Simple and high efficient optical trapping using a cylindrical lens and a single plane wave of incidence

We suggest a simple and high efficient method for trapping particles in the evanescent field. In this method, a single plane wave is normally incident on the cylindrical surface of a cylindrical lens and then incident on the plane surface of the lens at an angle larger than the critical angle. Multiple reflections of light within the cylindrical lens create two evanescent waves with different directions in the transmitted field. Interference of two evanescent waves comes into being a standing wave which can stably trap particles close to the top of the cylindrical lens. Based on the Rayleigh approximation, we obtain analytical expressions of optical force acting on a Rayleigh particle placed in the vicinity of the lens. We find that the trap stiffness and trap depth is dependent on the radius of the cylindrical lens, wavelength and polarization of light, and incident angle at the lens–liquid interface.     

Enhancement of light-emitting efficiency using combined plasmonic Ag grating and dielectric grating

Based on the analysis of the near-field evanescent wave in total internal reflection, the flip-chip light-emitting diode (LED) structure was proposed by placing a plasmonic Ag grating and a perforated sapphire grating in the substrate. The finite difference time domain (FDTD) method has been applied to study the spectral properties of the hybrid structure and the enhancement factor of light extraction efficiency of the LED model. From the computation examples, the effects of structure parameters on the extraction enhancement have been investigated. The results indicate that the plasmonic grating can enhance the near-field evanescent wave and couple it to propagation wave in the specific wavelength bands, which leads to the photons emitting out of the LED chip with high extraction efficiency. Due to the combined gratings used, the enhancement factor of the light extraction efficiency can reach approximately 4 times at a relatively longer wavelength.     

Cascade lasing with spherical microparticles

Lasing characteristics of dye-doped spherical solid microparticles produced by photopolymerization in aerosols are investigated. It is well known that dielectric spherical microparticles are optical resonators with very high quality. The resonant internal field is not completely confined to the interior of the particle. An evanescent wave extends a couple of micrometers into the surrounding area. This evanescent field decays exponentially. We demonstrate that cascade lasing can be excited by coupling of two spherical microresonators through this evanescent field.     

光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).     

Evanescent waves of an annular left-handed material lens

The imaging system formed by an annular left-handed material （LHM） lens as well as the evanescent waves in the lens are simulated numerically with a finite-difference time-domain （FDTD） method. For b - a 〉 λ （a and b are respectively the inner and outer radii of the annular lens, and λ is the wavelength）, when a point source is placed at an internal grid point, we demonstrate that the evanescent waves are produced around the internal interface, and cannot propagate outwards. As for b - a 〈λ ）,, the evanescent waves appear around both the internal and the external interfaces, which remarkably implies the coupling between the two interfaces. Hence it can be inferred that the evanescent waves around the external interface participating in the super-resolution imaging result from the coupling of the evanescent waves around the interface. Moreover, the partly uncomprehended properties of the evanescent waves in the LHM slab are also disclosed. It is conducive to understanding the evanescent waves in the LHMs further.     

石墨烯覆盖微纳光纤复合波导全光优先吸收特性

利用二氧化碳激光器加热法,将普通单模光纤拉制成微纳光纤,用湿法转移石墨烯覆盖在微纳光纤上构成复合波导,不同波长的光通过耦合器进入复合波导,以倏逝波的形式与石墨烯相互作用,开展石墨烯优先吸收特性的研究。当短波作为泵浦光时,随着入射强度的增长,测得输出端长波信号光光谱的变化,获得了约3.5 dB的调制深度,0.62 dB·mW-1的调制效率。当长波作为泵浦光并改变入射光强时,在输出端测得作为信号光的短波透过率变化约1.9%。实验结果表明,随着任意波长泵浦光入射光强的增长,复合波导对其表现出优先吸收的特性。实验还测试了长波和短波分别经过复合波导后透过率随输入功率的变化,得出长波的透过率增加速度比短波更快,并从能带和倏逝波两方面作出了对应的理论分析。     

Plasmon mechanism of light transmission through a metal film or a plasma layer

It is shown that a smooth metal film (or a plasma layer) can be made transparent for an electromagnetic wave when two identical subwavelength diffraction gratings are placed on both sides of the film. The electromagnetic wave transmission through the metal film is caused by excitation of evanescent surface waves (plasmons) and their transformation into propagating waves at the gratings. A model which is developed analytically shows that the problem of the wave transmission is physically equivalent to the problem of excitation of two coupled resonators of evanescent waves which are formed at the two film surfaces.     

