“…Energy transfer from Yb 3+ ions to Er 3+ ions enhances the efficiency of the upconversion emission drastically [20,21]. Although fluoride [22] and tellurite [23] with low phonon energy attracted more attention, compared to them, alumina as a host is of better thermal stability, durability and anti-chemical-corrosion [24]. The alumina would serve in worse environment, and at the same time, it was the main raw materials to prepare the garnet compositions such as YAG and Yb 3 Al 5 O 12 (YbAG) [25,26] which possess excellent optical property, so the alumina would be a nice host material for upconversion.…”