基于光学薄膜的高效超构表面研究

Study of High-Efficiency Metasurfaces Based on Optical Thin Films

超构表面(metasurfaces)是一种新型的人工二维平面阵列结构,通过合理设计亚波长纳米结构单元及排布可以实现对空间光场的调控,有望从原理层面上颠覆传统的透镜元件。然而,目前透射型超构表面普遍存在光学效率低的问题,制约着其应用和发展。首次提出了光学薄膜结合超构表面提高光能利用率的设想,期望超构表面结合光学薄膜的结构在不影响超构表面光学特性的基础上,可以显著地提高其光学效率。为了验证这一策略的有效性,仿真模拟了石英基底近红外宽波段的硅纳米块超构表面透镜,并与设计有光学薄膜的超构表面进行了对比。模拟结果表明:光学薄膜对超构表面的聚焦性能没有影响,但可以显著提高超构表面的光学透过率;在1450~1600 nm波段,平均透过率提高了10.5%以上,聚焦效率平均提高了8.3%以上。所提方法为超构表面器件设计带来了新的思路。

Objective The problem of low optical efficiency commonly exists on metasurfaces,which restricts their application and development.Although the efficiency of metasurfaces designed based on dielectric nanobricks structures is greatly improved compared to metal metasurfaces,the scattering and reflection losses of the unit structure are still relatively large.Metasurfaces are generally composed of high refractive index nanobricks to reduce their thickness and preparation process difficulty.Due to the high refractive index of the equivalent film layer on a high refractive index metasurfaces,it leads to significant interface reflection loss.In terms of improving the efficiency of metasurface devices,current research mainly focuses on improving the diffraction efficiency of metasurfaces and reducing scattering losses.However,there is no research focus on the reflection loss of metasurfaces currently,so it is necessary to study reducing the reflection loss of metasurfaces.Methods We propose an efficient design scheme for metasurfaces based on optical thin film theory to solve the problem of interface reflection loss caused by the mismatch between the equivalent refractive index of the metasurfaces and the substrate,as well as the mismatch between the equivalent optical thickness of the metasurfaces and the wavelength.First,we design the metasurface lens.Then,based on the equivalent medium theory,the metasurfaces are equivalent to a layer of dielectric thin films and serve as the outermost layer of the multi-layer antireflection coating system,with the equivalent layer thickness being the height of the metasurfaces.Finally,the optical thin film theory is adopted to design the antireflection coating that matches the substrate and incident medium.Results and Discussions We simulate the near-infrared broadband silicon nanobrick metasurface lens on the quartz substrate and compare it with the metasurfaces designed with optical thin films.The transmittance of the antireflection coating designed by the equivalent medium theory is much higher than that of the equivalent film layer on the metasurfaces,with an average transmittance of 12.4%higher(Fig.4).Comparison is made between the light field distribution patterns of a metasurface lens without optical thin films and with optical thin films(the antireflection coating structure of optical thin films combined with a metasurface)at different wavelengths(1460,1530,1600 nm).It can be seen that the focal spot size and focal length of the two types of structured metasurfaces at the same wavelength are basically the same.In the case of optical thin films,the light intensity at the focal point is significantly higher than that without optical thin films,whereas the focal point position is not affected by the antireflection coating and remains unchanged.This indicates that optical thin films only increase the transmittance of the metasurfaces and have little effect on their focusing performance(Figs.5‒7).The transmittance curves in the 1450‒1600 nm wavelength range and the focusing efficiency at 1450,1490,1530,1565,1600 nm wavelengths are simulated and calculated.From the transmittance curves,it can be seen that in the 1450‒1600 nm wavelength range,the transmittance of the metasurface lens designed with optical thin films remains around 94.0%,with the highest peak reaching 95.5%,which is much higher than that of metasurface without optical thin films,with an average increase of more than 10.5%(Fig.8 and Fig.9).The results of simulation calculations indicate that our proposed idea of combining optical thin films with metasurfaces is reasonable and has the potential to be applied to the actual production of metasurfaces.Conclusions We propose the concept of using optical thin films to improve the efficiency of metasurfaces.The characteristics of metasurface lens are studied in the near-infrared,and based on the properties of additional functional optical thin films on metasurfaces,the influence of the antireflection coating on the transmittance and focusing performance of metasurfaces are studied.Research has shown that combining the structure of optical thin films with the metasurfaces can significantly improve the optical efficiency of metasurfaces without affecting their optical properties.The idea of combining metasurfaces with the proposed optical thin films is expected to solve the problem of low efficiency of metasurfaces,bringing new ideas for the design of metasurface devices.