate lidar signal and sunlight

Sunlight contamination dominates the backscatter noise in space-based lidar measurements during daytime. The background scattered sunlight is highly variable and dependent upon the surface and atmospheric albedo. The scattered sunlight contribution to noise increases over land and snow surfaces where surface albedos are high and thus overwhelm lidar backscatter from optically thin atmospheric constituents like aerosols and thin clouds. In this work, we developed a novel lidar remote sensing concept that potentially can eliminate sunlight induced noise. The new lidar concept requires: (1) a transmitted laser light that carries orbital angular momentum (OAM); and (2) a photon sieve (PS) diffractive filter that separates scattered sunlight from laser light backscattered from the atmosphere, ocean and solid surfaces. The method is based on numerical modeling of the focusing of Laguerre-Gaussian (LG) laser beam and plane-wave light by a PS. The model results show that after passing through a PS, laser light that carries the OAM is focused on a ring (called "focal ring" here) on the focal plane of the PS filter, very little energy arrives at the center of the focal plane. However, scattered sunlight, as a plane wave without the OAM, focuses at the center of the focal plane and thus can be effectively blocked or ducted out. We also find that the radius of the "focal ring" increases with the increase of azimuthal mode (L) of LG laser light, thus increasing L can more effectively separate the lidar signal away from the sunlight noise.

Optical trapping and moving of microparticles by using asymmetrical Laguerre-Gaussian beams.

We considered a generalization of the Laguerre-Gaussian (LG) laser beam family by using a complex shift of the beam complex amplitude in Cartesian coordinates. In this case, LG-beams lose their axial symmetry. The normalized orbital angular momentum is the sum of the beam topological charge and the term which is in square dependence on the asymmetry parameter. By optical trapping and moving the polystyrene microspheres in the focus of the asymmetric LG-beam, it is proven that the velocity of the microspheres increases with increasing the asymmetry parameter and constant topological charge.

Transfer of orbital angular momentum from asymmetric Laguerre-Gaussian beams to dielectric microparticles

Transfer of the orbital angular momentum to dielectric microparticles optically trapped using recently discovered asymmetric Laguerre-Gaussian (LG) laser beams is demonstrated. We experimentally show that at a fixed topological charge, increasing beam asymmetry parameter leads to increasing velocity of the polystyrene beads moving along a crescent-shaped path.

Conversion of Laguerre–Gaussian beams into Gaussian beams of reduced focal spot by use of a circular echelon

Abstract In this article we present numerical calculations for the diffraction of a Laguerre–Gaussian laser beam (TEMp0), with “0” radial mode number and “p” azimuthal mode number, by circular echelon (CE) which is an annular multi-phase level Diffractive Optical Element (DOE) with a phase variations of Δφ. We apply the super resolution technique for transforming a symmetrical TEMp0 Laguerre–Gauss beam into a quasi Gaussian intensity distribution. The beam shaping is modeled on the p light rings of the incident beam. We present the diffracted intensities distributions in the back focal plane of a converging lens for different mode TEM01, TEM02, TEM03, TEM04, TEM05. The advantage of this technique is that the rectified TEMp0 beam at focus has a focal volume very smaller than that of a Gaussian beam and smaller than the rectified TEMp0 beam at focus of an annular binary Diffractive Optical Element (DOE) and its intensity is greater than that of an annular binary (DOE), also its fabrication is simpler.