Excited state relaxation dynamics and up-conversion phenomena in Gd3(Al,Ga)5O12 single crystals co-doped with holmium and ytterbium

Abstract

Single crystals of Gd3(Al,Ga)5O12 single doped with Ho3+ or Yb3+ ions and co-doped with Ho3+ and Yb3+ ions were grown by the Czochralski technique and investigated employing methods of optical spectroscopy. Rates of radiative transitions, branching ratio values and radiative lifetimes for excited multiplets of incorporated Ho3+ ions were determined based on the Judd–Ofelt treatment of optical absorption spectra recorded at room temperature. Absorption spectra and luminescence spectra recorded at 5 K made it possible to determine crystal field splitting for several low energy multiplets of Ho3+ ions and for the 2F5/2 multiplet of Yb3+ ions. Lifetimes of luminescent levels of incorporated holmium and ytterbium ions and the Yb3+ − Ho3+ energy transfer efficiency were determined based on analysis of recorded luminescence decay curves. Up-conversion of infrared radiation at 980 nm into yellow emission in crystals co-doped with holmium and ytterbium was observed and discussed. It was found that the structural disorder of the Gd3(Al,Ga)5O12 host resulting from a disparity of ionic radii of Al and Ga ions induces an inhomogeneous broadening of spectral lines of incorporated luminescent ions but the crystal field splitting of their multiplets is similar to that encountered in ordered garnet hosts. Based on analysis of gathered spectroscopic data it was concluded that the Gd3(Al,Ga)5O12:Ho3+,Yb3+ crystals are promising materials for the design of laser-diode pumped infrared lasers operating near 2000 nm.