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电磁编码超表面是一种通过数字编码来设计并调控远场方向图的编码超表面,其单元结构由二进制数值“0”和“1”来描述(1bit情况),分别代表0°和180°反射相位[22]。超表面对电磁波的调控原理不再是空间相位上的累积,而是电场以及磁场在单元两侧产生的相位以及幅度的突变特性,来调控电磁波在空间中的相位以及幅度分布[23]。通过对“0”和“1”两个数字单元在二维平面上进行编码,在入射电磁波作用下,产生符合设计要求的远场方向图。
编码超材料调控电磁波的原理可以用传统的相控阵天线理论来解释[15]:对于一般的方形超表面,如图1所示。包含N×N个大小相等边长为D的栅格,每个栅格D是由相同的“0”单元或“1”单元构成的子阵列。“0”单元和“1”单元的分布可以是任意设计的。第(m,n)单元的散射相位设为φ(m,n),其值可取0°或180°。上述的相位并非绝对相位,其值大小并不影响编码超表面的功能和性能,两个数字态所对应的基本单元在工作频率下相位差约为180°。
在垂直平面波的照射下,编码超表面散射的远场函数可由公式(1)表示:
$$ \begin{split} {f}\left( {{\rm{\theta }},{\rm{\varphi }}} \right) = & {f_e}\left( {\theta ,\varphi } \right)\mathop \sum \limits_{m = 1}^N \mathop \sum \limits_{n = 1}^N exp\{ - i\{ \varphi \left( {m,n} \right)+ \\ & kD\sin \theta {\rm{[}}\left( {m - \frac{1}{2}} \right)\cos \varphi + \left( {n - \frac{1}{2}} \right)\sin \varphi {\rm{]}}\} \} \end{split} $$ (1) 式中:θ和φ为俯仰角和方位角;fe(θ,φ)为单个栅格远场的辐射函数,方向图函数可由公式(2)表示为:
$$ {\rm{Dir}}\left( {{\rm{\theta }},{\rm{\varphi }}} \right) = \frac{{4\pi {{\left| {f\left( {\theta ,\varphi } \right)} \right|}^2}}}{{\displaystyle\int \nolimits_0^{2\pi }\displaystyle\int \nolimits_0^{\frac{\pi }{2}} {{\left| {f\left( {\theta ,\varphi } \right)} \right|}^2}\sin \theta \rm{d}\theta \rm{d}\varphi }} $$ (2) 因为“0”和“1”编码单元相位为0°或180°,两单元的散射特性相消,fe(θ,φ)的辐射特性基本为0。从公式(1)和(2)看出,控制电磁编码超材料远场散射特性主要通过编码栅格单元的不同序列方式来实现[24-25]。
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图2展示的是一种基于聚酰亚胺薄膜的人工单元。聚酰亚胺薄膜为柔性材料,易于弯曲和共形,从而增加了编码超表面在实际应用的灵活性。文中把含有圆环的结构作为“1”单元,不含圆环的结构作为“0”单元,如图2(a)和(b)所示。“0”单元整个结构由两层组成,从下到上,依次为金属背板-聚酰亚胺介质,“1”单元整个结构由三层组成,从下到上,分别为金属背板-聚酰亚胺介质-金属圆环。
图 2 编码超表面的基本单元结构。(a)“0”单元结构;(b)“1”单元结构
Figure 2. Basic unit structure for coding metasurfaces. (a) "0" unit structure; (b) "1" unit structure
金属背板为0.2 μm厚的金,保证结构在透射率为0的同时具有高反射率。介质部分为40 μm厚的聚酰亚胺薄膜。金属圆环为金,厚度为0.2 μm,宽度w=5 μm,内半径r=31.5 μm。单元周期a=110 μm,“0”单元和“1”单元的反射相位及其相位差随频率的变化曲线如图3所示。
由图3可知,在0.85~1.45 THz频带范围内,“0”单元和“1”单元的相位差接近180°,反射相位差浮动值为±20°,相对工作带宽可由公式(3)计算得到:
$$ {ffoc} = \frac{{2\left( {{f_H} - {f_L}} \right)}}{{\left( {{f_H} + {f_L}} \right)}} = 52\% $$ (3) 式中:fH和fL分别表示上限和下限频率。
图 3 “0”单元和“1”单元的反射相位及其相位差
Figure 3. Reflection phase and phase difference of "0" element and "1" element
研究表明,当相位差处于160°~200°范围内时,编码超表面依旧可表现出较好的功能和性能,基于聚酰亚胺薄膜的人工单元满足设计需求。
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文中所选编码超表面的尺寸为10λ×10λ,相邻两条边的编码位数均为10,因此编码超表面被划分成10×10个栅格,每个栅格的大小为λ×λ,由一个编码位占据。每个栅格是由相同的“0”单元和“1”单元构成的子阵列。由单元反射相位图可知,以1.15 THz为中心,栅格尺寸D=λ=c/f≈260 μm,单元结构周期a=110 μm,D/a≈2,则一个栅格包含2×2个“0”单元或“1”单元的子阵列,这种由“0”单元和“1”单元组成的子阵列又叫做超级子单元。如图4(a)和(b)所示。
图 4 超级子单元2×2排布示意图。(a)“0”超级子单元;(b)“1”超级子单元
Figure 4. Schematic diagram of super subunit 2×2 arrangement. (a) "0" super subunit; (b) "1" super subunit
