Nano-Silica-Based Double-Layer Antireflective Coatings with Mildew Resistance and Moisture Resistance for KDP Crystals.

Sol-gel nano-silica antireflective (AR) coatings with moisture resistance are widely used for optical elements, such as potassium dihydrogen phosphate (KDP) crystals, but their mildew resistance is often disregarded. This work reports a double-layer AR coating with moisture resistance and mildew resistance for KDP crystals. A polydimethylsiloxane-modified dense silica coating and a quaternary ammonium salt (QAS) modified nanoporous silica coating are selected as the bottom layer and top layer, which effectively serve as a moisture barrier and an antireflection layer, respectively. The coated KDP crystal shows excellent antireflection properties with a maximum transmittance of 99.1% at 532 nm. Perfluorooctyltriethoxysilane vapor treatment is performed further to improve the resistance to moisture and mildew. The resultant double-layer coating exhibits superior moisture resistance with almost no change in optical transmittance after a 3-month exposure to a high-humidity environment. The introduction of QAS and hydrophobicity in the top layer provides exceptional resistance against mildew, achieving an antimicrobial rate of 99.9% against E. coli and A. flavus. Moreover, the laser-induced damage threshold reaches 17.0 J cm^-2 (355 nm, 4.5 ns). This work imparts moisture resistance and mildew resistance to AR coatings, providing valuable insights for designing multifunctional AR coatings on optical components.