Spin-to-orbital angular momentum conversion in a strongly focused optical beam

As a fundamental property of light, the angular momentum of photons has been of great interest. Here, we demonstrate that optical spin-to-orbital angular momentum conversion can occur in a homogeneous and isotropic medium. This Letter presents both theoretical and experimental studies of this conversion in a tightly focused beam and shows that the orbital rotation speeds of trapped particles are altered because of this conversion as predicted by theory.     

Orbital angular momentum and nonparaxial light beams

The simple relationship between total angular momentum and energy and the seemingly natural separation of the angular momentum into spin and orbital components in the paraxial approximation, are investigated for a general nonparaxial form of monochromatic beam with near cylindrical symmetry.     

Delocalized correlations in twin light beams with orbital angular momentum

We generate intensity-difference-squeezed Laguerre-Gauss twin beams of light carrying orbital angular momentum by using four-wave mixing in a hot atomic vapor. The conservation of orbital angular momentum in the four-wave mixing process is studied as well as the spatial distribution of the quantum correlations obtained with different configurations of orbital angular momentum. Intensity-difference squeezing of up to -6.7 dB is demonstrated with beams carrying orbital angular momentum. Delocalized spatial correlations between the twin beams are observed.     

Measuring orbital angular momentum superpositions of light by mode transformation

We recently reported on a method for measuring orbital angular momentum (OAM) states of light based on the transformation of helically phased beams to tilted plane waves [Phys. Rev. Lett.105, 153601 (2010)]. Here we consider the performance of such a system for superpositions of OAM states by measuring the modal content of noninteger OAM states and beams produced by a Heaviside phase plate.     

Electro-optically forbidden or enhanced spin-to-orbital angular momentum conversion in a focused light beam

We demonstrate that the conversion of spin-to-orbital angular momentum in a focused vectorial light beam can be forbidden or enhanced by the electric-field-erased or increased spatial anisotropy of a uniaxial crystal undergoing the Pockels effect. A focused right-handed circularly polarized quasi-Gaussian beam incident on an electro-optic crystal strontium barium niobate is taken to illustrate the effect, and a conversion-forbidden electric field E(0)=-16.87 kV/cm is found. Of special interest is that single photons generated in this effect exhibit an entanglement of 2 deg of freedom of spin and orbital angular momentum, and it is electro-optically controllable.     

Four-wave mixing of light beams with engineered orbital angular momentum in cold cesium atoms

We report an experimental demonstration that shows that the spatial structure carried by engineered coherent superpositions of light beams with orbital angular momentum can be mapped into the nonlinear polarization induced in a cloud of cold cesium atoms. The structure of such polarization was revealed by nearly degenerate four-wave-mixing processes.     

Orbital angular momentum of partially coherent beams

The definition of the orbital angular momentum established for coherent beams is extended to partially coherent beams, expressed in terms of two elements of the beam matrix. This extension is justified by use of the Mercer expansion of partially coherent fields. General Gauss-Schell-model fields are considered, and the relation between the twist parameter and the orbital angular momentum is analyzed.     

The limited angular momentum in strongly deformed rotational bands

The moments of inertia of rotational bands in the cranked harmonic oscillator are explored. The important role played by the maximum spin is pointed out. A qualitative agreement is found with the properties of recently observed superdeformed bands.     

一种角动量算符的简便推导方法

在量子力学教本中推导(L)2及(L)z的球坐标表达式一般都采用由直角坐标系到球坐标系的变量代换的方法,运算过程十分复杂,本文利用了球坐标系中坐标变量r、θ、ψ对正交基矢er、eθ、eψ微分性质,采用球坐标系直接进行推导,简化量子力学中关于角动量算符(L)2及(L)z的的推导.     

Effects of atmospheric turbulence on the orbital angular momentum of multi-Gaussian Schell model beams

The effects of multibeams and partially coherent source on the orbital angular momentum transmission of multi-Gaussian Schell model beams in an atmospheric turbulence channel are analyzed. An effective crosstalk probability model of the orbital angular momentum is derived which leads to crosstalk probability function for eigenmodes of different orbital angular momentum states and beamlets. The work shows that the evolution of the effective spatial coherence radius with growing the rms width of the correlation of the partially coherent source and the number of the beamlet in multi-Gaussian model beams. The crosstalk probability of the orbital angular momentum of an effective Gaussian-Schell model beam decreases as increasing of the degree of the spatial coherence of the partially coherent source and the beamlet number of the multi-Gaussian Schell model beams.     

Measuring orbital angular momentum of vortex beams in optomechanics

Measuring the orbital angular momentum (OAM) of vortex beams, including the magnitude and the sign, has great application prospects due to its theoretically unbounded and orthogonal modes. Here, the sign-distinguishable OAM measurement in optomechanics is proposed, which is achieved by monitoring the shift of the transmission spectrum of the probe field in a double Laguerre–Gaussian (LG) rotational-cavity system. Compared with the traditional single LG rotational cavity, an asymmetric optomechanically induced transparency window can occur in our system. Meanwhile, the position of the resonance valley has a strong correlation with the magnitude and sign of OAM. This originally comes from the fact that the effective detuning of the cavity mode from the driving field can vary with the magnitude and sign of OAM, which causes the spectral shift to be directional for different signs of OAM. Our scheme solves the shortcoming of the inability to distinguish the sign of OAM in optomechanics, and works well for high-order vortex beams with topological charge value±45, which is a significant improvement for measuring OAM based on the cavity optomechanical system.     

