Ultraviolet-blue upconversion emissions of Ho^3+ ions

Ultraviolet-blue upconversion (UC) radiations of Ho3+ ions at 240, 290, 360, 385, 418, 445, 485 nm were observed in hexagonal NaYF4:Yb3+∕Ho3+ powders under diode laser excitation of 970 nm. UC mechanism analyses illustrate that successive energy transfers from Yb3+ to Ho3+ generate emissions at 240, 360, 385, and 418 nm, while cross relaxations between Ho3+ ions evoke UC emissions at 290 and 445 nm. Power dependence analyses indicate that these UC emissions already have intense saturation effects even at low-power density range of 0.3–38 W∕cm2. Theoretical calculations based on steady-state equations demonstrate the proposed UC mechanisms and explain well the observed saturation effects.