Growth Mechanism And Physical Properties Of Several Important Laser Crystals

Since the operation of the first Ruby laser device in 1960, laser has been proven to be one of the most important innovations and has been widely used in the fields of microelectronics, communication, medical treatment, military, reaserch, education and exploration, due to its unique optical characteristics, such as monochrome, orientation, coherence, which have been used to generate ultrabright and ultrashort pulses. Laser Materials, especially the laser crystals, played a vital role in the development of laser techniques and it can be expected that within 21st century laser and laser techniques will continue to support the fast progress of optoelectronics. At the same time, people will also propose newer and higher criteria for the laser crystals, which make them become the leading frontier and hot topic both in the fields of material science and engineering development.Rare-earth orthovanadate LnVO4 is an important functional crystal family, which has two polymorphs at ambient pressure:tetragonal zircon-type (ZrSiO4) structure with space group of I41/amd and monoclinic monazite-type (CePO4) structure with space group P21/n. Generally, larger Ln cations preferentially form the monazite type owing to the higher oxygen coordination number of 9 as compared to 8 for the zircon type. During the past two decades when the vast development has taken in the techniques of laser-diode pumping solid-state laser (LDPSSL), LnVO4 has been paied much attention to. Recently, a variety of tetragonal LnVO4 with large sized and high optical quality has been successfully grown by using the Czochraski method and many trivalent activated ions, such as Nd3+, Yb3+, Tm3+, Er3+ and Ho3+, have been doped in. YVO4, member of this family, has become one of the hottest stars in the laser crystals for its excellent performance. Nevertheless, just in this family the importance of LaVO4, one with the largest cation radius, and ScVO4, one with the smallest cation radius, has still not been valued, especially in the bulk size.Compared to the traditional Nd3+ ions, Yb3+ has a series of merits:a relatively simple electronic leve-ground state 2F7/2 and excited state 2F5/2, which means Yb-doped laser crystals have a quasi-three energy structure and can avoid any parasite process in the upper level, such as excited state absorption and fluorescence self-quenching; a long fluorescence lifetime and a high quantum efficiency; a broad absorption band in the wavelength of 900-980 nm. Thus with the development of InGaAs diodes, nowadays quasi-three level Yb, as a important activated ion, has been widely accepted. However, the quasi-three level properties of Yb ion will lead to the thermal population at the lower laser level, and thus a inevitable reabsorption will emerge at that band, which could make the properties of Yb laser strongly depends on the temperature. Therefore, the thermal performance of Yb-doped laser crystals is of key importance. For monoclinic silicates Lu2SiO5 and borates GdCa4O(BO3)3, low-symmetric structure will even yield a prominent anisotropy and thus for the design of a efficient laser device it should get to know those relationships. In this thesis, the growth mechanism and anisotropic physical properties of the above-mentioned laser crystals have been fully investigated, by a combination of experimental and theoretical means. It can be categorized in the following fileds:1. Monoclinic Nd:LaVO4 crystals with large size and high optical quality have been successfully grown in the Czochralski method by employing a variety of seeds orientations. It was found that Nd:LaVO4 crystal exhibits a typical bulk spiral growth habit when grown along an arbitrary direction or perpendicular to the (010) crystal face, and a severe footing growth when grown perpendicular to the (101) crystal face. Experiments also show that bulk spiral growth can be greatly reduced if growth proceeds perpendicular to the (001) or (001) crystal face.2. The crystal structure of monoclinic LaVO4 was determined by the x-ray single crystal diffraction. A morphology prediction based on the AE model of HP theory was made for the two representative crystals of the lanthanide orthovanadates:the monoclinic monazite type LaVO4 and the tetragonal zircon type YVO4. The effects of constituent supercooling and evaporation have also been evaluated, and the melting point of LaVO4 was reported for the first time: 2122.24 K at N2 atmosphere. The theoretical growth morphology was sketched using the Wulff construction and the shape of the ideal crystal-melt interface for various seed orientations was determined from the Jackson's theory. Making comparison with the actual growth morphology reveals a triggering mechanism of different seed crystal face orientations on the formation of bulk spiral. A highly axially symmetric crystal-melt interface consisting of large facets with similar growth velocities is much preferable according to our morphological analysis, and the use of such an appropriate seed crystal is advisable because it can greatly reduce the chance of bulk spiral formation. This result will also be beneficial for the selection of seed crystals in the Czochralski growth of other low-symmetry oxide crystals, a process that in the past has usually done by the method of trial and error.3. The growth of tetragonal ScVO4 crystals doped with 0.5 at.