Highly efficient up-conversion luminescence in Er/Yb co-doped NaLuF single crystals by vertical Bridgman method.

Er/Yb co-doped NaLuF single crystals with different concentrations of Yb ions were prepared to investigate their phase structure, up-conversion (UC) properties and mechanism of UC luminescence by Bridgman method. Under 980 nm near-infrared (NIR) excitation, three sharp UC emission bands topping at green ~525 nm, ~548 nm and red ~669 nm were obtained in Er/Yb doped NaLuF single crystals which are attributing to the transitions of H → I, S → I and F → I, respectively. The quadratic dependence of pump power on UC emission indicated that two-photon process is in charge of the transition from excited state of Yb ions to lower state of Er ion in NaLuF single crystals. The long-accepted mechanism for the production of red and green emissions through up-conversion (UC) under 980 nm excitation in Er/Yb co-doped materials apply in the NaLuF host was displayed. The enhancement of the red emission was observed due to a cross-relaxation (CR) process of the form: F + I → F + F Furthermore, an ideal yellowish green light performance could be achieved with 1.0 mol% Er doped certain Yb concentrations samples, and its external quantum efficiency approached to 6.80% under 5.5 Wcm 980 nm excitation which can be applied in developing UC displays for electro-optical devices.