藉由光學捕捉協同光散射及光干涉效應形成具有方向性拓展至焦點外之金奈米粒子聚集體

Abstract

藉由近紅外連續波雷射(波長1064奈米)聚焦於樣品固/液界面上，並透過暗視野的散射成像以及共軛焦之光譜術，我們探討金奈米粒子(直徑200奈米)經過光學捕捉以及聚集化之動態過程。單一的金奈米粒子聚集體形成在此界面上，並且這個聚集體沿著垂直於線性偏振光的方向拓展至焦點外，然而以傳統上的光學捕捉能力僅能在雷射焦點內捕捉奈米粒子。對於圓形偏振光的光學捕捉條件下，聚集體呈現圓形並拓展至焦點外。實驗顯示光學偏振性、雷射功率以及奈米粒子尺寸對於奈米粒子聚集化具有相依性，我們推斷聚集化的特性與雷射捕捉金奈米粒子所產生的散射光有關，而在消光光譜的實驗結果表示奈米粒子聚集體透過散射光進行長程交互作用下產生光學耦合。 更進一步的觀察聚集化之早期動態過程，並進行單一粒子級的影像分析，奈米粒子沿著垂直於線性偏振光的方向排列成直線，並以特定的粒子間距離排列，此距離可相應於雷射的波長，而在圓形偏振光的光學捕捉條件下，則是產生二維緊密堆積型的排列結構。這樣的粒子間距離蘊含著光學綁定的作用力，其作用力起因於金奈米粒子導致的散射光之干涉。綜合以上的實驗結果，我們提出奈米粒子聚集化的機制。

We demonstrate optical trapping and assembling of 200 nm gold nanoparticles at glass/solution interface with tightly focused 1064 nm continuous wave laser through the measurement on dark-field scattering imaging and confocal spectroscopy. A single gold nanoparticle assembly is formed at the interface, and this assembly is directionally and dynamically extended outside the focus along the direction perpendicular to the linear polarization, while the nanoparticles are trapped only inside the focus with conventional optical trapping. With circular polarization, the assembly shows circular shape extending outside the focus. The polarization dependence, laser power dependence, and nanoparticle size dependence are examined. We consider that these assembling behaviors are relative to scattered light of the trapping laser by the gold nanoparticles. The results of extinction spectra represents that the assembly is optically coupled through long-range interaction by light scattering. Furthermore, the single nanoparticle-level analysis is conducted to observe the initial stage of these assembling dynamics. The nanoparticles are aligned in straight line perpendicular to the linear polarization with certain interparticle distance which corresponds to laser wavelength, while two-dimensionally closed-pack structure is formed with circular polarization. The interparticle distance corresponds to optical binding force generated by interference of light scattering of gold nanoparticles. By integrating the results, the assembling mechanism is proposed.