微纳螺旋凹槽结构检测光子自旋角动量

建立螺旋凹槽结构模型,具有不同自旋角动量的光束入射到该结构后激发表面等离激元,螺旋凹槽结构的螺旋性与光子自旋角动量耦合,使得不同自旋偏振光激发的表面等离激元具有不同的强度分布.通过螺旋凹槽激发的表面等离激元的强度分布获得入射光的自旋角动量.利用有限元方法计算了左旋偏振光与右旋偏振光激发的表面等离激元在螺旋凹槽中心的光场强度比,最大消光比达到168,实现对光子的自旋角动量的检测.在数值仿真中,分析了不同入射光波长的消光比,入射光波长在600～740nm范围内消光比高于50,其中入射光波长为670nm时的检测效果最佳;此外,研究螺旋凹槽结构参量对消光比的影响,当凹槽宽度为200nm,凹槽深度为70nm,匝数为2时,消光比最大,螺旋凹槽结构检测光子自旋角动量的能力最强.该研究可为集成光学中光子自旋角动量的检测提供一种新途径.     

基于塔姆激元-表面等离极化激元混合模式的单缝加凹槽纳米结构的增强透射

金属单缝纳米结构因为结构简单、易于集成,常用在基于表面等离极化激元(surface plasmon polaritons,SPPs)的纳米结构中构建光源.但是,金属亚波长单缝结构一直存在透射率低的问题,如何提高其透射率一直是研究的重点.为了更好地提高金属亚波长单缝的透射率,本文对之前文献提出的分布式布拉格反射镜(distributed bragg reflector,DBR)和金属银薄膜纳米缝结构进行改进,在金属银薄膜两侧设计凹槽.当TM偏振光由DBR侧入射至DBR-银纳米缝结构时,DBR-银膜界面上和银膜入射侧凹槽一起激发的塔姆激元(Tamm plasmon polaritons,TPPs)和SPPs,以及纳米缝和银膜出射侧凹槽对的SPPs同时激发,利用凹槽激发的SPPs和银膜表面处的TPPs-SPPs混合模式的干涉相长耦合作用,通过塔姆激元的局域场增强效应和两侧凹槽与单纳米缝的干涉相长耦合作用进一步提高了表面等离极化激元模式的激发效率,再加上纳米缝中的类法布里-珀罗腔共振效应,使纳米缝的透射率得到增强.本文采用有限元方法研究了DBR-银纳米缝结构上单纳米缝加凹槽的透射特性.经过一系列参数优化,使DBR-银纳米缝凹槽结构的最大透射率增加到0.22,相对于TiO_2-银纳米缝结构的透射率(0.01)提高了22倍,比文献[23]得到的最大透射率0.166有所提高.研究结果在纳米光源设计、光子集成电路和光学信号传输等相关领域具有一定的应用价值.     

带状波导表面等离子激元的透射特性(英文)

为了深入探讨带状波导的表面等离子激元透射特性,采用衍射光栅耦合方式实现表面等离子激元激发,通过改变金属薄膜厚度和入射光角度,得到薄膜厚度、入射光角度与透射率的变化特性.结果表明:金属薄膜厚度的减少会导致透射带宽迅速降低,同时透射率也随之降低;而入射光角度的改变导致透射光效率的变化.这一研究对于半导体设备的纳米等离子激元耦合具有一定的意义.     

金膜上亚波长小孔阵列表面等离激元颜色滤波器偏振性质

金属薄膜上制备的表面等离激元颜色滤波器具有很强的颜色可调性. 在200 nm厚的金膜上, 通过聚焦离子束刻蚀, 制备一系列周期逐渐变化的圆形、方形、矩形亚波长尺寸小孔方阵列表面等离激元颜色滤波器, 改变入射光的偏振方向, 观察其超透射滤波现象. 研究发现: 对于矩形小孔阵列, 其透射光颜色随入射光偏振方向的变化而改变; 而对于圆形、方形的小孔阵列, 其透射光颜色对入射光的偏振方向并不敏感. 分析表明, 对于金膜上刻蚀的小孔结构, 虽然结构的周期性导致的表面等离激元极化子会对透射光的颜色变化产生一定影响, 但是随小孔形状变化的局域表面等离激元共振才是影响透射光颜色的决定性因素. 如果入射光没有在小孔中激发出局域表面等离激元, 则表面等离激元极化子对透射光的影响也会消失. 根据不同形状小孔周期结构透射光颜色随入射光的偏振变化特点, 制备出了包含两种小孔形状的复合周期结构. 随着入射光偏振方向的改变, 该结构会显示出不同的颜色图案. 关键词： 表面等离激元极化子 局域表面等离激元 颜色滤波器 亚波长小孔阵列     

