Analysis of Trapping Force and Torque for Fiber-optical Trap by the Finite-differential Time-domain (FDTD) Method

In this paper,the FDTD method is used to calculate the light field distribution in an uncoated fiber probe with a small dielectric particle.By integrating Maxwell′s stress tensor in the FDTD calculation,the time-averaged force exerted on the dielectric particles with different sizes and at different positions is obtained.The calculation results are also verified experimentally.The force near the axis in the vicinity of the tip is about 10-14 N for the incident 10 mW laser of 980 nm.Furthermore,the torque exerting on an ellipsoidal dielectric particle by focusing light field with two fiber probes is calculated and the dependence of this torque on both the declination angle of the ellipsoidal particle and the incident laser power ratio of the two probes is obtained.The detailed intensity map within the particle is also given.     

PSTM成像的FDTD数值模拟

尝试用时域有限差分法(FDTD)分析采用不同探针时光子扫描隧道显微镜(PSTM)成像的分辨率和效率。对四种探针,即不镀膜,带菱形金尖,镀金膜有孔径,镀金膜无孔径的探针,用二维p极化波,满足全反射条件,并用样品台上方平行于界面的隐失波作为等效入射波源,用探针距离尖顶一定高度截面的玻印亭矢量计算散射场强。数值模拟结果表明,带纳米尺度金尖的探针具有最好成像的分辨率和效率,镀膜无孔径和镀膜孔径探针次之,不镀膜探针最差。     

Optical near-field distribution in an asymmetrically illuminated tip–sample system for laser/STM nanopatterning

In surface nano-patterning using an atomic force microscope (AFM) tip in scanning tunnelling microscopy (STM) mode and illuminated by a laser, two controversial physical mechanisms exist in the literature: the field-enhancement (FE) model and the thermal-induced mechanical contact (TMC) model. Due to the presence of evanescent waves in the optical near-field, the exact calculation of the field distribution of the tip–sample system in micro/nano scales becomes complicated. There is a lack of understanding of the asymmetrically illuminated tip–sample system. In this paper, full 3D finite-difference time-domain (FDTD) analysis was carried out to investigate the field distribution in different tip–sample systems. The effects of different tip/sample materials (either dielectric or plasmonic material), the gap distance, and laser incidence angles on the field distribution/enhancement have been studied. For the first time, we have demonstrated two new effects which are helpful in distinguishing the controversial mechanisms: (1) on the sample surface, the field peak position has a shift away from the tip-axis at large angles of incidence, and (2) the field enhancement could depend strongly on the horizontal component (perpendicular to tip-axis) of the incident wave instead of the vertical component (along tip-axis). The optimal incident angle is around 30° for the maximum field under the tip. The existence of field-distribution nodes on the 3D tip surface that leads to the in-homogenous heating of the tip is also predicted. PACS 81.16.Mk; 61.80.Ba; 81.16.Rf; 81.65.Cf     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

Near field localization mediated by a single gold nanoparticle embedded in transparent matrix: Application for surface modification

In this work the near field properties of a single gold nanoparticle embedded in transparent host medium are investigated theoretically. The analysis of the electromagnetic field in the near field zone is obtained by finite difference time domain (FDTD) simulation technique. The nanoscale system consists of a transparent layer in which a gold particle with diameters of D = 200 or 80 nm is embedded, is situated on a substrate surface. Laser pulse at wavelength of 800 nm irradiates normally this system. It is found that the field in the vicinity of the particle is enhanced, and at a certain condition the zone with the highest enhancement is localized on the substrate surface. Furthermore, the near field characteristics are found to be controllable by the dielectric properties of the host material, substrate, parameters of the incident irradiation and particle size. With the increase of the refractive index of the host medium, both the magnitude of the near field on the substrate and the characteristic size of the field enhanced zone decrease. The influence of the particle size and polarization of the incident laser irradiation on the near filed properties of the system are also presented. The proposed configuration can be applied for a multiple nanoprocessing and an integrated near field source with a spatial resolution of D/3.     

