新穎混合式奈米棒二聚物/奈米環電漿結構於粒子捕獲及環境感測之應用

Abstract

本研究提出了一種新穎混合式電漿結構-奈米棒二聚物/奈米環。透過奈米棒二聚物，可將奈米環內的電荷引入至奈米棒中，並藉由兩尖端結構之電漿子相互耦合，進而產生出超強近場於間隙之中，藉此以大幅的能量梯度來形成高強度的光學作用力。同時，奈米棒二聚物/奈米環屬於開放性結構，更能使場強散佈在空間中，使該結構用於生物、化學等感測更具前瞻性。透過相關模擬與實驗結果指出，此電漿結構主要具有兩種模態─對稱模態及反對稱模態；其中對稱模態可以感應出較大的電偶極矩，能在間隙中產生較大的場強。此外，間隙間距、奈米棒寬度及奈米環外徑之幾何改變對結構之電漿特性的影響也被仔細探討。結果顯示，共振波長、場強分佈等基本特性與結構幾何呈現高度相關，顯示此結構之電漿特性具有高度可調性。為了評估此結構對粒子補獲與環境感測之能力，我們執行相關的實驗與模擬研究。在粒子捕獲的模擬中，當間隙為30奈米的結構下，透過1.58微米波長激發下，對20奈米的聚苯乙烯粒子可產生高達10.894 nN/W的光學箝制力。在實驗中我們採用易觀測的1微米聚苯乙烯粒子，並成功觀測到粒子被結構捕獲的行為。在環境感測的實驗中，我們在結構間隙50奈米的尺度下得到靈敏度為634奈米每單位折射率。藉由結構的參數優化設計，我們期盼新穎奈米棒二聚物/奈米環電漿結構可提供超高場強於環境中，使粒子捕獲及環境感測應用上更具發展潛力。

We propose a novel hybrid plasmonic nanostructure, rod-dimer/ring (RDR). The nanorod is regarded as a bridge pulling the charges from nanoring to nanorod, which induces large near-field in the gap region for its coupling effect and lightning effect. Besides, RDR can spread the field to the environment due to its open structure. Thus, optical properties of RDR are particularly suitable for relevant sensing applications. From the experimental and simulated results, we found that there are two dominant modes, symmetric mode and anti-symmetric mode. We specifically paid attention to the symmetric mode because it obtained stronger dipole moment, thereby gaining stronger field intensity. Besides, the plasmonic behaviors of RDR are dependent on the structural geometry such as gap distance, rod width, and outer ring radius. These results indicate that the plasmonic behaviors of RDR possesses high tunable ability. In the trapping simulation, the optical force is up to 10.894 nN/W to trap 20 nm polystyrene particle under the excitation at 1.58 μm wavelength. We also successfully trapped 1 μm polystyrene in experiment. Meanwhile, the sensitivity of 634 nm/RIU in bulk media sensing was be observed. Via structure optimization of RDR with extremely large field intensity in environment, we expect that it has more potential in trapping and sensing applications.