Abstract The surface optical loss in optical windows is primarily attributed to Fresnel reflection at the interface between air and substrate of high refractive index materials such as zinc sulfide (ZnS). In this study, the concave antireflective subwavelength structures (ASS) on ZnS have been numerically and experimentally investigated to obtain high transmittance over a wide bandwidth in the far-infrared range from 7 μm to 14 μm. The mechanism for transmittance enhancement is explored by localized field selective enhancement in structures based on Wood-Rayleigh law, also being confirmed by the light energy flowing in Poynting vector distribution. The geometric parameters including shapes, period, and depth of as-designed array have been specifically optimized by finite-difference time-domain (FDTD) method and effective medium theory (EMT). Hereby, three-dimensional direct femtosecond laser writing (3D fs-DLW) is employed to fabricate the ASS in inverted conical and pyramidal array with the largest transmittance reaching to 85.2% in 9 μm wavelength and robust mechanical characteristics (hardness >2.39 GPa), expecting to be of great potential applications in infrared optical windows.

中文翻译：

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用于远红外宽带抗反射的倒置阵列直接飞秒激光写入

摘要 光学窗口中的表面光损耗主要归因于空气与硫化锌 (ZnS) 等高折射率材料基体之间界面处的菲涅耳反射。在这项研究中，已对 ZnS 上的凹面抗反射亚波长结构 (ASS) 进行了数值和实验研究，以在 7 μm 至 14 μm 的远红外范围内的宽带宽内获得高透射率。基于伍德-瑞利定律的结构中局部场选择性增强探索了透射率增强的机制，也通过坡印廷矢量分布中流动的光能得到了证实。通过有限差分时域（FDTD）方法和有效介质理论（EMT）对设计阵列的形状、周期和深度等几何参数进行了专门优化。