Terahertz plasmonics: The rise of toroidal metadevices towards immunobiosensings

This work reviews fundamentals and the recent state-of-art achievements in the field of plasmonic biosensing based terahertz (THz) spectroscopy. Being nonpoisonous and nondestructive to the human tissues, THz signals offer promising, cost-effective, and real-time biodevices for practical pharmacological applications such as enzyme reaction analysis. Rapid developments in the field of THz plasmonics biosensors and immunosensors have brought many methodologies to employ the resonant subwavelength structures operating based on the fundamental physics of multipoles and asymmetric lineshape resonances. In the ongoing hunt for new and advanced THz plasmonic biosensors, the toroidal metasensors have emerged as excellent alternates and are introduced to be a very promising technology for THz immunosensing applications. Here, we provide examples of recently proposed THz plasmonic metasensors for the detection of thin films, chemical and biological substances. This review allows to compare the performance of various biosensing tools based on THz plasmonic approach and to understand the strategic role of toroidal metasensors in highly accurate and sensitive biosensors instrumentation. The possibility of using THz plasmonic biosensors based on toroidal technology in modern medical and clinical practices has been briefly discussed.

這項工作回顧了基于等離子生物感測的太赫茲（THz）光譜學領域的基礎知識和最新技術成就。 THz信號對人體組織無毒無害，可為酶學分析等實用藥理應用提供有前途，具有成本效益的實時生物設備。太赫茲等離子生物傳感器和免疫傳感器領域的飛速發展帶來了許多方法來采用基于多極子和非對稱線形共振的基本物理學原理工作的共振亞波長結構。在不斷尋求新的和先進的THz等離子體生物傳感器時，環形超傳感器已經作為優秀的替代品出現，并被介紹為THz免疫傳感應用中非常有前途的技術。在這里，我們提供了最近提出的用于檢測薄膜，化學和生物物質的THz等離子體元傳感器的示例。這項審查允許比較基于THz等離子體方法的各種生物傳感工具的性能，并了解環形元傳感器在高精度和靈敏的生物傳感器儀器中的戰略作用。簡要討論了在現代醫學和臨床實踐中使用基于環形技術的THz等離子體生物傳感器的可能性。

Introduction

Surface plasmon resonances (SPRs) are the coherent d-band electron oscillations, occurring at metal-dielectric interfaces, when exposed with intense light of certain frequencies [1,2]. As a promising counterpart of optical physics and nanophotonics for extreme light confinement and manipulation [3], plasmonics research field has been acknowledged as a motivating principle for developing advanced and practical photonics technologies, devices and applications [4–6]. Possessing a deep fundamental role in tailoring all-optical and optoelectronic devices, plasmonics has enabled designing of efficient photovoltaic devices [7–10], long-decay range hybrid waveguides [4,11,12], ultrafast modulators [13–15], photodetectors and transistors [16–20], polarization beam splitters [21–23], Mach-Zehnder interferometers [24–26], metamaterials [27–32], superlenses [33,34], nanolasers [35–37], quantum devices [38–40], solar water splitting [41,42], biodevices and clinical tools [43–45], etc. For the latter instance, plasmonics is a reliable technology with exquisite? benefits applicable in both in-vitro and in-vivo assays. The subwavelength plasmonic platforms have extensively been utilized for developing several types of label-free diseases diagnosing devices [46–55], cancer and tumor therapies [56–60], targeted drug delivery [61–64], nanowelding [65,66], hyperspectral nano-imaging [67–69], optoacoustic imaging [70,71], real-time pharmacology [72–74], vapor and micro-bubble generation [75– 77], laser nanosurgery [78–80], photothermal heat spectroscopy [81–85], photothermally controlled fluidics [86–89], heatassisted magnetic recording [90–93], and neuron stimulation [94,95]. Extreme localization of SPRs using subwavelength metallic objects leads to robust enhancement in the optical absorption, resulting in thermal heating of the free electron gas via electron– electron scattering in a hundreds of femtoseconds [96]. Of particular interests are the plasmonic biological and biochemical sensors, which possess a vital role in commercial and advanced clinical and pharmaceutical applications [43,44,97,98]. High accuracy, real-time response, being label-free, operating in room-temperature, cost-effective, and fast response, all these advantages prompted researchers to work on enhanced plasmonic biosensors for decades [99,100].

