Dual-band and ultra-broadband photonic spin-orbit interaction for electromagnetic shaping based on single-layer silicon metasurfaces
Single-layer silicon metasurfaces that can achieve high-efficiency dual-spectrum or ultra-broadband photonic spin-orbit interaction and geometric phase modulation.
Metasurfaces have enabled rapid development of ultrathin optical devices that can modify the light wavefront by altering its phase and amplitude. Since metasurfaces open a new route to redirect a reflected wave around the object, numerous structures have been put forward to reduce the reflection and scattering of objects, resulting in desired camouflage or invisibility. However, most current phase-gradient metasurfaces are designed in only a single spectrum with narrow bandwidth. Though some dual-band and wideband approaches are achieved by the vertical stacking of metasurfaces, the volume and fabrication difficulty are inevitably increased. In addition, these low-reflection metasurfaces generally cannot achieve thermal invisibility at the same time due to their high infrared absorption/emission arising from the complex metal–dielectric composites.
In order to provide a solution to this challenge, a team of researchers from the State Key Laboratory of Optical Technologies on Nano-Fabrication and Micro-Engineering, Institute of Optics and Electronics, Chinese Academy of Sciences presents a kind of ultrathin silicon-based metasurfaces to simultaneously implement low infrared specular reflection and emission in dual-band and ultra-broadband ranges, respectively. Related research results are published in Photonics Research, Vol. 7, Issue 5, 2019 (Xin Xie, et al., Dual-band and ultra-broadband photonic spin-orbit interaction for electromagnetic shaping based on single-layer silicon metasurfaces).
Both the metasurfaces comprise a monolayer of amorphous silicon gratings with the same geometry but diverse spatial orientations tiled on a metal mirror, which can generate high-efficiency dual-band and ultra-wideband photonic spin-orbit interaction and geometric phase. The first one is designed to suppress the specular reflectances in dual-band of 1.05-1.08 μm and 5-12 μm below 10%. The second one is for an ultra-broadband of 4.6-14 μm. At the same time, the presented structures exhibit low thermal emission due to the low absorption loss of silicon in the infrared spectrum, which can be regarded as an achievement of laser-infrared compatible camouflage.
Dr. Xin Xie from the research team believes that this work provides a new idea for multispectral and multifunctional electromagnetic wave modulation. Further work will focus on how to extend this strategy to tunable metasurfaces to achieve dynamic electromagnetic camouflage.
封面|单层硅超表面实现双光谱和超宽带的电磁波波前调控
利用单层硅超表面实现双光谱或超宽带的电磁波波前调控。
控制目标特征信号在光电磁对抗技术中具有重要意义。传统的方式是通过改变目标的几何形状来调控反射，但是，在实际装备应用中，考虑到其他物理条件的限制，如空气、水等方面的动力学问题，人们通常不希望改变目标的几何形状。
作为一种人工结构，超表面因其超薄、设计灵活、易集成等特点在近些年来引起了人们广泛的关注。超表面是由亚波长单元阵列构成的二维平面结构，通过调整基本单元的大小、形状及排列方式，可以在亚波长尺度内实现对电磁波相位、振幅、偏振等特性的调制。利用超表面的局域相位控制，可以突破传统反射定律的限制，在不改变目标几何形状的条件下重塑其反射波波前，从而降低反射信号以实现隐身。
然而，目前超表面大多只能作用于单个频谱中，并且带宽受限。因此，如何在多光谱、宽带范围内实现电磁波操控成为了亟待解决的问题。尽管通过垂直堆叠超表面可以实现宽带或多频段的调制，但这会使得厚度增加，且加工难度显著提高。另一方面，现有的低反射超表面多由金属-介质多层结构组合而成，难以获得较低的红外辐射，因此无法兼容热伪装。
为了解决这个问题，中国科学院光电技术研究所微细加工光学技术国家重点实验室的研究团队提出了一种超薄的硅基超表面，分别在双光谱和超宽带范围内同时实现了红外低反射和低辐射。相关研究结果发表于Photonics Research2019年第7卷第5期上。
课题组所设计的超表面由平铺在金属反射镜上的单层非晶硅光栅构成，通过合理地排布空间取向，可以产生特定的几何相位。在这里，每个硅柱作为一个各向异性的截断波导，能够在两个平行于主轴的偏振方向上产生大的相位延迟。通过优化硅柱的尺寸，可以在近红外波段（1.05-1.08 μm）和中红外波段（5-12 μm）分别得到π和-π的相位差，即在两个光谱内同时满足了半波片的条件，这为设计双光谱高效率的几何相位超表面奠定了基础。此外，利用硅柱在短轴方向上磁共振响应的急剧变化，可以在4.6-6.1 μm和6.1-14 μm波段同时得到π相位延迟。巧妙的是，这两个波段可以完美地连接起来，从而得到了一个超宽带的半波片，这为实现超宽带的几何相位超表面提供了条件。
基于以上的分析和方法，课题组设计了两个棋盘结构排布的超表面，通过散射电磁波来缩减镜面反射信号：
• 第一个超表面同时作用于近红外和中红外谱，可以将1.05-1.08 μm和5-12 μm波段的镜面反射率均抑制在10%以下。
• 第二个超表面则能够在4.6-14 μm的超宽带范围内将镜面反射率减小至10%以下。
此外，得益于硅材料在红外波段损耗低的特性，所设计的结构在降低电磁反射的同时表现出了低的热辐射，这为实现激光-红外主被动兼容伪装提供了可能。
本课题组的谢鑫博士认为该工作为实现多光谱和多功能的电磁特征调控提供了新的思路。下一步的工作将聚焦于如何将该方法拓展到可调谐超表面中以实现动态的电磁伪装。