Crystal Structure And Optical Properties Of Holmium Doped Barium Yttrium Fluoride

As a new type of near and mid infrared laser crystal material,Ho:BaY2F8(Ho:BYF)crystal has the advantages of long fluorescence lifetime,low threshold and low thermal effect.The energy level of Ho3+ in the crystal transition from 5I7→5I8,and the 2 μm band laser can be obtained.The laser has the advantages of eye safety,good atmospheric transmission characteristics and high confidentiality,and is widely used in many fields such as military,medical and environmental monitoring.As an ideal pump source for 3-5 μm mid infrared laser output,the near-infrared laser has become one of the research hotspots at home and abroad.However,in the growth process of Ho:BYF crystal,macro defects such as crystal cracking,growth stripes and micro defects such as vacancy structure and gap structure are easy to appear,which seriously affect the growth quality and optical properties of the crystal.The first principles method based on density functional theory can be used to study the microstructure of thecrystal,so as to clarify the inherent physical laws of the whole system.The main contents of this paper are as follows:(1)The construction of ideal BYF crystal model and ideal Ho:BYF crystal model with different doping concentration.An ideal Ho:BYF crystal model is constructed,and its electronic and optical properties are calculated by using different hybrid functional methods.Compared with the existing theoretical results,it is determined that HSE hybrid functional will be used to calculate the crystal properties;the ideal Ho:BYF crystal models of 16.33 mol%and 30.36 mol%are obtained by substituting the Y atom with Ho atom.(2)The influence of Ho3+on the optical properties of Ho:BYF crystal is calculated and analyzed.By calculating the optical properties of Ho:BYF crystal,it is found that the new absorption peak in the optical absorption spectrum of Ho:BYF crystal is caused by the common transition from 2p occupied orbit of F atom to 4f unoccupied orbit of Ho atom and from 4f occupied orbit of Ho atom to 4f unoccupied orbit of Ho atom.Due to the influence of crystal field,the parity forbidden condition of Ho3+is broken,and the f→f orbital transition is realized.It is theoretically explained that Ho:BYF crystal may realize the luminescence with wavelength of 2 μm.(3)The influence of intrinsic defects of Ho:BYF crystal on the crystal structure and optical properties is calculated and analyzed.All possible defect structures in Ho:BYF crystal are modeled and the formation energies of these structures are compared.The results show that the main defect structures in Ho:BYF crystal are F vacancy and F/Ba/Y gap defects;Through the first principles calculation of the geometry,electronic and optical properties of the above structure,it is found that the existence of defect structure does not cause serious lattice distortion,but the density of states of the crystal changes obviously;and there is obvious 2 μm absorption peak in the optical absorption spectrum of F vacancy defects.The results show that the luminescence of Ho:BYF crystal is the result of the interaction of Ho3+ and F vacancy defect structure.From the microscopic point of view,based on the first principles of density functional theory,this paper reasonably constructs the ideal Ho:BYF crystal and the crystal structure model with intrinsic defects for simulation calculation,which provides theoretical support for the preparation of high-quality crystals,the improvement of crystal growth,the optimization of matrix materials,the improvement of Ho:BYF crystal performance and the promotion of the application of the laser crystal.