| As one of the most important functional materials,rare earth oxide crystals play an extremely important role in daily life,military,defense,etc.Among these fuctional crystals,laser crystals and ferroelectric crystals have attracted much attention due to their remarkable applications.Hence,it is meaningful to do the research about these two kinds of crystals,which includes lots of aspects,for example the technology of crystal growth,exploration of new materials with good properties,and characterization of crystals.This thesis focused on the growth and characterization of several rare earth oxides crystals,which can be devided into two parts:growth and characterization of disordered laser crystals;crystal growth and characterization of novel geometric ferroelectric RInO3.A.Growth and characterization of disordered laser crystalsPulsed lasers have the characteristics of high peak power,short pulse width,large pulse energy,etc.They provide new measurement methods and extreme physical conditions for humans to carry out scientific research.Hence,pulsed lasers are the frontier of all-solid-state lasers,and one of the key projects is laser gain media.Laser gain media with high energy storage,broad spectrum line and excellent thermomechanical properties are ideal for pulsed lasers.The generation of short-pulse laser requires a wide emission linewidth of the gain medium.Crystal usually has excellent thermal properties but narrow fluorescence linewidth,the narror linewidth is not favor of ultroshort pulse.Glass has the wide emission linewidth but inferior thermal properties.The disordered crystal possesses the excellent thermal properties and wide emission linewidth,which has been a hot research topic.In the disordered crystal,the active ions will occupy different sites,the lattice fields around them are different.Different active ions will have different fluorescence linewidth,they overlap with each other and the total fluorescence linewidth will be broadened,which is called inhomogeneous broadening.Therefore,the disordered crystal also has a wider fluorescence line,and is suitable for generating pulse laser.In this thesis,Gd3Ga5O12(GGG)crystals with excellent thermal properties have been selected as the host materials for study.A novel disordered structure crystal has been designed and grown by doping different cations into the GGG lattice.Its short-pulse laser performance was studied through Q-switching and mode-locking techniques.The results showed that the disordered structure brought great improvements to the spectrum and laser performance of the crystal.The main contents and conclusions are as follows:1.Crystal growth,crystal composition and segregation coefficientThe disordered Yb:GAGG and Yb:LGGG laser crystals were grown by the Czochralski method.Some problems in the growth process were also discussed.The experimental results showed that the volatilization of Ga2O3 can be effectively suppressed by using 50%Ar + 50%CO2 gas as the growth atmosphere.By X-ray fluorescence method,the concentration of doped ions was measured and the corresponding segregation coefficient was calculated.2.Comparison of thermal,spectral and SESAM mode-locked laser performance ofYb:GAGG crystals with different degrees of disorderThe specific heat,thermal expansion and thermal diffusion of Yb:GAGG crystals were studied.By contrast,it was found that the Al3+ ion doping concentration has little effect on the basic thermal properties of the crystal in the Al3+ ion concentration range of 12 at.%to 22 at.%.In other words,an increase in the degree of disorder didn't decrease its thermal properties.Through spectral measurements,it was found that the increase of Al3+ ion doping concentration led to an increase in the degree of disorder in the crystal structure,causing an inhomogeneous broadening of the spectrum,which is consistent with the spectral inhomogeneous broadening theory.Through the SESAM mode-locked technique,a pulsed laser output of 1.6 ps was obtained by the 1#Yb:GAGG crystal and a pulsed laser output of 920 fs was obtained by the 4#Yb:GAGG crystal with a higher degree of disorder.The results showed that an increase in the degree of disorder is benefical to get shorter laser pulses.Yb:GAGG crystals have great potential for obtaining short-pulse lasers using SESAM mode-locking techniques.3.Disordered Yb:LGGG crystal spectrum and acousto-optical active Q-switched laser performanceThe fluorescence emission line width of Yb:LGGG crystal is 14.7 nm at 1025 nm,which is wider than that of Yb:GGG crystal.The doping of Lu3+ ions leads to an increase in the disorder degree of the crystal structure,which causes the fluorescence inhomogeneous broadening.High-efficiency acousto-optic Q-switched laser output of