Upconversion luminescence in Er3+/Yb3+ codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y2CaGe4O12 doped with Er3+ and Er3+/Yb3+ reveal upconversion emission in visible spectral range under near-infrared excitation, λex = 980 nm. For the solid solution ErxY2−xCaGe4O12 concentration dependencies for the green and red lines of the visible emission around 526 nm (2H11/2 → 4I15/2), 545 nm (4S3/2 → 4I15/2) and 670 nm (4F9/2 → 4I15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y2CaGe4O12:Er3+ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y2CaGe4O12:Er3+ and Y2CaGe4O12:Er3+, Yb3+ has been proposed. Conditions for visible emission occurrence under quasi-resonance λex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition 4I13/2 → 4I15/2 in erbium ions has been detected.

Research highlights▶ Visible upconversion emission under near-infrared excitation was shown in Y2CaGe4O12:Er3+ and Y2CaGe4O12:Er3+, Yb3+ for the first time. ▶ Y2CaGe4O12:Er3+, Yb3+ samples reveal green and red emissions’ increase several times more owing to energy transfer Yb3+ → Er3+. ▶ A scheme of excitation and emission routes implying ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in erbium and ytterbium ions has been proposed. ▶ Conditions for visible emission under quasi-resonance 1064 nm excitation depending on pump power values have been considered.