利用超材料來實現多功能生物感測器及細胞影像與低損耗、高對稱性的人工電磁特性

Realizing multi-functional plasmonic biosensor/bio-imaging and artificial electromagnetic response with low-loss and high-symmetry properties by metamaterials

賴岳軍(Yueh-Chun Lai)

指導教授 ： 嚴大任

國立清華大學/工學院/材料科學工程學系/博士(2012年)

摘要

超材料（metamaterials）為人工電磁材料，係由單位尺寸小於入射光波長所組成的結構。超材料的性質來自於內部結構產生共振的集合響應，所展現出物理與光學特性是自然界材料所無法展現的性質。其中，Pendry等科學家提出用來展現負透磁率（negative permeability）與高頻磁子（high magnetism）的人造磁性原子¬-隙環共振器（split-ring resonators）ㄧ直是最常設計來產生磁特性的超材料。ㄧ般而言，其基態的共振模態行為是透過等效的電容¬-電感電路概念來解釋並且其複數個共振響應的特性可以藉由駐波式電漿共振響應模型（standing-wave plasmonic resonance model）來闡釋並預估其所對應的響應波長。更重要的是，此共振模態對隙環共振器周圍介質環境很敏感，特別是當生物分子結合於隙環共振器的表面時，其共振模態的頻率位置會移動，使其可作為即時的、不需標定的折射率式細胞生物感測器。

因此我們對隙環共振器多模態共振生物感測器的相對靈敏度與偵測長度做深入調查，依據我們模擬與實驗的結果，藉由在隙環共振器上加上不同厚度的介電層，我們可以對隙環共振器上反射光譜中各個共振模態的靈敏度與偵測長度作一定量描述，使奈米尺寸下的隙環共振器可以作為無需耦合、可調控頻率範圍的多模態生物感測器。由於隙環共振器的共振模態具有無需標定、無需耦合、可調控頻率範圍（從中紅外線到可見光）、深的偵測長度等特性，我們因此發展出隙環共振器顯微影像（split-ring resonators microscopy），與表面電漿共振顯微影像做比較，此隙環共振器顯微影像更可以用來觀測生物分子。我們的實驗結果顯示可以利用此技術來觀測人類間葉幹細胞內部的折射率分布圖，且同時可以得到待測細胞官能基的資訊。因此，我們預期此隙環共振器顯微影像可以實現更簡單的光學組態與更佳的偵測深度來作全細胞影像的應用。

除此之外，我們也利用高介電常數的陶瓷材料例如二氧化鋯，氧化鋁等在微波頻段下來設計使傳統電磁光學規則相反的負折射率介質。從週期排列的商業用二氧化鋯（氧化鋁）立方體塊材，由於結合其displacement current與Mie resonance我們可以設計人造磁偶極與電偶極。藉由改變這些介電質共振器的大小與週期，其對應的磁性反應與電性反應可控制在設計的頻率。若更進一部的調整電性反應與磁性反應在相同頻率下，我們可以從單一種類的介電質共振器創造出負折射率介質（negative refractive index media; NRIM） 。另一方面，我們也混成商業用的二氧化鋯與氧化鋁獨自的電性反應與磁性反應來產生負折射率介質。

最後我們也討論一對介電質共振器的Mie resonance的耦合效應，特別是在幾何非對稱排列下其可以耦合出獨特的電磁反應例如超材料誘發透明現象（metamaterials-induced transparency; MIT），當混成一對相同的介電質共振器或是一對不同的介電質共振器，與金屬型式超材料比較，我們可以得到較大的group index，較佳的頻寬延遲乘積（bandwidth-delay product; BDP~0.9）。能有這些特色的關鍵在於藉由控制介電質共振器之間的介電常數對比，我們可以激發trapped mode or the suppressed mode共振。與金屬式的超材料相比，介電質式的超材料擁有低損耗，高對稱性等優點，使其有優勢於通訊元件，完美透鏡，隱形斗篷與其它電磁元件等相關應用。

