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

Polarized excited-state absorption spectra for the 4I13/2 → 4I9/2 transition in Er:KPb2Cl5 were determined from the measured 4I9/2 → 4I13/2 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 CDA = 2.6 × 10−42 cm6/s for (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and CDD = 6.9 × 10−39 cm6/s for (4I13/2, 4I15/2) → (4I15/2, 4I13/2). The macroscopic upconversion coefficient was measured by comparing fluorescence intensities from the 4I13/2 and 4I9/2 levels, and found to be Cup = (2.6 ± 0.4) × 10−20 cm3/s, in good agreement with the value calculated from the microparameters CDA and CDD. Energy transfer upconversion between two ions in the 4I13/2 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.