Design and Fabrication of luminescent antireflective sol-gel coatings with a Bi-layer structure

Transparent coatings consisting of a Y₂O₃:Eu ³⁺,Bi³⁺ layer (bottom) and a porous SiO₂ layer (top) with high and low refractive index, respectively, were fabricated on quartz glass substrates by a sol-gel dip-coating method. The thickness of both the layers and the refractive index of the SiO₂ layer were designed aiming at the realization of luminescent antireflective (AR) coatings. The target thickness of 140 nm for the Y₂O₃:Eu ³⁺,Bi³⁺ layer was obtained by controlling the withdrawal speed in dip-coating. The target refractive index of 1.30 for the porous SiO₂ layer was achieved by controlling the composition of coating solutions, which were composed of tetraethylorthosilicate, an aqueous ammonia solution, and ethanol, as well as the withdrawal speed. Bi-layer structured coatings with the bottom Y₂O₃:Eu³⁺,Bi ³⁺ layer and the top SiO₂ layer could reduce Fresnel reflection considerably, showing an excellent AR effect. Moreover, photoluminescence intensity from the Y₂O₃:Eu ³⁺,Bi³⁺ layer was increased by stacking the porous SiO₂ layer. This phenomenon was explained with the increased absorption of excitation light by decreasing Fresnel reflection at the surface (excitation enhancement) and the reduced total internal reflection of emission light by easing the difference in the refractive index at the interface (emission enhancement).