Abstract
The up-conversion processes due to the continuous wave excitation of the holmium 5 I 4 multiplet in LiYF 4 crystal are investigated. Despite a very low absorption cross-section for the transition from the holmium 5 I 8 ground state to the 5 I 4 multiplet, a bright green up-converted emission originating from the 5 S 2 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 5 I 7 multiplet. Some features of the up-converted green emission are characteristic for the photon avalanche process.
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