Geometric Phase Based Circular Array for Multimode Vortex Beam Generation

A geometric phase model for electromagnetic radiating elements is proposed. By rotation of the radiating element, a frequency‐independent geometric phase occurs for circularly polarized components of radiation field along every direction in far field. In addition, the geometric phase is equal to the rotation angle for a circularly polarized source, which enables phase modulation ranging from 0 to 2π. In contrast, the Pancharatnam–Berry phase for circular polarization conversion components brought by optical element rotation is twice the rotation angle and is applicable only for the scattering waves propagating along the rotation axis. As a proof of principle, an antenna array is designed and fabricated in microwave regime to verify the phase modulation approach. Both the calculated and measured results verify that three different orbital angular momentum modes are generated simultaneously at 8.5 GHz and 11.5 GHz.     

Dependence of the circular polarization of backscattered light in random media on anisotropy of scatterers

The scattering of linearly or circularly polarized light from a semi-infinite randomly inhomogeneous medium is considered. Using the Monte Carlo method, it is shown that, in the case of a wide front of incident and scattered optical radiation and irrespective of the degree of scattering anisotropy, the copolarized component of backscattered light dominates the cross-polarized component for the linear polarization and the cross-polarized component dominates the copolarized component for the circular polarization. If the beams of incident and scattered radiation are spatially separated and the size of scatterers exceeds the wavelength, the circular copolarized component dominates the cross-polarized one. A similar effect of the change in direction of the rotation of the plane of polarization in relation to the size of scatterers is revealed for pulsed radiation.     

手性介质中的自诱导非线性旋光

采用经典电磁场理论对非导电型各向同性手性介质光学非线性性质进行了描述,着重研究线偏振光自诱导非线性旋光现象。给出了在特定条件下非线性旋光附加角的表达式。     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Manifestation of gyrotropy upon light scattering in paratellurite

Upon scattering of laser radiation at different wavelengths, three optical effects—rotation of the plane of polarization of linearly polarized light, dependence of the specific rotation on the radiation wavelength, and the dependence of the scattered light intensity on the angle between the scattered and incident beams—are simultaneously observed. The intensity distributions of the scattered laser light at 0.488, 0.531 nm, and 0.633 μm along the optical axis of a thick sample of a TeO₂ single crystal are visualized. Based on the patterns obtained, the values of the specific rotation are calculated, and the curve of the optical rotation for the visible range is obtained.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Rotation of the polarization ellipse under multiple light scattering in disordered media

A study is performed of the multiple small-angle scattering of circularly polarized waves in random media with large-size transparent inhomogeneities that are less dense than the surrounding medium. In these conditions, a new effect is observed—rotation of the polarization ellipse of the multiply scattered light.     

Manipulating Spin-Dependent Wavefronts of Vortex Beams via Plasmonic Metasurfaces

Conventional metasurfaces based on geometric phase are constrained to spin-locked phase profile, resulting in mirrored functionalities for different spins. A single flat device that enables independent manipulation of wavefronts in two orthogonal circularly polarized channels is of paramount importance in wireless and optical communications. In this work, by tuning the dimension and rotation angle of H-shaped meta-atoms to synthesize propagating phase and geometric phase, spin-dependent plasmonic metasurfaces are presented to manipulate circularly polarized waves in the visible band. To verify the capability of spin-dependent wavefront manipulation, three metasurfaces are implemented. The first metasurface generates vortex beams with orbital angular momentum (OAM) l = 1 under left-handed circularly polarized (LCP) incidence and l = 2 under right-handed circularly polarized (RCP) incidence. By introducing convolution operation, the second metasurface is capable of producing vortex beams with different OAMs and different directions for two spins. The third metasurface produces dual-beam and quad-beam with different OAMs for different circular polarizations. This scheme can provide a new pathway in ultracompact nanophotonic devices and systems.     

Measuring small wavelength differences by heterodyne interferometry and optical activity of chiral crystal

The optical activity of a chiral crystal and common-path heterodyne interferometry are used in a simple measurement technique that was developed to measure small wavelength differences. When circularly polarized heterodyne light passes through a chiral crystal, the plane of polarization rotates. The phase difference between the right- and left-circular lights is directly proportional to the angle of rotation. The rotation angle depends strongly on the wavelength. The phase difference can be accurately detected and substituted into specially derived equations to estimate wavelength variations. The feasibility of this method was demonstrated and the wavelength sensitivity was about 0.00812 nm. This method provides the advantages of a simple structure, ease of operations, a large wavelength measurement range and high sensitivity.     

