Mechanistic investigation of upconversion luminescence in Er3+-doped BaCl2, BaF2 and NaYF4 phosphors

Near infrared to visible up-conversion luminescence in Er3+ single doped BaCl2, BaF2 and NaYF4 phosphors were compared. Intense blue emission at 410 nm due to 2H9/2 → 4I15/2 transition and greenish-blue emission at 496 nm due to 4F7/2 → 4I15/2 transition were only observed in BaCl2:Er3+ when excited by 808 nm and 976 nm laser diodes, respectively. The phenomena can be ascribed to the direct two-step excitation processes which occur in the chlorides where multiphonon relaxation is rapidly suppressed. In addition, the intensity of upconversion emission in BaCl2:Er3+ is greatly enhanced upon broadband near-infrared excitation.