| Lasers have long entered all aspects of human productions and lives since their inventions in the mid-20th century,and they have becoming the irreplaceable high-technologies for the development and progress of human society.Ultrafast lasers with high peak power and short pulse duration,have been used in various industries such as material processing,scientific research,and biomedical industries.All-solid-state lasers based on laser diode-pumped Yb3+doped laser gain materials are the latest generation of ultrafast laser sources.The development of ultrafast lasers toward higher peak powers and shorter pulse durations,which puts forward higher requirements for laser gain materials.Exploring the laser gain materials with wide fluorescence spectra and analyzing their spectral broadening mechanisms are the bases for achieving materials with narrower pulse width and higher peak power,which has been a hot spot for the researches in the field of laser materials.Among various ultrafast laser host materials,sesquioxides are considered as one of the important host materials because of their high thermal conductivity,high stability,and wide transmission band.However,it is not obvious of the advantage of the spectral width of the Yb3+doped sesquioxide crystals.It could satisfy the requirements of the development of ultrafast laser through broadening the emission spectra and revealing the broadening mechanism.Previous studies on sesquioxide crystals have shown that solid-solution mixed crystals formed by two or more sesquioxide hosts can substantially broaden the fluorescence and absorption spectra of Yb3+ ions,which is expected to solve the shortcomings of single-host materials.According to the basic principles of coordination chemistry,the energy level splitting of the activated ions and their optical properties are mainly influenced by the potential field(crystalfield)generated by the surrounding ligands,which is determined by the structure and constituent elements of the materials.The study of the crystal-field of the laser crystals is essential to understand the structure-function relationship and explore and design new materials.However,so far,it is still absent on the study of crystal-field and spectral broadening mechanism of Yb3+-doped sesquioxides due to their high melting point.In addition,the researches of the laser performance of the mixed crystals were mainly focused on the cubic phase structure,and it was little on the other structure of the orthorhombic phase crystals.It undoubtedly hindered the understanding of the structure-function relationship and the performance optimization of the sesquioxide crystals,which in turn affected their new crystal exploration and ultrafast laser applications.In this paper,the Yb3+-doped mixed crystals of cubic phase and orthorhombic phase sesquioxide solid solutions were taken as the research object.Starting from the material preparation and focusing on the spectral broadening mechanism under the effects of crystalfield and lattice vibration,the 19 different components of Yb:LuxSc2-xO3,Yb:LuxY2-xO3,Yb:GdScO3 and Yb:LaLuO3 crystals were studied,involving crystal growth,doping occupancy,spectral performance characterization,crystal-field theory calculation,and spectral broadening mechanism.The specific research contents are as follows:1.Cubic phase crystals Yb:LuxSc2-xO3The optical floating zone method for the growth of Yb:LuxSc2-xO3 crystals was summarized.The experimental energy levels of all Yb3+ ions were obtained by spectral experiment and decomposition for 11 component of Yb:LuxSc2-xO3 crystals.The crystal-field parameters of each component of crystals were calculated based on crystal-field theory,and the influence of host component varing on the crystal-field parameter values was revealed.It is found that the intrinsic crystal-field parameter Bk had an extreme point at x=0.50,and the crystal-field parameters Bq4 play a major role in the crystal-field under the point charge contribution.The lattice vibrations were analyzed by experiments and first-principles calculations.Corresponding to the lattice vibrations involved in the vibrational transitions found by the spectral decomposition,the vibrational modes involved in the spectral broadening in the form of electron-phonon coupling were identified.It is found that the vibrational modes with the largest Raman intensity and energy had the major effects in spectral broadening.2.Cubic phase crystals Yb:LuxY2-xO3For the 6 component of Yb:LuxY2-xO3 crystals,the crystal-field parameters under each component were calculated,the variation patterns of the ligand ionic pole and azimuth angle caused by the distortion of the ligand structure were found,which in turn revealed its influence on the sign of the crystal field parameters,the crystal-field intensity was calculated,the linear relationship between the radius of the attached cation,the crystal-field intensity and the energy level cleavage was found,and the first summary a semi-empirical formula that can be used for high-throughput spectral prediction was fitted:Nv(cm-1)=6707.42-36.88rion(pm)、ΔE7/2=0.279Nv、ΔE5/2=0.225Nv.It was found that due to the mass of Lu3+ions in the host was greater than that of Y3+ions and the radius was relatively small,the high-frequency and low-frequency lattice vibration energy moved in the opposite direction with the increase of x,which in turn caused the broadening of the spectra in different wavelength bands.With the participation of crystal-field effect and lattice vibration,the x=1.19 component of mixed crystal possessed the widest spectrum.3.Orthorhombic phase crystals Yb:GdScO3 and Yb:LaLuO3Yb:GdxSc2-xO3 and Yb:LaxLu2-xO3 crystals were grown by optical floating zone method.By optimizing the composition and controlling the growth conditions,high quality orthorhombic Yb:GdScO3 crystals were obtained when the proportion of Gd2O3 was 47.65%,and high quality orthorhombic Yb:LaLuO3 crystals were obtained when the proportion of LaLu was 0.9:1.1.The crystal structure and composition were characterized.It was verified that the grown Yb:GdScO3 and Yb:LaLuO3 crystals were orthorhombic phase structures with space group of Pnma,which have two types of cationic sites,where Gd/La occupied the eightcoordinated hendecahedron site with symmetry of Cs,Sc/Lu occupied the six-coordinated octahedral site with symmetry of Ci.It was found that x≥ 0.9044 in LaxLu2xO3 crystal was the cracking component point of crystal growth,and about 10%of La3+sites were occupied by Lu3+ions to form antisite-defects.Through spectral characterizations,radius analyzes and crystal-field calculations,the site occupation of Yb3+-doped orthorhombic Yb:GdScO3 and Yb:LaLuO3 crystals was analyzed.It was found that Yb3+ions occupied only Gd3+site in GdScO3 crystal,while they occupied both La3+ and Lu3+sites in LaLuO3 crystal.Due to the existence of antisite defects,the energy level splitting and spectral broadening were caused by the effect of crystal-fields at different sites.Combining the crystal-field effect and lattice vibration analysis,it was determined that the weak crystal-field strength caused by the large ligand leaded to the decrease of energy level splitting and the connection of electron transition peaks,resulting in the obvious broadening of the emission spectrum.The results of this paper revealed the physical mechanism of spectral broadening in Yb3+doped cubic and orthorhombic sesquioxide solid solutions under the combined action of crystalfield effect and lattice vibration,and proposed a universal semi-empirical formula.The occupations of Yb3+doping in the crystals were analyzed,which had important guide and reference for understanding the spectral broadening mechanism of Yb3+-doped ultrafast laser gain materials and exploring and designing novel sesquioxide crystals. |
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