Crystal field disorder effects in the optical spectra of Nd~(3+) and Yb~(3+)-doped calcium lithium niobium gallium garnets laser crystals and ceramics

摘要

The optical spectroscopic properties of RE~(3+) (Nd, 1 at. % or Yb, 1 to 10 at. %)-doped calcium-lithium-niobium-gallium garnet (CLNGG) single crystals and ceramics in the 10K-300K range are analyzed. In these compositionally disordered materials, RE~(3+) substitute Ca~(2+) in dodecahedral sites and the charge compensation is accomplished by adjusting the proportion of Li~+, Nb~(5+), and Ga~(3+) to the doping concentration. The crystals and ceramics show similar optical spectra, with broad and structured (especially at low temperatures) bands whose shape depends on temperature and doping concentration. At 10 K, the Nd~(3+) ~4I_(9/2)→~4F_(3/2, 5/2) and Yb~(3+) ~2F_(7/2) →~2F_(5/2) absorption bands, which show prospect for diode laser pumping, can be decomposed in several lines that can be attributed to centers with large differences in the crystal field. The positions of these components are the same, but the relative intensity depends on the doping concentration and two main centers dominate the spectra. Non-selective excitation evidences broad emission bands, of prospect for short-pulse laser emission, whereas the selective excitation reveals the particular emission spectra of the various centers. The modeling reveals that the nonequivalent centers correspond to RE~(3+) ions with different cationic combinations in the nearest octahedral and tetrahedral coordination spheres, and the most abundant two centers have 4Nb and, respectively, 3Nb1Li in the nearest octahedral sphere. At 300 K, the spectral resolution is lost. It is then inferred that the observed optical bands are envelopes of the spectra of various structural centers, whose resolution is determined by the relative contribution of the temperature-dependent homogeneous broadening and the effects of crystal field disordering (multicenter structure, inhomogeneous broadening). The relevance of spectroscopic properties for selection of pumping conditions and of laser design that would enable utilization of the broad optical bands for efficient laser emission and reduced heat generation is discussed.