Fundamental differences between micro- and nano-Raman spectroscopy

Electric field polarization orientations and gradients close to near-field scanning optical microscope (NSOM) probes render nano-Raman fundamentally different from micro-Raman spectroscopy. With x -polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano-Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near-field of a NSOM probe lead to a breakdown of the assumption of micro-Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano-Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra-red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.     

基于双环形旋涡相位调制的多焦点产生

为提高光学显微成像及微粒控制的效率,提出一种通过双环形旋涡相位板对入射光为拉盖尔高斯柱矢量光束进行波前相位调制的多焦点产生方法。通过Debye矢量衍射理论分析了该相位板对焦点区域光强分布的影响。分析表明,经调节内外环旋涡相位拓扑数及内环的半径,可以对焦点区域光强分布进行整形,在焦平面上产生多焦点,而且通过改变入射柱矢量光束的偏振态,可以进一步得到光场由轴向分量为主的多焦点及由横向分量组成的多焦点。通过改变旋涡相位板的拓扑数及光束的参数,可以达到对光斑的数量的控制。这对于捕捉折射率高于周围环境折射率的微粒具有重要的应用价值。     

Study of the focused laser spots generated by various polarized laser beam conditions

In this work, three‐dimensional near‐field imaging of the focused laser spot was studied theoretically and experimentally. In the theoretical simulation, we use the electromagnetic equivalent of the vectorial Kirchhoff diffraction integral to calculate the intensity distribution of the focal region, and a high depolarization is found in high numerical aperture systems (NA = 0.85). The experimental set‐up is based on a near‐field scanning optical microscope (NSOM) system. A high‐NA objective lens is used to focus incident light of various polarizations, and a tapered near‐field optical fibre probe of the NSOM system is used to determine the intensity of the focal field. The results show an asymmetric distribution of the focused intensity with the linear polarized laser beam.     

新型光子纳米射流器件的设计

光子纳米射流是一种高强度,极窄的亚波长电磁场区域,它是由介电微球或微柱体的Mie散射对电磁场的聚焦作用产生的。光子纳米射流广泛应用于激光加工、纳米光刻、光学高密度存储以及超分辨率显微镜。从径向偏振光的角度出发,使用一种介电圆环结构对光束进行聚焦,由于径向偏振光在焦点区域可以产生较强的纵向场,通过优化圆环的尺寸、折射率以及与物镜焦点的相对位置,可以得到超过90%光束质量的纵向光子纳米射流,而且强度相比于未使用圆环时可以提高约一个数量级,并在高折射率下可以获得半高全宽小于衍射极限尺寸的光斑,因此该结构预计可以在粒子加速、光镊以及拉曼光谱学中有所应用。     

保偏光纤模场直径和数值孔径测试研究

基于光纤测量远场法原理测量光纤远场光强分布。通过改进远场法得到光纤的远场光强分布后,同时计算得到保偏光子晶体光纤的模场直径和数值孔径,使测量装置实现集成化和简便化。通过光束测试仪测量光纤的出射光强分布,光束测试仪测量的是光纤中心最大光强点的两个垂直方向上的光强分布。保偏光子晶体光纤的出射光斑是椭圆的,每个方向的模场直径、数值孔径分布并不相同。传统的测试方法不能解决这个问题。通过旋转光纤测量光纤各个方向上的光强分布,然后计算各个方向上的模场直径,最后通过拟合各个方向上的模场直径得到保偏光子晶体光纤椭圆形的模场分布。实验测得保偏光子晶体光纤椭圆光斑长短轴的模场直径分别为7.5μm和4.2μm,数值孔径分别为0.159和0.276。     

Probing highly confined optical fields in the focal region of a high NA parabolic mirror with subwavelength spatial resolution

Parabolic mirrors with a high numerical aperture can be conveniently used to produce highly confined optical fields in the focal region. Furthermore, these fields can have interesting polarization behaviour due to the high numerical aperture. In particular, if the mirror is illuminated with a size matched radially polarized or azimuthally polarized doughnut mode, the electric field has in the focal region almost exclusively a longitudinal or a transverse polarization component. Such field distributions are interesting for applications in confocal or near‐field optical microscopy. Here we present experimental results where we have probed some of these field distributions by raster scanning a fine gold tip in nanometer steps through the focal region and detecting the scattered light intensity. The measured intensity patterns are compared with corresponding vector‐field calculations.     

