时域有限差分法数值仿真单光镊中微球受到的光阱力

本文采用三维时域有限差分法(FDTD)和Maxwell应力张量法建立了单光镊在焦点附近俘获球形微粒的光阱力模型,采用基于球矢量波函数(VSWF)的五阶高斯光源作为仿真光源,得到了准确的光场传播.讨论了光源的波长、束腰、偏振态和微球的半径、折射率对光阱力的影响,分析了在单光镊俘获微球时,邻近微球对光阱力的影响.特别研究了光源的偏振态对微球所受光阱力的作用效果,仿真结果表明圆偏振光比线偏振光对微球的俘获力更大;被光镊稳定俘获的微球,会受到邻近微球干扰,失去平衡状态,改变光源的偏振态可以改变微球的受力状态. 关键词： 光镊 光阱力 介质微球 时域有限差分法(FDTD)     

FDTD方法对固体浸没透镜的光场分析

采用二维时域有限差分(2D-FDTD)方法研究了高斯光束通过固体浸没透镜(SIL)的光场分布特性.模拟结果表明,SIL的折射率越大,底面出射的光斑越小,但随离底面距离的增大光束很快展宽;当只有入射角大于全反射临界角、具有高空间频率的部分高斯光束聚焦到SIL底面金属膜中心的微孔时,针对不同直径的微孔,模拟了出射光场的分布,发现微孔直径在某一特定值时,出射光斑半径在距SIL底面近半个波长范围内无明显改变,该结果为增大SIL和存储介质间距的方案提供了理论依据.     

PML-FDTD法在分析负折射率材料中的应用

将时域有限差分(FDTD)法引入了对负折射率材料物理现象的仿真研究.给出了二维TM波在负折射率材料中的时域差分方程,并且在吸收边界处使用了理想匹配层(PML).为了避免在迭代过程中出现的不稳定现象,在差分方程的推导中引入了Drude模型, 并对Pendry提出的由负折射率材料构成的平板透镜具有的完美成像现象进行了数值仿真验证.由仿真结果发现,完美成像现象只在平板透镜的折射率n=－1时出现,当n≠－1时则会出现近轴聚焦效应.     

两种单光纤光镊捕获效果的数值仿真与实验研究

采用一种基于时域有限差分(FDTD)的数值算法,仿真计算了抛物线形和大锥角形两种新型单光纤光镊的出射光场,并在稳态场下通过对麦克斯韦应力张量积分求得介质球在两种光场中受到的光阱力,得到大锥角型光纤端产生的光阱力较大的结论;讨论了不同介质球大小、折射率,光纤探针形状对光阱力的影响.在实验中这两种光纤探针都实现了对水中酵母菌细胞的捕获,且采用流体力学法对抛物线形和大锥角形二种新型单光纤光镊产生的光阱力进行了标定.结果表明:基于FDTD数值仿真方法计算受力与实验结果一致,并且这种计算光纤光镊产生的光阱力的方法简单.适用;且抛物线形和大锥角形光纤探头都具备构成单光纤光镊的条件.     

激光对含偏心核球形粒子的辐射俘获力

利用偏心球形粒子对任意角度入射有形波束散射的理论,从广义米理论出发,根据电磁场的动量守恒及麦克斯韦张量,推导了任意入射波束对偏心球形粒子辐射俘获力的级数表达式,并以高斯波束为例,就离轴入射有吸收偏心球形粒子时的辐射俘获力进行了数值模拟,讨论了束腰半径、吸收系数、内核的相对大小及位置对俘获情况的影响. 关键词： 广义米理论 偏心球 辐射俘获力 光镊     

纳米半球微镜阵列结构对GaN基LED光提取效率的影响

为了分析表面纳米半球微镜结构对GaN基发光二极管（LED)光提取效率的影响,利用时域有限差分法(FDTD)分别对GaN、ZnO、SiO2、聚苯乙烯组成的半球微镜结构进行了分析和比较,同时用模式分析方法从理论上对FDTD计算结果进行了进一步验证。研究发现,在亚波长范围,折射率较小的材料不利于导模与表面结构层的耦合,不会对光提取效率的提高产生明显影响。相比之下,折射率较大的材料会使更多的模式耦合到半球微镜阵列层,更有利于光提取效率的提高;当材料选定,纳米半球半径增加时,光提取效率也逐渐增加,优化后半径为600 nm的半球微镜阵列结构GaN基LED,其光提取效率比没有结构的普通平板LED增强5.66倍,在以上波导材料结构中最为优化。在此基础上,通过等效折射率的计算得到半球微镜结构的等效折射率模型,并利用非对称平板模式分析的办法对以上得到的结论进行了分析和验证。这些结果对实际的高性能GaN基LED的设计与优化具有重要意义。     

