Concentration dependence of the fluorescence decay profile in transition metal doped chalcogenide glass

In this paper we present the fluorescence decay profiles of vanadium and titanium doped gallium lanthanum sulphide (GLS) glass at various doping concentrations between 0.01 and 1% (molar). We demonstrate that below a critical doping concentration the fluorescence decay profile can be fitted with the stretched exponential function: exp[-(t/τ)\b{eta}], where τ is the fluorescence lifetime and \b{eta} is the stretch factor. At low concentrations the lifetime for vanadium and titanium doped GLS was 30 μs and 67 μs respectively. We validate the use of the stretched exponential model and discuss the possible microscopic phenomenon it arises from. We also demonstrate that above a critical doping concentration of around 0.1% (molar) the fluorescence decay profile can be fitted with the double exponential function: a*exp-(t/τ1)+ b*exp-(t/τ2), where τ1and τ2 are characteristic fast and slow components of the fluorescence decay profile, for vanadium the fast and slow components are 5 μs and 30 μs respectively and for titanium they are 15 μs and 67 μs respectively. We also show that the fluorescence lifetime of vanadium and titanium at low concentrations in the oxide rich host gallium lanthanum oxy-sulphide (GLSO) is 43 μs and 97 μs respectively, which is longer than that in GLS. From this we deduce that vanadium and titanium fluorescing ions preferentially substitute into high efficiency oxide sites until at a critical concentration they become saturated and low efficiency sulphide sites start to be filled.