多顆金屬奈米粒子在圓形極化高斯光束照射下的自組織之光力學行為

Abstract

本文研究多顆銀奈米粒子受右旋圓形極化之高斯光束照射下，所形成的自組織排列之穩定平衡圖形及其逆極化方向之公轉的光力效應。數值分析採用多重中心展開法(multiple-multipole expansions method)計算電磁場，再以馬克斯威爾應力張量計算出作用在每顆奈米粒子的光力與光力矩，以模擬實驗所觀察之多顆奈米粒子受高斯光束照射後的自組織排列之穩定平衡的圖形及逆極化方向之公轉，並探討該穩定圖形所處之平面位置與焦平面之相對距離(hf)的關聯，此外也分析各奈米粒子的三軸自轉。 模擬結果顯示，hf值必須在特定範圍內，不同數目的銀奈米粒子才可能形成穩定平衡排列之圖形，並且以光軸為旋轉軸呈逆極化方向公轉，此行為像剛體旋轉運動，每顆奈米粒子又因吸收光子的角動量，各自做三軸自旋運動，且相鄰兩顆奈米粒子的間距接近在水中之波長。該穩定圖形多發生於hf為負值時的平面上，但單顆奈米粒子則有順及逆極化方向公轉差異且發生在平面hf為正值時。若波源為平面波時，三顆粒子以上的整體排列形狀與高斯光束之結果差異不大，皆呈逆極化方向公轉，只是公轉速度較小。另外也分析多顆金奈米粒子的光力行為，其穩定平衡的圖形之hf 範圍僅稍微減小，因吸收光子能力較佳，多顆金奈米粒子的圖形公轉速度與各粒子的自旋速度相對於銀奈米粒子的結果較大，但圖形則無異。

In this thesis, the optomechanical stable self-organization patterns of multiple silver nanoparticles (NPs) with a reverse rotation induced by a right-handed circularly polarized (CP) Gaussian beam are studied. We adopt the multiple-multipole expansion method to calculate the electromagnetic field, and then use the Maxwell's stress tensor to obtain the optical forces and torques upon these NPs. Utilizing this method, we can simulate the stable patterns of these NPs and their reverse rotation excited by a CP Gaussian beam, which have been observed by previous experiment; the patterns are formed at an off-focal plane. The relationship of this stable pattern versus the position (hf) of the off-focal plane with respect to the focal plane of Gaussian beam is also studied. In addition, the three-axes spin of these NPs are analyzed. From the results of simulation, we find that there are specific stable patterns with different numbers of NPs; these stable patterns can rotate along the optical axis reversely with respect to the polarization direction of CP light; this orbital motion is like a rigid-body rotation. There is a certain range of the off-focal plane hf to obtain these stable patterns; the most cases occur in a range of hf  0. Since each NP absorbs the angular momentum of photons, NP can spin individually. Additionally, the distance of two adjacent NPs is nearly a wavelength in medium. We also analyze the stable patterns induced by a CP plane wave; the results of plane wave are almost the same with those of Gaussian beam. The reverse rotation of the stable patterns induced by CP plane wave is also observed, if the number of NPs is more than two. In addition, we investigate the 2D self-organized patterns of multiple gold NPs induced by CP Gaussian beam. Again, we find the same behaviors, except that the rotation and spin are relatively large than those of silver NPs. This is because that the absorption for photons by gold is stronger than silver at the NIR regime.