Competition between two- and three-photon upconversion in Er3+-doped microcrystals

The emission spectrum of lanthanide-rich upconverters consists of optically or thermally excited transition bands involving different pathways. We investigate multiphoton processes in Yb3+-Er3+ codoped beta - NaGdF4 to demonstrate that the emission from the phosphor involves both two- and three-photon mechanisms, where the sample temperature, and the laser excitation power play crucial roles in determining the leading pathway. The two-photon processes dominate at room temperature, and the effect of higher-order (three-photon) processes becomes increasingly important as the temperature is lowered. The Yb3+-Er3+ systems are reported to be among the best performing ratiometric luminescent thermometers in a very wide range. Our experimental findings demand a careful elimination of errors that may appear from the laser-induced heating of the sample and contributions from higher-order multiphoton processes when using such thermometers below room temperature.