Porous Antireflective Coatings Research Articles

Porous antireflective coatings with controlled thickness and refractive index on glass

Porous antireflective (AR) coatings were prepared by the neutral solution leaching of borosilicate glass. The textured surfaces typically exhibit broadband and wide-angle AR behavior with an extremely low scattering over 300–1400nm spectral range. The coating thickness increases with increase in the solution concentration, temperature and etching time, leading to the tunable transmittance spectra in the entire wavelength range. And the thickness and corresponding refractive index of AR coatings can be calculated using transmittance simulation. The relationship of experimental variations, coating thickness and etching rate was obtained and discussed. These porous AR coatings with controlled thickness and refractive index would have broad potential applications.

Investigation of etch pits on KDP crystals with porous sol-gel coatings

We have investigated etch-pit formation on potassium dihydrogen phosphate (KDP) crystals with porous anti-reflective coatings. Etch pits develop beneath the sol–gel coatings after exposure to ambient humidity. The etch pits are homogeneously distributed with a density and an average size governed by the relative humidity and the coating thickness. Furthermore, the etch pits are self-similar in shape and possess facets corresponding to low-energy planes of KDP. Results from optical microscopy, light scattering, and atomic force microscopy support the following model for etch-pit formation in this system. Water adsorbed from the environment into the porous sol–gel coating contacts the crystal surface, causing etch-pit nucleation at high undersaturation. The plume of KDP rising from an etch pit slowly diffuses laterally, reducing the undersaturation and shutting off nucleation in the surrounding area. Because surface kinetics are rapid compared to mass transport through the sol–gel, etch pits continually undergo equilibration to maintain a low-energy geometry and generate an average spacing. Growth continues until the reservoir of water in the sol–gel is saturated with KDP. Coarsening is only observed in high relative humidity environments.