Light scattering from cells: finite-difference time-domain simulations and goniometric measurements.

We have examined the light-scattering properties of inhomogeneous biological cells through a combination of theoretical simulations and goniometric measurements. A finite-difference time-domain (FDTD) technique was used to compute intensity as a function of scattering angle for cells containing multiple organelles and spatially varying index of refraction profiles. An automated goniometer was constructed to measure the scattering properties of dilute cell suspensions. Measurements compared favorably with FDTD predictions. FDTD and experimental results indicate that scattering properties are strongly influenced by cellular biochemical and morphological structure.     

Numerical analysis of variability in ultrasound propagation properties induced by trabecular microstructure in cancellous bone

The manner by which the trabecular microstructure affects the propagation of ultrasound waves through cancellous bone is numerically investigated by finite difference time-domain (FDTD) simulation. Sixteen 3-D numerical models of 6.45times6.45times6.45 mm with a voxel size of 64.5 mum are reconstructed using a 3-D microcomputed tomographic (muCT) image taken from a bovine cancellous bone specimen of approximately 20times20times9 mm. All cancellous bone models have an oriented trabecular structure, and their trabecular elements are gradually eroded to increase the porosity using an image processing technique. Three erosion procedures are presented to realize various changes in the trabecular microstructure with increasing porosity. FDTD simulations of the ultrasound pulse waves propagating through the cancellous bone models at each eroded step are performed in 2 cases of the propagations parallel and perpendicular to the major trabecular orientation. The propagation properties of the wave amplitudes and propagation speeds are derived as a function of the porosity, and their variability due to the trabecular microstructure is revealed. To elucidate an effect of the microstructure, the mean intercept length (MIL), which is a microstructural parameter, is introduced, and the correlations of the propagation properties with the MILs of the trabecular elements and pore spaces are investigated.     

柱坐标系下浅海传播声场时域有限差分预报方法

直角坐标系下的时域有限差分方法在水声传播仿真时由于计算量太大而较少应用。文章利用时域有限差分方法推导了柱坐标系下的声波方程的近似表达式,结合复频移完全匹配层建立浅海声传播模型,在减少计算量的同时准确预报了传播信号声压的时域波形、传播声场的时空演变过程以及频域传播损失曲线。在柱坐标系下使用时域有限差分方法仿真Pekeris传播环境声场并与简正波和波数积分模型进行对比,分析了时域有限差分水声传播模型的适用范围。结果表明,时域有限差分方法仿真浅海中近程传播声场精度较高。模型的稳定性与时间和空间网格大小有关,声源频率越高,空间和时间网格划分越小,计算量越大。数值离散带来的频散误差累积会导致远场传播声场计算不准确,因此时域有限差分水声传播模型更适用于低频中近程声场计算。     

Analysis of optical waveguide structures by use of a combined finite-difference/finite-difference time-domain method

We present a method for full-wave characterization of optical waveguide structures. The method computes mode-propagation constants and cross-sectional field profiles from a straight forward discretization of Maxwell's equations. These modes are directly excited in a three-dimensional finite-difference time-domain simulation to obtain optical field transmission and reflection coefficients for arbitrary waveguide discontinuities. The implementation uses the perfectly-matched-layer technique to absorb both guided modes and radiated fields. A scattered-field formulation is also utilized to allow accurate determination of weak scattered-field strengths. Individual three-dimensional waveguide sections are cascaded by S-parameter analysis. A complete 10(4)-section Bragg resonator is successfully simulated with the method.     

Two-wave propagation imaging to evaluate the structure of cancellous bone

The two-wave phenomenon reflects not only bone mass but also the complex bone structure of cancellous bone. We propose a new simple imaging technique based on the two-wave phenomenon for investigating the anisotropic structure of cancellous bone. A cylindrical specimen of cancellous bone was obtained from a bovine femur. The structure (alignment of trabeculae) of the specimen was obtained from 3-D X-ray micro computed tomography imaging. Using a conventional ultrasonic pulse technique, we rotated the receiver around the specimen to investigate the ultrasonic fields after propagation within the specimen. The ultrasonic propagation image clearly showed the effect of the bone structure. We found that the fast wave showed apparent refraction, whereas the slow wave did not. Fast-wave propagation imaging is thus a simple and convenient technique for easy interpretation of the anisotropic structure. This imaging technique has the potential to become a powerful tool to investigate the structure of trabeculae during in vivo measurements.     

