维基百科

电磁超表面

电磁超表面、磁超表面（electromagnetic metasurface）、超表面（metasurface）是指具有亚波长厚度的人造片材。亚波长尺度的图案，可以有结构化，也可以没结构化。[1][2][3]

液态可调谐电磁超表面

电磁理论中超表面通过特定边界条件而不是三维空间的本构参数调制电磁波，这在天然材料和超材料很常见。超表面也可以指超材料的二维对应物。[4][5]

还有2.5D超表面涉及三维自订其额外自由度功能。[6]

定义

研究人员以多种方式定义了超表面。

1,

“An alternative approach that has gained increasing attention in recent years deals with one- and two-dimensional (1D and 2D) plasmonic arrays with subwavelength periodicity, also known as metasurfaces. Due to their negligible thickness compared to the wavelength of operation, metasurfaces can (near resonances of unit cell constituents) be considered as an interface of discontinuity enforcing an abrupt change in both the amplitude and phase of the impinging light”.[7]

近年涉及具亚波长周期性的一维二维等离子体阵列一一“超表面”骎骎日上。由于与操作波长相比，超表面厚度可忽略不计，因此超表面（接近晶胞成分的共振）可视为不连续的界面，迫使撞击光的振幅和相位骤变。[7]

2,

“Our results can be understood using the concept of a metasurface, a periodic array of scattering elements whose dimensions and periods are small compared with the operating wavelength”.[8]

我们的结果可用超表面的概念理解，超表面是周期性散射元素阵列，其尺寸和周期与工作波长相比很小。[8]

3,

“Metasurfaces based on thin films”. A highly absorbing ultrathin film on a substrate can also be considered as a metasurface, with properties not occurring in natural materials.[3] Following this definition, the thin metallic films such as that in superlens（英语：superlens） are also the early type of metasurfaces.[9]

“基于薄膜的超表面”。基板的高吸收性超薄膜也可认为是超表面，天然材料不存在其特性。[3]根据这定义，超透镜（英语：superlens）的金属薄膜也是早期超表面类型。[9]

历史

电磁超表面的研究由来已久。早在1902年，罗伯特·威廉斯·伍德（英语：Robert W. Wood）就发现亚波长金属光栅反射光谱具有暗区。这现象命名为伍德异常，并引领在金属表面激发的特定电磁波，表面等离子体极化激元的发现[10]。随后，另一重要现象，列维-奇维塔关系发现了亚波长厚膜可导致电磁边界条件骤变。，[11]

一般来说，超表面可以包括一些传统微波频谱概念，例如频率选择表面（FSS）、阻抗片、甚至欧姆片。微波条件下，这些超表面的厚度可以远小于操作波长（例如，波长的1/1000），因为对于高导电金属来说，集肤深度可能很小。最近，一些新现象证明，如超表面能超宽频相干。结果表明，0.3nm薄膜可吸收射频、微波和太赫兹频率上的所有电磁波。 [12][13][14]

光学应用中，抗反射涂层（英语：anti-reflective coating）也可以被视为一种简单的超表面，正如瑞利勋爵首先观察到的那样。

近年开发了几种新超表面，包括等离子体超表面[15][4][7][16][17]、基于几何相位的超表面[18][19]、基于阻抗片的超表面[20][21]和滑动对称超表面[22]。

应用

超表面一重要应用是通过向入射波赋予局部梯度相移控制电磁波的波前，推广古代反射和折射定律。[18]这样，超表面可以用作平面透镜[23][24]、照明透镜[25]、平面全息图[26]、涡旋发生器[27]、光束偏转器、轴心等。[19][28]

除了梯度超表面透镜外，基于超表面的超透镜通过使用倏逝波提供了波前的另一种程度控制。利用超薄金属层中的表面等离子体，可以实现完美的成像和超分辨率光刻，这打破了所有光学透镜系统都受到衍射限制的普遍假设，这种现象称为衍射极限（英语：diffraction limit）。[29][30]

另一有前景的应用是在隐形技术领域。目标的雷达截面（英语：radar cross-section）通常透过辐射吸收材料（英语：radiation-absorbent material）或透过目标的特定形状缩小，以将散射能量重新引导远来源。不幸是辐射吸收材料（英语：radiation-absorbent material）频带功能很窄，限制了目标空气动力学性能。但已合成超表面可使用任一阵列理论或广义斯涅耳定律[31][32]将散射能量从源重定向[33][34][35]。这使得目标具有空气动力学上有利的形状并减少降低目标雷达截面（英语：radar cross-section）。

超表面还可以与光波导集成，控制引导电磁波。[36][37]可以支援元波导（英语：meta-waveguide）的应用，例如集成波导模式转换器[37]、结构光生成[38][39]、多功能多路复用器[40][41]和光子神经网络[42]。

此外，超表面还应用于电磁吸收器、偏振转换器和光谱滤光片。超表面赋能的新型生物成像和生物感测设备也已出现并被报导。[43][44][45][46]

对于许多基于光学的生物成像设备，其体积和沉重的物理重量限制了它们在临床环境中的使用。[47][48]

模拟

为有效地分析这种平面光学超表面，基于棱镜的算法允许平面几何形状最佳的三角棱柱空间离散化。与传统四面体方法相比，基于棱镜算法具更少元素，带来更高计算效率。[49]模拟工具已在线发布，用户能用自定义图元图案高效分析超表面。[50]

光学特性

因为所涉及光学特性通常包括相位和偏振特性，表征光域中的超表面需要先进的成像方法。2020年的研究表明，向量叠印法（英语：ptychography）成像计算成像方法似乎非常相关。即使在大型标本上也将钟斯矩阵映射与微观横向分辨率相结合。[51]

参见


参考

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