一种计算和分析二维光子晶体缺陷模式的方法

通过改进时域有限差分(FDTD)法,计算和分析了二维光子晶体的缺陷模式。运用一维时域有限差分算法和线性插值法在总场散射场(TF-SF)连接边界引入入射平面波,采用完全匹配层(PML)技术对外行波进行了有效吸收。计算和分析结果表明,在光子晶体非对称方向入射的平面波能激发所有的缺陷模式,选取合适的探测点位置收集电场值,经快速傅里叶变换(FFT)能得到所有的共振峰值。另外,采用该方法研究了二维正方介质柱光子晶体缺陷模的共振频率与缺陷介质柱半径和介电常量之间的关系。结果表明通过改变缺陷的半径和介电常量大小可以在光子晶体禁带中一定的范围内调节缺陷模式的共振频率大小。     

数值分析在不同偏振时复杂样品的近场强度分布

采用三维时域有限差分法(FDTD),模拟计算了光子扫描隧道显微镜(PSTM)系统中的介质和银质金属样品在不同偏振模式光源下的近场强度分布。使用介质小样品检验探针性能,在探针不同位置计算得到了相似的近场强度分布图,说明编写的程序是可信的。给出了等高扫描时p偏振和s偏振条件下,“PSTM”字样介质样品和银质金属样品上方5nm处的近场强度分布图,结果显示:对介质样品,p偏振波能较好的反映样品的形貌,这是由入射电场的方向决定的;银质金属有表面增强作用,对不同偏振波均能在某种程度上反映样品的形貌,仍有待近一步的研究。     

用于飞秒激光纳米加工的TiO_2粒子阵列诱导多种基底表面近场增强

利用纳米粒子辅助对飞秒激光能量进行空间局域化,使其在基底表面诱导产生纳米尺度的近场增强,这对超衍射极限微结构加工具有重要意义.目前对于粒子阵列诱导飞秒激光纳米孔加工的研究仅限于金属Au粒子及低折射率聚苯乙烯介电粒子等,本文提出并开展了应用高折射率TiO_2介电粒子阵列作为辅助诱导激光近场增强从而进行飞秒激光超衍射纳米孔加工的研究.对TiO_2介电粒子阵列在Si,Pt及SiO_2表面的近场强度分布进行了数值模拟,研究其基底表面近场增强的规律及物理过程.研究结果发现,使用硅基底时,阵列与单一TiO_2球形粒子相比其近场增强仅下降约30%;相对于入射激光强度而言,在直径约为100nm的空间范围内获得140倍的近场增强,这一现象可用于百纳米孔的激光加工.同时在其他典型基底的理论计算结果中也表明,几乎在所有金属及介电材料表面均可以实现良好的百纳米空间范围内的近场增强,并且具有近场随着基底折射率变大而增强的规律.这些现象的产生归因于TiO_2粒子中磁四极振荡产生的激光前向场增强及粒子与基底的耦合作用.进一步引入镜像电荷模型对基底光学参数对其表面近场增强的影响规律进行了分析和解释.本文的模拟结果对飞秒激光近场超衍射极限纳米加工的应用有着重要的意义.     

Superfocussing in a metal-coated tetrahedral tip by dimensional reduction of surface-to edge-plasmon modes

Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions, and in particular as probes for scanning near-field optical microscopy. Numerical investigations using the Finite-Difference Time-Domain (FDTD) method are used to explore the operation of a T-tip in extraction mode. A dipole source in close proximity to the apex excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metal-coated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for illumination-mode numerical investigations by a Volume Integral equation method, which compute the field distribution that develops in a T-tip when a Gaussian beam is incident into the tip, and which show that a highly confined electric field is produced at the apex of the tip. The process of light confinement can be considered as a superfocussing effect, because the intensity of the tightly confined light spot is significantly higher than that of the focussed yet much wider incident beam. The mechanism of superfocussing can be considered as a dimensional reduction of surface plasmon modes, where an edge plasmon is the most important link between the waveguide-modes inside the tip and the confined near field at the apex.     