介紹

表面等離子體激元共振（SPR）是相干d波段電子振蕩，當暴露于某些頻率的強光下時，會發生在金屬-電介質界面上[1,2]。作為極有可能的光學物理學和納米光子學的極限光限制和操縱方法[3]，等離子子學研究領域被認為是開發先進和實用的光子學技術，設備和應用的動力原理[4-6]。 plasmonics在定制全光學和光電設備中具有深遠的基礎性作用，已使設計高效的光伏設備[7-10]，長衰減范圍混合波導[4,11,12]，超快調制器[13-15]，光電探測器和晶體管[16–20]，偏振分束器[21–23]，馬赫曾德爾干涉儀[24–26]，超材料[27–32]，超透鏡[33,34]，納米激光[35–37]，量子設備[38–40]，太陽能分水器[41,42]，生物設備和臨床工具[43–45]等。對于后一種情況，等離子體技術是一種可靠的技術，在體外和體內均具有出色的益處。體內測定。亞波長等離激元平臺已被廣泛用于開發幾種類型的無標記疾病診斷設備[46-55]，癌癥和腫瘤療法[56-60]，靶向藥物輸送[61-64]，納米焊接[65,66] ，高光譜納米成像[67–69]，光聲成像[70,71]，實時藥理學[72–74]，蒸汽和微氣泡的產生[75–77]，激光納米外科手術[78–80]，光熱熱光譜學[81-85]，光熱控制流體學[86-89]，熱輔助磁記錄[90-93]和神經元刺激[94,95]。使用亞波長金屬物體將SPR極端地定位會導致光吸收的強烈增強，從而導致自由電子氣通過數百飛秒的電子-電子散射而熱加熱[96]。等離子體生化和生化傳感器特別令人感興趣，它們在商業和先進的臨床和制藥應用中具有至關重要的作用[43,44,97,98]。高精度，實時響應，無標簽，可在室溫下運行，具有成本效益和快速響應，所有這些優勢促使研究人員致力于增強型等離激元生物傳感器的研究已有數十年[99,100]。

Among all plasmonic biodevices, the terahertz (THz) plasmonic structures with the operating bandwidth between the world of transistors and lasers (0.1 THz < f < 10 THz) have received significant attention recently. It should be underlined that the plasmonic terminology has been used to distinguish the reviewed metallic metamaterials and metasensors from the classical nanophotonic platforms. As potential substitutes for traditional optical biosensors, THz spectroscopy has been acknowledged as a promising approach for advanced biological sensing applications with exotic features and advantages that have not been experienced in optical nanostructures [101]. THz plasmonic biodevices facilitate on-site detection, low-invasiveness, nondestructive, non-poisonous interaction with biological tissues, and high signal to noise (S/N) ratio [102]. In addition, the vibrational modes of various macro-molecules (i.e. proteins, DNA) are traced across the THz spectrum, which make this bandwidth interesting for biosensing purposes [101–105]. Furthermore, the development of micro-scale THz biosensing chips have successfully been realized by cost-effective and traditional single- or multi-step photolithography techniques. In recent years, THz plasmonic metamaterials composed of periodic arrays of artificially engineered building blocks with electromagnetic (EM) properties beyond natural materials have extensively been utilized for developing biosensing platforms [106–108]. Although current THz plasmonic metasensors are promising and provide substantial sensitivity and reasonable limit of detection (LOD), these technologies are still quite faraway from very early stage diagnosis of ultra-low weight infections and biomarkers at lowlevel densities. Generally, most of the THz plasmonic biosensors operating base on resonant-structures, and Fano-resonant metamaterials are highly popular in this field of sciences. However, due to the size of the THz plasmonic metamaterials, in most of assays, the nanoscopic molecules and microorganisms are transparent to the THz radiation and showing very low scattering cross-sections due to having sizes in the order of _k/100 [109].

