Study On The Local Structure And Spectral Properties Of Nd-doped Alkaline Earth Fluoride Crystals

As one of the most important research directions in the field of laser science,ultrafast and ultrashort lasers have great application promising in the fields of laser acceleration,attosecond science,laser fusion,and high energy physics.Trivalent Nd³⁺ion is a four-level system with small laser threshold.Alkaline earth fluoride crystals have high thermal conductivity,low phonon energy,and special fluorite cubic structure.Trivalent rare earth ions can form multiple lattice structures in the crystals.After the divalent alkaline earth metal ions are replaced by the trivalent rare earth ions,there will be excess positive charge in the system,interstitial F^-ions will be generated for charge compensation.The interstitial F_i^-ions can occupy many kinds of sites,and the local environment of rare earth ions will vary with them.Codoped with Y³⁺,Gd³⁺and La³⁺ions can eliminate fluorescence quenching and enhance luminescence intensity.In the paper,the spectral properties of Nd³⁺ions doped alkaline earth fluoride crystals are systematically studied,and the effects of matrix crystals and codopant ions on the spectral parameters are investigated.First-principles calculations of the local structure of Nd³⁺ions in Ba F₂ crystals are conducted,and the internal correlation between the local structure and the laws of spectral performance would be studied.With 0.5%Nd³⁺:Ba F₂ crystal codoped with Y³⁺,Gd³⁺and La³⁺ions,the largest absorption cross section is up to 1.92?10^-20 cm²,the fluorescence intensity can be increased up to 30 times,and the spectral performance of the crystal was obviously improved.The linewidth of 11.27 nm is obtained in crystals doped 8%La³⁺ions.The linewidth of the 1053 nm band doped 8%Y³⁺ions and 8%Gd³⁺ions is narrower about20 nm.The average lifetime of Nd³⁺,Y³⁺:Ba F₂ and Nd³⁺,Gd³⁺:Ba F₂ crystals is about510?s.First-principles calculation results show that[Nd-Nd]clusters and[Gd-Gd]clusters are easily formed in Ba F₂ crystals,Y³⁺ions and La³⁺ions are not easy to form high polymer clusters.Although the kinds of lattice sites in the crystals codoped with Gd³⁺ions are rich,most of them are quenching clusters,which do not contribute to luminescence,and also cause the crystal emission linewidth to be narrow.Due to combined with Nd³⁺ions for La³⁺ions much easier,which distorts the structure,and there are various of non-quenching luminescence centers,including cubic and square antiprism centers,resulting in the wider linewidth at 1053 nm and the longer average lifetime.The kind and concentration of the regulating ions greatly affect the local environment of the Nd³⁺ion in the crystals.The matrix crystal of Ca F₂ has the small lattice parameter,and it is easy to form clusters of Nd³⁺ions.The effect of the addition of codopant ions on absorption intensity is small,and the kinds of lattice sites will increase,leading to the absorption linewidth increased.After codoped La³⁺ions in Sr F₂ crystals and Gd³⁺ions in Ba F₂ crystals,the absorption cross section of the crystals are smaller and the linewidth of these crystals are wider.Codoped Y³⁺ions in these three matrix crystals will greatly reduce the linewidth.The combination of Y³⁺ions and Nd³⁺ions forms the non-quenching luminescence centers.The type of clusters is relatively single,resulting in the narrow linewidth.The spectral parameters of the crystals vary with the kinds of the conditioning ions and the matrix crystals.By adjusting the doping ratio of Sr²⁺ions in the crystals,the effects of the ratio of Sr²⁺ions and the kinds of regulate ions on the spectral properties of the crystal are researched.The shape and value of absorption crosss section and linewidth of Nd³⁺,Y³⁺:Ca_1-xSr_xF₂（x=0～1）mixed crystals are similar to these of Nd³⁺,Y³⁺:Sr F₂ crystals,and changing the host crystal has little effect on the spectral performance.Doping Sr²⁺ions will weaken the absorption cross section of the Nd³⁺,Gd³⁺:Ca_1-xSr_xF₂ crystals and reduce the linewidth.Compared with Nd³⁺,La³⁺:Ca F₂/Sr F₂,the absorption cross section and fluorescence intensity of the mixed crystal codoped with Nd³⁺and La³⁺ions will be increased,and the linewidth will become narrower.