Bi Lei, Yang Ping, Kattawar George W
Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, USA.
Appl Opt. 2010 Aug 20;49(24):4641-6. doi: 10.1364/AO.49.004641.
The extinction efficiency factor associated with the scattering of a plane electromagnetic wave impinging on a basal face of a dielectric disk or a cylindrical particle is investigated by employing the physical-geometric optics hybrid (PGOH) method and the discrete-dipole approximation (DDA) method. It is found that the derived extinction efficiency factor from the PGOH is a function of the thickness of the disk, or the length of the cylinder, and the refractive index, but is independent of the diameter and shape of the cross section of the basal face of the particle. Furthermore, the oscillations of the extinction efficiency factor versus the thickness or length of the particle do not diminish if the particle is not absorptive. The values of the extinction efficiency factor simulated from the DDA method are quite different from those computed from the PGOH, although the size parameter of the particle is in the commonly recognized geometric optics regime. To explain the difference, the concept of the edge effect associated with the tunneling rays in the semiclassical scattering theory is generalized from the case of spherical particles to that of nonspherical particles based on the localization principle. Accordingly, the edge-effect contribution can be distinguished and removed from the extinction cross section calculation by the DDA method. The remaining part of the extinction cross section, associated with the interference between the transmitted rays and incident rays, agrees well with the results computed from the PGOH, and the agreement illustrates the presence of the edge effect in the case of nonspherical particles with surfaces that have no curvature along the incident direction. It is found that the asymptotic extinction efficiency factor may not necessarily converge to 2, but it depends on the specific physical processes of the interference between diffracted and transmitted light and of the edge effect.
采用物理几何光学混合（PGOH）方法和离散偶极子近似（DDA）方法，研究了与平面电磁波撞击介质圆盘或圆柱粒子底面散射相关的消光效率因子。结果发现，由PGOH得出的消光效率因子是圆盘厚度或圆柱长度以及折射率的函数，但与粒子底面横截面的直径和形状无关。此外，如果粒子不吸收，消光效率因子随粒子厚度或长度的振荡不会减弱。尽管粒子的尺寸参数处于公认的几何光学范围内，但DDA方法模拟的消光效率因子值与PGOH计算的值有很大不同。为了解释这种差异，基于定位原理，将半经典散射理论中与隧穿光线相关的边缘效应概念从球形粒子的情况推广到非球形粒子的情况。因此，在通过DDA方法计算消光截面时，可以区分并去除边缘效应的贡献。消光截面的其余部分，与透射光线和入射光线之间的干涉有关，与PGOH计算的结果吻合良好，这一吻合说明了在具有沿入射方向无曲率表面的非球形粒子情况下边缘效应的存在。结果发现，渐近消光效率因子不一定收敛到2，而是取决于衍射光和透射光之间干涉以及边缘效应的具体物理过程。
