First-principles Study Of ?-TeO₂ And Er:Lu₂O₃ Materials In Laser Application

In recent years,the mid-infrared laser has shown strong application potential in the fields of atmospheric research,environmental science,communications,biomedical,and military.In the current technical solutions for achieving laser output in the mid-infrared region,acousto-optic Q-switched solid-state lasers based on rare earth Er³⁺doped Lu₂O₃ transparent ceramic gain medium materials have attracted much attention.As the overall performance improvement of the laser is dependent on the explore of the dielectric material in addition to the improved design,the study of the acousto-optic dielectric material?-TeO₂ and the laser gain medium Er:Lu₂O₃ is of great significance for the improvement of the laser system.Based on the first-principles method under the framework of density functional theory,the mechanical properties,electronic structure and optical properties of?-TeO₂ at isostatic pressure 0¹0 GPa and the crystal structure,electronic structure and optical properties of c-Lu₂O₃ under intrinsic and Er³⁺-doped conditions have been systematically studied.A set of E-V data was obtained by using the CASTEP module to optimize the fixed volume geometry of?-TeO₂,and the E-V curve was fitted by the third-order Birch Murnaghan-EOS equation.The equilibrium lattice constant of?-TeO₂ under two exchange-correlation functionalities of PBE-GGA and LDA was obtained.The calculated elastic constant C_ijj indicates that?-TeO₂ cannot maintain its mechanical stability when the hydrostatic pressure is greater than 5.1 GPa,and a slight structural phase transition occurs.The ratio of bulk modulus to shear modulus,B/G,indicates that the material changes from brittleness to toughness as pressure increases.From the elastic anisotropy index A^U,the three-dimensional Young's modulus and its two-dimensional projection,the mechanical anisotropy of?-TeO₂also increases.The results of the study on the electronic structure and optical properties of?-TeO₂ at 0¹0 GPa show that the band gap of?-TeO₂ increases with the increase of pressure,and the distribution and transfer of electrons in the system change.The orbits that do not hybridize under zero pressure hybridize under pressure,and the bond strength of the Te-O bond increases.The absorption coefficient,reflectivity and EELS of?-TeO₂ at 0¹0 GPa indicate that?-TeO₂ has obvious optical anisotropy in the x and z directions and strong light absorption and reflection in the ultraviolet region.As the pressure increases,the peaks of the absorption peak and the reflection peak increase.The peak of EELS decreases in the x direction and the z direction increases.The absorption peak,the reflection peak and the EELS peak all show a blue shift trend.The effects of different Hubbard U values on the crystal structure and electronic structure of c-Lu₂O₃ were tested using the VASP software package.A set of E-V data of c-Lu₂O₃ under different methods of PBE and PBE+U were fitted by the third-order Birch Murnaghan-EOS equation,and the corresponding equilibrium crystal structure parameters were obtained.The calculation results of the electronic structure of the system show that:Hubbard U has a great influence on the band gap and lattice constant of c-Lu₂O₃ system.The energy band gap E_g=3.957 eV is calculated by PBE+U_eff(U_eff=4.4 eV).The CBM part is mainly derived from the contribution of the 5d orbital of Lu atom,the contribution of Lu 4f and O 2p is small,and the part of VBM is mainly derived from the contribution of O 2p orbital.The analysis of the absorption coefficient and the reflectance spectrum shows that the absorption coefficient of Lu₂O₃ reaches a maximum value of 2.24×10⁵ cm^-11 at 30.6 eV,and the reflectance is maintained at a low level in the mid-infrared and visible-light?0³ eV?range,starting to increase significantly after 4 eV and reaching a maximum of 49%at 10.1 eV.The absorption of Lu₂O₃ host material is mainly in the ultraviolet region.There are transparent windows in the commonly used LD pump emission wavelengths?808 nm,815 nm and 980nm,etc.?,which can effectively improve pumping efficiency and laser efficiency.Lu₂O₃ is a promising host material for laser gain media.The crystal structure,electronic structure and optical properties of different concentrations of Er-doped Lu₂O₃ were analyzed by VASP software package.Two substitution doping configurations of C2 and C3i were established,respectively.The calculation results on the formation energy of the doping system indicate that Er³⁺preferentially substitute the Lu atom in the C3i site under the Lu-rich and O-rich conditions,and this forms the most stable structure.The bond length of the Lu_2-xEr_x O₃?0<x?0.09375?system increases slightly due to the difference of Lu³⁺and Er³⁺ionic radius.The calculation results of the electronic properties of the doping system show that the Er 4f electrons form impurity energy levels in the forbidden band,and the band gap value decreases with the increase of the doping concentration,which is more favorable for the transition of valence band electrons.The CBM of Lu_2-xEr_x O₃?0<x?0.09375?is mainly derived from the contributions of Lu 5d,Er5d and O 2p,and the VBM are mainly derived from the contributions of Er 5d,Er 4f and Lu 4f orbitals.The calculation results of the optical properties of the doping system show that the doping of Er³⁺can significantly increase the absorption coefficient in the ultraviolet region and increase the static dielectric constant of Lu₂O₃.As the doping concentration of Er³⁺increases,the energy of the valence band electrons excited to the conduction band decreases,and the transition is more easily to occur.The absorption coefficient,reflectance and EELS are red-shifted.The absorption coefficient is slightly reduced,and both the reflectance and the EELS are red-shifted.The Lu_2-xEr_x O₃?0<x?0.09375?system still retains a low absorption coefficient and reflectance in the mid-infrared and visible regions.The doping of Er³⁺can effectively control the electronic structure and optical properties of Lu₂O₃.