Optical spectroscopy of holmium doped K2LaCl5

Spectroscopic results on the infrared emission properties of trivalent holmium (Ho3+) doped potassium lanthanum chloride (K2LaCl5) are presented. Using ~900 nm excitation, Ho3+ doped K2LaCl5, which has a maximum phonon energy of 235 cm−1, exhibited the infrared emissions at ~1660, ~1995, and ~3900 nm at room temperature. The mid-infrared emission at 3900 nm originated from the 5I5 → 5I6 transition of Ho3+ ions with an exponential decay time of ~7.8 ms at room temperature. The fluorescence lifetimes were almost independent of temperature, indicating a negligibly small non-radiative decay rate for the 5I5 excited state, as predicted by the energy gap law for low phonon energy hosts. The Stark level energies of the lower 5IJ manifolds in Ho:K2LaCl5 were resolved from spectroscopic results performed at cryogenic temperatures. The transition line-strengths, radiative lifetimes, and fluorescence branching ratios were investigated by using the Judd-Ofelt method. In addition, the peak emission cross-section of the 5I5 → 5I6 transition was calculated to be 1.8 × 10-20 cm2 at ~3890 nm.