Effect of Ce co-doping on fluorescence dynamics of Er:NaLa(Wo4)2 crystals

Er³⁺ ion, due to its energy level structure, is a very attractive activator for several laser applications. The main transitions are the ⁴S_3/2rarr⁴I_5/2 around 550 nm, the ⁴I_11/2rarr⁴I_13/2 around 3 mum and the ⁴I_13/2rarr⁴I_15/2 around 1.5 mum. One of the limiting factor in the use of Er³⁺ as active ion for the development of efficient 1.5 mum laser sources is the upconversion processes between Er ions (⁴I_13/2,⁴I_13/2)rarr(⁴I_15/2,⁴I_9/2), that depopulates the ⁴I_13/2 level. To improve the efficiency of Er³⁺ emission at 1.5 mum it is very important, to enhance the population inversion between the ⁴I_13/2 and the ⁴I_15/2 levels. A possible solution consists in the introduction of another rare earth ion that increases the population of Er³⁺ upper laser level through energy transfer mechanisms. For this reason we studied the emission properties of NaLa(WO₄)₂ crystals (NLW) in case of Er-Ce co-doping. The first effect consists in the increasing of the ⁴I_11/2rarr⁴I_13/2 branching ratio through the cross relaxation: ⁴I_11/2 (Er), ²F_5/2 (Ce) rarr ⁴I_13/2 (Er), ²F_7/2 (Ce).