Mapping electromagnetic fields near a subwavelength hole

We study, both experimentally and theoretically, the scattering of electromagnetic waves by a subwavelength hole fabricated in a thin metallic film. We employ the scanning near-field optical microscopy in order to reconstruct experimentally the full three-dimensional structure of the electromagnetic fields in the vicinity of the hole. We observe an interference of all excited waves with an incident laser beam which allows us to gain the information about the wave phases. Along with the well-known surface plasmon polaritons propagating primarily in the direction of the incident beam polarization, we observe the free-space radiation diffracted by the hole. We compare the experimental results with the fields of pure electric and pure magnetic dipoles as well as with direct numerical simulations. We confirm that a single hole in a thin metallic film excited at the normal incidence manifests itself as an effective magnetic dipole in the visible spectral range.     

Self-organized nanogratings in glass irradiated by ultrashort light pulses

Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser. Backscattering electron images of the irradiated spot reveal a periodic structure of stripelike regions of approximately 20 nm width with a low oxygen concentration, which are aligned perpendicular to the laser polarization direction. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from approximately 140 to 320 nm by the pulse energy and the number of irradiated pulses. The phenomenon is interpreted in terms of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass.     

Cassegrain激光发射系统的光路设计

为了提高激光探测系统激光的发射功率,在出射激光为基模的高斯光束的光路设计中,根据高斯光束经过光学系统的变换与传输特性,分析了系统采用Cassegrain望远镜时,其遮拦比以及入射光束束腰半径对光功率透过率的影响。通过具体的数值计算得出,随着遮拦比及入射光束束腰半径的增大,系统光功率透过率将减小,给出了望远镜遮拦比与发射孔径处光斑尺寸之间的最佳匹配关系,最后讨论了离焦误差（安装误差）为0.2mm时对出射光斑尺寸的影响。     

多层球对高斯波束散射的德拜级数研究

基于广义洛伦兹-米氏理论,利用多层球粒子散射系数的德拜级数展开公式,提出了一种新的研究多层球粒子对高斯波束散射的方法。计算结果与已有的广义洛伦兹-米氏理论算法的计算结果吻合得很好。利用该方法有效分离了折射率分布满足指数变化规律的多层球粒子对高斯波束散射的远区散射场中多阶彩虹的干涉强度分布。数值模拟了双层球的归一化双一阶彩虹强度分布以及各层的一阶彩虹艾里结构。最后分析讨论了高斯波束的入射位置和束腰半径对多层球单阶彩虹强度分布的影响。     

Numerical study of optical trapping properties of nanoparticle on metallic film with periodic structure

Based on the three-dimensional dispersive finite difference time domain method and Maxwell stress tensor equation,the optical trapping properties of nanoparticle placed on the gold film with periodic circular holes are investigated numerically. Surface plasmon polaritons are excited on the metal-dielectric interface, with particular emphasis on the crucial role in tailoring the optical force acting on a nearby nanoparticle. Utilizing a first order corrected electromagnetic field components for a fundamental Gaussian beam, the incident beam is added into the calculation model of the proposed method. To obtain the detailed trapping properties of nanoparticle, the selected calculations on the effects of beam waist radius, sizes of nanoparticle and circular holes, distance between incident Gaussian beam and gold film, material of nanoparticle and polarization angles of incident wave are analyzed in detail to demonstrate that the optical-trapping force can be explained as a virtual spring which has a restoring force to perform positive and negative forces as a nanoparticle moves closer to or away from the centers of circular holes. The results of optical trapping properties of nanoparticle in the vicinity of the gold film could provide guidelines for further research on the optical system design and manipulation of arbitrary composite nanoparticles.     

Detailed characterization of polycapillary focusing x-ray lenses by a charge-coupled device detector and a pinhole

A method to measure the detailed performance of polycapillary x-ray optics by a pinhole and charge coupled device (CCD) detector was proposed in this study. The pinhole was located between the x-ray source and the polycapillary x-ray optics to determine the illuminating region of the incident x-ray beam on the input side of the optics. The CCD detector placed downstream of the polycapillary x-ray optics ensured that the incident x-ray beam controlled by the pinhole irradiated a specific region of the input surface of the optics. The intensity of the output beam of the polycapillary x-ray optics was obtained from the far-field image of the output beam of the optics captured by CCD detector. As an application example, the focal spot size, gain in power density, transmission efficiency, and beam divergence of different parts of a polycapillary focusing x-ray lenses (PFXRL) were measured by a pinhole and CCD detector. Three pinholes with diameters of 500, 1000, and 2000 μm were used to adjust the diameter of the incident x-ray beam illuminating the PFXRL from 500 μm to the entire surface of the input side of the PFXRL. The focal spot size of the PFXRL, gain in power density, transmission efficiency, and beam divergence ranged from 27.1 μm to 34.6 μm, 400 to 3460, 26.70% to 5.38%, and 16.8 mrad to 84.86 mrad, respectively.     