基于等效层理论的薄膜滤光片中心波长消偏振膜系设计

针对薄膜滤光片倾斜入射时产生的偏振光中心波长分离现象和偏振相关损耗,本文利用等效层理论设计了倾斜入射下中心波长消偏振的100GHz信道间隔滤光片膜系结构,实现了薄膜滤光片的角度和波长调谐。首先通过相位关系分析,计算了滤光片间隔层的消偏振等效折射率,实现了不同偏振光中心波长的对准。然后,根据等效层理论,设计三层对称膜层结构实现了对间隔层中心波长消偏振等效折射率的替换。与原有的五层规整低偏振薄膜滤光片相比,本文提出的膜系结构更为简单,对消偏振等效折射率的替换更为精确。仿真和实验结果表明:该膜系结构的薄膜滤光片能实现0~20°倾斜入射的偏振光中心波长对准,偏振光波长偏离度小于0.03nm,波长调谐范围能达到35nm。     

均匀光强分布的5kW半导体激光硬化光源研制

随着半导体激光自身输出功率和转换效率的提升,半导体激光已经广泛的应用于激光加工领域。本文针对目前激光加工领域对半导体激光硬化光源的需求,研制了波长为976nm的连续输出半导体激光硬化光源。该光源采用空间/偏振合束工艺达到了较高的合束效率,采用柱面微透镜阵列分割与聚焦镜复合较好地匀化了巴条激光器慢轴方向固有的光强起伏,使聚焦光斑的光强呈平顶分布。最后对该光源进行了实验装调和测试。结果表明,在工作电流为93A时,光源的最大输出功率为5 120W,电光转换效率达47%,光斑尺寸为2mm×16mm,光斑分布为平顶分布,平整度大于90%,满足工业中对大面积、高效率激光硬化的要求。     

拉盖尔-高斯光经超表面的聚焦特性

为了研究椭圆偏振拉盖尔-高斯光经超表面的聚焦特性,重点分析了聚焦光场的电场强度以及相位分布特性。利用时域有限差分(FDTD)方法仿真椭圆偏振拉盖尔-高斯光经超表面的聚焦光场。研究表明,椭圆偏振拉盖尔-高斯光经超表面聚焦后会在聚焦光场的纵向分量电场中出现自旋角动量转化为轨道角动量的现象,通过改变超表面的数值孔径大小、拉盖尔-高斯光的偏振类型等参数可以调控涡旋光场形态分布。该现象可应用于光镊、量子加密、光学扳手等技术中。     

A Bloch wave analysis of optical sectioning in aberration-corrected STEM

The reduction in the focal depth of field that occurs through the use of larger apertures in aberration-corrected STEM allows three-dimensional information to be retrieved by optical depth sectioning. This paper explores depth sectioning in zone-axis crystals using Bloch wave calculations. By decomposing the calculation into the contribution from individual states and from individual partial plane waves in the convergent cone of illumination, we explain the form of the electron intensity in the crystal as a function of depth. Two separate effects are found that can cause the intensity maximum to deviate from that of the expected defocus value. Firstly it is found that the unbound, high angle excited states give rise to a behaviour similar to that of the probe focusing in the vacuum, but with a prefocusing effect due to the lensing effect of the potential of the atomic column. Superimposed upon this prefocused peak is an oscillation due to interference between the channelling 1s state and the rest of the wavefunction. This oscillation can actually prevent an intensity maximum being formed at certain depths in the crystal, and will complicate the interpretation of optical sectioning data.     

高度局域线偏振时空光涡旋的生成方法

携带横向轨道角动量的时空光涡旋是一种新型光脉冲波包,受到越来越多的关注。本文报道一种经过高数值孔径透镜聚焦,在透镜焦平面上产生偏振态可控的线偏振时空光涡旋的方法。为克服聚焦透镜对入射时空涡旋产生的时空像散效应,需对入射时空波包作预劈裂处理。基于Richards Wolf矢量衍射公式仿真了不同偏振态下的高度局域时空涡旋的三维时空场分布,并对其强度和相位特性进行了分析。生成的水平偏振、垂直偏振以及45°偏振高度局域时空涡旋证明了本文所提方法的有效性。     

基于光场相机的深度面光场计算重构

光场相机与传统相机不同,通过在主镜组和传感器间特定位置设置微透镜阵列,实现在采集物方光强的同时记录光线方向。为了重构物方光场,研究了基于光场相机的深度面计算重构算法,对该算法所采用的焦点堆栈、投影切片定理进行研究。首先,对四维光场定义及光场相机工作原理进行分析;建立了相机内部深度面重构模型,分析了投影切片定理在深度面获取的应用,并推导出不同深度面图像表达式;根据得到不同深度位置图像,研究了计算重构算法,搭建了含有微透镜阵列的微型单相机光场采集系统,采集原始光场,利用本文方法实现了物方光场的逆向重构。实验结果表明:利用本文重构算法,光场相机采集的物方光场可通过滤波计算方法逆向重构,通过重构光场数据可获得物方场景深度信息。本文深度面获取算法较其他算法节省30%以上的时间,各深度面图像峰值信噪比在25~30dB之间,实现了高精度、稳定可靠的计算光场重构。     

基于渡越-切伦科夫辐射原理太赫兹横向场频率特性的实验研究

基于渡越-切伦科夫辐射原理,单色飞秒激光脉冲聚焦到空气中形成等离子体进而辐射出径向偏振太赫兹波,径向偏振太赫兹波经过紧聚焦后在焦点处产生太赫兹波横向偏振分量。为了有效调控太赫兹横向分量的频率分布和振幅特性,首先,通过理论分析太赫兹横向场分量在外加电压下的分布规律,给出不同角度的外加电压对太赫兹横向场频率强度的影响。然后,采用对等离子体施加外部电场的方法,得到一个最佳的纵向电场角度产生高强度太赫兹横向偏振分量。对发展太赫兹波特性的基础研究以及太赫兹技术的应用具有重要的参考意义。     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