% and 1 at.% Nd ions have been explored by using Czochralski method and floating-zone method, respectively. Through optimizing the growth parameters, a transparent bulk Nd:ScVO4 crystal (3×3×1 mm3) with no macro-defects has been obtained by using the floating-zone method for further laser experiments.4. The thermal transition of ScVO4 has been determined by the in-situ and ex-situ thermal analysis experiments, respectively. The reason why single crystal can not be obtained by the Czochralski method was thus given. It was found that incongruent vanadium oxide vaporization brings about a more significant change in the Sc-V stoichiometry of the ScVO4 melt, where a novel tetragonal metallic phase Sc2VO5 was detected within the scandium excess region, while crystalline ScVO4 is obtained primarily near the vanadium excess region. Further structural analysis shows that the high-temperature instability of ScVO4 originates directly from the mismatch between the zircon-type structure and the small size of the scandium cation. Moreover, two distinct topotactic structural transitions from the rutile lattice to the anion-deficient fluorite lattice are triggered in the melt when crystallization begins. Electronic structure analysis further indicates that such structural transitions are driven by a strengthening of the covalency in the Sc-O bonds. The theoretical results on calculations of the standard thermodynamical functions of formation also tend to lend support to this conclusion.5. Within the framework of density functional theory, the elastic stiffness constants of ScVO4 was calculated as follows:C11=240.93, C12=77.14, C13=100.76, C33=277.87, C44=23.22, C66=35.55; anisotropic optical properties, such as constants, absorption spectrum, refractive index, reflectivity and energy loss spectrum were also calculated and evaluated for future applications. The refractive index in the [100]/[010] and [001] directions at 1064 nm is 1.8295 and 2.0374, respectively. All the calculated results tend to support the experimental data. The laser output at a wavelength of 1068 nm has been achieved in the 0.5 at.% Nd:ScVO4 crystal, with a maximum average output power of 240 mW and a threshold of 230 mW.6. The structure of the LSO crystal was determined by using single-crystal XRD data. The unit-cell parameters are a=10.2550(2), b=6.6465(2), c=12.3626(4) A, andβ=102.4220(10)°in space group I2/a. The principal coefficients of the thermal expansion tensor areαⅠ=-1.0235×10-6 K,αⅡ=4.9119×10-6 K andαⅢ=10.1105×10-6 K over the temperature range of 303.15 to 768.15K. The change of unit cell dimensions and monoclinic angle with temperature is also evaluated. The specific heat capacity of LSO is 139.54 J mol-1 K-1 at room temperature. The thermal diffusivity of the crystal was also measured over the temperature range of 303.15 to 572.45 K and the principal components of the thermal conductivity are kⅠ=2.26 W m-1 K-1, kⅡ=3.14 W m-1 K-1 and kⅡ=3.67 W m-1 K-1 at 303.15 K. The relationship between the crystal structures and the anisotropic thermal properties has been fully investigated. These results illustrate that LSO has relatively large thermal conductivity, which makes LSO quite suitable for laser applications.7. The thermophysical properties of GdCOB with a high Yb dopant content were thoroughly investigated. The crystal melts at 1772.35 K at N2 atmosphere and possesses an enthalpy of fusion equal to 106.55 kJ mol-1. The anisotropy in the thermal expansion is much weaker than that of other Yb doped monoclinic crystals and an anomalously linear temperature dependence in the thermal conductivity was observed above 373.15 K. The overall thermal expansion decreases while the overall thermal conductivity kept almost unchanged at room temperature. Such features and the anisotropy in thermal behavior including the thermal expansion and conductivity is much smaller with the doping, which have a great influence on crystal growth and processing and greatly affect the possible application of this material in high average power lasers.