水中目标强度的一致性时域有限差分计算

本文应用一致性时域有限差分法（FDTD）和近场-远场变换计算散射声目标的反射声压强度，找出反射声压级与目标声中心到换能器间距离的关系。数值计算中考虑了自由场和有海面及海面波浪影响时的不同情况。根据声源和目标的几何特征对入射波和散射波分别作了球面和柱面扩散修正，并将计算结果与水池实验结果进行了比较。本文的讨论可对水下目标强度测量方法提供参考。     

Thin-wire finite-difference time-domain model for insulated and resistively loaded cylindrical antennas driven by coaxial lines

A thin wire-based finite-difference time-domain (FDTD) model for a simple analysis of insulated and resistively loaded cylindrical antennas fed by coaxial lines is proposed. The resistive loading and the coaxial feed line are approximated to equivalent resistors along the antenna axis and the equivalent source over the infinitesimal feed gap, respectively. The effects of the insulation are corrected by employing thin-wire approximation and the boundary condition. A full coarse-grid FDTD algorithm is then implemented without additional grid refinements for the insulation, the resistive loading and the feed line. As a function of insulation properties and resistive loading profiles, the transient reflected feed voltage and the input impedance of antennas are calculated numerically. The validity of the proposed model is proved by comparing it with the results of the full fine-grid FDTD.     

Boundaryless finite-difference method for three-dimensional beam propagation

A two-dimensional optical field paraxial propagation scheme, in Cartesian and cylindrical coordinate systems, is proposed. This is achieved by extending the method originally proposed by Ladouceur [Opt. Lett. 21, 4 (1996)] for boundaryless beam propagation to two-dimensional optical wave fields. With this formulation the arbitrary choice of physical window size is avoided by mapping the infinite transverse dimensions into a finite-size domain with an appropriate change of variables, thus avoiding the energy loss through the artificial physical boundary that is usually required for the absorbing or the transparent boundary approach.     

超长空间激光传输数值模拟研究进展

文章主要围绕空间引力波探测中超长空间链路传输部分进行介绍,概述了目前国内外星间传输仿真时采用的计算方法,以及指向抖动引起的相位噪声分析方法。相较于地基引力波探测,空间引力波探测可以有效降低噪声,增加干涉臂长度,从而实现更高精度、更低频率的探测。在长达数百万公里的传输距离,以及皮米量级数值模拟的精度要求下,需要考虑指向角变化引起的相位噪声。研究表明,在2.5×10⁹ m的传输距离下,离焦和像散是影响指向抖动噪声的主要像差。通常情况下,相位驻点位置与原点位置存在一定偏离,需要对望远镜角度进行调整,才能使相位噪声最小化。在相位驻点位置进行引力波探测,可以有效降低相位噪声,并降低望远镜出瞳波前的质量要求。而大的离焦像差与小的彗差可以使相位驻点接近光轴,提高接收到的激光功率。

     

Dual-grid finite-difference time-domain scheme for the fast simulation of surrounded antennas

A new finite-difference time-domain (FDTD) multiresolution strategy for surrounded antenna analysis is presented. The dual-grid FDTD (DG-FDTD) is divided into two FDTD simulations using different grids. Indeed, the antenna is firstly simulated without its environment using a finely discretised FDTD in order to determine its main characteristics and save its primary radiation. In a second step, this saved field is used as the excitation of a coarse FDTD to simulate the antenna with its environment. The application of the DG-FDTD to an ultra wide-band problem is discussed, and the DG-FDTD turns out to be an accurate and fast tool to simulate various antenna configurations. Furthermore, this method remains stable along the computational time, and is easy to implement in a classical FDTD scheme     

Computational modeling of the propagation of light through liquid crystals containing twist disclinations based on the finite-difference time-domain method

The finite-difference time-domain (FDTD) method is used to compute propagation of light through textured uniaxial nematic-liquid crystal (NLC) films containing various types of twist disclination (defect) lines. Computational modeling by the FDTD method provides an accurate prediction of the optical response in multidimensional and multiscale heterogeneities in NLC films in which significant spatial optic axis gradients are present. The computations based on the FDTD method are compared with those of the classic Berreman matrix-type method. As expected, significant deviations between predictions from the two methods are observed near the twist disclination line defects because lateral optic axis gradients are ignored in the matrix Berreman method. It is shown that the failure of Berreman's method to take into account lateral optic axis gradient effects leads to significant deviations in optical output. In addition, it is shown that the FDTD method is able to distinguish clearly different types of twist disclination lines. The FDTD optical simulation method can be used for understanding fundamental relationships between optical response and complex NLC defect textures in new liquid-crystal applications including liquid-crystal-based biosensors and rheo-optical characterization of flowing liquid crystals.     

