高效能光捕捉奈米粒子之電漿子奈米鑷夾設計

Design of plasmonic nanotweezers for highly efficient optical trapping of nanoparticle

林宜錩(Yi-Chang Lin)

指導教授 ： 李柏璁

國立交通大學/電機學院/光電工程系所/博士(2019年)

摘要

光學鑷夾提供了一個革命性的方法能對微小物體的運動進行控制，其基於光學力的操控具有精確、非接觸與非破壞的特性，這項技術已發展了超過三十年並廣泛的運用在生物、物理、化學等領域；除了使用強力聚焦雷射光束進行捕獲，人們也發現介電或金屬結構周遭的高聚集光場能對微小物體產生可觀的光學力，因此，這些運用近場的光鑷夾不僅能以低耗能進行更加精確與穩固的光操控，元件尺寸也大幅的縮小；更進一步，這些鑷夾元件能與其他功能性元件或微流道整合來進行更多元的應用；許多近場鑷夾設計已被提出來達到對奈米粒子進行有效捕捉，但會發現他們的光學模態大多位於捕捉對象無法接觸到的區域，而我們相信，進行有效捕捉的鑷夾設計應同時考慮如何增加場強與粒子光場間的重疊程度。
在此論文我們提出兩種電漿子奈米鑷夾來達成對奈米粒子進行有效捕捉，並且徹底研究所有會對捕捉能力影響的因素；第一種設計叫作電漿子領帶結凹槽，其具有緊緻尺寸200 nm × 220 nm，並能以超低激發場強0.64 mW/μm2來穩定捕捉100 nm粒子；此設計中領帶結形狀的空洞與底部金屬層扮演了重要角色，他們能使共振模態具有增強的區域場強與易接觸的場分佈來達到優異的捕捉能力；而凹槽的共振特性能對被捕捉的粒子進行高靈敏感測，其靈敏度為每當粒子尺寸增加1 nm便會對反射消光頻譜的峰值波長產生71 pm的紅移；第二種設計叫作波導耦合混合型電漿子奈米楔形，其具有緊緻尺寸400 nm × 625 nm，並能以超低注入能量3.57 mW來穩定捕捉100 nm粒子；此優異的捕捉能力來自於波導與奈米楔形間的有效能量耦合，與位於奈米楔形前方尖端之奈米聚焦電漿子模態具有易接觸的場分佈；我們期待這些能對奈米粒子進行有效捕捉與高靈敏感測的電漿子奈米鑷夾能夠成為基礎奈米科學發展的有用工具。

關鍵字

光學鑷夾 ； 電漿子光學 ； 領帶結形狀孔洞 ； 波導耦合 ； 奈米粒子 ； 光學感測

並列摘要

The optical tweezers introduce an evolutionary method to control the dynamics of tiny objects, which is based on optical forces to provide manipulation with features of precision, contactless, and nondestructive. This technique has been developed for over three decades and widely applied in the fields of biology, physics and chemistry. Beside the tweezing utilizing strongly focused laser beam, people found that the highly concentrated optical fields surrounding dielectric or metallic structures can exert considerable forces on tiny objects. Therefore, the tweezers using near fields are able to perform much precise and rigid manipulation under low energy consumption, and their devices are much miniaturized. Furthermore, they can be integrated with other functional devices or microfluidic channel to conduct diverse applications. Many designs of near-field tweezers have been proposed to pursue efficient trapping of nanoparticles, but their optical modes are mostly located at the regions which are hard to be accessed by trapping targets. We believe that the device design for efficient trapping should consider both increasing field intensity and degree of particle-field overlap.
In this thesis, we propose two designs of plasmonic nanotweezers to achieve efficient trapping of nanoparticle. The factors influencing their trapping capabilities are thoroughly studied, too. The first design, named plasmonic bowtie notch, has compact footprint of 200 nm × 220 nm and provides stable trapping of single 100 nm particle under an ultralow excitation intensity of 0.64 mW/μm2. The bowtie-shaped void and bottom metal film play important role of creating a resonance mode with both enhanced local intensity and accessible field distribution, which lead to great trapping capability. The resonance characteristic of notch also provides a high sensitive detection to trapped particle, which shows 71 pm redshift in peak wavelength of reflection extinction per 1 nm increase of particle size. The second design, named waveguide-coupled hybrid plasmonic nanotaper, has compact footprint of 400 nm × 625 nm and provides stable trapping of single 100 nm particle under an ultralow input power of 3.57 mW. The great trapping capability is owing to the very efficient energy coupling from waveguide to nanotaper and the accessible field distribution of nanofocused plasmonic mode at taper front tip. We anticipate these plasmonic nanotweezrs offering efficient trapping and sensitive detection of nanoparticle could be useful tools in the development of fundamental nanoscience.

並列關鍵字

Optical tweezers ； Plasmonics ； Bowtie aperture ； Waveguide coupling ； Nanoparticle ； Optical sensing

參考文獻

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