Optical study of La3Ga5.5Ta0.5O14 single crystal co-doped with Ho3+ and Yb3+

Single crystal of La3Ga5.5Ta0.5O14 (LGT) containing intentionally 0.5 % of Ho3+ and 1 % of Yb3+ was grown by the Czochralski method. Examination of chemical composition of the grown crystal revealed that luminescent holmium and ytterbium ions are preferably retained in the melt and their actual concentrations are 0.12 and 0.24 %, respectively. Spectroscopic investigation performed encompassed IR host absorption spectra and Raman spectra at room temperature, optical absorption and luminescence spectra of Ho3+ and Yb3+ at room temperature and at 5 K, and luminescence decay curves at room temperature. It was found that all spectral bands recorded show important inhomogeneous line broadening. This feature was attributed to structural disorder inherent to the crystal lattice in which pentavalent Ta5+ ions occupy octahedral Ga(1) sites together with trivalent Ga3+ ions. Despite small concentrations of luminescent ions, the occurrence of nonradiative interaction that feeds the I-5(6) and I-5(7) levels of Ho3+ ions by transfer of an excitation from the F-2(5/2) level of Yb3+ ions was evidenced. Based on examination of spectroscopic parameters evaluated, it was concluded that LGT:Ho, Yb may be considered as a potential intermediate-gain laser active material able to emit infrared radiation from the I-5(7) -> I-5(8) transition of Ho3+ around 2 micrometres upon laser-diode pumping into Yb3+ absorption band.