Scattering response of featured uniaxial anisotropic chiral medium-coated twisted PEC cylinder

Scattering of electromagnetic waves from a perfectly electrical conducting (PEC) cylinder of circular cross-section, coated with uniaxial non-magnetic chiral anisotropic medium, was discussed. A conducting sheath helix structure with adjustable pitch angle exists over the outer surface of coated cylinder, which is interfaced with the free-space. Under the uniform parallel polarized plane wave with fixed oblique incidence angle, the echo widths were determined emphasizing the other relevant scattering-related parameters (e.g. scattering/extinction efficiency etc.), taking into account different pitch angle values (of the sheath helix) and the kinds of coating mediums. The results revealed that the coating of negative uniaxial chiral medium causes enhancement in scattering efficiency (of the scatterer). Also, the scattering efficiency was found to be the same as the extinction efficiency, thereby yielding vanishing absorption of the medium. The obtained results were also compared with the situation of vanishing chirality of coating medium over the PEC cylinder; the echo width and scattering efficiency both decrease in this case.     

偏振相移与相位共轭结合的散射光聚焦技术研究

散射介质对光的随机散射作用是制约其光学聚焦和成像的重要因素，光学相位共轭技术能够通过对散射光场共轭还原实现透过散射介质的光学聚焦和成像，其中散射光场相位的获取是共轭还原的核心。阐述了偏振相移的基本原理，将偏振相移与相位共轭技术相结合，设计了新的基于偏振相移的数字光学相位共轭系统。采用633 nm的HeNe激光器作为系统光源，毛玻璃作为散射介质开展散射光聚焦实验研究。实验结果表明:该装置能够成功实现透过散射介质的光学聚焦，其中聚焦点与背景光强的比值可达约400倍。     

Measurement of optical rotation and phase retardance of optical samples with depolarization effects using linearly and circularly polarized probe lights

This paper presents an experimental technique for measuring the optical rotation, depolarization, and phase retardance of optical samples. In the proposed approach, the optical properties of the sample are derived using a Stokes–Mueller matrix formalism in conjunction with linearly and circularly polarized probe lights. For a compound sample comprising a half-wave plate positioned in front of glucose solutions with concentrations ranging from 0 to 1.2 g/dl, the average normalized error in the measured rotation angle is determined to be 3.11% when using a linearly polarized light. The average surviving linear and circular polarization fractions of the glucose solutions are determined to be 1.0252 and 0.9945. The average normalized error in the measured retardance of the half-wave plate is 3.45%. When measuring a compound sample comprising a half-wave plate positioned in front of the scattered glucose solutions with turbidities from 0% to 50% by the addition of milk, experimental results show that the induced rotation angle increased as the turbidity increased, whereas both surviving linear and circular polarization fractions decreased as the turbidity increased. The effect of the turbidity on rotation angle is more significant than that on both surviving linear and circular polarization fractions. The average normalized error in the measured retardance of the half-wave plate is 1.43%. Consequently, a simplified geometry of the polarimeter is proposed to independently estimate the rotation angle, surviving linear and circular polarization fractions, and retardance from the derived viable algorithm.     

Ring-resonator-based frequency-domain optical activity measurements of a chiral liquid

Chiral liquids rotate the plane of polarization of linearly polarized light and are therefore optically active. Here we show that optical rotation can be observed in the frequency domain. A chiral liquid introduced in a fiber-loop ring resonator that supports left and right circularly polarized modes gives rise to relative frequency shifts that are a direct measure of the liquid's circular birefringence and hence of its optical activity. The effect is in principle not diminished if the circumference of the ring is reduced. The technique is similarly applicable to refractive index and linear birefringence measurements.     

Vector statistics of multiply scattered light in a random medium with large inhomogeneities

The distribution function of the scattered radiation intensity is calculated in the approximation of the basic polarization modes. In such an approximation, the distribution function is represented in terms of the degree of polarization of the initially circularly and linearly polarized waves transmitted by the sample. The effect of the difference of the decay rates for the circular and linear polarization modes on the statistics of the light waves that are multiply scattered in the medium with large inhomogeneities is analyzed.     

Light transmission in inhomogeneous birefringent medium as a function of propagation and observation angles

We use the Mueller matrix for inhomogeneous linear birefringent media derived in Savenkov et al. [Mueller-matrix model of an inhomogeneous linear birefringent medium: single scattering case. JQSRT 2007;106:475–86] to generate new results on the forward scattering of light using Cloude's coherence matrix method. We show that the intensity of scattered light (m₁₁) as a function of observation angle depends on the difference between refractive indices along the eigen polarizations resulting in intensity lost when unpolarized light propagates parallel to the optical axes compared to propagation orthogonal to the optical axis. For a given inhomogeneity (roughness), depolarization strongly depends on the direction of light propagation in the medium. The depolarization at 90° propagation angle is minimal for any value of inhomogeneity. Sample calculations are based on calcite.     

