C波段携轨道角动量螺旋波微带阵天线设计

本文以同轴馈电的矩形微带天线作为阵单元,将8个相同的单元天线等间隔分布在一个同心圆面上以组成微带阵天线,并采用等幅但相邻单元间相位差为一常数的方式进行激励。基于三维高频结构电磁模拟软件仿真优化,在中心频率为6GHz处,根据不同的相位延迟得到不同模式的携有轨道角动量的方向图。仿真结论表明,该新颖的微带阵天线能产生携有轨道角动量的螺旋电磁波。     

Spin-to-orbital angular momentum conversion and spin-polarization filtering in electron beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.     

基于圆微带天线阵的射频涡旋电磁波的产生

射频涡旋电磁波等相位面呈涡旋状,是一种携有新自由度-轨道角动量的电磁波。在轨道角动量模式理论分析的基础上,提出了在中心频率6 GHz处产生携有轨道角动量的涡旋电磁波的一种圆微带天线阵新结构,设计了以双层圆形微带天线为阵元组成的圆形阵列天线,通过控制馈源的相位差,得到模式量子数为0,1,2, 3, 4的轨道角动量。仿真结果表明:携轨道角动量的电磁波矢量电场图具有涡旋波阵面的特性,合适的阵列半径和馈线排列分布将产生携有良好轨道角动量特性的涡旋电磁波,而不当的阵列半径或馈线排列分布将出现能量的分散或者相互耦合的问题。     

利用干涉光场的相位涡旋测量拉盖尔-高斯光束的轨道角动量

对拉盖尔-高斯光束经多圆孔衍射屏在远场平面上形成的干涉光场的相位和零值线进行了计算模拟.当入射光束的轨道角动量量子数为零时,实部零值线与虚部零值线在干涉光场中心点不相交,因而在该点上不能形成相位涡旋.当入射光束的轨道角动量量子数为+1和-1时,实部零值线与虚部零值线在干涉光场中心垂直并相交,干涉光场相应位置处的相位涡旋的符号相反.当入射光束的轨道角动量量子数为±2和±3时,有四条零值线相交于干涉光场的中心点上,并且实部零值线和虚部零值线交替分布,该交点处形成的相位涡旋的拓扑荷的值恰好与拉盖尔-高斯光束的轨道角动量量子数相等.这种结果可以用来测量涡旋光束的轨道角动量.     

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.     

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.     

Double-slit interference with Laguerre-Gaussian beams

The interference of Laguerre-Gaussian beams carrying orbital angular momentum was demonstrated in Young's double-slit geometry. Double-slit interference is shown to be affected by the azimuthal phase dependence of a Laguerre-Gaussian beam. This interference provides new insight into the helical phase structure of the Laguerre-Gaussian beam and has potential applications for measuring the orbital angular momentum of an arbitrary wavefront.     

Poincaré-sphere equivalent for light beams containing orbital angular momentum

The polarization state of a light beam is related to its spin angular momentum and can be represented on the Poincaré sphere. We propose a sphere for light beams in analogous orbital angular momentum states. Using the Poincaré-sphere equivalent, we interpret the rotational frequency shift for light beams with orbital angular momentum [Phys. Rev. Lett. 80, 3217 (1998)] as a dynamically evolving geometric phase.     

Geometrical phase shift of the extrinsic orbital angular momentum density of light propagating in a helically wound optical fiber

We calculate the intrinsic spin and extrinsic orbital angular momentum densities of an electromagnetic plane wave propagating in a helically wound optical fiber. The geometrical phase shift of the extrinsic angular momentum density of light traveling in such a fiber is derived analytically and discussed in comparison with that of the intrinsic angular momentum density.     

Generation of light carrying orbital angular momentum via induced coherence grating in cold atoms

We report on the generation of light carrying orbital angular momentum through Bragg diffraction into an electromagnetically induced coherence grating in a degenerate two-level system of cold cesium atoms. The induced Zeeman coherence grating is shown to contain the spatial phase structure of the incident beams. The exchange of phase information between a light beam with orbital angular momentum and a long-lived atomic coherence opens up the way to process quantum information encoded in a multidimensional state space.     

Optical orbital angular momentum from the curl of polarization

We predict a new category of optical orbital angular momentum that is associated with the curl of polarization and a kind of vector field with radial-variant hybrid states of polarization that can carry such novel optical orbital angular momentum. We present a scheme for creating the desired vector fields. Optical trapping experiments validate that the vector fields, which have no additional phase vortex, exert torques to drive the orbital motion of the trapped isotropic microspheres.     