Laser diode feedback interferometry in flowing Brownian motion system: a novel theory

A novel theory is developed for the laser-diode feedback interferometry in the flowing Brownian motion system irradiated by a focused Gaussian incident field. The back-emitted power from the laser cavity and the corresponding time autocorrelation function are derived with the aid of both the modified Lang–Kobayashi rate equations and dynamic light scattering theory. As a result, the time autocorrelation function of the back-emitted power from the laser diode includes a new character parameter dependent on the nature of laser diode and a new Gaussian term dependent on the transit time of the illuminated target through the Gaussian beam waist, which may also be applied to solid-state lasers.     

梯度特异材料表面对入射高斯光束的反射研究

最近人们[1-3]提出了一类反射式梯度特异材料表面系统,此系统可以高效地将入射波转换成被束缚于材料表面的表面波,这个系统成为连接行波和表面波的一座桥梁。以往人们研究梯度特异材料表面的反射特性时多是以入射平面波为主,这里我们结合实际,以入射的高斯光束作为研究对象,讨论梯度特异材料表面对其的反射特性,尤其是高斯光束的主入射角和束腰对反射行为的影响,得到了与平面波入射不同的反射现象。对于临界角为17.2°的梯度特异材料表面,束腰为4的高斯光束以主入射角分别为0°、10°、30°入射时,我们发现随着主入射角的变大,入射高斯光束转变为表面波的比例增大;而固定主入射角为30°,我们通过改变束腰,发现束腰越大,波矢分布越窄,越接近于平面波,转变为表面波的效率就越高。在数值模拟中,反射光束出现了明显的Goos-Hansen位移现象。同时还出现了类似光栅反射的特点,即分叉现象,这是由于模拟计算时网格不能无限小,梯度特异材料表面不能模拟为折射率连续变化的材料而应该是阶跃式变化的材料。由于实际制备的梯度特异材料表面是人工微结构的排列,因此微小的阶跃式变化的梯度特异材料表面更为实际。我们的工作对于利用梯度特异材料表面将入射高斯光束转变为表面波具有指导意义。     

双球粒子对任意入射单波束及双波束的散射

基于广义Lorenz-Mie理论,研究了双介质球粒子对任意角度入射波束的散射.将入射波束用球矢量波函数展开,推导了双介质球形粒子对任意入射单波束的散射方程;将该方法进一步推广,研究了双波束任意入射时的情况.以Gauss波束为例,对以上理论进行了数值验证,比较分析了单波束及双波束任意入射时散射特性随入射方向、球心距等参数的变化关系.     

双球粒子对任意入射单波束及双波束的散射

基于广义Lorenz-Mie理论,研究了双介质球粒子对任意角度入射波束的散射.将入射波束用球矢量波函数展开,推导了双介质球形粒子对任意入射单波束的散射方程;将该方法进一步推广,研究了双波束任意入射时的情况.以Gauss波束为例,对以上理论进行了数值验证,比较分析了单波束及双波束任意入射时散射特性随入射方向、球心距等参数的变化关系. 关键词： 广义波束因子 双球粒子 Gauss波束     

Spontaneous periodic structures induced in thin AgCl-Ag films by the continuum in the case of optical rotation of the plane of polarization

The formation of spontaneous periodic structures in thin AgCl-Ag films subject to the continuum (λ≈450–700 nm) in the case of rotation of the plane of polarization with the help of crystalline quartz is studied. The periodic structures are formed due to the excitation of waveguide TE₀ modes in spontaneous Rayleigh scattering of radiation with a wavelength λ incident on the film. The structure periods are smaller than λ and vary in proportion with λ. After special treatment of the irradiated sample (fixing and deposition of an Al layer), periodic structures form a thin reflection hologram capable of reproducing the direction of linear polarization, the angle of optical rotation, and the color of the spectrum recorded. The study of diffraction patterns and small-angle scattering from small portions of a hologram, which were exposed to light with various wavelengths in the recording process, with the help of a probing UV beam (λ_p=337 nm), made it possible to reveal special features of the manifestation of Rayleigh and Wood anomalies associated with the domain structure of PS.     

