Broad-band anti-reflective pore-like sub-wavelength surface nanostructures on sapphire for optical windows

• Main Authors: Lin, Zhao-Qing, Wang, Gui-Gen, Tian, Ji-Li, Wang, Li-Yan, Zhao, Dong-Dong, Liu, Zheng, Han, Jie-Cai
• Other Authors: School of Materials Science & Engineering
• Format: Article
• Language: English
• Published: 2019
• Subjects: Sub-wavelength Structure; Engineering::Materials; Sapphire
• Online Access: https://hdl.handle.net/10356/86224; http://hdl.handle.net/10220/49271
• Institution: Nanyang Technological University

Compared with conventional anti-reflective film, an anti-reflective sub-wavelength surface structure provides an ideal choice for a sapphire optical window especially in harsh environments. However, it is still a challenge to obtain a sapphire anti-reflective surface microstructure because of its high hardness and chemical inertness. In this paper, combined with optical simulation, we proposed a facile method based on the anodic oxidation of aluminum film and following epitaxial annealing. Al thin film was deposited on a sapphire substrate by magnetron sputtering, and anodic oxidation was then performed to prepare surface pore-like structures on the Al film. Followed by two-step annealing, both the anodic oxidized coating and underlying unoxidized Al film were transformed totally into alumina. The parameters of anodic oxidation were analyzed to obtain the optimal pore-like structures for the antireflection in the mid-infrared and visible spectrum regions, respectively. Finally, the optimized surface sub-wavelength nanostructure on sapphire can increase the transmittance by 7% in the wavelength range of 3000–5000 nm and can increase 13.2% significantly for visible spectrum region, respectively. Meanwhile, the surface wettability can be also manipulated effectively. The preparation of surface pore-like sub-wavelength structure by the annealing of anodic oxidized aluminum film on sapphire is a feasible, economical and convenient approach and can find the applications for various optoelectronic fields.