| Laser,which is the amplified stimulated radiation,has the extensive applications in the fields of military countermeasures,mechanical processing,laser display and laser medical treatment because of the characteristics of high brightness,monochromaticity,good coherence and directionality.Since the first ruby laser in the world emerged in 1960,laser technology has achieved great progress with decades of development,thereby affecting human lives profoundly.In the future,laser technology will be an important component in the technology that promotes the development of human society and science,and will play an irreplaceable role in more application fields.Generally,a laser is composed of three parts:a pump source,a gain medium and a laser resonator.The crux of laser generation is the gain medium,in which the type of activated ion determines the output wavelength and application field of the laser.The gain medium of all solid-state lasers is solid,mainly includes laser crystals,glass and ceramics.It has the advantages of compact and simple structure,small size and long life.It conforms to the development direction of high integration,miniaturization,"integration of materials and devices" and has gradually become the research hotspots of lasers.Moreover,rare-earth-doped laser crystals are an important gain medium and have received widespread attention.Ytterbium ion(Yb3+)is an important rare earth activating ion and its emission peak is concentrated near 1 ?m in the near infrared.Although the research on Yb3+doped laser crystals started early,the pump source conditions limited its development.Until the 1990s,thanks to the advancement of high-power InGaAs laser diode technology,the output power and efficiency of Yb3+doped laser crystals have been significantly improved,making it to become one of the most important laser gain media at present.Yb3+ion has unique advantages as an active ion because that the energy level structure is simple,there is no excited state absorption and up-conversion,concentration quenching effect is small and high doping can be achieved.In addition,the 4f electrons of Yb3+ions are seldom shielded by the 5s and 5p electrons in the outer layer,the electronic transition is easily affected by the vibration of the matrix lattice,that is electron-phonon coupling,which changes the energy transfer process of radioluminescence,and produces phenomena such as spectral broadening and new radiation wavelengths,which is conducive to the realization of ultra-short and ultra-fast laser output.Therefore,it is of great significance to research the broad-spectrum Yb3+doped crystal and its spectral line broadening mechanism.Borate ion(BO3)3-has the shortest bond length among the anion groups,and the scandium(Sc3+)has the smallest ion radius among the rare earth ions.Therefore,the scandium borate crystal(ScBO3)has the strong crystal field,which makes the activated ion generate the large energy level split.In addition,the ionicity of B—O bond is larger than others and the Yb:ScBO3 crystal has obvious electron-phonon coupling effect,so it has a large spectral width and is expected to realize near-infrared ultrashort pulse laser output and sideband emission.Yb:SrWO4 crystal has a variety of charge compensation defects caused by unequal substitution,which causes a non-uniform broadening of its spectrum,and there is a uniform broadening caused by the electron-phonon coupling effect at the same time,which makes it have a broad emission and is conducive to near-infrared broadband tuning and ultra-short and ultra-fast laser output.However,limited by the higher melting point of the crystal and the strong volatilization of B2O3,the growth of optical grade Yb:ScBO3 crystals is still a challenge.There are only relevant reports on physical and spectral properties about Yb:SrWO4 crystals,but no in-depth studies and reports on issues such as the spectrum widening mechanism.Therefore,it is of great significance to grow high-quality crystals and research the broadening mechanism of these two broad-spectrum Yb3+ion-doped crystals.This work takes Yb:ScBO3 crystal and Yb:SrWO4 crystal as the research objects and optimize crystal growth parameters.The Yb:ScBO3 and Yb:SrWO4 crystals with different doping concentrations were grown by the optical floating zone method and the Czochralski method,respectively.The structure and thermal properties of the crystal were characterized,the absorption and emission spectra of the crystal were tested,the spectral broadening mechanism of the crystal was analyzed,the influence of electron-phonon coupling and charge compensation on the optical properties was clarified.The main work is as follows:(1)Crystal growth and spectral properties research of Yb:ScBO3The Yb:ScBO3 crystal was grown by optical floating zone method.The influence of parameters such as speed,temperature and atmosphere on the crystallization performance of the crystal was analyzed in detail.Under the conditions of the excess of H3BO3 is 6 wt.