[The upconversion mechanisms for the red and green emissions in Er3+ doped and Er3+ -Yb3+ codopoed fluoride materials...

The rare earth Er3+ doped and Er3+ -Yb3+ co-doped fluoride samples were prepared. Under 980 nm laser excitation, the intense red (650 nm) and green (545 nm) upconversion luminescence was obtained corresponding to the transitions 4S3/2, 2H11/2--4I15/2 and 4F9/2--4I15/2, respectively. For the Er3+ -Yb3+ co-doped fluoride samples, it was found that the ratio of the red upconversion luminescence to the green one decreases as the Er3+ concentration increases. The dependence of upconversion intensities on excitation power indicates that two-photon absorption processes are responsible for the upconverted luminescence. The possible upconversion mechanisms of the red and green emissions are discussed based on the energy-matching conditions and the quadratic dependence on excitation power.