三维时变等离子体目标的电磁散射特性研究

基于电流密度拉普拉斯变换方法改进的时域有限差分(LTJEC-FDTD)算法, 研究时变等离子体目标的电磁散射特性.由Maxwell方程和等离子体本构方程出发, 利用拉普拉斯变换和拉普拉斯逆变换, 推导出计算三维时变问题的时域有限差分(FDTD)算法的迭代式. 采用模式匹配方法验证了FDTD迭代式的正确性, 并通过计算等离子体球的雷达散射截面(RCS)验证了算法相关边界的正确性. 最后用LTJEC-FDTD算法分析了涂覆时变等离子体的战斧式巡航导弹的RCS. 关键词： 时变等离子体 雷达散射截面 模式匹配方法 时域有限差分方法     

信息光学 光学记录、存储与器件

O438 2005021197 FDTD方法对固体浸没透镜的光场分析=Analysis of the optical field of solid immersion lens by FDTD[刊,中]／张 东玲(河南大学物理与信息光电子学院．河南,开封 (475001)),白永林…//光子学报．-2004,33(7).-884- 888 采用二维时域有限差分(2D-FDTD)方法研究了高斯 光束通过固体浸没透镜(SIL)的光场分布特性。模拟结果     

有源微型环腔谐振器的数值仿真

结合载流子速率方程以及时域有限差分(FDTD)法,通过将增益因子等效引入电导率,成功实现了有源微型环腔谐振器的数值仿真,并模拟了有源微环的传输特性.仿真结果表明,有源微环存在谐振波长漂移的现象.由于增益的作用,有源微环的谐振光场建立时间比无源微环要短,直通端口消光更快,表明有源微环比无源微环具有更高的开关速度.     

理想同心介质球纳米喷流特性

利用有限时域差分方法,对核壳结构进行了模拟研究,得到内核折射率以及内核半径对聚焦光斑纳米喷流特性的影响。研究表明:在内核半径不变的情况下,当内核相对折射率较低时,入射光能量主要集中在内核与外壳层交界区域,此时光波能量有很大一部分处于非聚焦的旁瓣中;当内核折射率超过外层,可以在核壳结构背光面形成双增强点;在内核相对折射率不变的情况下,通过改变内核半径实现了对内、外核聚焦点位置的主动控制。     

Theoretical comparison of optical traps created by standing wave and single beam

We used generalised Lorenz–Mie scattering theory (GLMT) to compare submicron-sized particle optical trapping in a single focused beam and a standing wave. We focus especially on the study of maximal axial trapping force, minimal laser power necessary for confinement, axial trap position, and axial trap stiffness in dependency on trapped sphere radius, refractive index, and Gaussian beam waist size. In the single beam trap (SBT), the range of refractive indices which enable stable trapping depends strongly on the beam waist size (it grows with decreasing waist). On the contrary to the SBT, there are certain sphere sizes (non-trapping radii) that disable sphere confinement in standing wave trap (SWT) for arbitrary value of refractive index. For other sphere radii we show that the SWT enables confinement of high refractive index particle in wider laser beams and provides axial trap stiffness and maximal axial trapping force at least by two orders and one order bigger than in SBT, respectively.     

光镊力影响因素的计算分析

运用基于T矩阵算法的开源光镊计算工具包对可能影响光镊力的微粒尺寸、相对折射率以及光束模式进行了研究,计算结果表明,这三方面因素都会对光镊力产生显著影响,微粒直径与波长相等、相对折射率尽可能大时选择恰当的光束模式能够产生最佳的光镊捕获效果.     

光梯度力与激光捕获

从电偶极子在非均匀电磁场中所受梯度力出发,用光的电磁理论和电解质小球的电偶极子模型,论述了介质小球在光场中所受光梯度力与光强梯度成正比,并以此来定量说明激光对介质小球的捕获原理.     