编码超表面中,编码单元的相位不仅与自身结构有关,同时受到单元之间电磁耦合的影响。当其相邻编码单元具有不同的结构尺寸时,反射相位将偏离设计值,导致其性能发生不可预测的恶化;另一方面超级子单元的引入可以有效地增加编码序列的物理周期长度,使散射波处于可见角度范围内(θ<90°)[22]。
Design of new terahertz beam splitter
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摘要: 近年来,随着射电望远镜外差式阵列接收机的发展,基于相位光栅技术的波束分离器在亚毫米波长范围内得到了重要的应用,它能够将单个本地振荡器信号经过分束同步传送到超导SIS/HEB混频器阵列接收机中。由于太赫兹频段相位光栅的特征尺寸在亚微米级,其加工精度直接影响器件性能,给微加工技术带来巨大挑战。基于此,笔者所在课题组结合相位编码超材料技术设计了一种新型的太赫兹四波束分离器,仅需利用单层超材料编码单元便可实现宽带电磁波束的分离,波束转换效率高,结构简单且易于加工,同时反射波束的方向可灵活调节。为了与实际测试系统相匹配,着重研究了不同入射角度下的波束分离,并得到了最佳的斜入射角度范围(小于30°),相对工作带宽可达52%,反射的四个波束功率相差不超过10%,这为太赫兹频段射电望远镜超导混频器阵列接收机的本振信号功率分配提供了新的解决方案,也有利于其他新型太赫兹功能器件的设计和发展。Abstract: In recent years, with the development of heterodyne array receivers of radio frequency (RF) telescope, beam splitters based on phase grating technology have gained important applications in the sub-millimeter wavelength range. It is capable of transmitting a single local oscillator signal to the superconducting SIS/HEB mixer receiver array by splitting beam synchronization. Due to the characteristic size of the terahertz phase grating at sub-micron level, its machining precision directly affects the performance of the device, which brings great challenges to the micro-machining technology. Based on this, a new terahertz four beam splitter combined with phase coding metamaterials was designed. Only one single-layer metamaterial coding unit can be used to realize the separation of broadband electromagnetic beam and high beam conversion efficiency. The structure of THz beam splitter is simple and easy to process, while the direction of the reflected beam can be flexibly adjusted. In order to match the actual test system, we focused on beam separation at different angles of incidence and obtain the best range of oblique incidence angles (less than 30°). The relative bandwidth is 52%, and the reflected four beam powers differ by no more than 10%. This provides a new solution for the local oscillator signal power distribution of terahertz superconducting mixer array receivers of RF telescope, and also facilitates the design and development of other new terahertz functional devices.
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Key words:
- phase coding /
- metamaterial /
- terahertz beam splitter /
- array receiver
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图 10 1×4波束分离的远场图:(a) 10°入射,(b) 20°入射,(c) 30°入射,(d) 40°入射;2×2波束分离的远场图:(e) 10°入射, (f) 20°入射, (g) 30°入射, (h) 40°入射
Figure 10. Far field pattern of 1×4 beam separation: (a) 10° incidence, (b) 20° incidence, (c) 30° incidence, (d) 40° incidence; Far field pattern of 2×2 beam separation: (e) 10° incidence, (f) 20° incidence, (g) 30° incidence, (h) 40° incidence
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