Characterization of vapour-transport-equilibrated Yb(/Er):LiNbO3 crystals

Abstract

Visible and infrared absorption, Raman scattering, x-ray powder diffraction and scanning electron microscope (SEM) have been used to characterize Er/Yb (0.6 mol%/0.3 mol%) codoped and singly Yb-doped (1.2 mol%) Z-cut LiNbO3 crystals that were vapour-transport-equilibrated (VTE) at 1110 and 1120 °C for 100, 120 and 220 h in a Li-rich atmosphere. Optical absorption edge and 153 cm−1 E(TO) phonon linewidth have been used to roughly evaluate the composition of the crystals. The results obtained show that the VTE treatment has brought all crystals towards stoichiometric composition. As a relatively strong VTE condition was adopted, the treatment results in the formation of submicron-sized ErNbO4 (+YbNbO4) and YbNbO4 flat polyhedron-like precipitates in the Er/Yb:LiNbO3 and Yb:LiNbO3 crystals, respectively. On the other hand, a relatively weak VTE procedure cannot induce the precipitation of both the singly and doubly doped crystals. The characteristic optical absorptions, characteristic x-ray diffractions, crystalline structure and crystallographic morphology of the ErNbO4 and YbNbO4 precipitates are discussed, summarized and compared with those of singly Er-doped LiNbO3 crystals reported previously. The absorption characteristics of the unprecipitated Er/Yb codoped and singly Yb-doped crystals are also summarized and compared with the case of singly Er doping. An atomic force microscope has been used to further verify the SEM results and to evaluate the roughness of the surface of precipitated crystals. Optical absorption measurement was also carried out on a precipitated Er/Yb codoped crystal with polished surfaces to verify that the precipitates grow not only on the surface of the crystal but also in the bulk.