藉由電漿奈米結構增益圓二色及光學掌性

Enhanced circular dichroism and optical chirality by plasmonic nanostructures

林展鴻

指導教授 ： 黃哲勳

國立清華大學/理學院/化學系/博士(2014年)

摘要

圓二色光譜的靈敏度由於波長與分子尺寸的不匹配所以通常很弱。在這個工作中我們利用時域有限差分法模擬計算以確認奈米天線可以增強圓二色及光學掌性。我們證實當使用線偏振光激發時，不對稱的奈米結構可以視為奈米四分一波片且而可以在間隙中產生高強度圓偏振態的近場。相較於單一結構，週期性的奈米結構可以增加有效區域，因此我們設計出一系列週期性的結構。最後為了製作方便，我們使用菱形天線並製作出實際的結構。另一方面由於反相結構具有方便分子進入有效增強區域的優勢，於是我們將之與不對稱結構結合設計出奈米橢圓洞，使奈米橢圓洞不僅具有光學力來捕捉小粒子也可以產生圓偏振近場來增強圓二色光譜的訊號。
實驗部份我們改裝商業化的圓二色光譜，但是由於光學元件對不同偏振光反射率不同的問題，目前無法得到可信的結果。未來我們希望在聚苯乙烯球上修飾掌性分子，用線偏振光激發奈米橢圓洞時可以捕捉聚苯乙烯球得到分子的圓二色光譜或是螢光偵測圓二色光譜。

關鍵字

電漿奈米天線 ； 圓二色性 ； 光學對掌性

並列摘要

The sensitivity of circular dichroism is usually low due to the mismatch between wavelength and molecule size. In this work we use finite-difference time-domain method to calculate and to make sure if the nanoantenna can enhance circular dichroism and optical chirality. It shows that upon linearly polarized excitation, an asymmetry cross antenna serves as a nanosized quarter waveplate and creates circularly polarized near field with highly enhanced intensity inside the gap. Compared with solitary cross antennas, periodic structures are able to enlarge the effective area. Therefore, we design a series of periodic structures. To easily fabricate the antennas, we use diamond shape antennas and fabricate it. On the other hand, the advantage of inverse structure is to make molecules enter the effective enhanced area. We combine the advantage of inverse structure with asymmetry nanostructures to design elliptical nanoholes. Our elliptical nanoholes not only provide plasmonic optical force for trapping small particles but also generate circularly polarized near fields to enhance the signal of circular dichroism.
We modify commercial circular dichroism spectrometer for experiment. However, we still haven’t got the reliable result due to the fact that optical elements respond differently to different reflectance of polarized light. In the future, we plan to modify chiral molecules on polystyrene spheres. When exciting elliptical nanoholes by linearly polarized light, we can trap the polystyrene spheres to get the molecular circular dichroism or fluorescence-detected circular dichroism.

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