柱形微腔回音壁激光光谱模式的精确标定

由柱形微腔中回音壁模式满足的本征值方程,得到确定回音壁模式位置和间距的近似解析公式。以此近似解析公式,首次对直径在215～328 μm间的5个柱形微腔回音壁激光光谱做了模式标定。在用近似解析公式对柱形微腔激光光谱的数值作拟合的计算中,除了回音壁模的径向模式数(l)和角动量模式数(n)外无需其他拟合参数,解析公式的拟合值和实验激光光谱波长值间的偏差小于0.05 nm,拟合结果精确可靠。柱形微腔回音壁激光光谱模式的精确标定在模式的场分布计算以及频移型微腔生物传感器的研究应用中具有重要作用,文章介绍的方法亦可应用于柱形微腔直径和折射率的精密测量。     

基于周期结构负反射的远场增强成像研究

远场高分辨率成像是近几年来声学和光学领域的研究焦点之一, 倏逝波无法在介质中传播成为将高分辨率成像带入远场的最大困难.本文提出了一种均匀排列的散射钢柱构成的超构散射体成像方式, 利用周期结构负反射现象将倏逝波信息转化为可传播波来增强成像.有限元数值模拟被用来研究和验证该方案的可行性, 结果显示波长3.4 mm的声波可以在20 cm外的远场获得大约0.6个波长的成像分辨能力. 通过减小散射体的晶格常数有希望达到更高分辨率成像.     

Manipulation of Particles with Counter-Propagating Evanescent Waves

Two counter-propagating evanescent beams are used to align and manipulate polystyrene particles on a prism surface. Since the radiation pressure transferred laterally from the evanescent wave is negated on both sides, particles can be stably aligned. By projecting a circular and a linear beam spot onto the interface, both multiple and single arrays of particles are achieved. Arrays of particles trapped on the interface can be easily moved adjusting the intensity of incident beams on either side. We also simulate electromagnetic distribution of scattering light that is converted from the evanescent wave using the FDTD method. The results show that scattering light converts from an evanescent wave propagating through a particle array and has a distance longer than that propagating from a normal evanescent wave.     

Simultaneous Measurement of Temperature and Refractive Index of Liquid Using Cladding-Etched Double-Core Fiber Bragg Grating

Abstract

This study proposed and demonstrated a fiber Bragg grating sensor for simultaneous detection of the temperature and refractive index of a liquid. The fiber Bragg grating is composed of three index layers: an inner core, an outer core, and a cladding. The cladding was etched for evanescent wave coupling between the outer core mode and an external liquid. The Bragg wavelength of the inner core mode is used for obtaining temperature data, while the refractive index for the liquid was determined by using the difference between the Bragg wavelength of the inner core mode and that of outer core mode.     

球冠型换能器声辐射指向性分析

为了研究球冠型换能器的声辐射特性,在分离变量法求解球面坐标系下波动方程的基础上,采用基于球谐基傅里叶变换及边界条件的求解模型,给出了球冠型换能器声辐射的远场声压计算表达式和远场指向性表达式;仿真计算了球冠换能器的远场指向性随球冠极角、球半径及振动频率变化的特性,球冠所在球障板的直径和介质中声波的波长比决定着球冠声辐射指向性,在低频或波长大于球障板直径时,球冠声辐射呈无指向性,随着频率的增高即波长的减小或者球障板直径的增大,球冠声辐射的指向性越明显,波束开角越趋向于球冠的开角,而且波束开角内出现波浪状起伏越明显;试制了高频球冠型换能器基阵,测试了换能器基阵300 kHz的指向性,测试结果与理论计算相符合,验证了理论计算表达式的正确性,可为设计球冠型换能器及基阵提供理论指导。      

Subdiffraction light focusing on a grating substrate

We describe a way to generate subdiffraction light spots that can be moved over a surface without resorting to near-field manipulation, nonlinear effects, or negative index materials. We use a periodically patterned substrate that converted efficiently, through scattering, the impinging propagative waves into evanescent ones. Then we optimize the wave front of the incident propagative beam so that the grating-scattered evanescent waves interfere constructively at the focal point. Numerical simulations show that focus spots as small as one-sixth of a wavelength can be obtained at any point on the substrate. One foreseen application is high resolution surface imaging.     

Simulated Output Images of Near-Field Optics by Volume Integral Equation: Object Placed on the Dielectric Substrate

We investigated near-field optical (NFO) imaging characteristics of a small object placed on a dielectric slab by a computer-code using a three-dimensional volume integral equation with an effective iteration technique called the generalized minimal residual method. A simplified three-dimensional NFO microscope that consists of a small dielectric object placed on the dielectric substrate and a small dielectric sphere as a scanning probe-tip was considered. Calculating two-dimensional output images obtained from scattered far fields, we studied the effect of the substrate on NFO output images, the comparison of NFO output images with electrostatic field around the small object, the dependence of output image characteristics on the wavelength and the difference of imaging characteristics between incident plane waves and incident evanescent waves.     

