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

Abstract

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.

Modeling and admittance recursive simulation of anti-reflective coatings for photothermal conversion: synergy between subwavelength structures and gradient refractive index layers

“…Among various anti-reflective coatings, subwavelength structures [17][18][19][20][21][22] and gradient refractive layers [23][24][25][26][27] are currently widely used. Hong Li et al 28 prepared PiTG with a GRIN coating, revealing that compared to single refractive index (SRIN) coatings, the luminous efficiency of PiTG with the GRIN coating increased from 9.51% to 23.12%.…”

Random nanostructured infrared window scatter analysis using the Harvey-Shack transfer function method

“…Anti-reflective (AR) treatments, such as thin-film coatings, suppress reflections by destructive interference along the propagating directional axis. Recent studies have demonstrated that random anti-reflective structured surfaces (rARSS) can be used as an alternative to thin film coatings, reducing Fresnel reflections in the visible and infrared (IR) [1][2][3][4]. Broadband response, high transmission across wide ranges of angle of incidence (AOI), and polarization insensitivity have also been reported [5,6].…”

Narrow-angle scatter of reflectivity-suppressing nanostructured surfaces

“…Recent studies have demonstrated that random antireflective structured surfaces (rARSS) can be used as an alternative to thin film coatings, reducing Fresnel reflections in the visible and the infrared (IR). [1][2][3][4][5][6][7][8] Broadband response, high transmission across wide ranges of angle of incidence (AOI), and polarization insensitivity have also been reported. 9,10 In general, the AR effects correlate with the density and depth of the nanostructures, simulating an effective medium gradient-index boundary profile, which reduces Fresnel reflectivity.…”