Mid-infrared Emission and Energy Transfer Properties of Sensitized Nd$^{3+}$ Ions in Low Phonon-Energy Hosts

Mid-infrared emission (4-6 $\mu $m) stemming from the first excited state of Nd$^{3+}$ can be generated in KPb$_{2}$Cl$_{5}$ by pumping its $^{4}$F$_{5/2}$ absorption band at $\sim $800 nm. It has been proposed that 4-6 $\mu $m emission of Nd$^{3+}$ could be enhanced by directly pumping the $^{4}$I$_{15/2}$ absorption band centered at $\sim $1650 nm. This pumping scheme could initiate a 3-for-1 cross relaxation, which ultimately increases the population in the first excited state of Nd$^{3+}$. Unfortunately, the $^{4}$I$_{15/2}$ absorption band of Nd$^{3+}$ is weak and diode laser pumping of this level is not practical. To more efficiently populate the $^{4}$I$_{15/2}$ level in Nd: KPb$_{2}$Cl$_{5}$, Tm sensitization of Nd$^{3+}$ via $\sim $1700 nm excitation is under exploration. Experimental results show that the Tm$\diamondsuit $Nd energy transfer was successful with energy transfer efficiencies ranging from 46{\%} - 98{\%}. The energy transfer was followed by strong 4-6 $\mu $m emission from co-doped Tm, Nd: KPb$_{2}$Cl$_{5}$ samples. Preliminary results on Tm, Nd: KPb$_{2}$Br$_{5}$ will also be discussed.