聚苯乙烯珠受金奈米結構的光力捕捉及金奈米粒子的光熱/光力形變之探討

Abstract

本論文主要分為兩個部分:第一部分探討金奈米結構的光力捕捉行為，第二部分則討論光造成金奈米粒子的變形。利用多重中心展開方法計算光與物體作用所產生的電磁場，再藉由馬克斯威爾應力張量，計算作用在物體的光曳引力及光力。 第一部分探討聚苯乙烯珠在受到不同極化方向之線偏振高斯光束照射的金奈米結構(二聚體、三聚體、四聚體)所產生的非接觸式的光力捕捉行為。模擬光力流線場的結果顯示，單組二聚體結構的光力捕捉行為與其擺置和光的極化方向之相對方向有極相關。若結構為三聚體或四聚體時，則對極化方向較不敏感，結構近場的靜滯點可隨結構相對光軸移動而變。陣列結構的結果更顯示，若是光束的極化方向可產生結構之間隙耦合，則其靜滯點可隨陣列移動，並出現階梯狀跳躍。 第二部分則討論在高斯光束照射下，單一金奈米粒子不但因光熱而軟化，同時受到表面光曳引力的作用而變形。模擬結果說明，當光熱作用階段，金奈米粒子因軟化而處於半固體狀態，此時其表面的光曳引力分布將其拉伸成一金奈米桿，甚至最後從中間斷裂成兩個較小的金奈米桿。若持續照射加熱，則表面產生融化現象，液態金之表面張力會使得金奈米桿變形而接近球型。

This thesis is divided into two parts: the first part is related to the optical trapping of gold nanostructure, and the second the photothermal and optomechanical deformation of gold nanoparticle (NP). We adopt the multiple-multipole expansions method to calculate the electromagnetic field of light-mater interaction, and then utilize Maxwell’s stress tensor to obtain the optical traction and force upon the object for both parts. In the first part, we study the optical trapping on a polystyrene bead by a gold nanostructure (dimer, trimer or tetramer), irradiated by a linearly polarized (LP) Gaussian beam. The streamline field of optical force for a dimer shows that the stagnation point of noncontact-mode trapping can follow the dimer to move if the polarization direction is not perpendicular to the orientation of dimer. However, the noncontact-mode trapping ability of a dimer depends on the polarization direction very much. In contrast, the trapping ability of a trimer and tetramer is not sensitive to the polarization. The results of a 2D dimer array or 1D trimer array also show that if the gap coupling in the cluster is induced by a LP light the stagnation point of noncontact mode will follow the array to move. The step-like jump of the trapped NP will occur as the array continuously moves away from the optical axis. In the second part, we study the photothermal and optomechanical deformation of a gold NP under the irradiation of a LP Gaussian beam. Due to the photothermal heating, the gold NP becomes softened. In addition, the optomechanical effect, the optical traction exerted upon the surface of NP, causes the deformation. Our results show that the photothermal effect softens NP to become semi-solid, and the optical stretching tractions at the two ends of NP elongate it. As a result, the NP becomes a nanorod (NR). In addition, optical pinching traction at the middle circumference of NR causes a necking. Subsequently, the elongated NR will be cut into two smaller NRs by the optical traction. If we continuously irradiate the NRs, liquid gold is induced, and the surface tension will spherify these NRs.