| The laser crystals used for generating mid-infrared laser such as 2μm and 3μm laser have attracted great attentions for their important applications in medicine,optoelectronic countermeasure,remote sensing and atmospheric monitoring in recent years.The rare earth ion holmium ion(Ho3+)has abundant electronic energy levels,its transition:5I7→5I8 can produces 2μm laser,and the transition:5I6→5I7 generates 3μm laser.Therefore,Ho3+doped crystals have great potential as gain medium for 2 and 3μm laser.However,its application in the mid-infrared band is limited by two factors:1)Lack of suitable commercial LD pump source,which made the pump efficiency is relatively low.2)For the transition:5I6→5I7,the lifetime of upper level:5I6is shorter than that of the lower level:5I7.such situation leads to the self-termination of 3μm laser.In this paper,we introduce the sensitizer Yb3+into the Ho-doped calcium niobium gallium garnet（Ho:CNGG）crystal,make Yb-Ho codoped CNGG crystal can well match the mature commercial LD pump source through the energy transfer between Yb3+and Ho3+.On this basis,the self-termination effect of 3μm laser was also restrain by introducing deactivated ions Eu3+,and make the transition:5I6→5I7became possible.High quality Yb-Ho/Yb-Ho-Eu:CNGG crystals was successfully grown for the first time by Czochralski method and the factors affecting quality of crystal were also discussed.the X-Ray powder diffraction pattern were used to determine the crystal structure and the results shown that the two crystals both belong to cubic with space group of Ia3?d（No.230）.In addition,the X-Ray rocking curve of Yb-Ho:CNGG crystal was measured,the full width at half maximum（FWHM）of the peak was about40.32′′（0.0112°）indicated that the grown crystals has good crystallinity and high optical quality.Using the X-ray fluorescence analysis,the doping concentration of Yb3+and Ho3+in Yb-Ho:CNGG crystal were measured to be 11.94 at.%,0.94 at.%,respectively;For Yb-Ho-Eu:CNGG crystal,they were 11.94 at.%,0.94 at.%and 0.2at.%,respectively.The optical properties of Yb-Ho:CNGG and Yb-Ho-Eu:CNGG crystals were studied.The absorption spectrum and emission spectrum were measured at room temperature.Based on the Judd-Ofelt theory,the oscillator intensity parameters,oscillator intensity,transition line intensity,fluorescence branch ratio and radiation lifetime were calculated respectively.For Yb-Ho:CNGG the oscillator intensity parameters were calculated to beΩ2=2.85×10-20 cm2,Ω4=2.35×10-20 cm2 andΩ6=0.76×10-20 cm2,respectively.In addition,the absorption and emission cross-sections of the two crsytals were also calculated.For Yb-Ho:CNGG crystal,The peak absorption cross section of 973 nm was were 0.97×10-20 cm2 and the FWHM was 50nm;for Yb-Ho-Eu:CNGG,they were 0.91×10-20 cm2 and 50.5 nm,respectively.All the results indicate that both Yb-Ho:CNGG and Yb-Ho-Eu:CNGG crystals can be pumped with commercial 975 nm LD.For Yb-Ho:CNGG crystal,the peak emission cross-sections at 2μm and 3μm were 0.63×10-20 cm2 and 0.57×10-20 cm2,and for Yb-Ho-Eu:CNGG crystal,the peak emission cross-sections at 2μm and 3μm were 0.55×10-20cm2 and 0.75×10-20 cm2.Besides,the lifetime of Ho3+:5I7 energy level in Yb-Ho:CNGG and Yb-Ho-Eu:CNGG crystals were 6.41 ms and 3.81 ms.The results mentioned above show that Yb-Ho:CNGG and Yb-Ho-Eu:CNGG crystals are promising mid-infrared laser crystals.Yb-Ho:CNGG crystal have more advantage for 2μm laser oscillation and Yb-Ho-Eu:CNGG crystal is more suitable for generates 3μm laser.The above research can enrich the range of gain medium for 2μm and 3μm lasers and promoting the development of mid-infrared lasers. |
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