關鍵字

超材料 ； 生物感測器 ； 介電質共振器 ； 隙環共振器

並列摘要

Metamaterials are artificial electromagnetic materials in which the size of building elements is smaller than the wavelength of illuminating light. Based on the collective resonances in internal designed structures, metamaterials enable physical and optical properties that have not been achieved in naturally existing materials. Among diverse metamaterials, it is the split-ring resonator (SRR) structure a pioneering design proposed by Pendry et al. as magnetic meta-atoms to achieve negative magnetic permeability and high-frequency magnetism. The fundamental resonant behaviors of SRRs are conventionally understood by the equivalent LC circuit model and the multiple resonant reflectance peaks under normal incidence can be elucidated by model of standing-wave plasmonic resonances. More importantly, such a resonance condition depends on the local dielectric environment so sensitively that the SRRs can be readily employed as refractive-index (RI) sensors, especially for real-time, label-free and cell-level bimolecular detections by monitoring the shifts of reflectance peaks as analytes binding to molecular receptors immobilized on the SRR surface.

Thus, we present a comprehensive understanding of the relative sensitivity and the detection length about the multi-mode plasmonic resonances in the planar SRR structure. By applying thin dielectric layers with different thicknesses on the SRR array, we demonstrate a quantitative interpretation to the distinct sensing behaviors (including sensitivity and detection length) of each resonance mode in the multi-resonance reflectance spectra based on both simulation and experimental results, present a coupler-free, scalable and multi-mode refractive index sensor based on nano-structured split ring resonators. Next, we develop a compact plasmonic bioimages based on SRRs. Owning advantages such as label-free, coupler-free, tunable spectrum range (from MIR to VIS) and longer detection length, the SRR microscopy (SRRM) is a strong competitor compared to the surface plasmon resonance microscopy (SPRM) for observing bio-target. Our experimental results has successfully demonstrated its capability of constructing the refractive index distribution images of human bone marrow-derived mesenchymal stem cells (hMSCs) and meanwhile, obtaining the information of functional groups from the target cells. Therefore, we expect that the SRR microscopy (SRRM) delivers much simple optical configuration and better penetration depth for truly whole-cell imaging applications.

In addition, we utilize high dielectric constant ceramic materials such as zirconia, alumina to design negative refractive index media (NRIM) in the microwave region that have attracted significant attention for their potential to revise conventional electromagnetic rules involving refractive indices such as inverse optical rules. From a periodic array of commercially available zirconia (Alumina) cubes, we demonstrate artificial magnetic and electric dipoles due to the combination of displacement currents and Mie resonance. By scaling the size and periodicity of these dielectric resonators, the corresponding magnetic and electric responses are shifted to the desired frequencies. To further overlap the magnetic and electric resonances in the same frequency, we create a negative refractive index medium from single-dielectric resonators. On the other hand, we hybridize commercially available zirconia and alumina structures to harvest their individual artificial magnetic and electric response simultaneously, presenting a negative refractive index medium.

Finally, we introduce the coupling of Mie resonances in the dielectric resonator pairs, especially in the asymmetric case that supports an extraordinary electromagnetic response such as metamaterials-induced transparency (MIT) phenomena. Using two hybrid structures of identical-dielectric-constant resonators (IDRs) and distinct-dielectric-constant resonators (DDRs), we demonstrate a larger group index (ng~354), better bandwidth-delay product (BDP~0.9) than metallic-type metamaterials. The keys to enable these properties are to excite either the trapped mode or the suppressed mode resonances, which can be managed by controlling the contrast of dielectric constants between the dielectric resonators in the hybrid metamaterials. Comparing with the conventional metamaterials-based applications constructed by metallic elements, the demonstrated all-dielectric metamaterials possesses low-loss and high-symmetry advantages, thus benefiting practical applications in communication components, perfect lenses, invisible cloaking and other novel electromagnetic devices.

參考文獻

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