Upconversion and excited-state absorption in the lower levels of Er:KPb2Cl5

Polarized excited-state absorption spectra for the 4 I 13/2 → 4 I 9/2 transition in Er:KPb 2 Cl 5 were determined from the measured 4 I 9/2 → 4 I 13/2 emission spectra using the reciprocity relation of [D.E. McCumber, Phys. Rev. 136 (1964) A954]. From these spectra, the microparameters for energy transfer were found to be C DA = 2.6 × 10 −42 cm 6 /s for ( 4 I 13/2 , 4 I 13/2 ) → ( 4 I 15/2 , 4 I 9/2 ) and C DD = 6.9 × 10 −39 cm 6 /s for ( 4 I 13/2 , 4 I 15/2 ) → ( 4 I 15/2 , 4 I 13/2 ). The macroscopic upconversion coefficient was measured by comparing fluorescence intensities from the 4 I 13/2 and 4 I 9/2 levels, and found to be C up = (2.6 ± 0.4) × 10 −20 cm 3 /s, in good agreement with the value calculated from the microparameters C DA and C DD . Energy transfer upconversion between two ions in the 4 I 13/2 level is orders of magnitude weaker than in other typical hosts, and this is predicted to lead to dramatically reduced heat loading in a resonantly pumped 1.5 μm Er laser.