Analysis of sub-bandgap losses in TiO2 coating deposited via single and dual ion beam deposition

We present a detailed study of sub-bandgap losses in TiO2 optical coatings prepared by using a single and a dual ion beam deposition. By extracting high-precision spectra of the losses in the TiO2 layers deposited under varied parameters we were able to study the losses in great detail. We were able to identify several spectral features and discuss their origin. Additional experiments allowed us to determine the contribution of absorption and scattering to the losses, as well as to discern whether the observed losses have volume or interface character. The results reveal that the dominant source of absorption in the high-quality samples is volume absorption originating from the so-called Urbach tail. We discuss the limitations imposed by the Urbach tail.