在所有等離子生物設備中，具有晶體管與激光世界之間的工作帶寬（0.1 THz <f <10 THz）的太赫茲（THz）等離子結構最近受到了廣泛關注。應該強調的是，已經使用等離激元術語來區分已審查的金屬超材料和超傳感器與經典的納米光子平臺。作為傳統光學生物傳感器的潛在替代品，太赫茲光譜已被認為是具有先進的生物傳感應用的有前途的方法，具有奇異的特征和在光學納米結構中未曾經歷過的優勢[101]。太赫茲等離子體生物設備有助于現場檢測，低侵入性，與生物組織的無損，無毒相互作用以及高信噪比（S / N）[102]。另外，在太赫茲頻譜上可以追蹤到各種大分子（即蛋白質，DNA）的振動模式，這使得該帶寬對于生物傳感而言很有趣[101-105]。此外，通過具有成本效益的傳統單步或多步光刻技術已成功實現了微型THz生物傳感芯片的開發。近年來，太赫茲等離子超材料由具有天然材料以外的電磁（EM）特性的人工工程構件的周期性陣列組成，已廣泛用于開發生物傳感平臺[106-108]。盡管當前的THz等離子體元傳感器前景廣闊，并提供足夠的靈敏度和合理的檢測限（LOD），但這些技術與超低重量感染和低水平密度生物標志物的早期診斷仍相距甚遠。通常，大多數基于共振結構的THz等離子體生物傳感器和Fano共振超材料在該科學領域中非常受歡迎。但是，由于太赫茲等離子超材料的尺寸，在大多數測定中，納米級分子和微生物對太赫茲輻射是透明的，并且由于其尺寸約為_k / 100，因此顯示出非常低的散射截面[109]。?

To address this inherent limitation in THz metasensors, very recently, an alternative technology has been introduced and experimentally validated for the precise detection of extremely small amount of biomarkers at ultra-low concentrations [110,111]. Such sensitive metadevices have been developed based on toroidal resonances that provide unique spectral properties [112]. Theoretically, toroidal resonances have been introduced for the first time in 1957, in the context of nuclear, atomic, and molecular physics [113]. On the other hand, dynamic toroidal dipole has been excited successfully by either linear or vortex beams illuminations [114,115]. Toroidal dipoleresonant metamaterials and structures have received growing interest in the last decade [116–121]. Moreover, possessing narrow lineshape, and ultrahigh sensitivity of toroidal metamaterials to the environmental perturbations stimulated researches to employ these subwavelength technologies for developing advanced plasmonic tools [117,122–124].

為了解決太赫茲元傳感器的固有局限性，最近，引入了一種替代技術，并進行了實驗驗證，可以精確檢測超低濃度的極少量生物標志物[110,111]。已經基于提供獨特光譜特性的環形共振開發了這種敏感的元設備[112]。從理論上講，在核物理，原子物理和分子物理的背景下，環形共振于1957年首次引入[113]。另一方面，動態環形偶極子已被線性或渦旋光束照明成功激發[114,115]。在過去十年中，環形雙極共振超材料和結構受到了越來越多的關注[116-121]。而且，具有超窄的線形和超材料的超環面材料對環境擾動的敏感性促使人們進行研究，以利用這些亞波長技術來開發先進的等離激元工具[117,122–124]。

The review article is organized as following: A detailed overview about various THz plasmonic metasensors based on perfect absorption techniques is presented in ‘THz plasmonic perfect absorbers for biological detection’. ‘Fano-resonant THz metamaterials for biological detection’ summarizes the recent advances in the field of label-free metasensors based on capacitive coupling in sub-microscale openings. The spectral properties and unique advantages of toroidal plasmonic metamaterials are demonstrated in ‘THz plasmonic biosensors based on extraordinary transparency’ with detailed key investigations about sensing performance of recently utilized biosensing tools. The use of toroidal plasmonics metadevices for biosensing that is explained in ‘Toroidal resonances for biosensing’, is a very novel aspect for both the plasmonics and biomedical technologies communities. Such a focused review reveals the recent advances in plasmonic biosensing aspects, and introduces the emergence of new technologies in the field for very broad interdisciplinary biomaterials community.

