電漿奈米天線增益光學對掌性及圓二色性

Plasmonic nanoantennas for optical chirality and circular dichroism enhancement

林晉平

指導教授 ： 黃哲勳

國立清華大學/理學院/化學系/碩士(2014年)

https://doi.org/10.6843/NTHU.2014.00026

摘要

由於圓二色光譜的訊號非常微弱，本論文利用時域有限差分法模擬透過電漿奈米天線來增益光學對掌性以提升圓二色光譜的訊號。我們設計擁有傾斜間隙的電漿奈米天線，藉由改變電漿奈米天線的長度使之與入射光達成共振時，可以在間隙中形成與入射磁場平行、相位差為π⁄2且高度增強的電場，以產生增益的光學對掌性。另一方面，改變間隙的傾斜角度亦會影響光學對掌性增益的大小。因此，透過改變電漿奈米天線的長度及間隙的傾斜角度，我們可以調控光學對掌性的增益情形。此外，我們也模擬非對稱十字型天線上方之光學對掌性增益值對介電物質的影響。
實驗方面，我們已自行架設了腔體震盪光譜系統。腔體震盪光譜擁有較高的靈敏度，且結合全反射還可以獲得更高的訊雜比。未來，結合腔體震盪光譜與我們所設計的傾斜間隙電漿奈米天線或非對稱十字型天線，我們可以不需要圓偏振光，透過線偏振光即可獲得增益之對掌性分子的圓二色光譜訊號。

關鍵字

電漿奈米天線 ； 光學對掌性 ； 圓二色性

並列摘要

Due to the weak signal of circular dichroism（CD） spectrum, this thesis uses finite-difference time-domain（FDTD） to simulate that enhanced optical chirality produced by plasmonic nanoantenna can improve CD signals. We present a design of plasmonic nanoantenna which has slant gap. When plasmonic nanoantenna at resonance by varying the total antenna length, the gap generates highly enhanced electric field that parallel to impinging magnetic field with a phase delay of π⁄2, lead to enhanced optical chirality. On the other hand, tuning the slant angle of the gap also has an influence on enhanced optical chirality. Thus, we can manipulate the enhancement of optical chirality by tuning the total length and the slant angle of plasmonic nanoantenna. Besides, we also show that asymmetric cross antenna can generate optical chirality above the antenna and interact with the dielectric material.
We build cavity ring-down spectroscopy（CRDS） for experiment. CRDS has high sensitivity and it can combine with total internal reflection to have higher signal to noise ratio. In the future, combine CRDS with slant gap plasmonic nanoantenna or asymmetric cross antenna, we are able to obtain enhanced circular dichroism by linearly polarized light instead of circularly polarized light.

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