基于SPPs-CDEW混合模式的亚波长单缝多凹槽结构全光二极管

全光二极管是集成光子回路上最基本的光子器件,如何有效增强全光二极管的单向透射性,提高消光比一直是学者们研究的重点.当前,利用表面等离极化激元(surface plasmon polaritons,SPPs)和复合衍射衰逝波(composite diffracted evanescent wave,CDEW)的亚波长金属微纳结构构建全光二极管器件还鲜有研究.因此,开发出一种可调制的全光二极管,对未来制备复杂的光子回路具有重要意义.本文提出了一种基于SPPs-CDEW混合模式设计全光二极管的方法和结构,该结构结合纳米缝中的类法布里-珀罗共振效应,利用结构参数对SPPs进行调控,实现了光束单向透过的功能.首先,利用理论推导和有限元算法分析了单缝-对称双凹槽纳米结构的透射增强现象,提出了透射增强和削弱的物理机理.其次,计算出规约化透射率随单狭缝和凹槽对之间距离变化的远场透射谱,给出的理论和数值计算结果符合得很好.最后,通过此透射谱精确确定凹槽的位置和数量,得出上表面对称分布五对增强透射凹槽、下表面六对抑制透射凹槽的最优全光二极管结构,有效增强了全光二极管的单向透射性,提高了消光比,最大消光比可以达到38.3 dB,即正向透射率是反向透射率的6761倍,比已有文献提高了14.6 dB,并在850 nm左右有70 nm宽的工作波长带宽(20 dB).本文提出的光二极管结构简单,宽带宽工作,易于集成,耦合效率高,研究结果对光学信号传输、集成光回路、超分辨率光刻等相关领域具有潜在的应用价值.     

基于表面等离激元交叉耦合作用的纳米金属双缝异常透射现象

利用二维时域有限差分方法研究了非对称纳米金属双缝结构在薄隔层情况下对光波的异常透射特性,以及狭缝长度、狭缝数目和入射角度对透射特性的影响.研究发现,该双缝结构中传导的表面等离激元波通过渗透中间隔层材料产生交叉耦合作用,形成对称和反对称耦合模式,导致其透射谱在特定波长位置处形成双共振峰传输和一个透射率为零的透射抑制现象;双缝结构中表面等离激元波交叉耦合作用的本质是其横向电场分量渗入中间隔层材料产生的相互干涉作用,而横向电场分量的初始相位差决定双缝结构中形成的表面等离激元波耦合模式的类型.由于双缝结构的透射极值与各狭缝腔内的法布里-珀罗共振效应密切相关,因此狭缝长度决定透射极值的波长位置,而狭缝数目和入射角度只影响透射峰的传输效率.该双缝结构具备光学滤波和空间分光功能,在新型纳米光子器件领域具有潜在的应用价值.     

表面等离激元传播的调制

光学衍射极限极大地制约了纳米光子器件的发展和应用.基于表面等离激元的纳米器件由于具有突破衍射极限传播和处理光信号的特性而受到广泛关注.通过调控表面等离激元的传播,可以实现纳米尺度上光信号的调制,对片上集成纳米光子回路和光信息处理技术的发展具有重要意义.本文主要介绍了表面等离激元传播调制的基本原理及近年来的研究进展,并分析了不同类型的表面等离激元传播调制的特点.     

银纳米线表面等离激元波导的能量损耗

金属纳米结构的表面等离激元可以突破光学衍射极限,为光子器件的微型化和集成光学芯片的实现奠定基础.基于表面等离激元的各种基本光学元件已经研制出来.然而,由于金属结构的固有欧姆损耗以及向衬底的辐射损耗等,表面等离激元的传输能量损耗较大,极大地制约了其在纳米光子器件和回路中的应用.研究能量损耗的影响因素以及如何有效降低能量损耗对未来光子器件的实际应用具有重要意义.本文从纳米线表面等离激元的基本模式出发,介绍了它在不同条件下的场分布和传输特性,在此基础上着重讨论纳米线表面等离激元传输损耗的影响因素和测量方法以及目前常用的降低传输损耗的思路.最后给出总结以及如何进一步降低能量损耗方法的展望.表面等离激元能量损耗的相关研究对于纳米光子器件的设计和集成光子回路的构建有着重要作用.     

内嵌矩形腔楔形金属狭缝的增强透射

金属薄膜上亚波长金属狭缝的非共振光增强透射对宽频带光子器件的设计具有重要意义. 为增强金属狭缝的光透射, 设计了内嵌矩形腔楔形金属狭缝阵列, 并用有限元方法研究了该结构的透射特性. 结果表明, 内嵌矩形腔的楔形金属狭缝阵列比无矩形腔的楔形金属狭缝阵列透射率高. 此外, 还研究了矩形腔的结构参数和位置对内嵌矩形腔楔形金属狭缝阵列透射特性的影响. 这些结果将对设计具有更高透射能力的金属狭缝具有一定的指导意义. 关键词： 光异常透射 金属狭缝 表面等离极化激元     

纳米银线波导中表面等离极化波激发和辐射的偏振特性研究术

在可见光波段（λ=750nm）,实验研究了在端面辅助情况下,细纳米银线波导中表面等离极化波激发和辐射的偏振特性．实验发现在细纳米银线中,不同偏振态的入射光对应的表面等离极化激元的激发和传输效率有明显不同,但对应的出射光始终为方向恒定的线偏振光．对于化学合成的纳米银线,端面的轴对称性普遍比较好,对此类纳米银线进行激发时,如果入射光偏振态与纳米线近似平行,则激发和传输表面等离极化激元的效率最高;如果正交,激发和传输效率则最低．对于某些端面轴对称性较差的纳米银线,如端面为尖端或类斜面,当入射光偏振态与纳米线有一定夹角时,激发和传输表面等离极化激元的效率最高．在入射光偏振改变的过程中出射光的偏振方向始终与纳米银线平行．最后结合有限元差分方法理论解释了纳米银线中这种偏振特性的物理机理．利用纳米银线中表面等离极化激元激发和辐射的偏振特性,可以在亚波长尺寸上实现对光强和偏振态的调控．     

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.     

