Barabanenkov Yurii N, Barabanenkov Mikhail Yu
J Opt Soc Am A Opt Image Sci Vis. 2017 Mar 1;34(3):321-330. doi: 10.1364/JOSAA.34.000321.
A new general formula is presented for a collective extinction cross section of a dielectric or a metallic nanoparticle ensemble in terms of incident electric field work on currents excited inside particles. The formula is obtained by identical transformation of the well-known expression for the summing power of electromagnetic field energy losses caused by particle ensemble scattering and absorption. The derived formula is applied to the problem of radiation losses at electromagnetic excitation transfer along a straight chain of particles. Our general formula predicts a zero collective extinction cross section for an infinite straight chain of nonabsorbing dielectric particles providing that the projection of the wave vector of an incident electromagnetic wave on the chain axis does not coincide with its counterpart of the Bloch wave vector of propagating excitation. In another case of a finite chain of particles, with only the first particle of the chain irradiated by an incident narrow electromagnetic wave beam, the derived formula shows that only the irradiated particle directly contributes to the collective extinction cross section despite how large the total number of particles can be, which makes a direct summing contribution of all other particles to wave scattering as if they were unviewed (dark mode). Using a recently developed quasi-separable T-scattering operator approach that leads to the equation system for self-consistent currents excited inside particles by an incident electromagnetic wave field and restricting ourselves to the electric dipole single scattering and neighbor coupling approximation, we revealed a few gigahertz transparency band in the terahertz frequency range (orange color) in the spectra of a straight chain of closely spaced gold nanospheres of a certain radius and a length of a few millimeters. A resonant mechanism of filtering the dark mode from radiation losses established in this work allowed us to reveal a few-fold-more narrow passband in the spectra of a longer gold particle chain with the full length of a few centimeters.
本文根据入射电场对粒子内部激发电流所做的功，给出了一种介电或金属纳米粒子集合体的集体消光截面的新通用公式。该公式是通过对由粒子集合体散射和吸收引起的电磁场能量损失的总功率的著名表达式进行恒等变换得到的。推导得到的公式被应用于沿直线排列的粒子链在电磁激发转移过程中的辐射损耗问题。我们的通用公式预测，对于无限长的非吸收性介电粒子直线链，只要入射电磁波的波矢在链轴上的投影与其传播激发的布洛赫波矢的对应投影不重合，其集体消光截面就为零。在另一种情况，即有限粒子链中，当只有链中的第一个粒子被入射的窄电磁波束照射时，推导得到的公式表明，尽管粒子总数可能很大，但只有被照射的粒子直接对集体消光截面有贡献，这使得所有其他粒子对波散射的直接求和贡献就好像它们没有被看到一样（暗模式）。利用最近开发的准可分T散射算子方法，该方法可得到由入射电磁波场在粒子内部激发的自洽电流的方程组，并将我们的研究限制在电偶极子单散射和相邻耦合近似下，我们在一定半径且长度为几毫米的紧密排列的金纳米球直线链的光谱中，发现了太赫兹频率范围内的几个吉赫兹透明带（橙色）。这项工作中建立的从辐射损耗中滤除暗模式的共振机制，使我们在全长为几厘米的更长金粒子链的光谱中发现了窄几倍的通带。