电介质/半导体结构样品电子束感生电流瞬态特性

电子束照射下电介质/半导体样品的电子束感生电流(electron beam induced current,EBIC)是其电子显微检测的重要手段.结合数值模拟和实验测量,研究了高能电子束辐照下SiO2/Si薄膜的瞬态EBIC特性.基于Rutherford模型和快二次电子模型研究电子的散射过程,基于电流连续性方程计算电荷的输运、俘获和复合过程,获得了电荷分布、EBIC和透射电流瞬态特性以及束能和束流对它们的影响.结果表明,由于电子散射效应,自由电子密度沿入射方向逐渐减小.由于二次电子出射,净电荷密度呈现近表面为正、内部为负的特性,空间电场在表面附近为正而在样品内部为负,导致一些电子输运到基底以及一些出射二次电子返回表面.SiO2与Si界面处俘获电子导致界面附近负电荷密度高于周围区域.随电子束照射样品内部净电荷密度逐渐降低,带电强度减弱.同时,负电荷逐渐向基底输运,EBIC和样品电流逐渐增大,电场强度逐渐减小.由于样品带电强度较弱,表面出射电流和透射电流随照射基本保持恒定.EBIC、透射电流及表面出射电流均随束流呈现近似正比例关系.对于本文SiO2/Si薄膜,透射电流随束能的升高逐渐增大并接近于束流值,EBIC在束能约15 keV时呈现极大值.     

Optimal x-ray beam formation using a focusing mirror at the Langmuir synchrotron station

A beam focusing method is proposed for the Langmuir station of the Kurchatov synchrotron radiation source. This method can be implemented during experiments on recording the fluorescence yield intensity as a function of the angle between an incident beam and a sample surface. The x-ray beam is focused by a cylindrical mirror in such a way that the dimensions of the spot illuminated by the beam on a Langmuir film remain within the input angular aperture of an energy dispersive detector during scanning in the angle of incidence. The relationships for calculating the dimension of the illuminated spot in the scattering plane and the angular divergence of the focused x-ray beam are derived taking into account the dimension of the synchrotron radiation source and spherical aberration. Recommendations for optimizing station parameters and experimental conditions are presented.     

Photoinduced transformations in a thin-film waveguide AgCl-Ag composition under a green (532 nm) laser beam

A linearly polarized (E ₀) laser beam (λ = 532 nm) causes photoinduced transformations in an AgCl-Ag composition consisting of a thin waveguide AgCl film on glass covered by a layer of Ag nanoparticles. Before the illumination the sample exhibits an absorption band due to localized plasmons in nanoparticles. The illumination excites plasmons and leads to scattering of waveguide TE₀ modes. The interference of modes with the incident light beam leads to the development of a periodic structure, the lines of which are formed by Ag particles and directed along E ₀. The measured structure period coincides with the result of calculation based on solving the dispersion equation for the TE₀-mode. Measurement of absorption in the E ‖ E ₀ polarization reveals dichroism and a spectral hole (at λ ≈ nm). It is shown that the structures formed remain on the substrate after removing AgCl in a fixing agent. The dichroism value and dispersion change after fixing. The character of dichroism prior to fixing is recovered after depositing an AgCl layer (with the parameters retained) on the fixed film.     

反射干涉光谱法测量固体薄膜的光学常数和厚度

本文报道一种简单的方法,从平原介质薄膜的反射干涉光谱来计算薄膜的光学常数和厚度。当一束光照射在基板上的介质膜上时,由于膜上下界面反射光的相干,会使反射光谱的曲线有一定的波动。我们对反射相干光谱进行理论分析,给出计算公式,从测量曲线中的实验值得出薄膜的光学常数ｎ、ｋ以及厚度等参数。此种方法简单可行,而且易于编程处理。     