用时阈有限差分方法研究光纤微探针近场分布特性

采用三维时间域有限差分（FDTD）方法研究了扫描近场光学显微镜中光纤微探针的近场分布特性,研究结果表明,锥形裸光纤探针的近场光中,以从探针锥变区侧面泄露的光即传播场为主,当在探针的侧面镀上一层金属膜对于从侧面泄露的光具有较好屏蔽效果,因而比裸光纤探针具有更好的空间束缚能力,无论是裸光纤探针还是金属膜探针,入射偏振光从针尖出射手发生了退极化现象,近场分布中产生与入射光偏振方向垂直的其它两个电场分量,而且金属膜探针的退极化分量对近场分布的影响比裸探针显著得多,如果在近场成像中接收退极化分量而不是总场则可以提高近场成像的对比度,还详细地分析了微探针的近场分布特征与退极化之间的关系。     

光隔离器法抑制Michelson干涉型光纤水听器中的SBS

光纤水听器是利用光的干涉信号进行探测的,但随着入射光功率的增加,易发生受激布里渊散射（stimulated Brillouin scattering,SBS）效应,并因而产生噪声,限制入射光功率的增加,极大地影响探测信号。为了抑制SBS效应,提高光纤水听器性能,以Michelson型光纤水听器为例,从实验和理论上研究了SBS及其阈值特性。根据阈值公式分析了提高阈值的方法,着重讨论了光隔离器法对SBS的抑制。最后利用时域有限差分方法对光纤内的入射光、Stokes光和声波的时空分布进行了分析。只要给出初始条件及光纤参数即可得出光纤内各波的时空分布。通过开展抑制此效应的研究,不仅为可调谐相干光的产生提供一个新途径,而且可作为一种有效手段来研究各种光纤系统中的散射特性。     

激光诱导金纳米棒图案化光纤SERS探针

研究了激光诱导沉积制备光纤表面增强拉曼散射(SERS)探针,并对探针的SERS性能进行检测。探讨光纤探针制备过程中金纳米棒溶液的浓度对探针灵敏度的影响。结果表明,将不同浓度的金纳米棒溶液进行激光诱导,在光纤端面会形成金纳米棒团簇和分散两种纳米结构。金纳米棒溶液的浓度、激光功率、诱导时间等因素都会对诱导沉积图案产生影响。实验利用功率为5 mW的激光进行诱导,在1.5×10^-9, 1.0×10^-9和7.5×10^-10 mol·L^-1的金纳米棒溶液中,经5 min沉积,制备出不同图案的光纤SERS探针。采用晶种法合成金纳米棒,用透射电子显微镜(TEM)观察金纳米棒形貌,并根据TEM图像分析计算了合成金纳米棒的长径比约为3.8。用扫描电子显微镜(SEM)观察金纳米棒的形貌以及激光诱导沉积后的纤维修饰端形貌,7.5×10^-10 mol·L^-1的金纳米棒溶液进行激光诱导,金纳米棒在光纤端面分布较为分散,而1.5×10^-9和1.0×10^-...     

Ray-wave approximation for the calculation of laser light scattering by transparent dielectric particles,mimicking red blood cells or their aggregates

A theoretical model is presented for the fast calculation of light scattering by transparent dielectric particles with sizes much larger than the wavelength. In this model, the light incident onto the particle and the light propagating inside the particle are represented by sets of rays while the scattered light is represented by a set of spherical waves emitted by different elements of the particle surface (ray-wave approximation, RWA). It is shown that RWA is comparable in precision with the discrete-dipole approximation but significantly exceeds the latter in the calculation rate for the particles with size parameters higher than 50.     

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.     

Large dielectrophoresis force and torque induced by localized surface plasmon resonance of Au nanoparticle array

A new approach is proposed for manipulating and rotating micro- or nano-objects by using polarized laser light with low intensity. The polarized light excites resonant dipoles on a cap-shaped Au nanoparticle array, which generates a highly nonuniform radiation field that induces large dielectrophoresis force on dielectric objects. The orientation control of the objects is realized by adjusting the polarization direction of the incident light. Theoretical modeling, fabrication, and characterization results for the cap-shaped Au nanoparticle array, as well as preliminary trapping results, are reported.     

