Micro-Pulling-Down Growth And Characterization Of Several Single-Crystal Fibers With Scheelite Structure

Artificial crystal is the driving force for the development of information technology.It is the basic substance and key material for the development and manufacture of optical,electronic,computer,communication,exploration,sensing and so on.Compared with natural crystals,artificial crystals have controllable growth laws and habits.According to requirements of the application performance,artificial crystals can be grown to meet the needs of specific applications depending on human desire,under appropriate environmental conditions and apparatus.Single-crystal fiber(SCFs)has the dual advantages of the bulk single crystal and the glass fiber,with a large length to diameter ratio and specific surface area,effectively reducing the overheating problem when an optical signal of high power density passes.L.ong-interacting paths can be obtained by the corresponding angular or mode,ideal for integrated optics applications.With the development of miniaturization 'and high-power lasers,the role of SCFs becomes increasingly prominent.The research of SCFs s is still in the stage of technical reserve in China,which will need a lot of work to be completed in theory and device application.The micro-pulling-down(?-PD)method is a rapid developing growth technology for SCFs with the advantages of shaped-crystal growth,and need less raw materials,and with high segregation coefficient.The international?-PD technology is still in the early stage of development,and ?-PD technology in China is in its infancy.The content of this thesis is based on the crystals of the Scheelite structure,exploring SCFs grown by the ?-PD for red-emitting and laser applications.The Scheelite structure belongs to the tetragonal system,the space group is I41/a.Two kinds of crystals with Scheelite structure are selected in this paper,CaMoO4 and LiYF4 types.Eu3+:CaMoO4 have the strong absorption in near ultraviolet and blue regions.The industry committee considers it most likely to replace the commercial red phosphor Eu3+:Y2O2S used in W-LEDs excited by UV chips[1].With the development of compact fluorescent lamps,the problems of poor heat dissipation and thermal stability appear more obvious after conventional phosphor packaging,however the SCF has excellent heat conduction and thermal stability.The growth of Eu3+:CaMoO4 single crystal is rare.Therefore,the growth and spectroscopy of Eu3+:CaMoO4 SCFs have been reported for the first time.Li YF4 single crystal is an excellent laser host with lower phonon energy so that the decay lifetime of rare earth energy is longer than oxides.The spectral transmission band of fluoride crystals is wider than oxides and high doping level can be achieved.However,LiYF4 crystal is soft,and the bulk single crystal is difficult to process into a small diameter(1 mm or less).Therefore the shaped-growth of fluoride SCFs by the ?-PD method has potential application value.The oriented-seed growth of the ?-PD is very important to improve the absorption from the pump light.However the oriented-seed growth of the fluoride SCFs by the ?-PD is difficult.In 2005,A.M.E.Santo et al.have reported the ?-PD growth of LiYF4[2],later D.Maier et al.[3]and T.Yanagida et al.have also reported the ?-PD growth of LiYF4.However,there has been no reports about the successful oriented-seed growth of the LiLuF4 and LiYF4 SCFs by the ?-PD method.We have first reported the successful oriented-seed growth of LiYF4 type SCFs by the ?-PD and studied their polarized absorption and emission spectra.The main research work and conclusions of this thesis are as follows:Growth and Spectroscopy of Eu3+:CaMoO4 SCFs0.01 at.%and 0.05 at.%Eu3+:CaMoO4 SCFs have been successfully grown by the?-PD method,and studied the atmosphere and thermal field required for growth.The serious cleavage plane(001)was avoided by using an a-cut seed and an additional after-heater.The high-quality Eu3+:CaMoO4 SCFs can be achieved by the ?-PD method.In the room and low temperature,the polarization absorption spectra of the 0.01 at.%Eu3+:CaMoO4 SCF have been characterized to analyze the relationship between the spectra and the lattice occupancy of coloring Eu2+ ions.The adjacent distance of Eu2+ions in ?-polarization is shorter than ?-polarization.The smaller effective distance of the adjacent Eu2+ makes the absorption stronger in the visible range.In room and low temperature emission spectra,the properties of the 0.01 at.