Infrared Absorption and Laser Spectroscopy of Ho$^{3+}$ Doped K$_2$YF$_5$ Microparticles

Abstract

High-resolution absorption and laser spectroscopy are used to determine electronic energy levels for Ho$^{3+}$ ions in K$_2$YF$_5$ microparticles. A total of 72 crystal-field energy levels, distributed among 8 multiplets, are assigned. This optical data is used for crystal-field modelling of the electronic structure of Ho$^{3+}$ in K$_2$YF$_5$. Partially-resolved hyperfine splittings are accurately reproduced by the model. The temperature dependence of the fluorescent lifetime of the $^5$F$_5$ multiplet is measured and the temperature dependence of the non-radiative relaxation is modelled by a five-phonon process. Preliminary measurements of infra-red to visible upconversion in microparticles co-doped with Ho$^{3+}$ and Yb$^{3+}$ is reported.