Investigation of rare earth-doped barium titanate thin films and their optical properties

Rare-earth-doped barium titanate thin films were studied for potential use as an optical amplification medium. Factors determining their luminescence efficiency and emission linewidth were investigated since these parameters govern the optical gain. Metal-organic chemical vapor deposition was used to prepare BaTiO₃ films on different oxide substrates using a liquid barium precursor, Ba(hfa)₂•PEB. The phase purity, degree of crystallinity, and epitaxy were determined by x-ray diffraction. The films were in-situ doped with the rare-earth erbium to concentrations as high as nine atomic percent.; The luminescent properties of the 1,540 nm emission were studied as a function of growth temperature and Er concentration with photoluminescence spectroscopy and transient photoluminescence. The intensity was linearly dependent on Er concentration up to 1 atomic percent, above which it saturated. The lowering of the emission lifetime from 8 msec to 3 msec indicated concentration quenching was partly responsible for the saturation. Vacuum annealing reduced the PL intensity by as much as 50 times, whereas oxygen annealing restored it, indicating the oxygen stoichiometry affects the luminescence efficiency and may be involved in concentration quenching.; Upconversion was investigated as a potential gain-limiting process. Visible emissions at 550 nm and 664 nm were observed with 980 nm pumping. From measurements of a bulk Er-doped BaTiO₃ crystal with two atomic percent Er, it was determined the green emission was 20 times stronger than the 1,540 nm emission, indicating upconversion is a strong gain-limiting factor.; The effect of temperature and strain on the emission linewidth was investigated. Thermal broadening due to ion-phonon interaction was the dominant line broadening mechanism. The 1,540 nm Stark transition width increased from 1 nm at 40K to 8 nm at 295K. A model incorporating direct and Raman phonon processes was used to describe the thermal broadening. The room-temperature linewidths of the Stark transitions were only weakly dependent on substrate mismatch.; Ridge waveguides were fabricated using the Er:BaTiO₃ films doped with approximately 1 at.% Er. Pump/probe measurements showed stimulated emission resulted in a 2.4 dB reduction in the 1,540 nm absorption. It was concluded that to achieve net optical gain, the background losses need to be further reduced.