The up-conversion of near-infrared excitation radiation in Ho3+-doped LiYF4

The up-conversion processes due to the continuous wave excitation of the holmium 5I4 multiplet in LiYF4 crystal are investigated. Despite a very low absorption cross-section for the transition from the holmium 5I8 ground state to the 5I4 multiplet, a bright green up-converted emission originating from the 5S2 multiplet is observed. The excitation and excited state absorption spectra indicate that the up-conversion process is based mainly on excited state absorption from the metastable 5I7 multiplet. Some features of the up-converted green emission are characteristic for the photon avalanche process.