Efficient upconversion luminescence in Er3+/Yb3+ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux

• Er3+/Yb3+ co-doped α-NaYF4 single crystal was grown by Bridgman method using KF as flux.
• Enhanced up-conversion green and red lights can be obtained.
• The mechanism of energy transfers from Yb3+ to Er3+ were analyzed.

Er3+/Yb3+ co-doped α-NaYF4 single crystals with various Yb3+ concentrations and ∼0.50 mol% Er3+ were grown by Bridgman method using Potassium fluoride as flux. The up-conversion (UC) luminescence and its pump power dependence on UC emission of the crystals under excitation of 980 nm laser diode were investigated. The UC emissions at green ∼522 nm, ∼541 nm and red ∼668 nm, which are attributed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively, were obtained in Er3+/Yb3+ doped α-NaYF4 single crystals. The quadratic dependence of pump power on the UC emission suggests that a two-photon process is responsible for the transition from the excited Yb3+ to the lower state of Er3+ ions in α-NaYF4 single crystals. A favorable performance of yellowish green light can be achieved with certain Yb3+ concentrations and 0.52 mol% Er3+ co-doped samples that makes further research valuable in developing UC displays for electro-optical devices.