1.Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China 2.Shandong Institute of Advanced Technology, Jinan 250100, China 3.School of Physics and Technology, Wuhan University, Wuhan 430072, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2019YFB2203400), the China Postdoctoral Science Foundation (Grant No. 2019M663467), and the Free Exploration Project of Science and Technology Department of Sichuan Province, China (Grant No. 20YYJC3609)
Received Date:03 June 2020
Accepted Date:30 June 2020
Available Online:14 October 2020
Published Online:20 October 2020
Abstract:The circular dichroism of chiral structure has been widely used in analytical chemistry, industrial pharmacy, biological monitoring, etc. However, the light-matter interaction between natural chiral structures is extremely weak. Plasmonic nanostructures can significantly enhance light-matter interaction. During the fabrication of the visible-to-near-infrared chiral plasmonic metamaterial absorbers, there exists usually a trade-off between the absorption and the sample area, that is, the circular dichroism signal of the large-area structure is small. Besides, the preparation of chiral absorbers working in the visible and near-infrared region usually requires expensive etching or lithography equipment, such as reactive ion etching or electron beam lithography. Therefore, preparing cost-effective chiral absorbers with large circular dichroism is attractive for practical applications. In order to improve the circular dichroism of large-scale chiral absorbers, a honeycomb-shaped elliptical hole absorber is proposed in this paper, and its absorption, circular dichroism, and optical g-factor are studied. By reasonable design, the numerical calculation results show that the circular dichroism can reach about 0.8 under the excitation of chiral polarized light, and the corresponding optical g-factor can reach about 1.7 at 920 nm. Compared with the reported absorber, our chiral absorber has a maximum g-factor value. The giant circular dichroism originates from the symmetry breaking of the structure by tilting ellipse structures, and the tilt angle has a significant influence on circular dichroism. To further explain the absorption difference, the electric profile, surface current distribution, and absorption loss of the chiral absorption at resonant wavelength are analyzed. Finally, we point out that the structure can be prepared by existing technologies, such as nanosphere photolithography: first, a layer of polystyrene (PS) balls is formed by self-organization, which can control the period of the structure; then the size of the PS balls is reduced to a suitable size and spacing by the reactive ion etching; finally, a metallic layer is deposited by oblique angle evaporation. This work provides useful guidance for fabricating the large-scale chiral plasmonic absorbers. Keywords:optical absorption/ circular dichroism/ plasmonics/ chiral
表1可见和近红外波段的手性超材料吸收器与本文吸收器的对比 Table1.Selected publications on chiral metamaterial absorbers at the visible and near-infrared region.
图 7 不同偏振的入射光照射下, 共振波长在920 nm附近的 (a)和 (b) 归一化电场E/E0分布图, 未归一化前, LCP和RCP光照射下E/E0最大值分别为11和36; (c) 和 (d) 表面电流分布图; (e) 和 (f) 吸收损耗(吸收密度)图. 图(a), (c)和 (e) 为LCP入射; 图(b), (d) 和 (f) 为RCP入射 Figure7. (a), (b) Normalized electric field E/E0; (c), (d) Surface current distribution; (e), (f) Absorption loss (absorption density) at the wavelength of 920 nm with different circularly polarized illuminations. (a), (c) and (e) For LCP; (b), (d) and (f) for RCP; the non-normalized maximum values of E/E0 under LCP and RCP light irradiation are 11 and 36, respectively.
通过CST Microwave Studio软件计算出的结构对LCP和RCP光的转换可以进一步从直观上理解手性吸收的现象(由于CST和FDTD计算方法的不同, 结果在共振波长处有少许差异). 图8显示了两种圆偏振光入射结构时, 反射波中两种圆偏振光的分量. 共振波长处, 入射光为LCP时, 反射波中的LCP接近0, RCP小于0.04; 入射光为RCP时, 反射波中RCP接近于0, LCP大于0.85. 因此在共振波长处, CD值可以达到约0.8. 其次, 可以看出未发生偏振转化部分LCP-LCP和RCP-RCP大小一样, 手性来源于两种圆偏振光具有不同的偏振转化效率. 除了在共振波长处, 反射波中既有LCP, 也会有RCP. 经典的手性参数提取方法并没有考虑偏振转化的情况, 认为LCP(RCP)入射时, 只有(RCP)的反射[25]. 然而, 真实情况下, 反射波中具有两种圆偏振光, 因此该方法在这里不适用. 图 8 LCP和RCP光入射结构时反射波中两种圆偏振光的分量 Figure8. The components of LCP and RCP light in the reflected wave.