On the Er3+ NIR photoluminescence at 800 nm.

Here, we study the Er3+ NIR 4I9/2-4I15/2 photoluminescence peaking at 800 nm. It can be detected with a good signal-to-noise for the prepared CaWO4:Yb3+,Er3+ phosphors upon excitation at 980 nm. When directly exciting the Er3+ green and red emitting states over the 333-773 K temperature range, the 800 nm photoluminescence for the samples is undetectable. It shows that the non-radiative relaxation from the upper excited states to the 4I9/2 emitting state is extremely inefficient. Moreover, the 800 nm photoluminescence decay curve is measured at high temperatures. It is found that the 800 nm emission always has a similar lifetime with the Er3+ 4I11/2-4I15/2 transition. This reminds us that the Er3+ 4I9/2 state is mainly populated by the adjacent lower 4I11/2 state by a thermally coupled way.