The inverse scattering transformation in the angular momentum plane

The inverse scattering transformation (IST) with the angular momentum () as a spectral variable turned out to be a useful method to deal with rotationally invariant problems in field theory at higher spatial dimensions [Refs. (1) and (2)]. We derive the direct and inverse scattering problems for thev-dimensional radial Schrödinger equation for variable and fixed energy (negative or positive). We determine the scattering data (SD) in one to one correspondence with the potential and derive the corresponding Gelfand-Levitan equation. A family of exactly solvable potentials for any and fixed energy is obtained. The trace identities associated to this IST are derived for both signs of the energy. They relate integrals of local polynomials in the potential and its derivatives timesr ^2n–1(n1) with the SD. The presence of this power ofr makes these relations useful in higher dimensions.Laboratoire associé au CNRS     

Self-homodyne detection of the light orbital angular momentum

A simple optical system for the self-homodyne detection of the orbital angular momentum (OAM) carried by optical beams is introduced. We propose two different schemes based on the use of optical hybrids, which could detect the OAM mode number, even when the input beam might be slightly distorted. A balanced receiver is used to perform a self-homodyne measure of the optical signal from two different locations at the beam wavefront.     

Angular momentum of gaussian light beams

Analysis of the orbital angular momentum of paraxial light beams shows that a key role in the formation of this quantity is played by phase relations between longitudinal and transverse radiation fields. When a light beam is circularly polarized or has a helical wave front, the azimuthal component of the Poynting vector and the density of orbital angular momentum prove to be non-zero. In the case of circularly polarized radiation, the azimuthal component of the Poynting vector and the density of the orbital angular momentum can change the sign at different points in the cross section of the light beam, while the total orbital momentum of the beam remains quantized.     

Beam moments and angular momentum in non-uniformly polarized beams

The angular momentum of non-uniformly totally polarized beams is investigated using methods from the beam characterization approach. The relationship between the elements of the beam matrix for the two components of the field and the angular momentum is given. The unconventional distribution of the polarization across the beam profile could result in contributions to both the spin and orbital terms of the angular momentum. To illustrate this, a particular example with a vortex beam is considered.     

Linear and angular momenta in tightly focused vortex segmented beams of light(Invited Paper)

We investigate the linear momentum density of light,which can be decomposed into spin and orbital parts,in the complex three-dimensional field distributions of tightly focused vortex segmented beams.The chosen angular spectrum exhibits two spatially separated vortices of opposite charge and orthogonal circular polarization to generate phase vortices in a meridional plane of observation.In the vicinity of those vortices,regions of negative orbital linear momentum occur.Besides these phase vortices,the occurrence of transverse orbital angular momentum manifests in a vortex charge-dependent relative shift of the energy density and linear momentum density.     

基于激光光束轨道角动量的8位数据信号产生与检测的实验研究

本文研究了利用激光光束的轨道角动量进行8位数据信息传输的技术. 建立了一套利用光束轨道角动量进行8位数据信息传输的系统, 在信息调制部分采用可选通控制的光源阵列和相位调制屏实现光束轨道角动量态的编码复用, 在信息解调部分采用二元振幅光栅进行轨道角动量态检测,实现了无误码的数据信息传输.     

Time-division multiplexing of the orbital angular momentum of light

We present an optical setup for generating a sequence of light pulses in which the orbital angular momentum (OAM) degree of freedom is correlated with the temporal one. The setup is based on a single q plate within a ring optical resonator. By this approach, we demonstrate the generation of a train of pulses carrying increasing values of OAM, or, alternatively, of a controlled temporal sequence of pulses having prescribed OAM superposition states. Finally, we exhibit an "OAM-to-time conversion" apparatus that divides different input OAM states into different time bins. The latter application provides a simple approach to digital spiral spectroscopy of pulsed light.     

Canonical momentum,angular momentum,and helicity of circularly polarized Airy beams

We report a study of the momentum, angular momentum, and helicity of circularly polarized Airy beams propagating in free space. By using the vector angular spectrum representation, the explicit analytical expressions for the electric and magnetic field components of circularly polarized Airy beams are derived in detail. To overcome the drawbacks of classical kinematics formulae when applied to structured light beams, a general canonical approach is introduced to describe the momentum, angular momentum and helicity of Airy beams. Numerical simulation results for the spatial distributions of the canonical momentum, spin and orbital angular momentum, as well as the helicity densities are presented and discussed. This study may provide useful insights into the dynamical properties of Airy beams that may be important in several applications, including the optical control, micromanipulation, and information processing.