Scattering theorems for dyadic chiral fields

A time-harmonic plane dyadic electromagnetic field is scattered by a chiral body in a chiral environment. The body is either a perfect conductor or a dielectric. The incident field is a linear combination of left-circularly polarized and right-circularly polarized dyadic electromagnetic fields, each of which has a different wave number. We prove reciprocity and scattering theorems in dyadic form, which incorporate as special cases the corresponding known theorems for vector electromagnetic waves. Specializing to the same direction of incidence and observation in the general scattering theorems we obtain forward scattering theorems.     

透过散射介质对直线运动目标的全光成像及追踪技术

光散射是限制光传输以及降低和破坏光学成像性能的主要因素,透过复杂散射介质对运动目标的全光成像是光学领域极具挑战性的技术之一.本文提出一种利用散斑差值自相关透过散射介质对运动目标进行实时追踪的方法.采用赝热光照明,基于光学记忆效应理论,通过对运动目标采集的两帧散斑做差值,然后做自相关运算,计算目标移动的距离,实现对目标的实时追踪,并且利用相位恢复算法进行简单处理就可以重建隐藏目标.对该方法进行了实验验证,成功地对隐藏的运动目标实现了成像与追踪.这种透过散射介质对运动目标的全光成像及实时追踪技术,在生物医学等领域具有重要应用潜力.     

颗粒溶液颗粒尺寸及浓度的差分偏振弹性散射光谱在线分析方法

偏振弹性散射光谱技术的基本原理为在偏振光入射条件下,根据出射光的偏振特性不同可以筛选出浅表层组织的单次散射光信息和深层组织的漫散射光信息。该研究的创新点在于将这种方法应用于颗粒溶液检测,目的是在颗粒溶液原始状态下实现对颗粒尺寸及浓度的同时检测。设计了一个共轴笼式光学系统,测量了聚苯乙烯微球颗粒溶液某一角度的背向散射信号,通过控制入射端和收集端偏振片的偏振方向获得了颗粒溶液的偏振平行光谱与偏振垂直光谱,两者之差即偏振差分光谱对应颗粒的单次散射信息,将该单次散射信息与Mie散射数据库进行比对获得颗粒的尺寸,然后在颗粒尺寸作为已知的条件下进一步分析偏振垂直光谱,将该垂直光谱对应的颗粒溶液的漫散射信息代入光漫散射下的近似表达式拟合得到颗粒的浓度信息。将实验结果与样品提供值进行了比对,并进一步分析了在获取颗粒数浓度时,颗粒直径的方差分布对结果的影响,最终验证了该实验方法的可行性。该方法的潜在应用包括对标准颗粒制造厂商的产品在线检测以及对牛奶制品中脂肪和蛋白质的浓度检测研究。     

Circularly polarized localized near-field radiation at the nanoscale

Diffraction-limited circularly polarized electromagnetic radiation has been widely used in the literature for various applications at both optical and microwave frequencies. With advances in nanotechnology, emerging plasmonic nano-optical applications, such as all-optical magnetic recording, require circularly polarized electromagnetic radiation beyond the diffraction limit. In this study, a plasmonic cross-dipole nano-antenna is investigated to obtain a circularly polarized near-field optical spot with a size smaller than the diffraction limit of light. A cross-dipole nano-antenna is composed of four metallic nano-rods placed at a perpendicular orientation with respect to each other. The performance of the nano-antenna is investigated through numerical simulations. In the first part of this study, the nano-antenna is illuminated with a diffraction-limited circularly polarized radiation. An optimal antenna geometry is specified to obtain an intense optical spot that satisfies two necessary conditions for circular polarization: a phase difference of 90° and a unit amplitude ratio between the electric field components in the vicinity of the antenna gap. In the second part of this study, the nano-antenna is illuminated with diffraction-limited linearly polarized radiation. It is shown that the phase difference between the electric field components can be adjusted by selecting either different antenna lengths or different gap distances in the vertical and horizontal directions. Due to the relatively short wavelength of surface plasma waves on the antenna, it is demonstrated that the phase difference can be sufficient to obtain circularly polarized light. An optimal physical configuration for the nano-antenna and the polarization angle of the incident light is identified to obtain a circularly polarized optical spot beyond the diffraction limit from diffraction-limited linearly polarized radiation.     

Synchronization of optical polarization conversion and scattering in chiral fibers

We demonstrate that polarization transformation and scattering in adiabatically twisted single-mode birefringent optical fibers is synchronized so that light in one evolving elliptically polarized mode is freely transmitted while orthogonally polarized light is scattered out of the fiber. Thus, linearly polarized radiation initially oriented along the fast axis of the untwisted fiber is transformed to circularly polarized light with the same sense of rotation as the twisted fiber and is scattered out of the fiber. When the fiber twist is first accelerated and then decelerated, the fiber becomes a broadband, low-insertion-loss, linear polarizer.