Yb:LGGG crystal is achieved.The maximum power of 3.26 W is obtained at a repetition rate of 1 kHz,the laser slope efficiency reaches 52%.The corresponding single pulse energy is 3.26 mJ,the pulse width is 14.5 ns and the maximum peak power is 225 kW.The beam quality factor is less than 1.2,which shows excellent beam quality.The results show that the disordered structure of Yb:LGGG crystals has great potential in the use of acousto-optic Q-switching technology to obtain high pulse energy and high peak power laser.B.Crystal growth and characterization of novel geometric ferroelectric RIn03Ferroelectric materials have a spontaneous electric polarization that can be switched by an applied electric field.They are widely used in many areas,including data storage,capacitors,and actuators.Most technologically ferroelectrics are perovskite structure oxides,but there is an increasing interest in the class of hexagonal ABO3 due to potential improper geometric ferroelectricity in these materials.The formation of improper geonetric ferroelectricity,which does not depend upon d0(Nb,Ta)or the lone-pair ions(Bi3+,Pb2+)like the conventional dielectrics,is driven by a non-polar but inversion-symmetry breaking crystallographic distortion,typically rotations or tilts of sub-unit polyhedral.Furthermore,the spontaneous polarization Ps occurs only through the nonlinear coupling to the primary order parameter.The geometric mechanism features novel functionalities such as charged domain walls with tunable conductance which are absent in conventional ferroelectrics.In addition,understanding of the geometric ferroelectrics will broaden research frontiers of ferroelectric materials,and offer a new route for designing magnetic ferroelectrics that have been historically difficult to achieve,but are appealing for 'spintronics'applications.In these systems,hexagonal RMnO3(R:rare-earth element)compounds have been well studied because of their multiferroic characteristics.If the Mn3+ ion is replaced by a nontransition metal In3+ ion,the electric or magnetic properties of the system are expected to arise only from the 4f-shell electrons of the rare-earth ion R.In this regard,it is easier to study the magnetic properties of R3+ in RIn03 than in RM1nO3.RInO3,which are isostructural with hexagonal RMnO3,the first principle study proposes that the spontaneous polarization in RInv3 reaches about 10 ?C/cm2,which is about 2 times as large as those of YMn03 and RMn03 families.Nevertheless,GdInO3 polycrystalline was reported to show a paraelectric behavior recently.Hence,the presence of geometric ferroelectricity in RInO3 is still under debate,and some other physical properties are still obscured due to the absence of RInO3 single crystals.To our best knowledge,there is no report on the growth of RInO3 single crystals up to now.Therefore,it is important to grow the RInO3 single crystal so as to well understand the structures and physical properties of this material.Furthermore,recent contributions reveal a fascinating domain structure featuring topologically protected domain intersections that wind through the h-RMnO3 crystal as vortex lines,which provides a possible pathway for the design and realization of non-volatile vortex memory devices and logical devices.Does the topologically protected domain exist in h-RInO3?In this thesis,crystal growth,structure and physical properties of h-RInO3 were investigated.The main contents and conclusions are as follows:1.h-RInO3 crystal growth by laser floating zone method was investigated for the first time in the world,and a brief introduction about the growth equipment and growth technics was given.Some problems encountered in growth process were also discussed.2.For the first time,the crystal structures of TbInO3,EulnO3 and GdInO3 were solved and refined,meanwhile,the data of crystal structures were submitted to the the cambridge crystallographic data centre(CCDC).Furthermore,the relationship between the crystal structures and the geometric ferroelectricity was studied.3.Electric polarization versus electric field P(E)hysteresis loops clearly show the existence of switchable polarization,which were the first time of experimental demonstration of geometric ferroelectricity in bulk single crystals of h-RInO3.we first study the ferroelectric domain structure across the h-RInO3 series by piezoresponse force microscopy(PFM)and reveal intriguing Z6 vortex topological defects domain structure,which demonstrates the potential application in future information-storage concepts.4.The magnetic properities of TbInO3 and GdInO3 crystal were investitated for the first time,all the results indicate that TbInO3 and GdInO3 are strong spin frustrated system at low temperature. |
PDF Full Text Request