非球形大气粒子对任意波束的电磁散射特性

研究大气中变形球状的气象粒子对电磁波以及激光的散射特性,采用广义米氏(Mie)理论,精确地求解在任意波束中大气粒子的散射强度,数值计算激光束腰位置对散射强度分布的影响,分析彩虹强度和散射角与变形球状粒子偏心率的关系,研究随着长短轴比例增加,彩虹强度峰值的偏移。计算结果表明雨滴在下降过程中,它在各个方向的光散射强度逐步减弱,即随着雨滴偏心率的增大,散射强度减少,彩虹角变大。对于激光入射,当粒子距离波束中心位置越远,粒子的散射就越弱,并且随尺寸参量增加,后向散射振荡的频率要大于前向散射,当束腰位置矢径的大小增加时,后向散射和前向散射强度均变小。     

Thickness Determination for a Two-Layered Composite of a Film and a Plate by Low-Frequency Ultrasound

We present an ultrasonic method for determining the thickness of a composite consisting of a soft thin film attached to a hard plate substrate, by resonance spectra in the low frequency region, The interrogating waves can be incident only to the two-layered composite from the substrate side. The reflection spectra are obtained by FFT analysis of the compressive pulsed echoes from the composite, and the thicknesses of the film and the substrate are simultaneously inversed by the simulated annealing method from the resonant frequencies knowing other acoustical parameters in prior. The sensitivity of the method to individual thickness, its convergence and stability against experimental noises are studied, Experiment with interrogating wavelength 4 times larger than the film thickness in a sample of a polymer film （0.054mm） on an aluminium plate （6.24mm） verifies the validity of the method. The average relative errors in the measurement of the thicknesses of the film and the substrate are found to be -4.1% and -0.62%, respectively.     

Radiation transfer in metallic powder beds used in laser processing

Two-dimensional radiation transfer in a powder layer backed with a substrate of the same material and normally irradiated with a narrow axially symmetric bell-like or the flat-top laser beam is numerically calculated. This corresponds to physical experiments with the powder layer of 50 μm thickness and the laser beam diameters 60–120 μm. The powder bed is treated as an equivalent homogeneous absorbing scattering medium, the radiative properties of which are estimated from the optical properties of the solid phase and the morphological parameters of the powder bed. The theoretical analysis shows that the absorptance of a semi-infinite powder bed of opaque particles is a universal function of the absorptivity of the solid phase being independent of the specific surface and the porosity. This is confirmed by literature experimental data. The radial transport of the radiative energy due to scattering of the incident laser beam in the powder layer can considerably reduce the deposited energy at the centre of the beam but the widening of the radial profile of the deposited energy is not pronounced. The fraction of laser energy deposited within the projection of the incident laser beam is evaluated. The efficiencies of laser heating the whole powder/substrate system and the substrate decrease with increasing the reflectivity of the material. More uniform heating of the powder layer can be attained at higher reflectivity.     

Numerical simulations of laser light guiding and focusing using an open hollow cone

An open hollow cone that can optically guide and focus a laser light into a small beam spot is proposed. The shaping and focusing effects are numerically demonstrated by finite-difference time-domain simulations. The results reveal that the cone-focused laser originates from the multi-beam interference and squeeze of two cone side walls acting like oblique targets. Moreover, the influences of oblique incident angle of laser light, cone angle and cone tip size on the focusing properties are discussed and the optimum cone geometry with cone angle of 16° and tip size of 4.5 wavelengths to achieve high energy concentration at the cone tip is also presented. It is shown that with an open hollow cone a laser light can be focused into a tiny highly localized beam spot of 1 µm diameter and keeps propagating forward for a distance of about 8 wavelengths with a tiny spot diameter remaining almost unchanged. The intensity almost increases up to twentyfold in a 1 µm focal spot.     

粗糙体目标激光束散射场的互相关函数统计特征

依据高斯波束体目标散射特征,研究了粗糙体目标高斯波束散射场量的互相关函数统计特征。在激光波束入射下,通过数值方法讨论圆球类目标、不同半径、不同材料、不同入射波束极化等条件下,散射场量的互相关函数随散射角变化情况。数值计算结果表明:金属薄膜材料比非金属镀漆材料互相关函数量值要大,金属材料球体目标的后向散射互相关函数值较大,而非金属镀漆材料球体互相关函数值较小。材料表面的粗糙度和目标尺寸对互相关函数影响较大,而入射光的极化方式对相关函数有影响但影响较小。