A simple method to control the microstructure and properties of sol-gel silica antireflective coatings

To control the microstructure and properties of sol-gel silica antireflective (AR) coatings, a simple PMHS (poly(methylhydrogen) siloxane) surface modification method was applied and investigated. This is accomplished simply by immersing the AR coating into the modifier solution containing PMHS, hexane and the Karstedt catalyst and then immediately withdrawing it at a desired withdrawal rate. A systematic study was carried out to investigate the effect of PMHS on the microstructure and optical, hydrophobic and mechanical properties of sol-gel silica AR coatings. It was found that PMHS surface modification greatly increased the hydrophobicity and mechanical properties of sol-gel silica AR coatings by increasing their water contact angles from 27.2 degrees to about 120 degrees and Young's modulus from 317 MPa to 482 MPa. Most importantly, the transmittance of AR coatings can be tailored well between 96.4% and 100% by controlling the PMHS chain length, PMHS concentration and withdrawal rate. Speculation and verification of the PMHS surface modification process was discussed in detail after SEM characterization and optical simulation by TFCalc (TM) software. Finally, with this simple method, a hydrophobic sol-gel silica AR coating with excellent transmittance of 99.9%, improved mechanical properties and AR stability was realized, which can find great applications in high power laser fusion systems.