The laser floating zone (LFZ) technique was used to produce polycrystalline fibers composed by P21/c monoclinic Y4Al2O9 (YAM) and P4¯21m tetragonal CaYAl3O7 (CYAM) phases embedded into I4/mmm tetragonal CaYAlO4 matrix. The scanning electron microscopy and X-ray diffraction patterns put in evidence the strong effect of growth rate on the microstructural and phases’ evolution. Besides the microstructural and structural analysis, complementary optical techniques as photoluminescence (PL), PL excitation (PLE), and lifetime measurements were used to characterize the produced fibers. The nonintentionally doped fibers were shown to exhibit an intense deep red emission likely due to Mn4+ trace impurities. From the PLE measurements, average crystal field strength was estimated with Dq/B ~ 2.94. Temperature-dependent PL measurements revealed that the red luminescence is due to the overlap of transitions from the almost electronic degenerate 2E and 4T2 excited states to the 4A2 ground state. The emission from the two excited states arises due to the breakdown of the adiabatic approximation. The overall luminescence intensity of the red emission was found to decrease from 11 K to RT, and the internal quantum efficiency, estimated from the ratio of the integrated luminescence at high and low temperatures, was found to be 60%. Time-resolved spectroscopy indicates a single decay time of ca. 2.0 ms at room temperature, corresponding to the spin and parity forbidden 2E → 4A2 transition.

This work is funded by FEDER funds (COMPETE 2020 Programme) and National Funds through FCT —Portuguese Foundation for Science and Technology (UID/CTM/50025/2019). F. Rey-García also acknowledges EU (Project SPRINT (EU H2020-FET-OPEN/0426)), Xunta de Galicia (ED431E 2018/08), and FCT (SFRH/BPD/108581/2015).

Peer reviewed