Energy transfer and laser gain cross-section analysis in Nd-Yb-Gd doped CaF2 crystals

Abstract

A detailed spectroscopic characterization of CaF2 co-doped with neodymium and ytterbium ions is presented. The higher Nd3+ absorption cross-section around 791 nm than Yb3+ around 980 nm enables an efficient excitation of Yb3+ ions by pumping Nd3+ with a subsequent energy transfer (ET) from Nd3+ to Yb3+ ions. By combining Nd3+ and Yb3+, along with Gd3+ buffer ions Nd3+,Yb3+,Gd3+:CaF2 crystals exhibit very large emission bands extending the usual Yb3+ emission spectrum by adding the Nd3+ emission contribution at longer wavelengths. The Nd3+ ⟶ Yb3+ energy transfer efficiency is estimated using two different approaches based on luminescence intensity measurements and lifetimes, giving consistent results. An energy transfer efficiency of 80 % is achieved in CaF2:0.5%Nd,3%Yb,2%Gd illustrating the ET strength within Nd3+-Yb3+ clusters. Finally, a detailed study of the gain cross-sections and laser emission spectra, and their dependence with Yb3+ doping concentration and population inversion is carried out, providing a detailed model of the laser spectral dynamics in such crystals. The gain cross-section is either dominated by one of the Nd3+ peaks at 1065 nm or 1048 nm or exhibits a flat profile covering the 1045–1067 nm range, in which case the laser oscillates randomly over this spectral range. The results highlight the specifics of the Nd3+ four level laser coupled by energy transfer to the Yb3+ quasi three level laser. Among others, this study shows how a large Nd3+ inversion ratio initiates the depletion of the Yb3+2F7/2 ground state through the ET. This depletion then has two effects as it saturates the energy transfer since the number of Yb3+ acceptors diminishes and it also shifts the laser wavelength towards shorter wavelength as the Yb3+ laser reabsorption is weakened.