光誘導的雙金桿之方向性附著

Abstract

本文研究光誘導的雙金桿之方向性附著，以及高斯光束下奈米粒子的力學行為。根據Maxwell電磁理論作為基礎，數值方法是利用多重展開中心法作計算，並利用Maxwell應力張量分析奈米粒子上的光力和光力矩。由數值結果得知，雙金桿之方向性附著會有兩種結合方式，一種為相互並排(side-by-side)，另一種為頭尾相接(end-to-end)，而金桿的行為與入射波長和初始姿態有關係。若在短波長要結合，雙金桿呈現相互並排；若在長波長要結合，雙金桿呈現頭尾相接。出現這兩種的結合方式是因為金桿處於不同模態，當入射波長在短軸共振波長(TSPR)與長軸共振波長(LSPR)之間為垂直模態(perpendicular mode)；當入射波長大於長軸共振波長則為平行模態(parallel mode)，而當波長為LSPR時光力矩為零，為兩種模態的轉捩點(turning point)。當然在某些初始姿態下金桿也會互相排斥。 高斯光束方面的研究，由於高斯光束的梯度力(gradient force)，當金桿往外移動時，金桿會被抓取到高斯光束之中間，且隨著電場極化方向旋轉。另一方面，研究金、銀奈米球的穩定點，由於入射光產生的光壓會使奈米粒子前進，但是在高斯光束作用下會被抓回來，而作用力互相抵消之後，即為穩定點。穩定點的位置與材料、高斯光束之腰寬、尺寸以及入射波長有關。結果顯示，在相同條件下，銀球比金球容易達到穩定點，因為銀的吸收比金的小。

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.