Optically transparent and refractive index-tunable ZrO₂/ photopolymer composites designed for ultraviolet nanoimprinting

Epoxy-based, acrylate-based, and urethane-based ultraviolet (UV)-curable resins containing acrylate-modified zirconia nanoparticles (ZrO₂-NPs) were prepared. UV-cured composite films fabricated using an acrylate-based resin and an epoxy-based resin exhibited a high transmittance >90% at visible and near-infrared wavelengths until the weight fraction of ZrO₂-NPs having an average zirconia core diameter of 4.0nm reached 0.66. Their refractive indices at 633 nm were widely controllable between 1.515 and 1.659 for the acrylate-based composite films and between 1.589 and 1.679 for the epoxy-based composite films. Thin films of the UV-curable composite resins could be transformed by UV nanoimprinting in pentafluoropropane using a fluorinated silica mold. Among the composite resins, the acrylate-based UV-curable composite resins showed facile workability by UV nanoimprinting. The epoxy-based UV-curable composite resins formed pattern shapes independent of the weight fraction of ZrO₂-NPs. These results indicated that the acrylate-based and epoxy-based composite films with optical transparency, refractive index controllability, and workability had potential as optical materials of optical devices and as resist materials for dry etching.