钼酸盐激光晶体的研究进展

对钼酸盐激光晶体材料的结构、生长技术、光谱特性等特点在国内外的研究进行了综述,认为作为激光晶体材料可以利用的钼酸盐分为类白钨矿型和钾钠铅矾型,在合成方法上以提拉法为主,但由于挥发及生长工艺等问题,晶体会出现色心、晶裂等现象.结合前人对该类材料的生长技术研究,我们重点对掺Yb3+离子的白钨矿型钼酸盐激光晶体光谱学特点进行了研究和总结,认为掺Yb3+离子的白钨矿型钼酸盐有望成为新一代的激光晶体材料.     

5英寸Nd:GGG激光晶体的生长

本文介绍采用提拉法,运用上称重自等径技术(ADC)生长5inch的掺钕钆镓石榴石(Nd:GGG)晶体.通过调整生长工艺参数,优化温场结构,我们成功生长了5inch的Nd:GGG晶体.晶体外观完整,无宏观缺陷,有望作为激光工作物质用于强固体热容激光器中.此外,我们对晶体的外形和生长界面作了讨论.     

溶液连续过滤快速生长KDP晶体及其品质分析

设计溶液实时连续过滤装置用于降温法快速生长KDP晶体.采用高亮度激光照射分析对比晶体内部的散射点密度,以同步辐射为光源的X射线形貌成像技术探测晶体内部的生长缺陷,并测定晶体三倍频激光(355 nm)的损伤阈值.实验结果显示,溶液连续过滤法生长的KDP晶体中包裹体和位错的密度明显降低,晶体的激光损伤阈值提高了30%.     

ADP晶体的全方位生长方法研究

采用一种全新的ADP晶体生长方法,使晶体首先恢复其理想外形,实现晶体的全方位生长,从而提高晶体的生长速度.并对所得晶体进行了透过率,激光散射,摇摆曲线测试及热重差热分析,与常规生长方法比较并分析了全方位生长方法的优势.     

提拉法生长直径8inch Yb∶YAG激光晶体

设计制造了适于生长直径8 inch Yb∶YAG激光晶体的生长系统和热场,采用感应加热提拉法成功生长出了直径8英寸Yb∶YAG激光晶体,晶体等径部分最小直径202 mm,等径长超过215 mm,总重63.7 kg,晶体外形完整,内部无气泡、包裹物、开裂等宏观缺陷,5 mW绿光激光照射下,未见散射颗粒.     

点籽晶定向生长70％DKDP晶体的研究

磷酸二氢钾(KDP)类晶体快速生长主要采用点籽晶快速生长方法,三维生长的点籽晶不可避免的会使晶体产生柱锥交界线,从而影响晶体的性能和质量.本文通过所设计的载晶架,改进传统点籽晶快速生长方法,采用点籽晶定向生长方法,成功生长出磷酸二氘钾晶体(70%DKDP),避开生长晶体中的柱锥交界线取片,并对其氘含量、透过率、抗激光损伤性能以及光学均匀性等指标进行了测试.结果表明:所生长的晶体氘含量符合设计要求,晶体透过率和抗激光损伤性能和传统点籽晶快速生长晶体保持一致.光学均匀性结果表明样片内部无柱锥交界线,对于大口径70%DKDP晶体的生长具有指导意义.     

Nd:LuVO4晶体缺陷的研究

采用提拉法生长的Nd:LuVO4晶体是一种适合二极管泵浦的新型激光晶体,运用化学腐蚀结合光学显微术和同步辐射白光X射线形貌术对Nd:LuVO4晶体缺陷进行观察,结果表明:晶体的主要缺陷为位错和小角晶界.利用高分辨X射线衍射仪进一步验证了这一结果.并初步讨论了缺陷形成的原因.     

2.7μm激光晶体Yb,Er,Ho∶GSGG的生长与光谱性能研究

用提拉法成功生长出了Yb,Er,Ho∶ GSGG晶体,对其光谱性能进行了研究.测量了晶体在320～3000 nm波段内的吸收光谱.用970 nm激光激发,测量了晶体的稳态和瞬态荧光光谱,拟合得到2.7～3 μm激光上下能级寿命,用F-L公式计算了晶体的发射截面.与Er∶ GSGG和Yb,Er∶ GSGG晶体的光谱参数进行了比较,结果表明,Yb,Er,Ho∶GSGG是一种更适合LD泵浦,实现低阈值、高效率2.79 μm激光输出的新型激光晶体.     