這篇綜述文章的組織方式如下：在“用于生物檢測的THz等離子體完美吸收體”中，詳細介紹了基于完美吸收技術的各種THz等離子體激元傳感器。 “用于生物檢測的共振FHz超材料”總結了基于亞微米級開口中電容耦合的無標記超傳感器領域的最新進展。 “基于非凡透明性的THz等離子體生物傳感器”展示了環形等離子體超材料的光譜特性和獨特優勢，并對最近使用的生物傳感工具的傳感性能進行了詳細的關鍵研究。環形等離子體電子元設備在生物傳感中的使用已在“用于生物傳感的環形共振”中進行了說明，這對于等離子體和生物醫學技術界都是一個非常新穎的方面。這樣的重點綜述揭示了等離激元生物傳感方面的最新進展，并介紹了針對非常廣泛的跨學科生物材料界的新技術的出現。

Thz plasmonic perfect absorbers for biological detection?用于生物檢測的Thz等離子體完美吸收體

Fundamental theoretical aspects?基本理論方面

Perfect metamaterial absorbers are subwavelength structures consisting of well-engineered unit cells, offering advantages based on both the inherent lossy behavior of plasmons. By forming of a sub-wavelength resonant cavity in these structure, (where the reflected beam is trapped) thus, any losses-particularly in metals or dielectric spacer, substrate, superstrate, analyte, etc.- will be amplified to diminish the reflection and boost the absorption spectra [125,126]. The perfect and broadband absorption of the incident beam has successfully been obtained by tailoring wellengineered plasmonic meta-atoms and metamolecules [125– 130]. The presence of a metallic backed layer in the design of a perfect absorber leads to tremendous absorption of both components of the incident EM field by minimizing the reflectance response. Such a technology has been broadly employed to address the typical limitations associating with the biological sensing surfaces. Generally, in perfectly absorptive metamaterials, the optimal goal is achieving the maximum possible absorption (almost unity), where this feature can be obtained by dramatic decay of transmitted and reflected beam from a given metamaterial (A = 1-T-R, where T = 0). Multilayer structures consisting of metal-dielectric-metal interfaces are the traditional designs for developing perfect absorbers (Fig. 1). Such a sandwich-type structure allows for strong confinement of the incident light and hinders escaping of light. This results in the formation of circulating magnetic field at the metal-dielectric interfaces, and enhances the absorptance of the entire structure. In the upcoming sections, the recent advances in the use of perfect absorbers based on THz plasmonic technology for biochemical and biological immunosensing applications will be explained

完美的超材料吸收體是由精心設計的晶胞組成的亞波長結構，基于等離激元固有的損耗特性，它們具有優勢。通過在這些結構中形成亞波長諧振腔（在其中捕獲了反射光束），任何損耗（尤其是金屬或介電墊片，基板，覆層，分析物等）中的任何損耗都會被放大，以減少反射和提高吸收光譜[125,126]。通過精心設計良好的等離激元亞原子和超分子，成功獲得了入射光束的寬帶吸收效果[125-130]。完美吸收體的設計中存在金屬背襯層，可通過最小化反射響應來極大地吸收入射EM場的兩個分量。這種技術已被廣泛采用以解決與生物感測表面相關的典型限制。通常，在完全吸收的超材料中，最佳目標是實現最大可能的吸收（幾乎統一），其中此特征可以通過給定超材料的透射和反射光束急劇衰減來獲得（A = 1-TR，其中T = 0 ）。由金屬-電介質-金屬界面組成的多層結構是開發完美吸收體的傳統設計（圖1）。這樣的夾心型結構允許入射光的強烈限制并且阻礙光的逸出。這導致在金屬-電介質界面處形成循環磁場，并增強了整個結構的吸收率。在接下來的部分中，將解釋基于太赫茲等離子技術的完美吸收體在生物化學和生物免疫傳感應用中的最新進展。

