Mechanisms of Upconversion Luminescence in BaF2–HoF3 Crystals under Excitation to the 5I5 Level of Ho3+ Ions

Abstract— We report a study of upconversion luminescence in BaF2–HoF3 crystals in the visible spectral region under excitation to the 5I5 level of Ho3+ ions by laser radiation with a wavelength of 890 nm. For these excitation conditions, we propose mechanisms responsible for upconversion luminescence from the 5F3, 5S2(5F4) and 5F5 levels of Ho3+ ions.