Cai Bin, Chen Haitao, Xu Gongjie, Zhao Hongwei, Sugihara Okihiro
Engineering Research Center of Optical Instrument and System, Ministry of Education, Shanghai Key Laboratory of Modern Optical Systems, University of Shanghai for Science and Technology, No. 516 Jun Gong Road, Shanghai 200093, China.
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China.
Polymers (Basel). 2017 Nov 3;9(11):574. doi: 10.3390/polym9110574.
Achieving an ultra-broadband range is an essential development direction in terahertz techniques; however, a method to cover the full terahertz band by using a highly efficient antireflection (AR) coating that could greatly increase the efficiency of terahertz radiation is still lacking. It is known that structures possessing a graded-index profile can offer a broadband AR effect, and such structures have been widely used, especially in the visible range. In this paper, first, we tuned the refractive index of a cyclo-olefin polymer (COP) by using a TiO₂ dopant, and a polymer⁻TiO₂ composite with a refractive index of 3.1 was achieved. We then fabricated a surface-relief structure with a graded-index profile by using a hot-embossing method. The structure on the silicon substrate can provide an excellent AR effect, but the working band is still limited by its scale of sag and swell. To obtain an ultra-broadband AR effect, we then proposed a flat six-layer structure; a graded-index profile was obtained by casting epoxy⁻TiO₂ composites in the order of a high index to lower indices. With a very well controlled refractive index and thickness of each layer, we achieved an AR effect of <2% in the ultra-broadband of 0.2⁻20 THz.
实现超宽带范围是太赫兹技术的一个重要发展方向；然而，目前仍缺乏一种通过使用高效减反射（AR）涂层来覆盖整个太赫兹频段的方法，这种涂层能够大大提高太赫兹辐射的效率。众所周知，具有渐变折射率分布的结构可以提供宽带AR效应，并且这种结构已经被广泛应用，尤其是在可见光范围内。在本文中，首先，我们通过使用TiO₂掺杂剂来调节环烯烃聚合物（COP）的折射率，从而获得了折射率为3.1的聚合物-TiO₂复合材料。然后，我们使用热压印方法制造了具有渐变折射率分布的表面浮雕结构。硅衬底上的这种结构可以提供优异的AR效应，但其工作频段仍然受到其起伏尺度的限制。为了获得超宽带AR效应，我们随后提出了一种平面六层结构；通过按高折射率到低折射率的顺序浇铸环氧-TiO₂复合材料来获得渐变折射率分布。通过很好地控制每层的折射率和厚度，我们在0.2-20 THz的超宽带范围内实现了<2%的AR效应。
