Influence of PEG content on optical band gap and laser damage characteristics of porous films

TiO₂ and SiO₂ films with different porosity were prepared on quartz substrate and Si substrate respectively by sol-gel technique combined with spin coating method, and the optical and laser damage characteristics of both films were investigated. The optical band gap of the films was calculated based on the transmission spectral curves. It was found that the optical band gap increased with the increase of porosity, the optical band gap size of TiO₂ films ranged from 3.75 eV to 3.97 eV, and that of SiO₂ films ranged from 3.52 eV to 3.78 eV. The ellipsometry measurement results show that when the mass fraction of polyethylene glycol (PEG) was increased from 0, 0.8%, 4% to 8% at a wavelength of 1 064 nm, the porosity of TiO₂ films increased from 11.5%, 14.1%, 30.9% to 38.7%, and the refractive index decreased from 2.063 5, 2.016 5, 1.748 1 to 1.640 9; the porosity of SiO₂ films increased from 4.04%, 4.6%, 5.7% to 13.9%, and the refractive index decreased from 1.438 6, 1.435 8, 1.420 4 to 1.387 9. The extinction coefficients of all TiO₂ and SiO₂ film samples are better than 10⁻³ except for the TiO₂ film with PEG mass fraction of 0.8%, which indicates that the absorption of the films is smaller. The laser induced damage threshold (LIDT) of the films is greatly influenced by the porosity, and the larger the porosity, the higher the LIDT of TiO₂ films, with a value up to 16.7 J/cm². However, the LIDT of SiO₂ film is not improved compared with that without PEG, and when the PEG mass fraction increased from 0.8% to 8%, the LIDT of SiO₂ film increased by 1.9 J/cm². In summary, improving the porosity helps to improve the laser damage resistance of the film.