Study On Defects And Mechanism Of Yb:YAG Crystal

Yb:YAG crystal is a favorable material for high-power solid-state laser output under laser diode pumping conditions due to its excellent optical,thermodynamic,mechanical properties and stable physical and chemical properties.It has broad application prospects in industrial and scientific research fields.Restricted by growth conditions and other factors,the Yb:YAG crystal obtained from the melt will inevitably have defects inside.Defects can scatter,reflect or absorb the magneto-optical-acousticelectric energy of the crystal itself or outside in a variety of ways,such as scattering,reflecting or absorbing,which has a serious impact on the practical application performance of the crystal.Therefore,the study of Yb:YAG crystal defects and mechanism can provide guidance for optimizing the growth process,improving the growth environment,and ultimately reducing or eliminating crystal defects to obtain large-size,high-quality Yb:YAG laser crystals.In Yb:YAG crystals,there are usually many defects such as coloring centers,scattering particles,dislocations and so on.This thesis mainly carried out the following work:Color center defects in Yb:YAG crystals were studied.In order to solve the problem of color centers in Yb:YAG crystal under pure nitrogen atmosphere,the crystals were annealed in high temperature air,and the basic properties were analyzed before and after annealing to investigate the mechanism of color center formation.It is found that the color center is an electronic defect caused by the defect trapping electrons in the YAG matrix.The lack of oxygen in the growth atmosphere greatly promotes the formation of oxygen ion vacancies,which can capture electrons to form F center.The electron paramagnetic resonance spectra of the samples before and after annealing are quite different,indicating that the oxygen vacancy can be effectively reflected.The absorption spectra of the crystals between 300 nm and 800 nm before and after annealing combined with the Raman spectra at 545 cm-1 and 1100 cm-1 indicate that the crystal does not form a Re-F center,which further confirms and verifies the formation mechanism of the color center defects of the crystal samples.The scattering particles in Yb:YAG crystals were studied.The Yb:YAG crystal with or without scattering particles under the Czochralski method was used as a comparison sample.The origin of the Yb:YAG crystal scattering particles was studied by positron annihilation technique.It was speculated that the crystal scattering particles were caused by vacancy groups or nanopores,and the main defects of the Yb:YAG crystal were the intrinsic defects in the YAG structure.The experimental results show that the technique can sensitively characterize the YAG crystal scattering particles.In addition,the quality of Yb:YAG crystal was discussed by X-ray diffraction,single crystal rocking curve experiment,double optical path transmittance measurement experiment and dislocation experiment.The difference is consistent with the positron experiment results,which has certain guiding significance for how to control the defect structure of Yb:YAG laser crystal and improve the crystal growth process.The dislocation defects in Yb:YAG crystals were studied.The diversity of etching morphology of different crystal planes of Yb:YAG crystal under chemical etching method shows the anisotropy of YAG single crystal,and then explains it theoretically by symmetry.Combined with the YAG crystal cell structure and the atomic arrangement configuration of YAG crystal planes,it is discussed and analyzed that the anisotropy is due to the different symmetry of YAG crystal planes and the different stability of surface atoms.The symmetry shown by the dislocation etch pits is also consistent with the projection symmetry of the space group,and the regular shape of the dislocation etch pits is closely related to the anisotropy of the crystal.