| Because of the important application in a variety of cutting-edge technology (such as laser guidance,military confrontation, materials handling,laser medicine, environmental monitoring, and so on), the marketdemand for mid-infrared laser crystals is growing greatly, especially the requirements about the ability toproduce sensitive band, large-size, high-quality and high-output power become much demanding.Materials scientists have been looking for high-quality laser crystals which can produce extended infraredradiation and worked long time in complex environment. This paper aims to study lithium yttrium fluoridelaser crystal with high properties that can produce enhanced3μm mid-infrared fluorescence radiation. Weused an improved Bridgman method to grow Ho3+single doped LiYF4crystals with1mol%molepercentage and the Ho3+/Pr3+co-doped LiYF4crystals with1%:0.5%mole percentage. What’s more,crystal structure, infrared transmittance, spectral parameters, the gain characteristics of infrared andfluorescence decay characteristics and other aspects are analyzed in detail in this work.In the first section of this paper, we summarized the principles of solid-state lasers, and the features ofthree laser material which are crystal, glass and ceramics; and summarized the research progress for lasertechnology in the3μm mid-infrared. We introduced the current domestic and international research aboutlaser crystal, and the physical and chemical properties of lithium yttrium fluoride crystals. In the secondsection of this paper, we described the common energy levels structure of rare earth ions, the detailedcharacteristics of the holmium and praseodymium ions, the Judd-Ofelt theory of rare-earth ions in thecrystal, and introduced the mod of the transfer between ion. In the third chapter, we told the commonlyused experimental materials and instruments, operating procedures and the process parameters which wasobtained in several experiments, by using which we could grow high-quality and large-size crystals. Wealso showed many technologies and method which can represent the spectroscopic properties of crystalsample.In the fourth and fifth chapters, we analyzes the luminescent properties of the Ho3+ion single doped,Ho3+/Pr3+co-doped LiYF4crystal prepared by Bridgman method, and evaluate the application value forpractical application in2~3μm mid-infrared solid-state laser. The value of Judd-Ofelt effective strengthparameters (6,4,2) of Ho3+ion were calculated to be1.33×10-20cm2,0.79×10-20cm2,1.14×10-20cm2in Ho3+ion single doped yttrium lithium fluoride crystal samples respectively, and1.05×10-20cm2,1.96×10-20cm2,1.52×10-20cm2inHo3+/Pr3+co-doped crystal samples, by which the spontaneous radiativetransition probabilities(A) of excited states, the excited state radiation lifetime(τrad), and the branchingratio(β) of corresponding level transition were obtained in two kinds of sample, respectively. The radiationlifetime(τrad) of Ho3+ion excited state5I7level which level is the up one in level transition of2μmfluorescence radiation was measured to reach17ms in Ho3+ion single doped sample, while in the Ho3+/Pr3+co-doped samples, some level lifetime become longer. What’s more, the lifetime ratio between5I6and5I7level in this sample is0.53which is greater than the value of0.48in Ho3+single doped sample, the resultreflect that the Pr3+ions can improve relatively the lifetime of level5I6level in2.9μm fluorescence. With640nm light excitation, The infrared emission intensity at1~2μm in Ho3+ion single doped sample is significant larger, but the one at3μm is weak relatively. The fluorescence lifetime1.2and2μm radiationwas measured to be2.13ms and17.23ms, respectively, which is very close to the level lifetime calculatedin J-O theory. Compared with other Ho3+ion doped glass solid materials, Ho3+ion single doped LiYF4crystal sample have a higher broad absorption band and emission cross-section at2μm band. The maximumof the stimulated emission cross-section at1.2and2.0μm is0.20×10-20cm2and0.51×10-20cm2, respectively,while in the Ho3+/Pr3+co-doped samples, the value around2950nm is up to0.68×10-20cm2, which is greaterthan0.53×10-20cm2in Ho3+single-doped samples, and the FWHM around3μm reach135nm which islarger than120nm in Ho3+single doped sample, however, there is a higher gain cross section at2μm bandin Ho3+: LiYF4crystal, the gain spectrum reflects that the particles population inversion level reachlower only30%that the gain become stronger at2μm wavelength, Hence, the single crystal Ho3+: LiYF4have low laser pumping threshold and it is much more efficiency to output2.05μm wavelength light.Compared with Ho3+ion single doped crystal, in the incorporation of Pr3+ions in Ho3+/Pr3+co-doped LiYF4crystal, there is a higher gain cross section at2.9μm band, and the inversion level reach a lower value only40%that the gain appears obvious around3μm band, but it need50%in Ho3+: LiYF4crystal, and there is ahigher gain cross section and the FWHM at3μm band. That indicate that Pr3+ions can effectively reducethe laser pump threshold when Ho3+/Pr3+co-doped LiYF4crystal is as a kind of laser material, and alsoimprove energy efficiency and increase significant laser efficiency at2.9μm wavelength. The radiativelifetime corresponding Ho3+:5I7'5I8transition in Ho3+/Pr3+co-doped samples was measured to be2.15mswhich is87.85%less than the value in Ho3+single-doped sample, which illustrates that Pr3+ions have thegenuinely quenching effect on Ho3+:5I7level, meanwhile it have little effect on the lifetime of5I6level.The fluorescence decay curve at2.0μm was fitting very well by using I-H model, that show the existentenergy transfer process (Ho3+:5I3+7, Pr3+:3H4)'(Ho3+:5I8, Pr:3F+2) between Ho3+and Pr3ions, and thevalue of the energy transfer efficiency was calculated to be87.53%, which indicate that the Pr3+ions withcurrent concentration can effectively quenched the lower2.9μm laser level Ho3+:5I7,and promote particlespopulation inversion between the upper and lower level of2.9μm laser. All that is beneficial to output laserlight at2.9μm wavelength by pumping the Ho3+/Pr3+co-doped LiYF4crystals.The results show that LiYF4crystals which doped with rare-earth as the mid-infrared laser hostmaterial has obvious advantages. By comprehensive evaluation and comparison with other solid material,we found that single-doped Ho3+: LiYF4single crystal have a more significant in application prospects insolid state laser working in range of1~2μm infrared wavelength; on the other hand, the Ho3+/Pr3+co-doped LiYF4single crystal will have a clear competitive edge as the laser materials in the solid statelaser working in mid-infrared about2.8~3.0μm wavelength. |
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