| Ytterbium-doped yttrium aluminum garnet crystal having become a new hotspot forits excellent thermal, mechanical, optical, laser performance, is regarded as one of themajor booming direction of high efficiency, high power solid state lasers, which isproved to be the most promising material. Large size, high quality Yb:YAG crystalshave been needed urgently owing to be applied widely in the fields of military, civilian,material processing, communication. But the traditional growth methods hardly meetthe requirements, Yb:YAG single crystal has been fabricated by horizontal directionalsolidification (HDS) method for the first time domestically in this paper.The Ukraine UKAK-2type single crystal furnace was reconstructed via amplestudies. Through having been obtained a series of preparatory operations and the moresuitable parameters of crystal growth technology, eventually, the large-plate8at.%Yb:YAG laser crystal with the size of170mm×150mm×30mm and fine integritywas successfully achieved.Yb:YAG crystals grown at vacuum atmosphere is blue and green due to corlorcenter, changed to colorless after annealing at1400?. Structure characterizationsshowed that the two kinds of crystals had nice quality and no impurity phase.Morphology analysis after corrosion indicated surface roughness had increased, togetherwith more quantity of corrosion pits under the microscope, which resulted from therelease of internal stress after annealing, and more dislocations reaching the surfacewhere were more susceptible to be corroded. Aiming at the problem of the ambiguousformation mechanism of corlor ceter in Yb:YAG, we investigated optical performanceof crystals by annealing temperature at900?,1000?,1100?,1400?for5hoursand annealing at900?for5hour,10hours,20hours,30hours and40hours,respectively. The results demonstrated that three absorption peaks which located at thenear infrared region900nm~1050nm range, and these peaks belonged to intrinsicabsorption peaks of Yb:YAG, which always existed under different conditions withchanged strength. Two absorption peaks at385nm and645nm were induced by colorcenter, which was caused by oxygen vacancies and transformations from Yb3+to Yb2+.After appropriately annealing, original crystals became transparent combined that colorcenters eliminated and two absorption peaks disappeared. There existed twofluorescence emission peaks near485nm and1029nm with weak intensity. In general,samples annealed at1100?exhibited maximum intensity. The fluorescence lifetimewas directly proportional to the temperature and time of annealing, as well as themaximum was annealed at900?for40hours. All the results above were corresponding with electron paramagnetic resonance spectra respectively.First principle was used for simulation and analysis of experimental phenomena innature. Models were Yb:YAG primitive cell of the doping concentration and one oxygenvacancy. Comparatively analyzing the changs of lattice constant, band structure, densityof States, optical performance, we confirmed that the oxygen vacancy brought about thedecrease of band gap and the movements of most electronic states in atomic orbitals.Combined with absorption spectroscopy experiments, we figured out that the absorptionpeaks of385nm were related to oxygen vacancy and the absorption peaks at645nmwere related to the transformation of Yb3+.50at.%-doped Yb:YAG cell was built inorder to provide theoretical guide for subsequent experiments. The results revealed thatnot only did band gap decrease sharply and did electronic states alter, but newabsorption bands and concentration quenching might also appear. |
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