Observation of Laser Cooling on an Electric‐Dipole‐Allowed Transition in Cr 3+ :LiSAF Crystal

Abstract In this work, an electric dipole allowed transition in two Cr 3+ :LiSAF samples is investigated for the purpose of optical refrigeration. A cooling efficiency approaching 10% is experimentally determined using a mode‐mismatched dual‐beam thermal lens method with excitation wavelength,λ exc ${\lambda}_{\textit{exc}}$ , of 1 µm in an unoriented 1% Cr 3+ :LiSAF, which has an average emission wavelength,⟨ λ em ⟩ $\langle{\lambda}_{\textit{em}}\rangle$ , of 850 nm. In an oriented 4% Cr 3+ :LiSAF sample, the efficiency reached (but does not exceed) the threshold for cooling when excited with π‐polarized light. These results suggest that Cr 3+ :LiSAF can approach an ideal cooling efficiency ∼( λ exc − ⟨ λ em ⟩ ) / ⟨ λ em ⟩∼ 20 % $({\lambda}_{\textit{exc}}- \langle {\lambda}_{\textit{em}} \rangle )/ \langle {\lambda}_{\textit{em}} \rangle \hspace*{0.28em}\sim \hspace*{0.28em}20\%$ with excitation ≈1µm. Additionally, this analysis indicates that parasitic background absorption rather than upconversion or excited state absorption limits the cooling at higher Cr 3+ concentrations.