Porous alumina-based broadband anti-reflection film for optical absorption with superhydrophilic to superhydrophobic surface

• NGLA is easy to prepare with excellent performance for enhancing light absorption. • Comprehensive characterization offered insights to the formation mechanism of NGLA. • Two light absorber with absorptivity up to 99.75 % and 98.76 % in wide wave range. • Superhydrophilic to superhydrophobic (contact angle 4.1° to 160.3°) surface. The light absorption of an optical element or material is limited by the surface reflectance, and an anti-reflection (AR) film is necessary to enhance the absorption. We propose a simple and convenient method to fabricate AR film of nano scale thickness with properties of low reflectivity, ultra-wide spectra and incident angular insensitivity, to enhance light absorption. A layer of refractive-index-gradient nano-grass-like alumina (NGLA) plays a key role, which was converted from an Al 2 O 3 layer by atomic layer deposition via a hot water bath. The optical properties of NGLA samples prepared by different processes were investigated. Black glass and black lacquer-sprayed fused silica, which are opaque in the solar spectral range, were chosen as substrates to act as the absorber device. One proposed coated absorber has an average absorptivity of 99.75 % in the wavelength range of 400–1100 nm and another one has an average absorptivity of 98.76 % in the wavelength range of 400–2500 nm. Meantime, we have hydrophobized the AR film to convert superhydrophilic surface to superhydrophobic surface. The proposed coated broadband absorber has great potential for application in the field of light absorption for its simple fabrication process, low cost, and other characteristics that facilitate industrial manufacturing.