Biochemical sensing application?生化感測應用

Here, we consider the optical and sensing properties of several multiresonant THz metamaterials with the perfect absorption feature [131]. Using both numerical and experimental studies, Yahiaoui and co-workers demonstrated the detection performance of the tailored perfect beam absorber by variations in the thickness and refractive index (RI) of an analyte layer. Fig. 2a illustrates the scanning electron microscope (SEM) image of the fabricated metamaterial, and the insets are the crosssectional schematic and description for geometrical components of a single metamolecule. In the experiments, the aluminum (Al) unit cells are deposited on a multilayer substrate consists of 50 mm dielectric spacer and an Al mirror. The multilayer geometry of the perfect absorber can be described in the way that antiparallel currents are excited in the top layer and the bottom metallic layer [132,133]. Actually, this is known a magnetic resonance due to the fluxing currents result in a magnetic mode which can strongly interact with the magnetic field of the incident beam [132–134]. At the resonance frequency, a strong enhancement of the localized EM field is established between the two layers. Thus, the EM energy can be efficiently confined in the intermediate dielectric spacer and hence no light is reflected back. This leads to a pronounced reflectance dip in the spectrum with nearly zero intensity, therefore giving rise to around _100% absorbance. Fig. 2b shows the experimentally and numerically defined reflection (R) amplitude as a function of frequency, confirming the excitation of multiple resonances around fR1 = 0.22 THz, fR2 = 0.48 THz, and fR3 = 0.76 THz. Perturbing the RI of the medium is a traditional method to demonstrate the sensing performance of the perfect absorber metamaterial. It is well-accepted that increasing the RI of the media gives rise to red-shifts in the position of the excited modes to the shorter frequencies [135]. In this work [131], the plasmonic metamaterial absorber is loaded by a thin analyte layer with the RI of n = 1.73. Fig. 2b exhibits red-shifts in the position of all reflection dips. The reason for this shift can be explained by the changes in the entire capacitance across the structure. Once the surface of the metamaterial is loaded with small amount of dielectric material, the capacitance value increases and the resonances shift towards the lower frequencies. The frequency shift (Df) of the metasensor is shown as a function of RI of the analyte in Fig. 2c(i). Obviously, the frequency shift of the resonances continuously increases linearly with the increase of the RI of the thin analyte layer. The amplitude modulation of the reflectivity (DR) is also demonstrated numerically in Fig. 2c(ii). Upon increasing the RI of the analyte and depending on the excited resonant mode, their amplitude variations as a function of the RI could be very different with a mutual nonlinear evolution. To define the sensitivity of the perfect metamaterial absorber, Yahiauoui and teammates artificially changed the thickness and RI of the analyte layer. The thickness of the overlayer is altered numerically in the range 1–50 mm in order to evaluate the frequency sensitivity of the sensor as a function of the analyte thickness. By increasing the analyte thickness, a distinct red shift of the resonances is observed by the authors. Based on the frequency shift with the change in analyte thicknesses, the frequency sensitivity of the sensor is estimated explicitly, as shown in Fig. 2d(i). Here, the third resonant mode is more sensitive than the first and the second resonances, since it induces remarkably larger frequency sensitivity and eventually reaches almost 140 GHz/RIU for an overlayer thickness of 50 mm. In this work, the researchers also performed further simulations to evaluate the effect of the dielectric spacer on the characteristic of the sensor. The results for the sensitivity variations based on reducing the thickness of the dielectric substrate to 15 mm are reported in Fig. 2d(ii), for the third resonant mode of the metamaterial absorber. Thus, when the thickness of the overlayer is less than 20 mm, the sensitivity of the sensor is not dramatically enhanced as compared to the nominal case (50 mm).?

