The use of Pr3+co-doping for great enhancement of mid-infrared(mid-IR) emissions at 2.9 μm and 2.4 μm is investigated in the Ho3+/Pr3+co-doped LiYF4 crystals.With the introduction of Pr3+ions,the fluorescence lifetime of Ho3+:5I7 level is 2.15 ms for Ho3+/Pr3+co-doped crystal,and the lifetime for Ho3+singly doped crystal is 17.70 ms,while the lifetime of Ho3+:5I6 level decreases slightly from 2.11 ms for Ho3+:LiYF4 to 1.83 ms for Ho3+/Pr3+:LiYF4.It is also demonstrated that the introduction of Pr3+greatly increases the mid-infrared emission of Ho3+:5I6 →5I7 which depopulates the Ho3+:5I7 level,while it has little influence on the Ho3+:5I6 level,which is beneficial for greater population inversion and laser operation.The analysis on the decay curves of the 2.0 μm emissions in the framework of the Inokuti-Hirayama model indicates that the energy transfer from Ho3+to Pr3+is mainly from electric dipole-dipole interaction.The calculated efficiency of energy transfer from Ho3+:5I7 to Pr3+:3F2 level is 87.53% for Ho3+/Pr3+(1.02%/0.22%) co-doped sample.Our results suggest that the Ho3+/Pr3+co-doped LiYF4 single crystals may have potential applications in mid-IR lasers.
The LiYF4single crystals singly doped Ho3tand co-doped Ho3t,Pr3tions were grown by a modified Bridgman method.The JuddeOfelt strength parameters(U2,U4,U6)of Ho3twere calculated according to the absorption spectra and the JuddeOfelt theory,by which the radiative transition probabilities(A),fluorescence branching ratios(b)and radiative lifetime(srad)were obtained.The radiative lifetimes of5I6and5I7levels in Ho3t(1 mol%):LiYF4are 10.89 and 20.19 ms,respectively,while 9.77 and 18.50 ms in Ho3t/Pr3tdoped crystals.Hence,the sradof5I7level decreases significantly by introduction of Pr3tinto Ho3t:LiYF4crystal which is beneficial to the emission of 2.9 mm.The maximum emission cross section of Ho3t:LiYF4crystal located at2.05 mm calculated by McCumber theory is 0.51 10 20cm2which is compared with other crystals.The maximum emission cross section at 2948 nm in Ho3t/Pr3tco-doped LiYF4crystal obtained by Fuchtbauere Ladenburg theory is 0.68 10 20cm2,and is larger than the value of 0.53 10 20cm2in Ho3tsingly doped LiYF4crystal.Based on the absorption and emission cross section spectra,the gain cross section spectra were calculated.In the Ho3tions singly doped LiYF4crystal,the gain cross sections for 2.05 mm infrared emission becomes positive once the population inversion level reaches 30%.It means that the pump threshold for obtaining 2.05 mm laser is probably lower which is an advantage for Ho3t-doped LiYF42.05 mm infrared lasers.The calculated gain cross section for 2.9 mm mid-infrared emission does not become positive until the population inversion level reaches 40%in Ho3t/Pr3t:LiYF4crystal,but 50%in Ho3tsingly doped LiYF4crystal,indicating that a low pumping threshold is achieved for the Ho3t:5I6/5I7laser operation with the introduction of Pr3tions.It was also demonstrated that Pr3tion can deplete rapidly the lower laser Ho3t:5I7level and has influence on the Ho3t:5I6level.The Ho3t/Pr3t:LiYF4crystal may be a potential media for 2.9 mm mid-infrared laser.
A high-quality Cr 3+:CdWO4 single crystal at a size of approximatelyΦ25×80 mm is grown using the Bridgman method with CdO,WO3,and Cr2O3 as raw materials and their molar ratio of 100:100:0.5.The temperature gradient of solid-liquid interface at growth is approximately 50?C/cm and the growth rate is 0.05 mm/h.The X-ray diffraction(XRD),absorption,excitation,and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated.Two strong broad optical absorption bands of about 472 and 708 nm are observed,and they are associated with the transitions 4 A2→ 4 T1 and 4 A2→ 4 T2.The weak 4 T2→ 2 E transition(the R-line)at 632 nm is also observed.The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4,776.8,and 3 427.6 cm? 1,respectively,according to the absorption spectra and crystal-splitting theory.A broadband fluorescence at about 1 000 nm due to 4 T2→ 4 A2 transition is produced by exciting the samples at 675 nm.After being annealed in an O2 atmosphere,the crystals become more transparent,while the effective light absorption of Cr 3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.
Mid-infrared(MIR) emissions of 2.4 and 3.5 μm from Tm3+:LiYF4 single crystals attributed to3H4 →3H5 and3H5 →3F4 transitions as well as MIR emissions of 4.2,4.3,and 4.5 μm from Nd3+:LiYF4 lasers attributed to4I15/2 →4I13/2,4I13/2 →4I11/2,and4I11/2 →4I9/2 transitions,respectively,are observed.LiYF4 single crystals possess high transmittance of over 85% in the 2.5-6 μm range.The large emission crosssections of Tm-doped crystals at 2.4 μm(1.9×10-20cm2) and Nd-doped crystals at 4.2 μm(0.84×10-20 cm2) as well as the high rare-earth doping concentrations,excellent optical transmission,and chemicalphysical properties of the resultant samples indicate that Nd3+and Tm3+singly doped crystals may be promising materials for application in MIR lasers.
