Single layer Al_2O_3·H_2O and multilayer Al_2O_3·H_2O–SiO_2 optical coatings prepared from colloidal suspensions

Single layer coatings of Al2O3·H2O have been prepared on fused silica and BK-7 substrates from aqueous colloidal suspensions of hydrated alumina at room temperature. These coatings were porous and had a refractive index of ∼1.44. Multilayer, high reflectivity, dielectric coatings were also prepared by laying down quarterwave-thick, alternating coats of this alumina with silica, also prepared from colloidal suspension, and having a refractive index of 1.22. To achieve 99% reflectivity, 30–35 layers were required. Single shot, laser damage threshold measurements at 1064 nm wavelength with a pulse length of 10 ns were carried out on both the single layer and multilayer system. The thresholds averaged 21 and 12 J/cm2, respectively.