在這里，我們考慮幾種具有完美吸收特性的多共振太赫茲超材料的光學和傳感特性[131]。 Yahiaoui及其同事使用數值研究和實驗研究，通過分析物層的厚度和折射率（RI）的變化，證明了量身定制的完美光束吸收器的檢測性能。圖2a示出了所制造的超材料的掃描電子顯微鏡（SEM）圖像，插圖是單個超分子的幾何成分的橫截面示意圖和描述。在實驗中，鋁（Al）晶胞沉積在由50 mm介電墊片和Al鏡組成的多層基板上。可以通過在頂層和底層金屬層[132,133]中激發反平行電流的方式描述完美吸收體的多層幾何形狀。實際上，由于通量電流導致的磁模式會與入射光束的磁場發生強烈相互作用，從而導致磁共振[132-134]。在共振頻率處，在兩層之間建立了局部電磁場的強烈增強。因此，可以將EM能量有效地限制在中間電介質間隔物中，并且因此沒有光被反射回去。這會導致光譜的反射率下降，強度幾乎為零，因此吸收率約為_100％。圖2b顯示了實驗和數字定義的反射（R）幅度隨頻率變化的情況，確認了在fR1 = 0.22 THz，fR2 = 0.48 THz和fR3 = 0.76 THz附近的多個共振的激發。擾動介質的RI是證明完美吸收體超材料的傳感性能的傳統方法。廣為接受的是，增加介質的RI會導致激發模式位置向較短頻率的紅移[135]。在這項工作[131]中，等離子超材料吸收體由薄的分析物層加載，RI為n = 1.73。圖2b顯示了所有反射傾角位置的紅移。這種偏移的原因可以通過整個結構上整個電容的變化來解釋。一旦超材料的表面負載了少量的介電材料，電容值就會增加，并且諧振會朝著更低的頻率移動。圖2c（i）中顯示了元傳感器的頻移（Df）與分析物的RI的函數關系。顯然，共振的頻移隨著薄分析物層的RI的增加而線性地連續增加。反射率（DR）的幅度調制也在圖2c（ii）中進行了數值演示。在增加分析物的RI并取決于激發的共振模式時，它們的幅度變化作為RI的函數可能會隨著相互非線性演化而非常不同。為了定義完美的超材料吸收體的靈敏度，Yahioauoui及其隊友人為地改變了分析物層的厚度和RI。為了評估傳感器的頻率靈敏度與分析物厚度的函數關系，覆蓋層的厚度會在1-50 mm范圍內進行數字更改。通過增加分析物的厚度，作者觀察到了共振的明顯紅移。如圖2d（i）所示，基于隨分析物厚度變化而產生的頻率偏移，顯式估計傳感器的頻率靈敏度。在此，第三諧振模式比第一和第二諧振更敏感，因為它引起明顯更大的頻率靈敏度，并最終在50 mm的覆蓋層厚度下達到近140 GHz / RIU。在這項工作中，研究人員還進行了進一步的仿真，以評估介電墊片對傳感器特性的影響。對于超材料吸收器的第三共振模式，在圖2d（ii）中報告了基于將介電基片的厚度減小到15 mm而引起的靈敏度變化的結果。因此，當覆蓋層的厚度小于20mm時，與標稱情況（50mm）相比，傳感器的靈敏度沒有顯著提高。

FIGURE 1 Schematic representation of a perfect absorber unit cell consisting of metal-dielectric-metal interfaces, where the transmittance (T) and reflectance (R) spectra suppress and the absorption (A) cross-section enhances.