%of the total mass,the growth rate of 0.2 mm/h and the rotation speed of 17.5 r/min,Yb:ScBO3 single crystal with the crystalline area of about 5 mm×3 mm×2 mm was grown in a 50%O2+50%Ar atmosphere by balancing the relationship between crystal growth rate,excess of H3BO3 and content of O2.The phase analysis of the obtained Yb:ScBO3 crystal was carried out by using X-ray powder diffraction method,the actual cell parameters of Yb:ScBO3 crystal were obtained through Rietveld structure refinement and the influence of Yb3+ ion doping on the cell parameters of ScBO3 crystal was analyzed.The absorption spectrum of Yb:ScBO3 crystal under room temperature was tested and the absorption cross section was calculated.Using a xenon lamp with a wavelength of 893 nm as the pump source,the fluorescence spectra under different temperature conditions were tested,the corresponding launch section was calculated by Fuchtbauer-Ladenburg(F-L)method.The gain cross section is calculated and the effective positive gain wavelength range is obtained.The Huang-Rhys factor S is calculated,whose result shows that there is a strong electron-phonon coupling effect in Yb:ScBO3 crystal,which plays an important role in the uniform broadening of the spectrum.(2)Crystal growth and physical properties research of Yb:SrWO4The Yb:SrWO4 crystals with Yb3+ion doping concentrations of 0.5 at.%,1 at.%,and 5 at.%were grown by the Czochralski method.The structure,composition,density,thermal and optical properties of the crystal were systematically analyzed.XRD test shows that the grown Yb:SrWO4 crystals possess high purity and good single crystallinity.The Rietveld structure refinement analysis shows that the variation of the cell parameters due to the different doping concentration of Yb3+ions.The components of the three groups of crystals were tested,the actual doping concentration and chemical formula of the crystals were obtained and the effective segregation coefficient was calculated.The rocking curve indicates that the crystal has high optical quality.The density and hardness of three crystals at room temperature were measured.The specific heat capacity,thermal expansion coefficient,thermal diffusivity and thermal conductivity of Yb:SrWO4 crystals with different Yb3+doping concentrations were measured.The variation of crystal thermal properties with temperature and doping concentration has been analyzed and summarized.The change curve of crystal density with temperature and the thermal shock coefficient were calculated.The room temperature non-polarization,polarization transmission spectrum,fluorescence lifetime and room temperature polarization fluorescence spectrum of Yb:SrWO4 crystals with different doping concentrations were measured.The polarization absorption cross section was calculated according to the transmission spectrum,the polarization emission cross section was calculated using the reciprocity method,fluorescence spectrum of three groups of Yb:SrWO4 crystals under the room temperature and low temperature(77 K)were tested using a fiber-coupled diode laser with the wavelength of 976 nm as the pump source and the Huang-Rhys factor was calculated.The results show that the absorption and emission spectrum of Yb:SrWO4 crystals with different doping concentrations have obvious polarization characteristics.The maximum absorption wavelength is 969 nm,the maximum absorption cross-section is 1.49×10-20 cm2,the maximum emission wavelength is 1003 nm and the maximum cross-section is 1.75×10-20 cm2.The emission bandwidth is 150 nm and the half-peak width is 50.84 nm,which is slightly wider than common tungstate laser crystals such as Yb:KLu(WO4)2,Yb:NaY(WO4)2 and Yb:NaGd(WO4)2.The spectral broadening mechanism of Yb:SrWO4 crystal can be attributed to the synergistic effect of non-uniform broadening caused by charge compensation defects and uniform broadening caused by electron-phonon coupling.The above results show that Yb:SrWO4 crystal has both broad spectrum and excellent thermal performance,indicating that it is a high-power ultrafast laser gain medium with application potential. |
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