Optical Properties and Mechanical Responses of Gold and Silver Nanorods
Date Issued
2014
Date
2014
Author(s)
Lo, Wei-Jiun
Abstract
In this thesis, we theoretically studied the optical properties and mechanical responses of gold (GNRs) and silver nanorods (SNRs). GNRs and SNRs have different kinds of polarizability: longitudinal surface plasmon resonance (LSPR) and transverse surface plasmon resonance (TSPR), because of the anisotropic geometry. Based on Maxwell’s equations, the multiple-multipole (MMP) method was used to investigate the electromagnetic field of a plane wave illuminating these nanorods.
The research is mainly divided into two parts. For the optical properties, we focused on the average optical effects of a plane wave illuminating a GNRs. The average scattering, absorption and extinction efficiency, and the average depolarization ratio of a randomly oriented GNR were analyzed. A simple model, three-principal-axes method, was also used to calculate these efficiencies. The result showed that the simple model can only be useful for slim GNRs which have only the first SPR mode.
For the mechanical responses, Maxwell stress tensor is used to investigate optical force and optical torque produced by the electromagnetic field on GNRs and SNRs. Simulation results show that there are two alignment modes of GNRs and SNRs. One is perpendicular mode; the range is between of LSPR and TSPR. The other one is parallel mode for those wavelengths longer than LSPR. The turning point between the two modes is at LSPR with a null optical torque. The two different modes can help us to explain why under the irradiance of the same NIR laser the alignments of nanorod and nanowire are different. For the former, the long axis is aligned to parallel to the polarization of a linearly polarized light. In contrast, the latter is aligned to perpendicular to the light polarization. This is because that the LSPR of GNR or SNR is red-shifted as the aspect ratio increases.
Subjects
奈米桿
表面電漿共振
長軸表面電漿共振
多重中心展開法
散射、吸收與消散截面積平均效率
去極化平均效率
Maxwell應力張量
光力矩
垂直與平行模態
Type
thesis
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