2.1μm激光晶体Cr,Tm,Ho∶YAG的生长、光谱和激光性能

采用提拉法成功生长出了优质的Cr,Tm,Ho∶ YAG晶体,并对其光谱及激光性能进行了研究.测量了晶体在350～2700 nm波段内的吸收光谱.用450 nm激光激发,测量了晶体的稳态和瞬态荧光光谱,拟合得到2.1 μm激光上能级的寿命为10.65 ms.用F-L公式计算了晶体在2.1 μm处的发射截面为6.92×10-20 cm2,并与其他Ho3+掺杂的激光晶体进行了对比.采用氙灯泵浦Cr,Tm,Ho∶ YAG晶体棒,实现2.1 μm的脉冲输出.在3 Hz和10 Hz时的激光阈值分别为33.11 J和33.88 J,斜效率均为4.5;,比市售成熟的激光棒的激光输出效率更高.     

高抗灰迹KTP晶体生长与性能研究

采用顶部熔盐籽晶法生长了具有高抗灰迹性能的KTiOPO4(HGTR-KTP)晶体,并对其透过光谱、光学均匀性、相位匹配角、抗激光损伤阈值等性能进行了表征和测试,与普通KTP晶体的对比和大功率激光实验表明该晶体具有高的抗灰迹性能。     

Measurement of three‐dimensional concentration gradients around a crystal growing from its aqueous solution using laser schlieren

Schlieren measurements of the gradients of the concentration field around a KDP crystal growing from its aqueous solution are reported. The measurement of the concentration gradient field is important for crystal growth because it controls the rate of solute transport from the bulk of the solution to the crystal surface. In the crystal vicinity, the concentration gradients have a three dimensional distribution. The concentration gradient field has been imaged using monochrome schlieren technique. Four view angles, namely 0, 45, 90 and 135° have been utilized. By interpreting the schlieren images as projection data of solute concentration gradient, the three‐dimensional concentration gradient field around the crystal has been determined using an algebraic reconstruction technique. At low supersaturation levels, the growth process is accompanied by weak fluid movement during which diffusion effects are significant. At higher levels of supersaturation and large crystal size, a well‐defined convective plume around the growing crystal is observed. Reconstruction of concentration gradients around the crystal explains the preferential growth rates of various faces of the crystal. The non‐circular shape of the crystal is seen to affect the symmetry of the distribution of concentration gradients in its vicinity. The effect of crystal morphology on the orientation of convection currents rising from the crystal surface has also been brought out on the basis of the reconstructed concentration gradients distribution in the growth chamber. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fourier analysis based phase shift interferometric tomography for three-dimensional reconstruction of concentration field around a growing crystal

The present work is concerned with the three-dimensional reconstruction of concentration field around a crystal growing from its aqueous solution using Fourier analysis based phase shift interferometric tomography. Projection data of convective field around a growing NaClO₃ crystal have been recorded using a Mach–Zehnder interferometer. A new numerical method based on the concept of real time phase shift interferometry has been employed for the analysis of the interferograms. By interpreting the interferograms as projection data, the 3-D distribution of salt concentration has been determined using principles of tomography. Study shows that the Fourier analysis-based phase shift method is a novel approach for crystal growth studies as it requires only one interferometric image for the determination of full phase information as opposed to the conventional real time phase shift interferometry wherein at least three or more phase-shifted interferograms are needed. Results have been presented in the form of interferograms, path-integrated concentration contours and three-dimensional concentration profiles over select horizontal planes above the growing crystal. Based on the reconstruction results, distribution of salt concentration in the crystal vicinity is determined and an appropriate mechanism of buoyancy-induced fluid movement in the growth chamber is discussed.     

Implementation of Temperature-Controlled Method of Protein Crystallization in Microgravity

An experimental scientific equipment for implementing temperature-controlled protein crystallization in capillaries under microgravity has been developed, fabricated, and tested. This crystallization method, providing on-line separate control of crystal growth both in the stage of nucleation of crystals and during their further growth, requires small amounts of protein solution. The equipment has been tested on board of Foton-M4 spacecraft (growth of lysozyme protein crystals of high structural quality in microgravity) using a cyclogram developed in ground-based experiments. The results obtained have demonstrated efficiency and importance of the developed equipment and method for growing biomacromolecular crystals of high-structural quality.

     

Analysis of the Dynamics of the Controlled Crystallization Process Using the Czochralski Method

A mathematical model proposed for the Czochralski method has been employed to analyse the stability, to calculate the transitional characteristics of open and closed systems for the case that the PID control is used in the feedback circuit. The method to control the crystallization process by weighing the crystal is analysed in detail. Based on the proposed criterion of quality of the transitional process, an optimum adjustment of the controller for one- and two-channel control is performed.     