Calculation of optical trapping forces on dielectric spheres at an oil-water interface with ray-optics model

The transverse trapping forces on a dielectric sphere located at an oil-water interface are theoretically investigated with the ray-optics model. The transverse trapping forces rely on the internal property of the particle-interface system, and increase with either the decrease of three phase contact angles at the oil- water interface or the use of oil phase with low refractive index. The numerical results also show that the transverse trapping forces can be improved by either decreasing the numerical aperture of the microscope objective or shrinking the diameter of the trapping laser beam.     

Radiation force exerted on nanometer size non-resonant Kerr particle by a tightly focused Gaussian beam

We calculate the radiation force that is exerted by a focused continuous-wave Gaussian beam of wavelength λ on a non-absorbing nonlinear particle of radius a ? 50λ/π. The refractive index of the mechanically-rigid particle is proportional to the incident intensity according to the electro-optic Kerr effect. The force consists of two components representing the contributions of the electromagnetic field gradient and the light scattered by the Kerr particle. The focused intensity distribution is determined using expressions for the six electromagnetic components that are corrected to the fifth order in the numerical aperture (NA) of the focusing objective lens. We found that for particles with a < λ/21.28, the trapping force is dominated by the gradient force and the axial trapping force is symmetric about the geometrical focus. The two contributions are comparable with larger particles and the axial trapping force becomes asymmetric with its zero location displaced away from the focus and towards the beam propagation direction. We study the trapping force behavior versus incident beam power, NA, λ, and relative refractive index between the surrounding liquid and the particle. We also examine the confinement of a Kerr particle that exhibits Brownian motion in a focused beam. Numerical results show that the Kerr effect increases the trapping force strength and significantly improves the confinement of Brownian particles.     

高度聚焦的余弦高斯光束对瑞利粒子的辐射力分析

通过数值计算,得出了聚焦后的余弦高斯光束在瑞利粒子上产生的辐射力在整个空间的分布情况.研究发现,利用余弦高斯光束操控粒子是可行的,且利用余弦高斯光束能同时对高折射率粒子和低折射率粒子进行俘获. 关键词： 余弦高斯光束 光陷俘 辐射力     

Calculation of acoustical radiation force on microsphere by spherically-focused source

Based on the ray acoustics approach, the trapping effects on a microsphere by an ideally spherically-focused ultrasound are discussed. The acoustical radiation force from a focused ultrasound beam on a spherical particle in a three-dimensional sound field is calculated considering the effect of the attenuation of the ultrasound beam both inside the particle and in the surrounding medium. The results show that as long as the particle is in the range of the ultrasound beam and as long as the appropriate parameters of the transducer are selected, the particle will be captured in the vicinity of the focus of the ultrasound beam. Also, the particle radius and different parameters of the transducer are analyzed for their affect on the radiation force.     

贝塞尔光束中瑞利粒子所受横向力的参数理论分析

基于电磁模型,数值计算了瑞利粒子在贝塞尔光束中所受横向光阱力,给出了粒子所受横向力与粒子的半径、折射率和波长的关系。结果表明,与高斯光束形成的光阱相比,贝塞尔光束既能捕获高折射率粒子也能捕获低折射率粒子,并且有多个平衡位置,因此应用更为广泛。     

Systematical study of the trapping forces of optical tweezers formed by different types of optical ring beams

The technique of optical tweezers has been improved a lot since its invention, which extends the application fields of optical tweezers. Besides the conventionally used Gaussian beams, different types of ring beams have also been used to form optical tweezers for different purposes. The two typical kinds of ring beams used in optical tweezers are the hollow Gaussian beam and Laguerre--Gaussian (LG) beam. Both theoretical computation and experiments have shown that the axial trapping force is improved for the ring beams compared with the Gaussian beam, and hence the trapping stability is improved, although the transverse trapping forces of ring beams are smaller than that of Gaussian beam. However, no systematic study on the trapping forces of ring beam has ever been discussed. In this article, we will investigate the axial and transverse trapping forces of different types of ring beams with different parameters systematically, by numerical computation in which the ray optics model is adopted. The spherical aberration caused by the refractive index mismatch between oil and water is also considered in the article. The trapping forces for different objectives that obey the sine condition and tangent condition are also compared with each other. The result of systematical calculation will be useful for the applications of optical tweezers formed by different types of ring beams.