On the Er^{3 +} 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.