Upconversion fluorescence of Ho3+ ions in a BaF2 crystal

Efficient upconversion luminescence from the Ho3+ C3v centre in a BaF2 crystal is reported. The red-to-green and red-to-blue upconversion is observed upon pumping with a single cw red dye laser. The upconversion mechanism is assigned as a sequential two-photon excitation process in which the excitation wavelength is resonant with either ground state absorption (GSA) or excited state absorption transitions. The efficient green upconversion with excitation into GSA results from a double-resonant excitation between the 5I8→5F5 and 5I7→5F3 transitions.