Terahertz broadband absorption spectrum enhancement based on asymmetric dielectric meta-grating on a metal substrate
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摘要:
太赫兹分子指纹谱是一种非常有潜力的无标记检测方法，在实际应用往往需要对微量甚至痕量样品检测。然而太赫兹波的波长远远大于待测分子的尺寸，导致波与痕量物质之间的相互作用较为微弱，需要额外结构来增强样品对电磁波的吸收。本文在金属基底上构造了倒置的非对称介质光栅结构，该结构利用导模共振(Guided-mode resonance，GMR)和连续域束缚态(Bound state In Continuum，BIC)显著提升了薄膜样品的太赫兹吸收谱。同时仅需测量反射吸收信号就可以得到薄膜增强吸收谱，而且样品涂覆于倒置的介质光栅平坦的背面，易于制备。当该结构用于0.2 μm厚的α-乳糖薄膜时，吸收谱幅度增强达到236倍。该结构为太赫兹波段痕量分析物的检测提供了一种新的方法。
Abstract:Terahertz molecular fingerprinting is a promising method for label-free detection, particularly for micro or trace amount samples in practical applications. However, the wavelength of terahertz waves is much larger than the size of the molecules to be tested, resulting in a weak interaction between the waves and the matter. To address this challenge, additional structures are needed to enhance the absorption of electromagnetic waves by trace amount samples. In this study, we constructed an inverted asymmetric dielectric grating structure on a metal substrate. By utilizing guided-mode resonance (GMR) and a bound state in the continuum (BIC) effect, the terahertz absorption spectrum of thin film samples was significantly enhanced. The enhanced absorption spectra can be easily obtained by measuring the reflected absorption signal. The samples are coated on the flat back of the inverted dielectric grating, which simplifies the preparation process. For instance, when the thickness of an α-lactose film is 0.2 μm, the absorption enhancement factor reaches 236. This study provides a new method for detecting trace analytes in the terahertz band.
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Key words:
- Terahertz /
- fingerprint spectrum /
- absorption enhancement /
- meta-grating /
- metal substrate
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图 1 金属基底上倒置全介质超光栅角度复用增强薄膜太赫兹吸收谱结构示意图。符号w1，w2，w3和w4分别表示四个光栅元件的宽度，p，h1，h2和d分别表示单元结构周期、光栅高度、波导层厚度和分析物厚度
Figure 1. Schematic diagram of the inverted metagrating on the metal substrate for THz absorption enhancing of a film analyte. The symbols w1, w2, w3, and w4 denote the widths of the four grating elements, while p, h1, h2, and d represent the unit cell period, grating layer height, WG layer height, and analyte thickness, respectively
图 2 当入射角从3°增加到20°时超光栅结构的反射谱。(a)未涂覆α-乳糖的反射曲线；(b)涂覆0.5 μm α-乳糖的反射曲线
Figure 2. Reflectance of the structure with angle of incidence increased from 3°to 20°. (a) Reflection curves for uncoated α-lactose; (b) Reflection curves for coated 0.5 μm α-lactose
图 3 (a)超光栅中TE波基模的色散关系；(b)覆盖有1 μm α-乳糖的结构吸收率(A)与频率和入射角度的关系
Figure 3. (a) Dispersion relation of the TE wave fundamental mode in the superlattice; (b) Absorbance (A) of the structure covered with 1 μm α-lactose as a function of the frequency and incident angle(θ)
图 4 本文超光栅结构增强乳糖薄膜的太赫兹吸收谱。(a)0.2 μm纯α-乳糖薄膜吸收光谱（蓝色实线），涂覆有0.2 μm α-乳糖的吸收光谱(灰色实线)，涂覆有0.2 μm α-乳糖吸收光谱包络线(红色虚线)；(b)0.48 THz~0.58 THz波段内α-乳糖介电常数的实部和虚部；(c)在不同的入射角θ下，涂覆有α-乳糖(d=0.2 μm)和未涂覆α-乳糖对应共振频率下的x-z平面归一化电场分布
Figure 4. Terahertz absorption spectra of lactose thin films enhanced by the structure in this paper. (a) Absorption spectrum of 0.2 μm pure α-lactose film (blue solid line), absorption spectrum coated with 0.2 μm α-lactose (gray solid line), and the envelope of the absorption spectrum coated with 0.2 μm α-lactose (red dashed line); (b) the real and imaginary parts of the α-lactose dielectric constants in the band of 0.48 THz~0.58 THz; (c) the absorption spectra of the thin-film coating of α-lactose (d=0.2 μm) and uncoated α-lactose (d=0.2 μm) under different incidence angles θ at corresponding resonant frequencies. lactose (d=0.2 μm) and uncoated α-lactose at different incidence angles θ. The x-z plane normalized electric field distributions at the corresponding resonance frequencies are shown below.
图 5 (a)不同入射角(θ)和频率下有无涂覆α-乳糖电场强度的变化情况。红色点上方标注的数字表示涂覆α-乳糖后结构中最大电场强度相较于无涂覆乳糖的降低百分比。(b)太赫兹波入射角度θ=5°时不同γ值的超表面反射谱。
Figure 5. (a) Changes in electric field intensity with and without α-lactose coating under different incident angles (θ) and frequencies. The numbers above the red dots represent the percentage reduction in the maximum electric field intensity of the structure with α-lactose coating compared to that without the coating.(b) Reflectance spectra of the metasurface at different γ values with a terahertz wave incident angle of θ = 5°.
图 6 吸收增强光谱与薄膜厚度及光栅参数的关系(h1=h2=140 μm,w1=w3=150 μm,w2=60 μm,w4=20 μm)。(a)超光栅上不同厚度的 α-乳糖的吸收包络曲线。(b)不同h1下的吸收率包络曲线。(c) 不同h2下的吸收率包络曲线。(d) 不同γ的吸收率包络曲线。
Figure 6. Absorption enhancement spectra versus film thickness and metagrating parameters (h1=h2=140 μm,w1=w3=150 μm,w2=60 μm,w4=20 μm). (a) Absorbance envelope curves for different thicknesses of α-lactose on the metasurface. (b). Absorbance envelope curves at different h1. (c) Absorbance envelope curves at different h2. (d) Absorbance envelope curves for different γ.
表 1 我们设计的结构与目前基于不同复用方法的元表面或光栅的吸收增强性能比较
Table 1. Comparison of absorption enhancement performances of our designed structure and present metasurfaces or gratings based on different multiplexing method
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