| The new lasers at wavelength range from 2.7 to 3.0 ?m.have been widely attracted by the requirements for medical applications,such as ophthalmology,dental,and the laser cosmetic surgery,due to their strong absorption in water,used for body soft tissue and bone incision surgery.It have major applications in infrared countermeasure,laser radar,environmental monitoring,remote sensing,laser spectroscopy,and so on.However,the fine spectral structure in the range of 2.7-3.0?m shows the water absorption at the wavelength of 2.911 ?m is weak.Therefore,the 2.911?m laser can be applied in the fields of the military and scientific research in space.Moreover,the 2.7-3.0 ?m wavebands are also suitable pumping source for 3-15?m optical parametric oscillator(OPO)laser system that can be used for environmental detection,optoelectrionic countermeasure,anti-terrorist etc.The Ho3+doped tantalate crystals are potential 2,7-3.0 laser materials.In this work,the crystal growth of Yb3+,Tm3+,Ho3+,Pr3+,and Y3+ doped GdTaO4 and the structure,spectroscopy,and thermal properties are studied in detail.The main contents and results of this work are as follows:(1)Yb3+,Ho3+ codoped GdTaO4 and YTaO4 polycrystal powder were prepared with high temperature solid state reaction.Their phase and structure were investigated and structure parameters were obtained by Rietveld full-profile fitting.According to the diffuse reflection absorption spectra,the strongest absorption peak was located at 955 nm,and the FWHM was about 75 nm,which was suitable to mature InGaAs LD pumping source without strict temperature control.There were strong emission at 2.9?m,which could be moved to the 2.911 ?m with concentration optimization of Y3+.Therefore,the Yb,Ho:Gd/YTaO4 will be a new 2.911 ?m laser material with LD pumping.(2)High quality Ho:Gd/YTaO4,Tm,Ho:Gd/YTaO4,Yb,Ho:Gd/YTaO4,and Yb,Ho,Pr:Gd/YTaO4 single crystals were successfully grown by Cz method.Through XRD patterns,four crystals were found to show coincident diffraction peaks as M GdTaO4.The crystallographic parameters,such as lattice constant,atomic coordinate,temperature factor,and so on,were obtained by Rietveld fitting.The X-ray rocking curves showed symmetric shape without split and small FWHM,which indicated a high crystalline quality.The effective segregation coefficient keff of Ho3+ in Gd/YTaO4 was calculated to be nearly 1,which was beneficial for high concentration and uniform doping.(3)Spectral properties of Ho:Gd/YTaO4,Tm,Ho:Gd/YTaO4,Yb,Ho:Gd/YTaO4,and Yb,Ho,Pr:Gd/YTaO4 single crystals had been studied.The transmission spectra along a,b,and c direction of four kinds of crystals showed that the biggest absorption coefficient was along the c direction,which could be the optimal direction of the pump,to improve efficient.The absorption coefficient of Yb,Ho:Gd/YTaO4 and Yb,Ho,Pr:Gd/YTaO4 at the pump wavelength was big and the FWHM was broad,which indicated that Yb,Ho:Gd/YTaO4 and Yb,Ho,Pr:Gd/YTaO4 would be more suitable for LD pumping 2.9?m laser materials.The absorption spectrum of Ho:GYTO crystal from 350 to 3000 nm was used for J-O analysis.The spectrum parameters,such as intensity parameters,oscillator strength,line strength,life time,fluorescence branching ratio,and so on,were calculated.The 2.9 ?m emission spectral were measured.The strong emission peaks of Ho:Gd/YTaO4 were located at 2848,2862,and 2906 nm,Tm,Ho:Gd/YTaO4 were located at 2895,2915,and 2932 nm.Yb,Ho,Pr:Gd/YTaO4 were located at 2865 and 2907 nm.Importantly,the strongest emission peak of Yb,Ho:Gd/YTao4 was located at 2911 Nm,with 21 nm broad FWHM,which will be a promise 2.911 ?m laser material.The lifetimes of upper level and low evel of 2.9 ?m laser were measured.The lifetimes of upper level 5I6 were 310.7,131.0,419.0,and 376.2 ?s,respectively.The lifetimes of low level 5I7 were 8.1,4.1,7.3,and 0.9 ms,respectively.The shorter lifetime of 5I7 and longer lifetime of 5I6 of Yb,Ho,Pr:Gd/YTaO4 were easier to realize population inversion and laser output,reduce the pumping threshold and improve the output powder.The emission cross section of four kinds of crystals at 2.9 ?m bands were calculated.The biggest cross section were 12.6×10-203 37.2×10-20,17.6×10-20,and 13.9×10-20 cm2 respectively,which indicated that Tm,Ho:Gd/YTaO4 was easiest to realize laser output,followed by Yb,Ho:Gd/YTaO4,Yb,Ho,Pr:Gd/YTaO4,and Ho:Gd/YTa04.On the basis of the absorption and emission spectral of Ho:Gd/YTaO4 at 8K,the Stark energy level of 5I8,5I7,5I6,5F5,5F4+5S2,and 5F1+5G6 were analyzed.(4)The thermal properties of Yb,Ho:Gd/YTaO4,including specific heat,thermal diffuse,thermal diffusivity,and thermal conductivity,were characterized.The value of specific heat is 0.32 J/(g·K)at 300 K.The thermal expansion coefficient along a axis was the smallest,which indicated that crystal growth along a axis was beneficial for large size without crack.The thermal conductivity of ka=5.550,kb=4.428,kc=5.728 W/mK were calculated with thermal conductivity of ?a=2.019,?b=1.611,?c=2.084 mm2/s.c axis was larger than those of the other two directions at the same temperature,which indicated that the Yb,Ho:Gd/YTaO4 crystal should be pumped along c axis more favorable to reduce the thermal lensing effect.This work provides new 2.7?3.0 ?m laser crystal materials. |
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