Polarization evolution characteristics of focused hybridly polarized vector fields

We investigate the focusing property and the polarization evolution characteristics of hybridly polarized vector fields in the focal region. Three types of hybridly polarized vector fields, namely azimuthal-variant hybridly polarized vector field, radial-variant hybridly polarized vector field, and spatial-variant hybridly polarized vector field, are experimentally generated. The intensity distributions and the polarization evolution of these hybridly polarized vector fields focused under low numerical aperture (NA) are experimentally studied and good agreements with the numerical simulations are obtained. The three-dimensional (3D) state of polarization and the field distribution within the focal volume of these hybridly polarized vector fields under high-NA focusing are studied numerically. The optical curl force on Rayleigh particles induced by tightly focused hybridly polarized vector fields, which results in the orbital motion of trapped particles, is analyzed. Simulation results demonstrate that polarization-only modulation provided by the hybridly polarized vector field allows one to control both the intensity distribution and 3D elliptical polarization in the focal region, which may find interesting applications in particle trapping, manipulation, and orientation analysis.     

基于可调谐复振幅滤波器的超长焦深矢量光场

根据矢量光场衍射积分理论和离散复振幅光瞳滤波原理, 通过一种由双λ/2波片和离散复振幅滤波器组成的可调谐复振幅滤波器, 研究了大数值孔径下超长焦深聚焦矢量光场的构建与调控. 给出了一个六环带区的离散复振幅滤波器, 对入射光场的偏振态、振幅滤波和相位滤波三者进行同步优化, 获得了焦深接近10λ的三维平顶光场; 通过调控双λ/2波片夹角来改变聚焦光场的矢量化结构, 使之在光针场、平顶光场、光管场及中间结构光场之间交替变化. 研究结果揭示了入射光场矢量化结构演化与聚焦光场矢量化结构变换之间的关系, 解决了获取动态的、可调控的超长焦深聚焦光场的问题. 两种基本的聚焦光场光针场、光管场的独自使用或三维平顶光场的调和使用, 将会在光学显微、光学微纳操控以及光学精细加工领域获得重要应用.     

Experimental verification on tightly focused radially polarized vortex beams

The theoretical and experimental results of tightly focused radially polarized vortex beams are demonstrated. An auto-focus technology is introduced into the measurement system in order to enhance the measurement precision, and the radially polarized vortex beams are generated by a liquid-crystal polarization converter and a vortex phase plate. The focused fields of radially polarized vortex beams with different topological charges at numerical apertures （NAs） of 0.65 and 0.85 are measured respectively, and the results indicate that the total intensity distribution at focus is dependent not only on the NA of the focusing objective lens and polarization pattern of the beam but also on the topological charge l of the beam. Some unique focusing properties of radially polarized vortex beams with fractional topological charges are presented based on numerical calculations. The experimental verification paves the way for some practical applications of radially polarized vortex beams, such as in optical trapping, near-field microscopy, and material processing.     

Sharper focus for a radially polarized light beam

We experimentally demonstrate for the first time that a radially polarized field can be focused to a spot size significantly smaller [0.16(1)lambda(2)] than for linear polarization (0.26lambda(2)). The effect of the vector properties of light is shown by a comparison of the focal intensity distribution for radially and azimuthally polarized input fields. For strong focusing, a radially polarized field leads to a longitudinal electric field component at the focus which is sharp and centered at the optical axis. The relative contribution of this component is enhanced by using an annular aperture.     

Longitudinal anisotropy of the photoinduced molecular migration in azobenzene polymer films

The effects of tightly focused, higher-order laser beams on the photoinduced molecular migration and surface deformations in azobenzene polymer films are investigated. We demonstrate that the surface relief is principally triggered by longitudinal fields, i.e., electric fields polarized along the optical axis of the focused beam. Our findings can be explained by the translational diffusion of isomerized chromophores when the constraining effect of the polymer-air interface is considered.     

