Influences of the ambient humidity and temperature on the material removal and wear behaviors on the soft-brittle KDP optics lithographed by the AFM probes

KDP (K₂HPO₄) crystals possess excellent nonlinear electro-optical properties and are often employed as Pockels cells and frequency converters. Ink-free DPN (IF-DPN) provides a promising method to lithograph surface nano-patterns in KDP optics to improve their laser damage resistances. However, material removal and wear behaviors are greatly affected by humidity and temperature. In this work, theoretical analyses at atomic- and electronic-scales are performed based on molecular dynamics and ab initio to study lithography behaviors (indentation and scratching) for IF-DPN. Lithography experiments with different humidity and temperatures are also executed on KDP substrate, which align well with theoretical results. During indentation processes, surface materials are squeezed into substrate, leading to deep subsurface damages and significant material recoveries. While the scratching processes would cause intense surface material stripping, and sharp-edge of AFM probe facilitates this process. Numerous K-related chemical structure defects would emerge during the lithography processes, which is the predisposition of microstructure change inside KDP crystal. The binding processes of AFM probe, water, and KDP crystal are spontaneous exothermic reactions. Bonding reactions of Si-O, Si-H, and O-H covalent bonds would expedite the wear of AFM probe and KDP crystal. The humidity affects lithography behaviors mainly through chemical dissolution actions, reducing surface integrity and structural strength of KDP crystal and facilitating material stripping. The high humidity would intensify the hydroxylation reaction occurring on AFM probe and KDP crystal surfaces, enhancing their hydrophilicity. The high temperature would result in a reduction of the condensed water meniscus and adsorbed water film, weakening chemical actions of IF-DPN. But it is beneficial for the penetration of AFM probe into KDP crystal surface, material stripping, and minimization of subsurface damages. The high temperature would further expedite the wear of AFM probe and KDP crystal. This work could provide theoretical guidance for revealing the lithography mechanisms of IF-DPN.