A mid-IR phosphor using thulium-praseodymium-doped potassium lead chloride

Efficient energy transfer at room temperature from Tm$^{3+}$ to Pr$^{3+}$ has been demonstrated in co-doped KPb$_{2}$Cl$_{5}$. The energy transfer processes result in the conversion of 805 nm laser diode pump energy to a broad band of mid-IR radiation between 3500 to 5500 nm. Spectroscopic measurements show that the material is suitable as a phosphor for the 4 to 5 micron spectral range that can be optically pumped with low-cost 0.8-micron laser diodes. The mid-IR emission is enabled by the low phonon energies in KPb$_{2}$Cl$_{5}$.$_{ }$Energy transfer pathways, rates, and quantum efficiencies are evaluated.