Probing Dual Asymmetric Transverse Spin Angular Momentum in Tightly Focused Vector Beams in Optical Tweezers

The spin-orbit interaction (SOI) of light generated by tight focusing in optical tweezers is regularly employed in generating angular momentum - both spin and orbital - the effects being extensively observed in trapped mesoscopic particles. Specifically, the transverse spin angular momentum (TSAM), which arises due to the longitudinal component of the electromagnetic field generated by tight focusing is of special interest, both in terms of fundamental studies and associated applications. This study provides an effective and optimal strategy for generating TSAM in optical tweezers by tightly focusing first-order radially and azimuthally polarized vector beams with no intrinsic angular momentum (AM) into a refractive index stratified medium. The choice of such input fields ensures that the longitudinal spin angular momentum (LSAM) arising from the electric (magnetic) field for the radial (azimuthal) polarization is zero. As a result, the effects of the electric and magnetic TSAM are exclusively observed separately in the case of input first-order radially and azimuthally polarized vector beams on single optically trapped birefringent particles. This research opens up new and simple avenues for exotic and complex particle manipulation in optical tweezers.     

Experimental Research on Focusing Property of Radially Polarized Beam Based on Solid Immersion Lens

Realization of a near-field optical virtual probe based on an evanescent Bessel beam is strongly dependent on a radially polarized beam; this makes it essential to study the focusing property of the beam. In this paper, two experimental setups based on a fiber device and a liquid crystal device, respectively, are built to generate a radially polarized beam. This beam and an annular radially polarized beam are focused by means of a high numerical aperture objective and a solid immersion lens (SIL). Near-field distribution of the focus spot, the evanescent Bessel field, is experimentally measured with a scanning near-field optical microscope (SNOM). The full width at half maximum (FWHM) of the central peak of the evanescent Bessel field is about 200 nm in the close vicinity of the bottom surface of SIL. This has potential for use as a near-field optical virtual probe.     

Controllable vector bottle-shaped fields generated by focused spatial-variant linearly polarized vector beams

We demonstrate that the optical bottle-shaped fields can be controllably generated by the focused spatial-variant linearly polarized vector beams. Based on the vectorial Rayleigh–Sommerfeld formulas under the paraxial approximation, we present theoretically the analytical expression for the focused field of the vector beam and predict the evolution of the sate of polarization (SoP) in the focal region. Experimentally, we observe the vector bottle-shaped field that is in agreement with the numerical simulations. In particular, we validate that both the SoP and the size of the optical bottle field are manipulated easily by varying the azimuthal topological charge and the radial mode index.     

Focusing property of a double-ring-shaped radially polarized beam

The intensity distributions of a tightly focused radially polarized beam that has a double-ring-shaped transverse mode pattern were calculated based on vector diffraction theory. The distribution of the longitudinal component near the focus varied drastically with the degree of truncation of the incident beam by a pupil. When the ratio of the pupil radius to the beam radius was approximately 1.3, the longitudinal component disappeared at the focal point, owing to destructive interference. This dark area surrounded by an intense light field was of the order of the wavelength, with excellent intensity symmetry.     

Two-photon fluorescence scanning near-field microscopy based on a focused evanescent field under total internal reflection

We present two-photon fluorescence near-field microscopy based on an evanescent field focus produced by a ring beam under total internal reflection. The evanescent field produced by this method is focused by a high-numerical-aperture objective, producing a tightly confined volume that can effectively induce two-photon excitation. The imaging system is characterized by the two-photon-excited images of the nanocrystals, which show that the focused evanescent field is split into two lobes because of the enhancement of the longitudinal polarization component at the focus. This feature is confirmed by the theoretical prediction. Unlike other two-photon near-field probes, this method does not have the heating effect and requires no control mechanism of the distance between a sample and the probe.     

