| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 103 === In this thesis, we theoretically studied the plasmon-mediated oriented attachments of two gold nanorods (GNRs) irradiated by a linearly polarized (LP) plane wave and mechanical responses of a single nanoparticle (NP) by Gaussian beam. Based on Maxwell’s equations, the multiple-multipole (MMP) method was used to calculate Maxwell stress tensor for the analysis of optical forces and torques exerted on these nanoparticles. Numerical results show that due to the short-range interaction the end-to-end or side-by-side coalescence of two nearby GNRs could be induced by a LP plane wave, depending on the wavelength. The short wavelength most likely induces the side-by-side oriented attachment, whereas the long wavelength most likely induces the end-to-end oriented attachment. The turning point between the two behaviors is at LSPR. This is because that GNR performs two alignment modes as irradiated by a LP light. One is the perpendicular mode; the range is between the longitudinal surface plasmon resonance (LSPR) and transverse surface plasmon resonance (TSPR). The other is the parallel mode as the wavelength is longer than LSPR. On the other hand, for some initial conditions the two GNRs could repulse each other.
Numerical result also shows that due to the gradient force of LP Gaussian beam a GNR tends to be trapped at the center of Gaussian beam and aligned by the polarization. Moreover, we investigated the stagnation point of Au or Ag NP along the axis of Gaussian beam, where the optical pushing force in the downstream direction vanishes. This is because that the optical gradient force and the optical radiative force are in balance. This behavior is sensitive to the waist of Gaussian beam and wavelength. We found that the stagnation point of Ag NP is induced easily compared to Au NP because the absorption of Ag NP is smaller than Au one of the same size.
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