Bistable upconversion emission in Yb-sensitized Tm:ZrO2 nanophosphors at room temperature

We report the strong bistability effect of multiphoton upconversion, observed at room temperature, in Yb3+-sensitized Tm3+:ZrO2 condensed-phase nanophosphors via the efficient upconversion excitation. The sizable bistability of upconversion from a collection of excited Tm3+ ions exhibit good controllability in polarity only by varying the density of Yb3+ sensitizer. The polarity of bistability converts regularly from clockwise into anti-clockwise cycles as the dopant density grows from 2% to 10% Yb3+. The results suggest a practicable way to control bistable emission of solids at room temperature.