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摘要:
本文利用亚波长硅圆柱组成的超表面设计了用于太赫兹探测器的聚焦透镜，通过改变硅柱的直径，实现对太赫兹波传输相位0~2π的调控。在1 THz频率下，所设计的单面超表面透镜，太赫兹波电场能量密度提升到入射平面波的32倍。基于工艺制备可行性和抗反射考虑，提出了一种双面超表面透镜，将电场能量密度提升到入射平面波的44倍。对比于传统超半球太赫兹硅透镜，超表面透镜具有厚度薄，体积小的优点，有利于太赫兹探测器组件的小型化，为实现与太赫兹探测器的集成提供了可能性。
Abstract:In this paper, a focusing lens for terahertz detection is designed using a metasurface composed of sub-wavelength silicon cylinders. By tuning the diameter of the silicon cylinder, the transmission phase of the THz wave is controlled from 0 to 2π. At 1 THz, the terahertz electric field energy density focused by the single-sided metasurface lens designed can be increased to 32 times that of the incident wave. After adding the anti-reflection, a double-sided metasurface lens is proposed, which is feasible in processing, increasing the electric field energy density to 44 times that of the original. Compared with the traditional hyper-hemispheric terahertz silicon lenses, our metasurface lens has the advantages of thin thickness and small volume, which is conducive to the miniaturization of the terahertz detector component and provides the possibility to realize the integration with the terahertz detector.
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Key words:
- metasurface /
- terahertz /
- focus
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Overview: Terahertz detector is an important device in the field of terahertz technology, and it is important to improve its sensitivity. The sensitivity of the detector can be improved in two aspects: one is to further optimize the antenna of the detector, and the other is to optimize the size of the detector and the spot size of incident terahertz wave. Due to the long wavelength of the electromagnetic wave in the terahertz band, the spot size is much larger than the effective acceptance area of the detector, which limits the effective absorption rate of the detector to the incident terahertz wave. In order to make the focus spot to be small, the lens aperture needs to be increased. At present, the commonly used method is to integrate the hyper-hemispheric silicon lens with the terahertz detector to reduce the spot size by one order of magnitude and increase the electric field energy density. However, hyper-hemispheric silicon lens is difficult to be ultra-thin and ultra-light, and is not planar, which is not conducive to the device integration, especially for array detectors. In this paper, a series of metasurface lenses for terahertz detectors are designed using sub-wavelength silicon cylinders. By tuning the diameter of the silicon cylinders, the transmission phase of the terahertz wave can be controlled from 0 to 2π with high transmission amplitude. At 1 THz, the backside integration of the designed single-surface lens with the terahertz detector can increase the electric field energy density in the core region of the THz detector to 32 times that of the incident plane wave, and reduce the focal spot to the same order of magnitude as the wavelength. Based on the feasibility of fabrication and anti-reflection considerations, we propose a two-sided metasurface lens, which further increases the energy density of the electric field to 44 times that of the incident plane wave. Compared with the traditional hyper-hemispheric silicon lenses, the size and thickness of the metasurface lens are smaller and more convenient for integration. Metasurface lenses have a great prospect for reducing the complexity of the terahertz system and improving the responsiveness of the detector, and provide a new idea for the integration and miniaturization of the terahertz device. However, the current metasurface lenses produce many side lobes after focusing, resulting in low focusing efficiency. Further research needs to further optimize the materials and unit structures of the metasurface lenses, to improve the focusing efficiency and electric field energy density.
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图 1 (a) AlGaN/GaN 太赫兹探测器与超表面透镜集成模型；(b) 超表面单元阵列的相位调控示意图；(c) 太赫兹天线结构示意图
Figure 1. (a) The integration model of an AlGaN/GaN terahertz detector and a metasurface lens; (b) Schematic diagram of phase regulation of metasurface element array; (c) Schematic diagram of terahertz antenna structure
图 2 (a)，(c) 分别为单面单元结构示意图及其能量分布；(b)，(d) 分别为双面单元结构示意图及其能量分布
Figure 2. (a) and (c) show the schematic diagram of single-sided cell structure and its energy distribution; (b) and (d) show the schematic diagram of double-sided cell structure and its energy distribution
图 3 超表面透镜单双面结构的相位和透射率随直径变化曲线
Figure 3. The phase and transmittance curves of the single and double structure of metasurface lens vary with diameter
图 5 (a) 单面超表面透镜布阵；(b) 双面超表面透镜布阵
Figure 5. (a) The array of single-sided metasurface lens; (b) The array of double-sided metasurface lens
图 6 (a)，(c) 分别为单面结构x=0时，y-z面场强分布和y=0时一维分布；(b)，(d) 分别为双面结构超表面透镜x=0时，y-z面场强分布和y=0时一维分布；(e)，(g) 分别为单面结构的超表面透镜z=1.026 mm时，x-y面场强分布和y=0时一维分布；(f)，(h) 分别为双面结构的超表面透镜z=1.067 mm时，x-y面场强分布和y=0时一维分布
Figure 6. (a) and (c) show the field intensity distribution on the y-z plane and one-dimensional distribution on the y=0 of a single-sided structure when x =0 ; (b) and (d) are the field intensity distribution on the y-z surface when x=0 and one-dimensional distribution when y=0 of the double-sided metasurface lens; (e) and (g) are the field intensity distribution on the x-y plane and the one-dimensional distribution on y=0 when z=1.026 mm of the single-sided metasurface lens; (f) and (h) are the field intensity distribution on the x-y plane and the one-dimensional distribution on y=0 when z=1.067 mm of the double-sided metasurface lens
表 1 单元结构详细参数
Table 1. The detail parameters of unit structure
参数 单位：mm d 0.03~0.1 P 0.1 H 0.35 h0 0.15 h1 0.04 h2 0.11 
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表 2 超表面透镜设计参数
Table 2. Design parameters of metasurface lens
参数 工作频率/THz 1 超表面透镜衬底厚度H/mm 0.35 单面超表面透镜口径Ls/mm 3 双面超表面透镜口径Ld/mm 3 设计焦距f/mm 1
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