-
自从文献[1]发现单一波束产生的梯度力能够吸引介质微粒以来,激光对颗粒的捕获效应,即“光镊”技术,由于其在物理、机械、化学和生命等学科的潜在应用,引发了对光学操纵等的研究热潮。为了得到负的光学力,文献[2-3]通过设计特殊的贝塞尔光束或者通过如非磁各向异性介质、梯度折射率介质、增益介质和手征介质等特殊材料来实现。文献[4]给出了排列成螺旋形状的25个金属球所受光力的解析解,文献[5]讨论了光场施加在介电系数虚部为负数时增益材料负的辐射光压。而对于激光与增益手征介质相互作用的复杂机理尚需要进一步的研究。
增益材料包括固体、液体、气体和半导体等。有些增益材料是非手征的,有些增益材料具有手征性,如手征异向介质[5]、细胞中的绿色荧光蛋白[6]、碳纳米管[7]等。目前手征介质光学力的研究大部分采用Mie理论[8]或通过设计特殊的结构光束[9]。Mie理论尽管具有准确度高和速度快等优点,但是不适合于求解非球形粒子,且Mie理论是基于Bohren的波分解技术,无法实时计算手征介质的磁电耦合效应。
与解析解相比[8],时域有限差分方法(finite-difference time-domain, FDTD)作为一种计算简单、表达直观的数值方法[10-19],具有广泛的适用性,能够模拟任意形状手征介质随时间和空间变化的电磁场分布情况。与其他数值方法相比,FDTD方法可以模拟天然有机分子、人工玫瑰花型和等效色散的手征介质[11, 13]。除了基于与Mie理论类似的波场分解技术的BI-FDTD方法[10],色散的FDTD方法还可以直接处理本构关系为磁电耦合的手征介质[12]。麦克斯韦应力张量(Maxwell’s stress tensor)和洛伦兹力(Lorentz force)是常用的两种光学力计算方法,均基于微粒的电磁场分布。麦克斯韦应力张量的方法,是在包含所计算结构的任意闭合曲面S上对麦克斯应力张量做面积分,能够获取宽频段的光学力分布情况;而洛伦兹力方法可以基于所计算结构的时谐电磁场分布,提取结构中任意位置的光学力分布情况。文献[3]基于FDTD方法,计算了两个手征介质板之间的作用力,无须考虑电荷和磁荷密度对手征介质辐射光压的影响;此外,与一维情况相比,二维FDTD方法能够模拟电磁波斜入射时,复杂形状手征介质的电磁场和洛伦兹力分布情况。
本文基于FDTD方法模拟了电磁波在二维增益手征介质柱的波传播和洛伦兹力密度的分布情况。首先给出了基于辅助差分方程(auxiliary differential equation, ADE) FDTD方法中手征介质的电极化和磁极化强度,推导了计算手征介质的波方程和洛伦兹力密度。模拟了二维普通介质板的场和洛伦兹力密度的分布,验证了本文方法和程序的正确性。最后分析了手征介质柱的同极化和交叉极化力密度分布情况,讨论其潜在工程应用。
Wave Propagation and the Lorentz Force Density of a Chiral Column Based on the FDTD Method
doi: 10.3969/j.issn.1001-0548.2017.06.010
- Received Date: 2016-09-09
- Rev Recd Date: 2017-01-17
- Publish Date: 2017-12-15
-
Key words:
- chiral media /
- dispersion /
- finite-difference time-domain method /
- force
Abstract: Based on the auxiliary differential equation (ADE) finite-difference time-domain (FDTD) method, distributions of electromagnetic fields and Lorentz force densities in a dispersive chiral column are simulated. Firstly, relationships between electromagnetic polarization densities and induced electromagnetic polarization densities, as well as coupled electromagnetic polarization densities of chiral media, are presented based on the constitutive relations. Wave equations and recurrence formula of electric are given. Secondly, the Lorentz force density in chiral media containing bound electric charge and electric current densities, as well as bound magnetic charge and magnetic current densities, is derived. Then, we verify the correctness of the ADE-FDTD method and the Lorentz force density method by comparing with literature's results. Finally, distributions of fields and optical forces for an active chiral cylinder are simulated. The contribution of electromagnetic current and electromagnetic charge densities to the Lorentz force density is discussed. The work in this paper provides some theoretical guidance for chiral media's potential engineering applications in optical tweezers and measurement of chiral parameter.
Citation: | LI Gui-ping, WANG Mao-yan, LI Hai-long, ZHANG Xiao-chuan, DONG Yu-liang, XU Jun. Wave Propagation and the Lorentz Force Density of a Chiral Column Based on the FDTD Method[J]. Journal of University of Electronic Science and Technology of China, 2017, 46(6): 850-855, 860. doi: 10.3969/j.issn.1001-0548.2017.06.010 |