Spectral Properties And Local Structure Study Of Nd-doped SrF₂ Laser Crystals

Ultrafast lasers have been envisioned in an increasing number of envisaged applications,such as extremely high energy triggered nuclear fusion,laser accelerated particles as light sources,and elucidating fast-evolving processes of chemical reactions in materials.As one of the most important factors,laser gains with broad amplification linewidths are desirable for generation of ultrafast lasers.Due to the charge compensation effect,various clusters were formed in Nd-doped fluorite-structure crystals,which contribute to the very broad emission.Besides,trivalent Nd ion is a true four-level system,and the fluorite crystals with high thermal conductivity and low nonlinear refractive index could be grown in large size.These advantages caused the Nd-doped fluorites to be a very promising candidate for generation of ultrafast lasers.However,the energy transfer process among Nd ions made the emission at 1?m deadly quenched.By codoped with Y,La and Gd ions,the quenching could be eliminated.However,what really happened to the local structures of Nd when incorporated with Y,La and Gd was still unclear.In the work,spectral properties and local structures of Nd ions in Nd,Y:SrF₂,Nd,Gd:SrF₂,Nd,La:SrF₂crystals were studied.The clusters were simulated,and the relations with spectral properties were investigated.The spectral properties of 0.5%Nd:SrF₂ crystal were highly improved when codoped with Y,La and Gd.The peak emission cross section,fluorescence lifetime and absolute quantum yields were increased exponentially with R/Nd ratios,and the parameters were enhanced to be 5.22×10^-2020 cm²,484.5?s and 92.3%,respectively.Simultaneously,the bandwidth of the crystals decreased exponentially to be 20 nm.For Nd,Gd:SrF₂ and Nd,Y:SrF₂ crystals,the photoluminescence parameters were manipulated to be varied following nearly the same pattern with R/Nd ratios.It is different with that of Nd,La:SrF₂ crystal.The low temperature TRES results show that a multisite crystal was changed to be quasi single center system.The first principles simulation results show,that Y ions prefer to agglomerate with Nd to be[Nd-Y]clusters.The formation energy of the aggregate decreased linearly with more Y ions within a cluster,and the first shell of Nd with cubic sublattice was changed to be lower symmetric square antiprism structures.The local environment of Nd was manipulated to be more distorted which contribute to the highly enhanced spectral properties in Nd,Y:SrF₂ crystals.The thermodynamic stable Gd and La centers were also computed,and compared with that of Nd and Y aggregates.The clustering characters of Gd and Y are nearly the same.Due to the strong repulsion of F_i^--F_latticeattice interaction in Gd:SrF₂ and Y:SrF₂ crystals,the cubic sublattice was transformed to be the square antiprism structures.While for La and Nd ions,the La-F_i^-attraction is stronger than that of F_i^--F_lattice,it is difficult to drive the transformation unless with higher doping concentrations.There are large amount of cubic sublattice clusters,as well as the square antiprism lattice aggregates in Nd,La:SrF₂ crystal,and the relatively weak intensity and broad emission was obtained.The Y,La and Gd ions were therefore worked as local structure regulators.By finely manipulating the local structures of the active ions,the spectral and laser properties of rare-earth-doped fluorite crystals could be designed artificially.The spectral properties of 0.5%Nd:SrF₂ were improved when codoped with Y,La and Gd regulators,and the high efficient laser output was also demonstrated in the work.The highest slope efficiency of 50.6%and output power of 1.234 W of continuous-wave laser were achieved in Nd,R:SrF₂ crystal.At the same time,the mode-locked pulses of 97 fs were performed.These results indicate that Nd-doped fluorites would be a very promising candidate for ultrafast lasers.