圖1是由金屬-電介質-金屬界面組成的理想吸收器單元電池的示意圖，其中透射率（T）和反射率（R）光譜受到抑制，吸收率（A）橫截面增強。

In recent years, several alternative and promising platforms have been introduced for RI sensing purpose by taking advantage of remarkable absorption cross-section in spoof plasmon metamaterials [136], and multilayer metamaterials combined with microfluidic channels for liquid sensing [137]. Focusing on these mechanisms, Ng et al. [136] developed a spoof plasmon metamaterial integrated with an Otto prism setup to utilize its surface sensitivity for RI sensing of various fluids. Prism coupling systems have previously been used to excite surface resonant modes on semiconductor and metallic arrays in the THz spectrum [138,139]. As plotted in Fig. 3a, the developed spoof plasmon-resonant system consists of a linear array of grooves based on 600 nm of gold (Au) layer deposited on a layer of photoresist. The optical microscope image of the fabricated groove array is exhibited in Fig. 3b and the employed geometries are specified in the caption of the figure. In this study, the researchers coupled to THz spoof plasmons via a wax prism and as a proof of principle, experimentally demonstrated RI sensing of different fluids by monitoring significant changes in both amplitude and phase of the THz radiation. A wax prism in the traditional Otto prism configuration has been used for (1) phase-matching, (2) coupling to the spoof plasmon mode evanescently at the base of the prism with a coupling gap (g), and (3) between the spoof plasmons and prism base (see Fig. 3c). Then the changes in the reflectivity (R) has been monitored, and phase change spectra were studied as the grooves are filled with different fluids: nitrogen (n = 1.00), gasoline (n = 1.41), liquid paraffin (n = 1.49), glycerin (n = 1.82) and water (n = 2.1). This led to a perfect spread of RI values, enabling to investigate the efficacy of THz spoof plasmon sensing with various sample fluids. As can be seen in Fig. 3d, there is a remarkable red-shift in the reflection dip as the RI of the fluid filling the grooves increases. The resonance points for nitrogen, gasoline, liquid paraffin, glycerin and water are 1.71, 1.53, 1.48, 1.30, and 1.17 THz, respectively. For lowloss fluids (e.g. gasoline and liquid paraffin), the width of the resonances are 90 GHz and 50 GHz, respectively, while for high-loss fluid (e.g. glycerin), the reflection dip broadens significantly to 280 GHz, giving rise to a Quality-factor (Q-factor) of approximately 4.6. Conversely, for a very high-loss fluid (e.g. water), the resonance lineshape further broadens to approximately 390 GHz. This implies that RI sensing with amplitude measurements is not suitable for highly absorbing fluids, since a larger RI change would be required to properly discern any spectral shifts in the resonance lineshape. These results were achieved in spite of dramatic attenuation on the radiated THz signal through the high-loss fluid layer. The profile in Fig. 3e demonstrates the resonance frequencies of the different fluids as a function of respective RIs. The blue solid line is a linear fit given by fsp = _0.49n + 2.21. This graph shows a sensitivity of 0.49 THz/ RIU with the corresponding LOD of 0.02 RIU at a detection resolution of 10 GHz. The insets are the electric-field density maps for the lowest and highest RIs at the plasmon resonance frequency. The computed figure of merit (FOM) values for nitrogen, gasoline, liquid paraffin and glycerin are 49, 15, 25, and 7, respectively. The relationship between resonance frequency (fsp) and n of an idealized spoof plasmonic sensor consisting of a linear array of square grooves with effective groove width weff (weff = wt = wb), is analytically given by:?

equation

?where c is the speed of light, n is the RI of the dielectric substance filling the grooves, h = 74_ is the angle of the incident beam, and np is the RI of the prism (1.44). This equation defines the spoof plasmon dispersion of the metamaterial with the prism light line and is solved for weff = wt = 37 lm and weff = wb = 25 lm with the RI variations between 1 and 2.1. Thus, the minimum sensitivity over the range of sampled values of n, quantified by taking the numerical gradient of the analytical curves, are 0.40 THz/RIU and 0.47 THz/RIU for weff = 37 lm and weff = 25 lm, respectively. Although the reported LOD and FOM values in this work are remarkable, it is shown that the performance and accuracy of plasmonic THz absorber sensors can be boosted [133].

近年來，通過利用欺騙性等離激元超材料[136]中的顯著吸收截面以及多層超材料與微流體通道相結合進行液體感測[137]，已經引入了幾種替代且有前途的平臺用于RI感測。 Ng等人著眼于這些機制。 [136]開發了一種與奧托棱鏡裝置集成在一起的欺騙性等離激元超材料，以利用其表面靈敏度對各種流體進行RI感測。棱鏡耦合系統以前曾被用來在THz頻譜中激發半導體和金屬陣列上的表面共振模式[138,139]。如圖3a所示，已開發的欺騙型等離子體激元共振系統由基于600 nm沉積在光刻膠層上的金（Au）層的溝槽的線性陣列組成。制成的凹槽陣列的光學顯微鏡圖像如圖3b所示，所用的幾何形狀在圖的標題中指定。在這項研究中，研究人員通過蠟棱鏡與太赫茲欺騙等離子激元耦合，并作為原理證明，通過監測太赫茲輻射的振幅和相位的顯著變化，實驗證明了RI對不同流體的傳感。傳統的Otto棱鏡配置的蠟棱鏡已用于（1）相位匹配，（2）在棱鏡的底部以耦合間隙（g）短暫耦合到欺騙的等離振子模式，以及（3）欺騙的等離激元和棱鏡底座（見圖3c）。然后，對反射率（R）的變化進行了監測，并研究了溝槽填充不同流體時的相變光譜：氮氣（n = 1.00），汽油（n = 1.41），液體石蠟（n = 1.49），甘油（n = 1.82）和水（n = 2.1）。這導致RI值的完美散布，從而能夠研究THz欺騙等離子體激元在各種樣品流體中的感應功效。從圖3d中可以看出，隨著填充凹槽的流體的RI的增加，反射傾角會出現明顯的紅移。氮氣，汽油，液體石蠟，甘油和水的共振點分別為1.71、1.53、1.48、1.30和1.17 THz。對于低損耗流體（例如汽油和液體石蠟），諧振的寬度分別為90 GHz和50 GHz，而對于高損耗流體（例如甘油），反射傾角顯著擴展至280 GHz，從而提高了質量。因子（Q因子）約為4.6。相反，對于損耗極高的流體（例如水），諧振線形會進一步加寬到大約390 GHz。這意味著具有振幅測量值的RI傳感不適用于高吸收性流體，因為需要較大的RI變化才能正確識別共振線形中的任何頻譜偏移。盡管通過高損耗流體層的輻射太赫茲信號發生了顯著衰減，但仍獲得了這些結果。圖3e中的曲線顯示了不同流體的共振頻率與相應RI的關系。藍色實線是由fsp = _0.49n + 2.21給出的線性擬合。該圖顯示了在10 GHz的檢測分辨率下靈敏度為0.49 THz / RIU，相應的LOD為0.02 RIU。插圖是等離激元共振頻率下最低和最高RI的電場密度圖。氮氣，汽油，液體石蠟和甘油的品質因數（FOM）值分別為49、15、25和7。理想頻率的欺騙性等離子體傳感器的諧振頻率（fsp）與n之間的關系由具有有效溝槽寬度weff（weff = wt = wb）的方形溝槽的線性陣列組成，解析式為：

