Er-Yb codoped CaF2 thin films grown by molecular beam epitaxy

Yb:Er: codoped CaF2 monocrystalline films have been grown by molecular beam epitaxy (MBE) using YbF3, ErF3, and CaF2 evaporation on CaF2 substrates. Luminescence and excitation of luminescence at 25 K are measured in order to investigate the role of Yb3+ codoping and the effect of the growth conditions during the MBE process on the incorporation of active rare-earth species in CaF2. It is found, from a comparison with earlier work on singly doped thin films, that the energies of the levels of Er3+ ions are not modified by the double doping, whereas mixed complex centers are formed. Growth conditions are found to be important controlling factors for the incorporation of Yb3+ as a sensitizer, and, consequently, for the transfer efficiency. From excitation and absorption spectra it is shown that the lowest component of the Er:I-4(11/2) manifold has an energy higher than that of Yb:F-2(5/2) and the consequences are discussed. (C) 1997 American Institute of Physics.