CO2 laser-assisted deposition of multilayer homogeneous TiO2/SiO2 nanocomposite films with varying refraction and absorption indices

Multilayer thin films of sintered TiO2 and SiO2 nanoparticles (NPs) are frequently used as antireflection coatings (ARCs) due to their excellent optical properties. We report CO2 laser-assisted deposition, i.e., spin-coating of wet films followed by laser processing, of multilayer transparent films using a mixture of TiO2 and SiO2 NPs. A simple heat transfer model is presented to optimize laser processing parameters for effective sintering while preventing the anatase-to-rutile phase transformation of TiO2. To minimize the reflectance, and enhance the transmittance, an ARC model is utilized to select the film material and determine the thickness of each layer. UV/Vis spectrophotometry was carried out to measure the optical reflectance. UV/Vis/NIR spectrophotometry analysis shows that the sintered homogenous films are highly transparent, with an average transmittance of above 90% in a certain range of wavelength. Due to the porosity of the films, an effective medium model was utilized to determine the optical constants, i.e., the refraction and absorption indices, of just the solid phase, which is formed by the sintered NPs in the nanocomposite TiO2/SiO2 coatings. The results demonstrate that the effective refraction or absorption index can be smaller than that of the solid phase. The effect of TiO2 and SiO2 concentrations on the optical properties of multilayer homogenous TiO2/SiO2 coatings is also investigated. The effect of laser processing on the microstructural properties was investigated using SEM and XRD techniques. Optical profilometry was conducted to determine the thickness of the sintered films.