公式

其中c是光速， n是填充凹槽的電介質的RI，h = 74_是入射光束的角度，np是棱鏡的RI（1.44）。該方程式定義了超材料在棱鏡光線下的欺騙性等離激元色散，對于RI = 1至2.1的weff = wt = 37 lm和weff = wb = 25 lm求解。因此，通過取分析曲線的數值梯度來量化的n采樣值范圍內的最小靈敏度分別對于weff = 37 lm和weff = 25 lm為0.40 THz / RIU和0.47 THz / RIU。盡管在這項工作中報告的LOD和FOM值非常可觀，但已表明可以提高等離子體THz吸收器傳感器的性能和準確性[133]。

FIGURE 2 (a) Scanning electron microscope (SEM) image of the fabricated metamaterial absorber with a schematic cross-section of the sample (top right) and the polarization of the incident plane wave (bottom left). The inset is a representation of a single unit cell with the relevant geometrical dimensions: a = b = 250 mm, c = d = g = 50 mm, e = 25 mm, and l = 155 mm. The unit cells are arranged in the periods of px = py = 300 mm. (b) Simulated and measured reflection spectra of the metamaterial absorber versus frequency without analyte and with 50-mm-thick analyte (n = 1.73). (c) Frequency shift (i) and amplitude modulation (ii) against analyte RI for the different resonant modes of the metamaterial absorber-based sensor device. (d) Frequency sensitivity of the resonant modes as a function of the analyte thickness (i), and frequency sensitivity of the third resonant mode as a function of the analyte thickness at dielectric spacer thicknesses of 50 mm and 15 mm, respectively (ii). The symbols represent the exact values, while the solid lines are the fitting functions [131]. Copyright 2015, American Institute of Physics (AIP).

圖2（a）所制造的超材料吸收體的掃描電子顯微鏡（SEM）圖像，其樣品的橫截面示意圖（右上方）和入射平面波的偏振態（左下方）。插圖表示具有相關幾何尺寸的單個晶胞：a = b = 250 mm，c = d = g = 50 mm，e = 25 mm，l = 155 mm。單位晶格的排列周期為px = py = 300 mm。 （b）模擬和測量的超材料吸收體的反射光譜與不帶分析物和使用50毫米厚分析物時的頻率的關系（n = 1.73）。 （c）針對基于超材料吸收體的傳感器設備的不同共振模式，針對分析物RI的頻移（i）和幅度調制（ii）。 （d）介電間隔層厚度分別為50 mm和15 mm時，共振模式的頻率靈敏度與分析物厚度（i）的函數關系以及第三共振模式的頻率靈敏度與分析物厚度的函數關系（ii） 。符號代表精確值，而實線是擬合函數[131]。美國物理研究所（AIP）版權所有2015。