Tight focusing of linearly and circularly polarized vortex beams; effect of third-order spherical aberration

Tight focusing of linearly and circularly polarized vortex beams is studied in the presence of third-order spherical aberration, using vectorial Debye–Wolf integral. Results for total intensity distribution are presented for both polarizations. In addition, results for x-, y-, and z-polarization components are presented for the circularly polarized beam. Generation of longitudinal optical vortex in the tightly focused left circularly polarized beam has also been demonstrated by plotting its phase distribution. Compensation for the effect of spherical aberration has been studied in the presence of defocusing. Effect of aberration on the dark core of a tightly focused azimuthally polarized beam is also investigated.     

Sub wavelength multiple focal spot generation of high NA objective lens using diffractive optical element

In this article, based on the vector diffraction theory, the effect of specially designed phase modulating optical element by means of an incident tightly focused azimuthally polarized Bessel Gaussian beam in the focal region of high NA lens is investigated numerically. It is observed, that a specially designed diffractive optical modulation element (DOE) can generate multiple focal spot segment of transversely polarized in the focal region by controlling the angles of DOE. Such kind of sub wavelength transversely polarized focal spots segment may find wide applications in multiple optical traps and optical manipulation technology.     

Second harmonic generation of metal nanoparticles under tightly focused illumination

The near-field and far-field second harmonic(SH) responses of a metal spherical nanoparticle placed in the focal region of a highly focused beam are investigated by using the calculation model based on three-dimensional finite-difference time-domain(FDTD) method. The results show that off-axis backward-propagating SH response can be reinforced by tightly focusing, due to the increase of the relative magnitude of the longitudinal field component and the phase shift along the propagation direction.     

阿基米德螺旋微纳结构中的表面等离激元聚焦

研究光在微纳结构中的分布与传播, 实现在纳米范围内操纵光子, 对于微型光学芯片的设计有着重要意义. 本文利用聚焦离子束刻蚀方法, 在基底为石英玻璃的150 nm厚金膜上刻制了不同参数的阿基米德螺旋微纳狭缝结构, 通过改变入射光波长、手性、及螺旋结构手性和螺距等方式, 在理论和实验上系统地研究了阿基米德螺旋微纳结构中的表面等离激元聚焦性质. 我们发现, 除了入射激光偏振态、螺旋结构手性之外, 结构螺距与表面等离激元波长的比值也可以用来控制结构表面电场分布, 进而在结构中心形成0阶、1阶乃至更高阶符合隐失贝塞尔函数的涡旋电场. 通过相位分析, 我们对涡旋电场的成因进行了解释. 并利用有限时域差分的模拟方法计算了不同螺距时, 结构中形成的电场及相应空间相位分布. 最后利用扫描近场光学显微镜, 观测结构中不同的光场分布, 在结构中心得到了亚波长的聚焦光斑及符合不同阶贝塞尔函数的涡旋形表面等离激元聚焦环.     

Near-field properties of diffraction through a circular subwavelength-size aperture

Analytical nonparaxial vectorial electric field expressions for both Gaussian beams and plane waves diffracted through a circular aperture are derived by using the vector plane angular spectrum method for the first time,which is suitable for the subwavelength aperture and the near-field region.The transverse properties of intensity distributions and their evolutions with the propagating distance,and the power transmission functions for diffracted fields containing the whole field,the evanescent field and the propagating field are investigated in detail,which is helpful for understanding the relationship between evanescent and propagating components in the near-field region and can be applied to apertured near-field scanning optical microscopy.     

