Spectroscopic Properties Of Ho³⁺ Doped And Ho³⁺/Pr³⁺ Co-doped Y₂O₃ Transparent Ceramics

The 3μm Mid-infrared lasers are widely used in gas detection,laser medicine,materials manufacturing,military,etc,with important application context and great scope for demand,and have become a hot research direction of research and development for national defense and civilians.Holmium ion(Ho³⁺)doped luminescent materials have received increasing attention because of their potential to achieve infrared luminescence near 2.85μm.However,the luminescence of Ho³⁺-doped luminescent materials near 2.85μm is produced by a radiative transition between the⁵I₆→⁵I₇ energy levels.However,the energy levels are in a"self-terminating"state in the normal state,with a short lifetime for the upper laser energy level ⁵I₆ and a long lifetime for the lower laser energy level ⁵I₇,resulting in the bottleneck type of population in the upper energy level with little and the lower energy level with a large number.These characteristics make the electrons in the ⁵I₆ energy level relax to the ⁵I₇ energy level mainly by transferring energy to the lattice phonons through the non-radiation transition,with the chance of radiative leap being very small.The key to achieving 2.85μm mid-infrared luminescence is to open the self-closing state between the ⁵I₆ and ⁵I₇ levels to make the non-radiative transition into a radiative transition,which requires low phonon energy of the matrix material.so that the upper energy level ⁵I₆ of the laser gets a longer lifetime,allowing more electrons to gather on it and better particle number inversion between the upper and lower energy levels ⁵I₆ and ⁵I₇ of the laser.The sesquioxide usually has relatively low phonon energy and serves as a potential matrix material for rare earth ions.Among the three half-oxides（Sc₂O₃、Y₂O₃ and Lu₂O₃）,Y₂O₃ has the lowest phonon energy,generally 597 cm^-1,while Y₂O₃ used in this paper has lower phonon energy of 563 cm^-1.Therefore,it is of certain significance and value to study the spectral properties of Ho³⁺in low phonon energy Y₂O₃.In addition,we select praseodymium ions(Pr³⁺)that can be coupled with Ho³⁺levels to reduce the lifetime of the lower laser level ⁵I₇ and promote the inversion of particle numbers at ⁵I₆ and ⁵I₇levels,so as to reduce the pump threshold of the gain material to improve the laser efficiency.Based on the above considerations,the spectral properties of Ho:Y₂O₃ and Ho/Pr:Y₂O₃ transparent ceramics with low phonon energy have been systematically studied in this research.The following achievements and progress have been made:1.Cooperate to prepare transparent ceramic samples with different concentrations of Ho:Y₂O₃ by vacuum sintering and hot isostatic pressing.The maximum phonon energy is only 563 cm^-1 by the FTIR test,with lower maximum phonon energy than normal Y₂O₃ transparent ceramics and lower phonon energy than other common oxide transparent ceramics,which is more conducive to the improvement of the energy level5I6 lifetime on the laser.By the XRD test shows that Ho³⁺will not change the structure of Y₂O₃,which ensures the excellent performance of ceramics and the efficient transition emission of Ho³⁺.2.Our Y₂O₃ transparent ceramics were found to achieve fluorescence at 2850 nm when doped with Ho³⁺ions alone at sufficiently low phonon energies for pumping at640 nm instead of 1100 nm.Fluorescence lifetime tests show that the upper energy level of the Ho³⁺laser has a lifetime of 0.90 ms,which is a significant increase compared to common Ho:YAP crystals（0.39 ms,subsection 3.3.7）due to the low phonon energy,while the lower energy level has little increase in lifetime,suggesting that the ⁵I₆ energy level of Ho³⁺can gather more electrons,facilitating particle number reversal between the upper and lower energy levels of the laser.This suggests that the ⁵I₆ energy level of Ho³⁺can concentrate more electrons,which is conducive to particle number reversal between the upper and lower energy levels and achieving～2.85μm laser output.3.According to the Judd-Ofelt（J-O）theory,the research models the spectral parameters of Ho³⁺transition process in the sample,such as vibrator intensity,J-O parameter,spontaneous radiation transition probability,radiation lifetime,fluorescence branching ratio and so on.The calculated J-O parameters areΩ₂=2.38×10^-20 cm²,Ω₄=1.92×10^-20 cm²,Ω₆=1.04×10^-20 cm².Compared with general matrix materials with a smallerΩ₂ and a larger mass factor,it is proved that Ho³⁺has a higher ionic property in Ho:Y₂O₃,which makes the inversion of particle number easier to realize,Therefore,it proves that our Ho:Y₂O₃ transparent ceramic has the possibility of realizing～2.85μm laser output.4.By testing and comparing the spectral properties of Ho³⁺/Pr³⁺co-doped Y₂O₃samples and Ho³⁺single-doped samples,it is found that the fluorescence of the Ho³⁺/Pr³⁺co-doped Y₂O₃ sample at 2094 nm was very weak compared to the Ho³⁺single-doped Y₂O₃ sample and the lifetime of the energy level under the laser decreased significantly from 8.65 ms for the Ho³⁺single-doped Y₂O₃ to 0.67 ms for the Ho³⁺/Pr³⁺co-doped Y₂O₃,the lifetime of the upper laser energy level did not change significantly to 0.72 ms,indicating that（1）the energy transfer between Ho³⁺:⁵I₇→Pr³⁺:³F₂ with a transfer efficiency of 92.26%（Eq.5-1 in the text）was generated between Ho³⁺and Pr³⁺;（2）the Ho³⁺/Pr³⁺co-doped Y₂O₃（2）the number of ⁵I₇ energy levels in Ho³⁺is greatly reduced,and the bottleneck electron distribution between the upper and lower energy levels of the laser is significantly improved or even disappears;（3）the upper energy level lifetime of the laser is slightly longer than the lower energy level lifetime,and the particle number reversal between the upper and lower energy levels is achieved.All this suggests that the doping of rare earth ions Pr³⁺effectively increases the potential of Ho/Pr:Y₂O₃ transparent ceramics to achieve～2.85μm mid-infrared laser output.