用波晶片相位板产生角动量可调的无衍射涡旋空心光束

本文提出了一种用波晶片产生无衍射涡旋空心光束的新方案. 根据晶体双折射的性质, 设计波晶片的厚度, 在一块晶体薄片上对o光和e光分别形成各自的四台阶相位板, 线偏振光入射到该相位板后, o光和e光衍射按强度叠加, 利用准伽利略望远镜系统聚焦, 得到近似无衍射涡旋空心光束. 光路简单, 调节方便. 在近轴条件下, 运用菲涅耳衍射理论和经典电磁场角动量理论, 数值模拟计算了周期数不同的两块波晶片相位板衍射光强和角动量的分布, 结果表明: 两块相位板都能在较长距离内产生近似无衍射涡旋空心光束, 光强和轨道角动量的分布与螺旋相位板产生的涡旋光束基本相同. 在衍射光路中加入相位补偿器, 调节o光和e光的相位差可以调节自旋角动量的大小, 从而可以调节总角动量密度和平均光子角动量的大小. 用这种空心光束导引冷原子或冷分子, 原子在与光子相互作用过程中可获得可调的转动力矩.     

Backscattered polarization patterns, optical vortices, and the angular momentum of light

We demonstrate that the polarization patterns observed in backscattering of linearly polarized light are a manifestation of the conservation of angular momentum of light. We will show that this phenomenon can be described in terms of phase vortices that are acquired by the right and left circularly polarized components. The helicity and orbital angular momentum of these components satisfy the requirement for conservation of angular momentum.     

Focusing of elliptically polarized Gaussian beams through an annular high numerical aperture

Based on the vectorial Debye theory,the focusing properties of the Gaussian beam through an annular high numerical aperture are studied numerically,including the intensity,the phase and the orbital angular momentum properties.Then the influence of certain parameters on the focusing properties is also investigated.It is shown that sub-wavelength elliptical light spots can be obtained.And there exists a vortex in the longitudinal component of the focused field even though the incident beam is Gaussian beam,indicating that the spin angular momentum of the elliptically polarized Gaussian beam is converted into the orbital angular momentum by the focusing.     

Tight focusing of spirally polarized vortex beams

The tight focusing of spirally polarized focused vortex beams is analyzed numerically based on the vectorial Debye theory. The expressions for the electric field and the orbital angular momentum of focused beams are derived. It is shown that the intensity distribution in the focal plane is dependent on the specific spirally polarized state and the coefficient of the spiral polarization function. By presenting the phase contours of the component polarized in the radial direction, it is found that the radii of dislocation lines will increase with the increase of the power of the spirally polarization function. It is reveled that the same orbital angular momentum can be obtained for different spirally polarized state at certain distance along the propagation direction in the focal region. Besides, the orbital angular momentum distributions for different polarized states have fewer crossover points with each other for higher topological charge. The influence of the spirally polarized state on the orbital angular momentum in the focal plane is also studied.     

Orbital angular momentum exchange in the interaction of twisted light with molecules

In the interaction of molecules with light endowed with orbital angular momentum, an exchange of orbital angular momentum in an electric dipole transition occurs only between the light and the center of mass motion; i.e., internal "electronic-type" motion does not participate in any exchange of orbital angular momentum in a dipole transition. A quadrupole transition is the lowest electric multipolar process in which an exchange of orbital angular momentum can occur between the light, the internal motion, and the center of mass motion. This rules out experiments seeking to observe exchange of orbital angular momentum between light beams and the internal motion in electric dipole transitions.     

Resistance of nondiffracting vortex beam against amplitude and phase perturbations

The revival of the nondiffracting vortex beam after its interaction with the 2D on axis obstacle is examined. We show that the phase topology and the spatial distribution of the orbital angular momentum of the beam transmitted through the obstacle regenerate to the initial form during further free propagation. We verify that the healing effect appears even if the interaction is accompanied by the exchange of the orbital angular momentum.     

Mechanical equivalence of spin and orbital angular momentum of light: an optical spanner

We use a Laguerre-Gaussian laser mode within an optical tweezers arrangement to demonstrate the transfer of the orbital angular momentum of a laser mode to a trapped particle. The particle is optically confined in three dimensions and can be made to rotate; thus the apparatus is an optical spanner. We show that the spin angular momentum of +/-?per photon associated with circularly polarized light can add to, or subtract from, the orbital angular momentum to give a total angular momentum. The observed cancellation of the spin and orbital angular momentum shows that, as predicted, a Laguerre-Gaussian mode with an azimuthal mode index l=1 has a well-defined orbital angular momentum corresponding to ? per photon.     

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.