Growth, Properties And Potential Application Of Series LiNbO₃ Crystals

Lithium niobate(LN) is an important multi-functional crystal,possessing good piezo-electric,nonlinear-optic,electro-optic and photorefractive properties.Lithium Niobate have been widely grown mainly by Czochralski method,and it is easy to be cut and polished for the fabrication of desired elements due to its excellent mechanical property.Based on the above merits,lithium niobate has been widely used in various areas and is still a promising crystal for new applications.Lithium niobate is a typical non-stoichiometry crystal.The deficiency of lithium ion in the crystal induced a number of vacancies reduction of some important function properties.Stoichiometric lithium niobate crystal(SLN) with low vacancy density has been grown and attracted many attentions recent years.Compared NSLN crystal with CLN crystal,the defect density of SLN crystal decreases obviously.In the meantime, many functional properties of SLN are better than those of CLN.The CLN crystal is widely used to fabricate the acoustic devices and called to be acoustic grade LN crystal. And those used to fabricate optics elements are called optical grade LN crystal.The application of acoustics grade LN crystal is very succeed,however,the applications of optical grade and doped CLN crystal has encountered some problems due to its component and refractive index homogeneous.In this thesis,we focused our attention to the crystal growth,determination of composition,observation of defects and basic properties of NSLN crystals.And we investigated the CLN and doped crystals for fabrication of electro-optic Q-switch.The main content of this thesis is described as follows:1.The synthesis of LiNbO₃ nanopowders through a wet chemical route:In order to reduce doped elementary inhomogeneity,undoped and doped LiNbO₃ powder have been successfully prepared by using commercial Nb(OH)₅ and nontoxic DL-Malic Acid as raw materials at relatively low temperatures through a wet chemical route.The powder prepared is suitable to be used as feeding powder in the double crucible technique growth of NSLN crystals due to its low melting point.2.Crystal growth:On the understanding of crystal growth methods of NSLN in this field,we adopted CZ technology to use a large crucible to grow a small SLN crystal.A modified suspend quazi double-crucible technology with continuous powder feeding system was designed so as to keep a composition homogeneity in grown crystal.By this technology,NSLN crystals with high optical quality were successfully grown from a rich lithium melt.The factors that determine quality of a crystal during the growth were analyzed,on the basis of thermodynamics and dynamics theories of crystal growth.In our understanding,the optimal thermal field,high quality seed,the optimal parameters and controlling lithium volatilization from the melt surface are the prerequisite for the crystal growth with high perfection.3.Crystal components:The ratio value of[Li]/[Nb]in LiNbO₃ crystal have great influence on crystal quality,And the components homogeneity of the large size LN crystal is very important in industrialization production.Therefore,components the analysis of crystal components is necessary.Here,crystals compositions were assessed by measuring the UV absorption edge and OH^- absorption.The reason of components inhomogeneity is volatilization of Li₂O.The absorption edge wavelength of Mg:SLN crystal have not obviously changed along Z axis,and the reason is discussed.The OH^- infrared absorption peak for NSLN crystal was located at 3466 cm^-1 with a shoulder peak at the high wavenumber side.The absorption peak for Mg:SLN (2mol%) and Mg:CLN(6 mol%) crystal were located at 3534 cm^-1,which indicated indirectly that the Mg²⁺ began to enter the Nb site as the substitiution of Nb⁵⁺.MgO doping concentration in the as-grown crystal has exceeded optical damage resistance threshold concentration. 4.Crystal structures and defectsHigh temperature X-ray diffraction,thermal expansion and Raman scattering measurements have been conducted to investigate the structural behavior of a nearly stoichiometric LiNbO₃.Abnormal expansion is observed within the temperature range of 1021 K to 1373 K.This abnormal behavior was discussed from E(TO1) and A1(TO1) modes Raman shifts at different temperature.5.Crystal defects:Defects of LN crystals have been observed by white-beam synchrotron radiation topography(WBSRT) technique,the chemical etching method and optical microscopy. The main defects in LN crystals were twins,inclusions and dislocations.Based on the observations of growth defects in LN crystals,the formation mechanisms of these defects were discussed.In order to avoid or decrease these defects,we take some corresponding steps according to the above discussions.The perfection of the crystal was assessed by high-energy X-ray diffraction.It could be helpful for the high quality LN single crystal growth.6.Crystal domain structure:Domain structure has also been observed by chemical etching method in acid solution of HF and HNO₃.The results showed that NSLN crystal possessed an incomplete single domain structure.The regional single domain structure and ring domain structure were found in Mg:NSLN crystal.Crystal components and polarization of Mg ion have an effect on the domain structure.7.Crystal thermal properties:The investigation of thermal properties including melt point,T_c,specific heat, thermal expansion coefficient,density changing along the temperature,thermal diffusion coefficient and thermal conductivity.The influence of these properties on growth and applications were also discussed.8.Basic physical properties:The transmission and absorption properties of NSLN crystals were studied.The refractive indices and optical homogeneity of LN crystals were measured.The ferroelectric hysteresis was obtained and the polarization mechanism was also discussed.At present,many of CLN crystal can only work stably at room temperature and light power laser.In order to suit the width temperature range high stable Q switch, we have analyzed the extinction ratio,half wave voltage,UV absorption edge and trace element of LN crystals.We reduced CLN slice to eliminate the pyroelectric discharge in SAW device manufacture.