Upconversion in the (Yb3+, Er3+) couple. The dashed lines represent energy transfer, the full lines represent the radiative decay, and the curly lines indicate multi-phonon relaxation processes. The main route is a two-step energy transfer after excitation around 980 nm in the Yb3+ ion that leads to excitation to the 4F7/2 state of the Er3+ ion. After relaxation from this state, emission is observed from the 2H11/2 level, the 4S3/2 level (green), and the 4F9/2 level (red).
van Sark et al. Nanoscale Research Letters 2013 8:81 doi:10.1186/1556-276X-8-81