Effect of porosity on the laser induced damage threshold of sol-gel SiO2 and ZrO2 single layer films

In this paper, we investigated the effect of porosity on the laser induced damage threshold (LIDT) of sol-gel SiO₂ and ZnO₂ single layer films. Sol-gel SiO₂ films were prepared with the dip-coating method from acid and base catalyzed SiO₂ sols, respectively. Some of the SiO₂ base films were subsequently treated in saturated ammonia gas for 20 hours. ZnO₂ sol-gel and physical vapor deposition (PVD) films were prepared by spin method and electron beam evaporation method, respectively. The films were irradiated by a pulsed Nd:YAG laser to obtain the LIDT of each film. In order to study the damage mechanism of films under laser irradiation, four types of tests were used. Thermal absorption of films was detected via Stanford photo-thermal solutions (SPTS). The porous ratio was derived via refractive index measured by ellipsometer. The surface morphologies of films were imaged by atomic force microscopy (AFM) before laser irradiation. Optical microscopy was used to characterize the defects and impurities of films before laser irradiation and damage morphology after laser irradiation. The experimental results showed that porous ratio is an essential factor to decide the LIDT for sol-gel films, which benefits the pressure exerted on the film or substrate by the moving particle to dissipate. The films with lower thermal absorption and higher porous ratio have higher LIDT.