Concentration-dependent upconversion emission in Er-doped and Er/Yb-codoped LiTaO3 polycrystals

Er-doped and Er/Yb-codoped LiTaO3 polycrystals were synthesized by the solid-state reaction method at 1200 °C. The structural properties of Er/Yb:LiTaO3 polycrystals were confirmed by the X-ray diffraction patterns. The upconversion emission spectra of Er:LiTaO3 polycrystals showed that the intensities of the green and red upconversion emissions enhanced with the increasing concentration of Er3+ ions. The increased intensity ratio of the red to green emission was attributed to the two cross relaxation processes 4S3/2+4I15/2→4I9/2+4I13/2 and 2H11/2+4I11/2→4F9/2+4F9/2. Upconversion emission tuning from green to red was observed in Er/Yb:LiTaO3 polycrystals under 980 nm excitation. It was proposed that the efficient cross relaxation process (4F7/2+4I11/2→4F9/2+4F9/2) and the energy back transfer (4S3/2 Er+2F7/2 Yb→4I13/2 Er+2F5/2 Yb) contribute the increased intensity ratio of the red to green emission in Er/Yb:LiTaO3 polycrystals. Studies on the pump power dependence indicated that the green emitting 4S3/2/2H11/2 states and the red emitting 4F9/2 state were populated via the energy transfer assisted UC process besides a two-photon process in Er/Yb:LiTaO3 polycrystals.