Spectroscopic Characterization of Thulium doped Potassium Lead Chloride for Potential Applications in Optical Cooling

Rare-earth doped solids have experienced increased attention for possible applications in anti-Stokes fluorescence cooling. Solid-state optical refrigeration offer several advantages over current bulky mechanical coolers including compact, lightweight, and vibration free. Most efforts have focused on optical cooling in Yb$^{3+}$ doped solids and cooling down to $\sim$155 K has been demonstrated. In this work, the optical properties of Tm$^{3+}$ doped KPC were evaluated as a potential solid-state material for laser cooling applications. Following 1907 nm excitation, Tm:KPC exhibited infrared emission with a center wavelength of 1806 nm arising from the $^{3}$F$_{4}\to $ $^{3}$H$_{6}$ transition of Tm$^{3+}$ ions. Under 1907nm pumping conditions, it was estimated that a quantum emission efficiency of at least 95{\%} is required to achieve a net cooling effect in Tm:KPC. Based on temperature dependent decay time studies the emission quantum efficiency of Tm:KPC was estimated to be only $\sim$75{\%}. Employing the energy-gap law, non-radiative decay through multi-phonon relaxation is predicted to be negligibly small in Tm:KPC. Concentration quenching effects and/or energy transfer processes to other defects seems most likely to be responsible for the low quantum efficiency.