Far-field characterization of small apertures

A far‐field method for characterizing apertures based on the relationship between the relative intensity of propagating modes in a given medium and a small aperture illuminated with a light beam is proposed. A simple optical set‐up based on computer‐generated holograms and spatial filters is suggested to measure the relative strength of modes and provide axial intensity measurements in the far‐field zone. It is shown that the minimal size of a spot that may be measured decreases with an increase in the refractive index of a medium into which light propagates and with the use of high‐order spatial mode filters. The intensities transmitted through tapered optical fibre tips have been measured and their aperture diameters determined using window‐type spatial filters. The results have been compared with measurements using scanning electron microscopy.     

MRT letter: Experimental verification of vectorial theory to determine field at the geometrical focus of a cylindrical lens

We provide experimental evidence supporting the vectorial theory for determining electric field at and near the geometrical focus of a cylindrical lens. This theory provides precise distribution of field and its polarization effects. Experimental results show a close match (≈ 95% using χ²‐test) with the simulation results (obtained using vectorial theory). Light‐sheet generated both at low and high NA cylindrical lens shows the importance of vectorial theory for further development of light‐sheet techniques. Potential applications are in planar imaging systems (such as, SPIM, IML‐SPIM, imaging cytometry) and spectroscopy. Microsc. Res. Tech. 77:105–109, 2014. © 2014 Wiley Periodicals, Inc.     

多角度偏振辐射计星上积分球结构设计及检测

为满足星载多角度偏振辐射计六孔径同时定标及全光路定标的要求,根据多角度偏振辐射计的光路特点和结构约束设计了六出孔椭球形积分球定标源,并对积分球辐射输出的一致性﹑均匀性及偏振特性进行了检测。其中,一致性的检测采用定标后的光谱辐射计直接测量;均匀性的检测考虑积分球出口小,采用了透镜加光阑扫描的方法;偏振度的检测使用了根据马吕斯消光定律设计的线偏振检测仪。实验和检测结果表明,采用的检测和数据处理方法提高了系统的信噪比和测量的准确性。积分球成对开孔输出一致性优于99.5%,面均匀性优于99%,偏振度低于1%,这些结果满足多角度偏振辐射计的辐射定标需求。     

考虑偏振片非理想性的可见光偏振成像修正模型

常用的偏振成像理论模型大多基于理想偏振片假设,即偏振片的消光比为无穷大且主方向已知,而实际偏振片的非理想性会对偏振成像系统的测量精度产生明显的影响,为降低这种影响,对考虑偏振片非理想性的偏振成像模型进行研究。以基于斯托克斯矢量的偏振成像模型为基础,通过分析实际偏振片对入射光偏振态的改变,提出了一种考虑偏振片非理想性的可见光偏振成像修正模型,给出了考虑实际偏振片性能及主方向误差的偏振度、偏振角修正公式。利用分时偏振成像系统对线偏振光的偏振度进行测量,实验结果表明:当偏振片消光比为100:1时,理想模型的线偏振光偏振度测量的平均相对误差为5.53%,修正模型的偏振度测量的平均相对误差降低到3.62%。应用该修正模型可在使用低消光比偏振片时达到与高消光比偏振片相当的偏振度测量精度,使偏振成像系统能够拥有更大视场,成本降低。     

基于几何相位的线偏振聚焦全介质超透镜的设计

超透镜凭借其独特的优势逐渐取代或补充了传统的折光和衍射透镜,从而领先小型化高性能光学设备和系统。这种小型化有望带来紧凑的纳米级光学设备,应用于相机、照明、显示器和可穿戴光学。但在传统的超透镜设计中,几何相位通常需要左右旋圆偏振光的手性限制。为了使其不再局限于左旋圆偏振光(LCP)和右旋圆偏振光(RCP)入射,设计了基于几何相位的全介质超透镜,实现了线偏振光的近场聚焦,其与超表面滤光片主要输出波长(可见光波段)一致。采用有效时域差分法研究并验证了超透镜各变量对光场聚焦特性的影响,超透镜等其他超表面光学元器件前所未有的设计自由度将极大地扩大微光学和集成光学的应用领域。     

Simulation of a hollow cone-shaped probe in aberration-corrected STEM for high-resolution tomography

In a simulation study, we found that focal depth extension using a hollow cone-shaped probe with an annular aperture is useful for three-dimension (3D) tomography of aberration-corrected scanning transmission electron microscopy (STEM). Our calculations showed that, for 200 kV STEM, a sub-angstrom sized probe could extend the focal depth from a few to more than several tens nm. We also examined the influence of obstructing bridges, including actual fabricated annular apertures, on focused probe intensity distribution. We found that, to avoid any distortion of probe intensity, the width of the bridges should be narrow. Quantitative evaluation showed that the ratio of obstructing area of the bridges to the area of the annular slit should be less than 0.11.