一种增强LED光提取效率的新方法

本文从分析全内反射中的近场倏逝波出发,提出了在近场倏逝波衰减长度内置一金属Ag光栅的LED结构模型,同时基于时域有限差分(FDTD)方法计算了此模型的光谱特性,分析了此LED模型的光提取效率。结果表明置入的金属Ag光栅在特定的波段可以将近场倏逝波耦合为传播波模式辐射出去,从而有效地增强了LED的光提取效率。     

An optical fibre pH sensor based on dye doped mesostructured silica

This paper describes the synthesis and optical pH sensing properties of a pH indicator dye activated mesostructured silica coated optical fibre. The pH-sensitive dye bromothymol blue was incorporated into the mesostructure sol precursor and thin films/coatings were used for pH-sensing applications without prior removal of the surfactant. The pH sensitivity range and operating wavelength were analysed by absorption measurements on layer dip-coated on glass slides. The pH sensing properties of coated optical fibre were tested by evanescent wave method. Experiments were carried out by using a series of solutions of known pH. No leaching was observed (except at very high pH) and an excellent reversibility was obtained between pH=2 and 5.     

隐失驻波全内反射荧光显微术的数值计算

从理论上详细计算了在全内反射的条件下，两束入射光产生的隐失干涉场(即隐失驻波)的强度分布，并分析了其不同于传统传播波干涉场的特点。同时使用数值模拟证明了利用隐失干涉场，即隐失驻波的激发方式，可以提高系统的分辨力，在横向实现超经典分辨的荧光成像。具体的分析表明，两束光以相等的角度入射，同时振幅相等，偏振态相同，所形成干涉条纹的反衬度最高，此时成像系统的有效点扩展函数最优化；入射介质1的折射率越大，隐失干涉场的空间周期越短(空间频率越高)，其对应的调制点扩展函数中心瓣的半峰全宽越小，可能分辨更小的物体，但同时旁瓣的强度也增强，最终成像的分辨力受两者的共同制约。     

用于双层存储的SIL近场分析

固体浸没透镜 (SIL)用于光存储光学系统增大了光学头物镜的数值孔径 ,利用近场倏逝波读写 ,减小了光斑的直径 ,提高了存储密度。文中建议 :(1)将半球SIL分割成两部分 ,其中一部分由存储媒质的保护层充当 ,避免直接污染记录层 ;(2 )在半球SIL单层读写的基础上 ,再将第一层记录媒质作为SIL的一部分形成超半球齐明SIL ,以实现双层存储。并对此时的近场进行了解析分折     

Analysis of acoustic directivity of spherical cap transducers

In order to study the acoustic radiation from the spherical cap transducer,a theoretical model is used by solving the wave equation in spherical coordinates using the method of separation of variables,based on the spherical harmonic Fourier transform and boundary condition.The calculation formulas for the far field radiated pressure and directivity of spherical cap are derived.Some theoretical results are presented in the form of far-field directivity patterns of the spherical cap transducer for various polar angles of the spherical cap,radii of sphere baffle and operating frequencies.Both the diameter of the sphere baffle and the wavelength determine the directivity of acoustic radiation from a spherical cap.When the frequency is lower or the wavelength is longer than the diameter of the sphere baffle,the acoustic radiation from a spherical cap is omnidirectional.With the increase of the frequency or the diameter of the sphere baffle,the acoustic radiation from a spherical cap is more directional and the beamwidth more tends to spherical cap angle.Furthermore,the ripple in the beam is more obvious.The high frequency spherical cap transducer was fabricated and the directivity pattern was tested and the measurement data is shown to coincide with the theoretical results.This research can provide a guideline for designing the spherical cap transducers and arrays.     

A scheme of plasmon excitation at the interface between metal and a photonic crystal

A one-dimensional photonic crystal and an incident light wave that is noncollinear with the permittivity gradient of the crystal are considered. On the basis of coupled-mode equations, eigensolutions for the fields are obtained; a special focus is placed on evanescent waves. Field transformation at the crystal-air and crystal-metal interfaces is analyzed. A condition for the resonance excitation of surface waves at the interface between a crystal and a metal is obtained, and an estimate is given for the efficiency of transformation of the original traveling wave into a surface wave.