Site selective analysis of Nd3+-Lu3+ codoped CaF2 laser crystals

Within neodymium-doped laser crystals, the codoping of fluorite-type single crystals with non-optically active buffer ions reveals the possibility for broadband laser operation around 1m, which makes these materials appealing for a number of applications such as amplifiers for new generation laser fusion facilities. In this work, a detailed spectroscopic investigation of CaF2:Nd3+ single crystals co-doped with various concentrations of Lu3+ ions is presented. The results show that the Nd3+ emission quantum efficiency increases drastically with the Lu3+ concentration, as a consequence of the replacement of Nd3+-Nd3+ clusters by two predominant types of Nd3+-Lu3+ clusters. These two main Nd3+ emitting centres are characterized in detail by identifying, for the first time to the best of our knowledge, their individual absorption and emission cross-section spectra, along with their fluorescence and radiative lifetimes and respective concentrations. The identification of the two predominant Nd3+-Lu3+ clusters ( labelled NL1 and NL2) is carried out at room temperature by carefully choosing the Lu3+ concentration, the time windows used in time-resolved spectroscopy and the excitation or emission wavelength. The determination of the different centre concentrations and respective absorption cross-sections is achieved by comparing selective timeresolved excitation spectra and absorption spectra encompassing contributions from Nd-Nd clusters and the two types of Nd3+-Lu3+ clusters. As the Lu3+ concentration increases, the NL2 centres become predominant and are characterized by an emission peak at 1054 nm and a stronger emission cross-section than that of their NL1 counterparts. Finally, the absorption cross-sections, concentrations and lifetimes are successfully used to calculate the contributions of NL1 and NL2 in the various emission spectra regardless of the Lu3+ concentration or excitation wavelength confirming the consistency of the spectroscopic description.