Waveform Distortions in Femtosecond Laser Pulses Transmitting through Coated Optical Fiber Probes

We calculate the light field in metal-coated optical fiber probes under illumination of femtosecond laser pulses with the three-dimensional finite-difference time-domain method. By choosing the maximum of the calculated light field data in the time period of one light wave cycle at each spatial point, we obtain the amplitude distributions instead of the conventional light field distribution of a time instant. It is found that with an incident plane wave pulse of y-polarization, the output amplitude distributions of y-polarization have roughly a circled-cross-like pattern with two arc-like zero-amplitude zones. By analyzing the y-polarized waveforms versus time at the points near the arc-like zero-amplitude zones, we find that waveform distortions appear at points in the neighborhood of the zero amplitude lines and in the time intervals when two wave-packets overlap.     

Finite-difference time-domain analysis of refractive index grating on planar light waveguide circuit with optically trapped gold particles

The super-resolution capability of scanning near-field optical microscopy (SNOM) with a gold particle is studied by the two-dimensional finite-difference time-domain (2D FDTD) method. We obtain SNOM signals by integrating the far field within the numerical aperture of an objective lens for a refractive index grating by scanning optically trapped gold particles with different diameters illuminated by focused laser light at the wavelength of 515 nm. The signal is strong at a high refractive index of the grating and exhibits similar behavior to that obtained in the experiment with the grating fabricated on a planar light waveguide circuit with a period of 1060 nm. Furthermore, the signal modulation increases as the gold particle diameter decreases and reaches 0.82 at a diameter of 50 nm.     

The optical properties between an incident wave and the active layer of a bubble-pit AgO<Subscript>x</Subscript>-type super-resolution near-field structure

The deformation and plasmon effects of collective localized surface plasmons between incident light and bubble-pit AgO_x-type super-RENS structure have been studied using finite-difference time-domain (FDTD) method. We find that the polarization, wavelength of incident light, and particle sizes of Ag nanoparticles are sensitive to the plasma resonance. The Ag nanoparticles inside the bubble-pit AgO_x-type super-RENS structure give the additional outer boundaries to the motion of the Ag nanoparticles, and excite more evanescent field which located in the far edge of the bubble from the optical axis of the incident beam. The optical properties between active layer and incident light with polarization direction, different wavelengths, and varied particle sizes of Ag nanoparticles exhibits nonlinear optical behavior in the near field. The far-field signals of different wavelength of incident light confirm the relation between highly localized near-field distributions and enhanced resolution of far-field signals. The subwavelength recording marks smaller than the diffraction limit were distinguishable since the Ag nanoparticles with high localized fields transferred evanescent waves to detectable signals in the far field. PACS 42.79.Vb; 71.15.Rn; 72.15.Rn; 73.22.-f; 73.22.Lp; 78.67.Bf; 73.20.Mf     

Electromagnetic momenta and forces in dispersive dielectric media

When the effects of dispersion are included, neither the Abraham nor the Minkowski expression for electromagnetic momentum in a dielectric medium gives the correct recoil momentum for absorbers or emitters of radiation. The total momentum density associated with a field in a dielectric medium has three contributions: (i) the Abraham momentum density of the field, (ii) the momentum density associated with the Abraham force, and (iii) a momentum density arising from the dispersive part of the response of the medium to the field, the latter having a form evidently first derived by Nelson (1991) [8]. All three contributions are required for momentum conservation in the recoil of an absorber or emitter in a dielectric medium. We consider the momentum exchanged and the force on a polarizable particle (e.g., an atom or a small dielectric sphere) in a host dielectric when a pulse of light is incident upon it, including the dispersion of the dielectric medium as well as a dispersive component in the response of the particle to the field. The force can be greatly increased in slow-light dielectric media.