Near-field effect in two-atom system

The self-consistent problem is solved for the interaction of two dipole atoms situated at arbitrary distance from one another with the field of quasiresonant light wave. Atoms are considered to be linear Lorenz oscillators. Polarizing fields inside the system include both Coulomb and retarding parts. The solutions obtained are investigated for the case when atoms have the same polarizabilities and interatomic distance is much less than external light wavelength. Formulas for electric fields inside and outside of small object are obtained. It is shown that longitudinal and transverse optical oscillations are possible to exist inside small two-atom object. Dispersion laws of these oscillations depend upon interatomic distance and upon angle between axis of the system and the direction of propagation of external wave. The field outside the small object in wave zone is linearly polarized with the choice of linear polarization of external field. However, the directions of polarization of these waves are different and depend essentially upon frequency. The amplitude of field outside small object in wave zone is shown to depend essentially on the frequency of external field and interatomic distance. The results obtained are treated as near-field effect in the optics of small objects making it possible to investigate the structure of small objects with optical radiation. Received 26 October 1998 and Received in final form 26 January 2000     

Near-field optical power transmission of dipole nano-antennas

Nano-antennas in functional plasmonic applications require high near-field optical power transmission. In this study, a model is developed to compute the near-field optical power transmission in the vicinity of a nano-antenna. To increase the near-field optical power transmission from a nano-antenna, a tightly focused beam of light is utilized to illuminate a metallic nano-antenna. The modeling and simulation of these structures is performed using 3-D finite element method based full-wave solutions of Maxwell’s equations. Using the optical power transmission model, the interaction of a focused beam of light with plasmonic nano-antennas is investigated. In addition, the tightly focused beam of light is passed through a band-pass filter to identify the effect of various regions of the angular spectrum to the near-field radiation of a dipole nano-antenna. An extensive parametric study is performed to quantify the effects of various parameters on the transmission efficiency of dipole nano-antennas, including length, thickness, width, and the composition of the antenna, as well as the wavelength and half-beam angle of incident light. An optimal dipole nano-antenna geometry is identified based on the parameter studies in this work. In addition, the results of this study show the interaction of the optimized dipole nano-antenna with a magnetic recording medium when it is illuminated with a focused beam of light.     

Investigation of Near-Field Imaging Characteristics of Radial Polarization for Application to Optical Data Storage

Radially polarized incident light can generate a more confined longitudinal electric field on a focal plane in near-field (NF) optics than focusing circularly polarized light. Using this phenomenon, it is feasible to reduce beam spot size on storage media to increase the areal density of optical data storage. A radially polarized beam generates a beam spot which is 20% more confined on the 1st surface of medium than that of circularly polarized light. However, the peak intensity of total electric field sharply decreases and its transverse component is much more dominant inside the media stack. This confirms that radially polarized optics can be a candidate not for an NF recording system but for an NF read-only memory (ROM) system. Potentially, the results could be useful to understand the effect of radial and circular polarizations inside and outside medium for various applications of NF optics.     

Focusing of evanescent vector waves

We study focusing of two and three-dimensional evanescent vector waves, with a particular emphasis on identifying suitable intensity structures for applications in optical data storage. For two-dimensional evanescent waves large transverse spatial wave vectors result in purely circularly polarized evanescent states. We suggest that these may have applications in all-optical data storage through the inverse Faraday effect. On the other hand, for three-dimensional evanescent waves longitudinally polarized modes are observed to give the most tightly focused spot, and this may be utilized to confine light behind a solid immersion lens.     

轴对称矢量光束聚焦特性研究现状及其应用

轴对称矢量光束是一种空间非均匀偏振光束, 中心光强为零, 经物镜聚焦后能在焦点附近产生空间场分量. 在高变迹系数光学系统成像情况下, 与线偏光、圆偏光相比, 径向偏振光与光瞳滤波技术及图像复原技术结合, 能获得较小焦斑, 提高横向分辨力. 介绍了轴对称矢量光束的特性, 基于电偶极子辐射模型和矢量衍射理论研究了轴对称矢量光束经高数值孔径物镜聚焦后的特性, 系统介绍了基于轴对称矢量光束实现光斑紧聚焦的几种方法, 并简述了轴对称矢量光束在差动共焦超分辨成像领域的研究设想. 关键词： 差动共焦显微技术 紧聚焦 光瞳滤波 轴对称矢量光束