Abstract
AbstractReduction of optical loss for dielectric high reflectance mirrors can promote the navigation accuracy of ring laser gyroscope, elevate the laser damage threshold of high-energy laser and enhance the sensitivity of laser interferometer. In application of visible wavelength region, titanium dioxide (TiO2) film is the most commonly utilized high refractive index material in the multi-layers dielettric mirror fabricated. Ion beam sputtering is the commond method to deposited these films, because it produces films with better quality such as high density with higher refractive index and lower scattering loss amorphous structure.In our study, we found that the optical loss of ion-beam-sputtered TiO2 thin film could be reduced by annealed, but when annealing temperature increased above 200oC there exited a phase transition from amorphous to polycrystalline anatase. The polycrystalline grain boundary and roughened surface of annealed TiO2 film cause increased scattering loss. Higher annealing temperature for further reducing the absorption loss of the film is therefore limited by increasing scattering from the appearance of the polycrystalline phase.The main achievement of our research are to provide a low-loss optical mixed films fabricated by ion-beam-sputtered deposition. The film can sustain higher annealing temperature without crystall-ization to reduce the absorption loss further. The optical mixed films therefore posess the property of low absorption loss and low scattering loss which can be used to produce low-loss high reflectance dielectric laser mirrors for such above optical systems application .In our study, we used ion-beam-sputtered deposition to deposit different fraction TiO2-SiO2 mixed films. We found that the structure of the as-deposited TiO2-SiO2 mixed films were all amorphous and posessed high refractive index. The ion-beam-sputtered TiO2-SiO2 mixed films have lower optical absorption than the ion-beam-sputtered pure TiO2 films and the optical absorption decreases with increasing SiO2 concentration. TiO2-SiO2 mixed films can endure higher annealing temperature to reduce the optical absorption and yet avoiding increasing in the scattering loss which is due to the appearance of the polycrystalline structure on high temperature annealing. With the properties of high temperature amorphous structure and high refractive index, the ion-beam- sputtered TiO2-SiO2 mixed films can be used as a low loss high refractive index material in optical thin film applications.In expermemtal proceeding, the ion-beam-sputtered TiO2-SiO2 mixed film was used as the high refractive index film in a multi-layer stack dielectric mirror. The total loss of the as-deposited mirror with mixed films was 34% lower than that of the mirror with pure TiO2 film as the high refractive index film, and can sustain 400oC annealing temperature for 24h of annealing time still with smooth amorphous surface. After annealing, the total loss of the mirror with the mixed films decreased 88% of the as-deposited value. Laser damage threshold for the annealed mirror with mixed films was above 6.37KJ/cm2 and was higher than the as-deposited mirror and the mirror with pure films under high power 1064nm Nd-YAG laser testing.