Two-dimensional finite-difference time-domain simulation for rewritable optical disk surface structure design

A novel two-dimensional finite-difference time-domain simulation for treating the interaction of a focused beam with a rewritable optical disk is detailed and experimentally validated. In this simulation, the real material properties of the rewritable multilayer stack and the aperiodic nature of the disk topography are considered. Excellent agreement is obtained between calculated and measured push-pull tracking servosignals for magneto-optical disks with pregrooves and infinite-length preformat pits. To demonstrate the utility of the simulation as a design tool, the design process for a 0.9-μm track pitch, continuous, composite servoformat magneto-optical disk is given.     

Design of metal-cladded near-field fiber probes with a dispersive body-of-revolution finite-difference time-domain method

A dispersive body-of-revolution finite-difference time-domain method is developed to simulate metal-cladded near-field scanning optical microscope (NSOM) probes. Two types of NSOM probe (aperture and plasmon NSOM probes) are analyzed and designed with this fast method. The influence of the metal-cladding thickness and the excitation mode on the performance of the NSOM probes is studied. We introduce a new scheme of illumination-mode NSOM by employing the plasmon NSOM probe with the TM01 mode excitation. Such a NSOM probe is designed, and we demonstrate its advantages over the conventional aperture NSOM probe by scanning across a metallic object.     

Light distribution in the erythrocyte under laser irradiation: a finite-difference time-domain calculation

In medical applications of low power laser irradiations, safety is one of the most concerning problems since the light focused by the biological object itself may cause damage of living organisms. The light distributions in an erythrocyte with the shape of native biconcave, oblate spheroid, or disk sphere under the irradiation of a plane light of 632.8 nm were studied with a numerical calculation method of finite-difference time domain. The focusing effect by either the biconcave erythrocyte, oblate spheroid, or disk sphere erythrocyte was found to be so remarkable that the light intensities at the focused areas close to the erythrocyte membrane were about 10 times higher than that of the incident light when the light irradiated along the erythrocyte plane. This focusing effect became weak and even disappeared when the irradiation direction deviated from the erythrocyte plane for more than an angle of 15 degrees. Because the highest light intensity in the erythrocyte can be about one order of magnitude higher than that of the incident light, this factor should be taken into account for laser safety in medical applications.     

Calculation method of reflectance distributions for computer-generated holograms using the finite-difference time-domain method

The research on reflectance distributions in computer-generated holograms (CGHs) is particularly sparse, and the textures of materials are not expressed. Thus, we propose a method for calculating reflectance distributions in CGHs that uses the finite-difference time-domain method. In this method, reflected light from an uneven surface made on a computer is analyzed by finite-difference time-domain simulation, and the reflected light distribution is applied to the CGH as an object light. We report the relations between the surface roughness of the objects and the reflectance distributions, and show that the reflectance distributions are given to CGHs by imaging simulation.     

Formulation of the finite-difference time-domain method for the analysis of axially symmetric metal nanodevices

A systematic finite-difference time-domain method, established in the cylindrical coordinate and integrated with the six-pole Lorentz–Drude model using the auxiliary differential equation method, is formulated. The model is appropriate for analyzing metal photonic devices with an axially symmetric nanostructure, such as metal nanowires, metal particles, and plasmonic lenses. As an example, an experimentally demonstrated plasmonic lens is analyzed based on the Drude model, the Lorentz–Drude model, and the Lorentz model. Depending on the different dispersion models, distinct electric field distributions for the plasmonic lens are obtained. The interesting numerical results, which are explained in this paper, show the high efficiency and accuracy of the simulation model.     

Preferential-order waveguide grating couplers: a comparative rigorous analysis using the finite-difference time-domain method

We rigorously analyze and compare preferential-order waveguide grating output couplers using the finite-difference time-domain method in the total-field/scattered-field formulation for TE and TM polarizations. Four kinds of preferential-order grating couplers are studied: volume holographic grating couplers, slanted parallelogrammic surface-relief grating couplers, double-corrugated surface-relief grating couplers, and reflecting-stack surface-relief grating couplers. The outcoupling efficiencies and branching ratios of the couplers, revealing their preferentiality, are calculated and compared with the rigorous coupled-wave analysis leaky-mode method. In addition, their performance is examined in terms of the main design parameters and the excitation wavelength.