Study of spontaneous and induced absorption in amorphous Ta2O5 and SiO2 dielectric thin films

Tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2) are common high-index and low-index materials used in dielectric optical coatings for high average-power lasers since high-density sputtered oxide films with absorption losses at near- and mid-infrared wavelengths of less than 1 ppm can be obtained. These oxides have been chosen to investigate the spontaneous and optically induced absorption at λ0 = 1064 nm that occurs due to simultaneous illumination at shorter wavelengths. The effect is measured using the photothermal common-path interferometric technique. This technique is capable of detecting sub-ppm levels of optical absorption and tracking its changes at a given wavelength when a second laser beam is also incident on a thin film oxide sample. In this work, dual beam experiments are employed to assess changes in the optical absorption at λ0 = 1064 nm in ion beam sputtered Ta2O5 and SiO2 thin films deposited on fused silica substrates, with stimulating illumination λ1 ranging from λ1 = 266 nm to λ1 = 780 nm. The power and wavelength of the stimulating radiation were found to affect the optical absorption at λ0 = 1064 nm. Furthermore, the relaxation dynamics of the induced infrared absorption was found to be wavelength dependent and is thought to be associated with various electron traps existing in the forbidden gap that depend essentially on the film's preparation conditions. The significantly greater effect observed in Ta2O5 at λ1 = 266 nm is attributed to band-to-band transitions.