二維金奈米陣列對介電質奈米粒子的光力學捕捉

Abstract

本論文研究二維金奈米陣列受線性極化之高斯光束照射後，對介電質奈米粒子產生的光力效應，採用多重中心展開法計算電磁場，並以馬克斯威爾應力張量計算出奈米粒子之光力場，再以此光力向量場分析其流線場分佈，藉此探討結構在向光側及背光側對介電質奈米粒子的光力學捕捉行為。 固定之單一金屬奈米結構(單顆、雙顆)的模擬結果顯示，在受線性極化之高斯光束照射下，對附近一顆自由移動的介電質奈米粒子所產生之三維運動，光力學捕捉分為接觸模式與非接觸模式兩種，後者之向光側有兩個靜滯點及背光側有一個靜滯點，即介電質奈米粒子懸浮處。而接觸模式之一接觸面上有最終接觸點。另外又利用蒙地卡羅隨機分佈統計法，在定義分佈空間中隨機選取數萬個點做為介電質奈米粒子的起始點，再統計此隨機點最後停留的位置，其中這些點分為代表接觸模式與非接觸模式捕捉的位置，本研究發現，由近場的非接觸模式占最大比例，即非接觸捕捉效應較明顯。 二維陣列在背光側僅有一個靜滯點在二維陣列正中心，即光軸正下方，移動陣列後，此點仍在光束下方，可知在背光側由高斯光束主導光力捕捉，非受結構影響。而在向光側，當陣列移動後，在靠光軸附近的兩個靜滯點隨之移動，近場的靜滯點之捕捉強度隨結構移動而呈現週期性消長現象，因而被捕捉之介電質奈米粒子出現階梯般地跳躍現象，此說明：在向光側由陣列結構之近場捕捉主導對介電質奈米粒子的光力捕捉行為。

The optomechanical trapping of a 2D gold nano-array on a freestanding dielectric nanoparticle (NP) is studied in this thesis. The near-field optical trapping is induced by a linearly polarized Gaussian beam irradiating the system. We used the multiple multipole method to calculate the electromagnetic field, and then calculated the optical force exerted on the NP. We can calculate the streamline field in terms of the vector field of optical force, depending on the relative position of NP, to investigate the optomechanical behaviors of two configurations; one is on the front side and the other on the back side of the nano-array with respect to the incident light. From the results of the simulation for a single nanostructure, two trapping modes (contact and noncontact modes) for trapping a freestanding dielectric NP in the proximity are observed. For the latter, there are two stagnation points, at which the NP floats, for the front-side configuration, and a stagnation point for the back-side one. For the contact mode, there are two terminal points on the contact surface. We also used Monte Carlo method to analyze the probability of these modes. We chose some points as the initial positon of the NP randomly in the selected space, and then followed the streamline to estimate its final location. Based on these calculations, we can obtain the probabilities of the contact mode and the noncontact modes at these stagnation points. The numerical results show that the noncontact mode has larger probability than the contact mode; i.e. the noncontact mode of the optical trapping is dominant over the contact mode. The results of 2D nano-array show that there are two stagnation points of the noncontact mode at the center zone of Gaussian beam in the near field of nano-array for the front-side configuration and a stagnation point for the back-side one. As the nano-array moves, these stagnation points on the front side follow. As the nano-array continuously moves away from the optical axis, the trapping strength of the old stagnation points decay and new ones grow periodically. Consequently, the periodic step-like jump of the trapped dielectric NP occurs. This phenomenon means that the near-field stagnation points of the nano-array dominate the trapping of the NP on the front side of irradiation, rather than the Gaussian beam. In contrast, the stagnation point at the optical axis on the back side does not follow the nano-array, and still stays near the optical axis. It means that Gaussian beam dominates the trapping of the NP on the back side.