%Eu3+:CaMoO4 SCFs as red-emitting single crystals have been measured showing the good color purity.In room temperature,the polarized emission spectra excited by a 395 nm LED or 532 nm laser,the relative intensity and splitting numbers of the em:ission peaks change.This was caused by the different symmetry sites of Eu3+ when some Eu3+ ions were surrounded by Oi2-ions.The average decay lifetime of the 1 at.%Eu3+:CaMoO4 SCF is shorter than the reported 1 at.%Eu3+:CaMoO4 phosphor,probably due to the lower Eu3+ ion content in single crystal.Origin of Eu3+:CaMoO4 SCFs ColorationThe origin of the coloration of Eu3+:CaMoO4 SCFs can be attributed to the charge compensations,which is the co-existence of Eu2+ and interstitial oxygen(Oi2-)ions.The existence of Eu2+ ion has been found by the Eu3d XPS of the crystal sample.In the annealing process Eu3+:CaMoO4 SCFs,the existence of O2-iions has been proved by the absorption spectra and XPS spectra before and after annealing.,By measuring the conductivity of pure CaMoO4 and Eu3+:CaMoO4 SCFs before and after annealing in air,it is clear that Eu3+ ion doped and air annealing process introduce Oi2-ions.The 1 at.%Eu3+ doped and 1 at.%Li+/Eu3+ co-doped Eu3+:CaMoO4 crystals have been achieved by the p,-PD method.The XRD results show that Li+ ions occupy the Ca2+ sites as the charge compensation when EuUW ions are doped.Compared the absorption spectra of the Li+/Eu3+ co-doped with the Eu3+ doped samples,it is found that the Li+/Eu3+ co-doped sample has a lower intensity of the absorption band of 400-750 nm than the Eu3+ doped one.It is clear that the Li+ doped ions decrease the absorption intensity of 400-750 nm that is caused by charge compensations.Li+ ions reduce the content of Eu2+ and O?"ions so that reduce the absorption intensity.The presence of Oi2-ion has also been verified by the Eu3+ probe spectra.The intensity ratio A21 of the 5Do?7F2 transition to the 5Do?7F1 transition indicates the change of Oi2-ions content.Compared the emission spectra,the Eu3+ doped sample has a higher A21 value and Oi2-ions content than the Li+/Eu3+ co-doped sample.Compared the as-grown Eu3+:CaMoO4 with the Eu3+:CaMoO4 samples annealed in air and hydrogen,the sample annealed in air has the largest A21 value and the most Oi2-ions,consistent with the charge compensations.Using DFT calculations,Eu2+ and Oi2-ions have been introduced successively in CaMoO4.When Eu2+ and Oi2-ions exist,the free energy of the system is the lowest.The band gaps of CaMoO4 and Eu2+/Oi2-:CaMoO4 are 3.1 and 1.5 eV,respectively.The electrons of Eu2+ and Oi2-ions contribute to forming the top of the valence band of the Eu2+/Oi2-:CaMoO4,where most of the electrons come from the Eu2+ ions.In low temperature emission spectra,the emission of 630-720 nm centered at 680 nm is 1.8 eV.It may be attributed to the transition from the conduction band to the Eu2+ level.In a high temperature of oxidizing atmosphere,molybdate matrix can undergo the reduction of Eu3+->Eu2+ ions.The Oi2-ion acts as a donor of electrons while Eu3+becomes an acceptor of electrons.At high temperature,electrons released from Oi2-could be captured by Eu3+ ions.As a consequence,the reduction of Eu3+to Eu2+ occurs.Growth and Spectroscopy of LiYF4 type SCFsThe oriented-seed growth of LiYF4(YLF)and LiLuF4(LLF)SCFs by the ?-PD method has been reported for the first time.The crystal diameter is controlled stably and the quality is high.Compared the polarization absorption and emission spectra of the 1 at.%Pr3+:YLF single crystal grown by the ?-PD with the Czochralski method,the peak shape and position are very close.The Pr3+:YLF SCF grown by the ?-PD method has a higher segregation coefficient than the Czochralski method.The cubic crystal KY3F10(KYF)SCF grown by the ?-PD method has been reported.The difference in the spectra of the KYF and YLF SCFs are compared.The absorption and emission spectra of the 1 at.%Pr3+:KY3Fio single crystal grown by the ?-PD and the Czochralski method are very similar in peak intensity and position.The Pr3+:KYF SCF shows a secondary absorption peak at 486 nm,while the Pr3+:YLF SCF doesn't,which may be related to the slight disorder of the KYF lattice.Compared the polarization absorption and emission spectra of the 10 at.%Yb3+:YLF single crystal grown by the ?-PD and Czochralski method,the peak shape and the peak position are very close.Yb3+:LiLuF4 SCF grown by the ?-PD method can achieve a high doping concentration of Yb3+ ions.