Investigation of allowed and forbidden electronic transitions in rare earth doped materials for laser cooling application by thermal lens spectroscopy

In this work Thermal Lens Spectroscopy (TLS) is used for sensitive characterization of absorption in optical materials with particular application for solid-state laser cooling (optical refrigeration). TLS is used to characterize key parameters involved in 4f-4f transition in Yb3+ :YLF and 4f-5d transition in Ce3+:YAG systems. The results confirm the excellent laser cooling results found in previous works on Yb-3+( ):YLF crystal. In addition, although no cooling effect was observed in Ce3+:YAG sample, the TLS showed that this material exhibits promising characteristics towards laser cooling applications, such as high external quantum efficiency ( > 92%), and low parasitic background absorption of (1.2 +/- 0.2) x 10(-4) cm(-1) at 885 nm. The key mechanisms that appear to inhibit net cooling in Ce3+:YAG are attributed to the intrinsic excited state absorption and/or 4f-4f transitions between the two lowest levels of Ce3+. This investigation was turned possible due to the high sensitivity of TLS for very low absorptions.