Upconverted emission spectra under low and high excitation density. For the low excitation power, the green state was not yet saturated. The intensities may be compared. New peaks (italic) are assigned: 2H9/2 → 4I15/2 transition at 410 nm, 4I9/2 → 4I15/2 transition at 815 nm, and the intermediate transition 2H9/2 → 4I13/2 at 560 nm.
van Sark et al. Nanoscale Research Letters 2013 8:81 doi:10.1186/1556-276X-8-81