Modelling the dynamics and control design for Czochralski,Liquid Encapsulated Czochralski and Floating Zone processes

The need for high quality crystals constantly rise. It is especially obvious in connection with the evolution of electronics and optoelectronics. At the present time all the basic methods for crystal growing are known. So the question arises – what further developments are needed to create further advances? Without doubt first of all it is necessary to speak about perfecting crystal growing equipment. Perfecting modern equipment will enhance and accommodate the results of our understanding of the crystallization physics and provide solutions to the various physical tasks at the atomic and macroscopic levels. Each new step in the process of perfecting the technology and production processes demands large intellectual and material inputs. The continual updating of pullers requires constructive solutions and control systems. Mathematical modelling of the methods of crystallization enables one to more rapidly create the software for the digital systems which are a feature of the latest achievements of physics, that is IT engineering and the modern theory of automatic control.Here we consider the problem of mathematical modelling of crystallization processes from the melt by Czochralski, Liquid Encapsulated Czochralski and Floating Zone methods based on linearization of three conservation laws: the heat, mass and the growth angle constancy is reviewed in depth. Special attention is given to the problem of the dynamical analysis of these processes in open and closed states and to the synthesis of the digital control of crystal diameter for the weight technique. The main problems discussed involve the determination of the parameters required for calculating such control systems together with the use of the multichannel parametric PID regulator involving the state variable observer concept. In addition briefly considered are the problems of digital filtering of the measurement noise based on multidimentional Kalman filters and the determination of mechanical stability limits for static menisci.     

Growth spirals of gadolinium gallium garnet (GGG) crystals

Slices cut from various locations in two GGG spirals grown by the Czochralski method have been studied using double crystal X-ray topography. In selected regions of the slices the lattice parameter has been measured by a modified Bond method. An increase of lattice parameter has been found in the outer part of the crystal where growth bands have smaller periodicity than those in the central region of the spiral. An analysis of crystallization front changes allowed us to suppose that the asymmetry of the heat field around the growing crystal, and the forced convection of the melt, are responsible for the crystal's growing in the shape of a spiral.     

Study of the possibility of growing germanium single crystals under low temperature gradients

The possibility of growing germanium single crystals under low temperature gradients in order to produce a dislocation-free material has been studied. Germanium crystals with a dislocation density of about 100–200 cm^?2 have been grown in a system with a weight control of crystal growth at maximum axial gradients of about 1.5 K/cm.     

Thermal radiation and low-temperature-vapour growth of HgI2 crystal in production furnace

Heat exchanges in a sealed ampoule in the LTVG (low temperature vapour growth) furnace have been modelled in order to compute temperature fields and control the growth of HgI₂ crystals from vapour phase at low temperatures. We use a coupled conductive-radiative model to determine the shapes of the source and the crystal at different equilibrium states (i.e. without growth rate). The model involves conductivity anisotropy in the crystal and radiative exchanges between grey and diffuse surfaces (source and crystal interfaces, Pyrex walls), which are considered as opaque. Internal buoyancy effect is not taken into account as the pressure inside the ampoule is very small. The source temperature is fixed. For different undercoolings, i.e. for different cold finger temperatures, the “equilibrium” isotherm between the source/gas and crystal/gas interface has been numerically obtained. This “equilibrium” isotherm, which is associated with the stop of the growing process, gives a crystal shape. This shape is compared with experimental results given by the ETH-Zürich group. The model would permit a better understanding and control of the future HgI₂ crystal growth experiment. The computations are performed using a finite element package (FIDAP).     

Influence of Gravitation on the Processes of Heat and Mass Transfer in Solution Crystal Growth by the Travelling Heater Method (THM) (II)

The two-dimensional quasi-stationary thermodiffusive hydrodynamic problem of the crystal PbTe growth from solution by the travelling heater method has been solved. Computation is given for the temperature field distribution in crystal and solution. The picture of fluxes in solution was obtained under free convection for different lengths of solvent zone. It is shown that in the absence of absence of gravitation at the growing interface in solution there occurs a constitutional supercooling, whereas the free convection removes it, which leads to greater morphological stability of a growing crystal face.     

Rim formation on crystal faces growing in confinement

A crystal face growing from solution while exerting a normal force on a confining surface is often observed to develop a growth rim surrounding a hollow core. The interpretation has been that this is a manifestation of steady state growth due to the balance between the concentration gradient and stress gradient along the confined crystal surface. In this paper, we present experimental results which show that the growth rim is instead formed as a consequence of faceted growth on the confined surface. Steady state growth is not ensured by a gradient in normal stress, but rather a gradient in step density along the crystal face. The loaded crystal surfaces display a high degree of roughness, and the stress is not uniformly distributed across the surface, but transmitted at discrete asperities. We discuss the implications of these findings for the interpretation of previous experimental results, and for the thermodynamics of crystal growth subject to normal stress.