Optical spectroscopy of Sm~(3+) in C_2 and C_(3i) sites of Y_2O_3 ceramics

摘要

The Sm~(3+) optical spectra in Y_2O_3 translucent ceramics are dominated by electric-dipole transitions connected to the centers of C_2 symmetry, similar to those reported for single crystals. However, quasi-selective time-resolved excitation and emission spectroscopy investigation evidenced, for the first time, additional lines in the magnetic dipole allowed ~6H_(5/2)→ ~4G_(5/2), ~4F_(5/2) and ~4G_(5/2)→ ~6H_(5/2,7/2) transitions, associated with the centrosymmetric C_(3i) centers. An energy level scheme for C_(3i) and an improved one for C_2 centers are proposed. The ~4G_(5/2) emission lifetimes of these two structural centers (1.48 ms for C_2 and 8.4 ms for C_(3i) at 300 K) as well as the emission quenching are quite distinct, whereas Sm~(3+) ions in C_2 centers are involved both in intra-center C_2→ C_2 cross-relaxation on intermediate levels and inter-center C_2 →C_(3i) energy transfers (that imply only Stark levels of the ground ~6H_(5/2) and metastable ~4G_(5/2) manifolds), Sm~(3+) ions in C_(3i) sites can participate only in the last processes. The spec-troscopic data determined in this work (energy level structures, absorption and emission cross-sections, de-excitation processes parameters) suggest the prospect of Sm-doped Y_2O_3 ceramics as phosphor or four-level laser active materials on the highest intensity line at 608 nm in the ~4G_(5/2) → ~6H_(7/2) transition under GaN diode laser (～405 nm) pumping or as suppressors of amplified one-micron spontaneous emission for the Nd:Y_2O_3 lasers.