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

Abstract

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 1 μm, 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 time-resolved 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.