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.
Single-layer silicon metasurfaces that can achieve high-efficiency dual-spectrum or ultra-broadband photonic spin-orbit interaction and geometric phase modulation.
