Modified antireflective thin films for optical crystals are developed by sol gel chemistry. Hexamethyldisilazane is added
to silica sol to prepare modified sol by the Stöber method. The particle size distribution of the modified sol is found to
have good uniformity, with an average particle diameter of about 42.14 nm. This sol can be used to prepare antireflective
films with low refractive index of about 1.2. Homogeneous double layered films of antireflective and moisture resistant
properties are demonstrated to coat small square sized KH2PO4/KD2PO4 crystals through a combination of dip and spin
coating. The modified antireflective films display residual reflectivity close to zero, and laser induced damage threshold
of 10.9 J/cm2 (pulse wavelength: 355nm, pulse width: 3ns) measured by raster scan method.