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Polymers 2017, 9(11), 574; https://doi.org/10.3390/polym9110574

Ultra-Broadband THz Antireflective Coating with Polymer Composites

1
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
2
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
3
Graduate School of Engineering, Utsunomiya University, Utsunomiya 321-8585, Japan
*
Author to whom correspondence should be addressed.
Received: 24 September 2017 / Revised: 23 October 2017 / Accepted: 26 October 2017 / Published: 3 November 2017
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Abstract

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. View Full-Text
Keywords: COP–TiO2 composite; epoxy–TiO2 composite; antireflection; THz; ultra-broadband; graded refractive index COP–TiO2 composite; epoxy–TiO2 composite; antireflection; THz; ultra-broadband; graded refractive index
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Cai, B.; Chen, H.; Xu, G.; Zhao, H.; Sugihara, O. Ultra-Broadband THz Antireflective Coating with Polymer Composites. Polymers 2017, 9, 574.

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