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

Abstract 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.