Highly efficient up-conversion luminescence in Er(3+)/Yb(3+) co-doped Na(5)Lu(9)F(32) single crystals by vertical Bridgman method

Er(3+)/Yb(3+) co-doped Na(5)Lu(9)F(32) single crystals with different concentrations of Yb(3+) 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(3+)/Yb(3+) doped Na(5)Lu(9)F(32) single crystals which are attributing to the transitions of (2)H(11/2) → (4)I(15/2), (4)S(3/2) → (4)I(15/2) and (4)F(9/2) → (4)I(15/2), 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(3+) ions to lower state of Er(3+) ion in Na(5)Lu(9)F(32) single crystals. The long-accepted mechanism for the production of red and green emissions through up-conversion (UC) under 980 nm excitation in Er(3+)/Yb(3+) co-doped materials apply in the Na(5)Lu(9)F(32) host was displayed. The enhancement of the red emission was observed due to a cross-relaxation (CR) process of the form: (4)F(7/2) + (4)I(11/2) → (4)F(9/2) + (4)F(9/2.) Furthermore, an ideal yellowish green light performance could be achieved with 1.0 mol% Er(3+) doped certain Yb(3+) concentrations samples, and its external quantum efficiency approached to 6.80% under 5.5 Wcm(−2) 980 nm excitation which can be applied in developing UC displays for electro-optical devices.