Highly twisted M-line of a vortex beam due to the coupling of ultrahigh-order modes

In modern optics, particular interest is devoted to the phase singularities that yield complicated and twisted phase structures by photons carrying optical angular momentum. In this paper, the traditional M-line method is applied to a vortex beam(VB) by a symmetric metal cladding waveguide chip, which can host numerous oscillating guided modes via free space coupling. These ultrahigh-order modes(UOMs) result in high angular resolution due to the high finesse of the resonant chip.Experiments show that the reflected pattern of a VB can be divided into a series of inner and outer rings, whilst both of them are highly distorted by the M-lines due to the UOMs' leakage. Taking the distribution of the energy flux into account, a simple ray-optics-based model is proposed to simulate the reflected pattern by calculating the local incident angle over the cross section of the beam. The theoretical simulations fit well with the experimental results, and the proposed scheme may enable new applications in imaging and sensing of complicated phase structures.     

Displacements and deformations of a vortex light beam produced by the diffraction grating with embedded phase singularity

Properties of vortex light beams produced by a diffraction grating with groove bifurcation (“fork” structure) are studied in the case of small diffraction angles. Analytical expressions are derived for the amplitude distribution of a diffracted beam generated from an incident Gaussian beam with arbitrary radius and wavefront curvature, transversely shifted and inclined with respect to the nominal axis (normal to the grating crossing it at the bifurcation point). In such situations, the output beam becomes asymmetric; the optical vortex core and the intensity maximum displace orthogonally to the incident beam shift direction. The nearest vicinity of the vortex core preserves its circular symmetry and the optical vortex remains locally isotropic. The effects of misalignment depend on the incident beam characteristics, the diffraction order and the propagation distance behind the grating. Experimental measurements support the results of calculations.     

Observation of orbital angular momentum transfer between acoustic and optical vortices in optical fiber

Acousto-optic interaction in optical fiber is examined from the perspective of copropagating optical and acoustic vortex modes. Calculation of the acousto-optic coupling coefficient between different optical modes leads to independent conservation of spin and orbital angular momentum of the interacting photons and phonons. We show that the orbital angular momentum of the acoustic vortex can be transferred to a circularly polarized fundamental optical mode to form a stable optical vortex in the fiber carrying orbital angular momentum. The technique provides a useful way of generating stable optical vortices in the fiber medium.     

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.     

Measuring the orbital angular momentum of elliptical vortex beams by using a slit hexagon aperture

The far-field diffraction pattern of an elliptical vortex beam by a slit hexagon aperture is investigated theoretically and experimentally. It is found that the number of the dark spots or stripes in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and that the centre of each dark spot or stripe is just a phase singularity point. Based on this property, it provides us a simple way to detect the orbital angular momentum (OAM) of an optical vortex beam.     

A crossed focused vortex beam with application to cold molecules

We report the generation of a crossed, focused, optical vortex beam by using a pair of hybrid holograms, which combine the vortex phase and lens phase onto a spatial light modulator. We study the intensity distributions of the vortex beam in free propagation space, and the relationship of its dark spot size with the incident Gaussian beam's waist, the lens's focal length, and its orbital angular momentum. Our results show that the crossed, focused, vortex beam's dark spot size can be as small as 16.3μm and adjustable by the quantum number of the orbital angular momentum, and can be used to increase the density of trapped molecules. Furthermore, we calculate the optical potential of the blue-detuned, crossed vortex beam for MgF molecules. It is applicable to cool and trap neutral molecules by intensity-gradient-induced Sisyphus cooling, as the intensity gradient of such vortex beam is extremely high near the focal point.     

Subwave spikes of the orbital angular momentum of the vortex beams in a uniaxial crystal

We have theoretically predicted gigantic spikes of orbital angular momentum caused by conversion processes of the centered optical vortex in the circularly polarized components of an elliptic vortex beam propagating perpendicularly to the crystal optical axis. We have experimentally observed the conversion process inside subwave deviations of the crystal length. We have found that the total orbital angular momentum of the wave beam is conserved.     

Wavelength-scalable micro-fabricated wedge for generation of optical vortex beam in optical manipulation

An optical wedge phase plate is fabricated for the study of vortex beam generation in optical manipulation. It is demonstrated that the wedge phase element provides scalability for generating the vortex beam at different wavelengths as long as the incident beam size is adjusted accordingly. The presence of singularity for different wavelengths and the associated orbital angular momentum are confirmed experimentally in optical manipulation. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. PACS 41.85.Ct; 42.82.Cr