奈米粒子在金奈米陣列的光力場作用下之渦漩運動

Abstract

本文研究奈米介電球與奈米金球在受線性極化之高斯光束激發的金奈米低聚物近場產生之光力效應，藉由多重展開中心法計算複雜的電磁場，再利用馬克斯威爾應力張量計算出奈米粒子所受之光力及光力矩，以模擬實驗中奈米結構製造出之光鑷子，增加對光力場的理解，可以更精準地控制此光鑷子效應。 從我們的模擬結果可以發現，不管是奈米介電球還是奈米金球，移動金奈米陣列之後皆有階梯般的跳躍現象，但只有奈米介電球在光鑷子抓取的過程中，有著渦漩軌跡且帶有與渦漩相反旋轉方向之自旋，且藉由改變奈米陣列位置可以精準控制光鑷子抓取奈米粒子的位置。奈米介電球的光力場行為不太受其尺寸所影響，而奈米金球只要尺寸不同，光力場的行為就差很多，較小顆的奈米金球才能夠被奈米結構所抓取。此外，三聚體的抓取能力明顯比二聚體強，且三聚體具有極化方向上的優勢，對極化方向較不敏感。

This study theoretically investigates the dielectric nanoparticle (NP) and the gold NP in the near field of the gold oligomer, which is irradiated by a linearly polarized Gaussian beam. In order to simulate the optical tweezers generated by the nanostructures (NS) in the experiment, we apply multiple-multipole expansions method (MMP) to compute the complicated electromagnetic field and Maxwell stress tensor to get the optical force and optical torque. A better understanding of the optical force field would help to control the optical tweezers more accurately. Numerical results indicated that both dielectric and gold NP move with a step-like pattern as the nano-gold array (NGA) moves. However, only dielectric NP has the opposite helicity of the spin and spiral orbit during the trapping process of the optical tweezers. Moreover, the trapping position of the NP can be controlled accurately by altering the position of NGA. There is an obvious difference between gold NP and dielectric NP in the optical force field. While the behaviors of dielectric NP barely affected by the size difference, the behaviors of gold NP seem to be extremely different due to the different size for the reason that the smaller the golden NP is, the easier it would be trapped by the nanostructure. In addition, trimer has a stronger trapping ability than dimer and it has an advantage in the direction of polarization which means that it’s less sensitive to the direction of polarization.