Les cristaux de KDP (KH2PO4) et de DKDP (K(H1-xDx)2PO4) sont largement utilisés pour leur propriété optique non linéaire comme convertisseur de fréquence ou cellule de Pockels. Le développement d’un système de croissance rapide par circulation de solution sous conditions stationnaire permet d’obtenir des cristaux de DKDP plus rapidement avec une distribution en deutérium homogène. Cependant, les premiers optiques obtenus par ce système montrent de fortes perturbations optiques en cartographie de surface d’onde ainsi que du crazing (formation de fissure sur le surface résultant d’un échange isotopique entre H/D).pouvant rendre leur utilisation problématique. Des défauts structuraux en topographie des rayons X ainsi qu’un abaissement de surface de 100 nm (mesuré en microtopographie de surface) ont été observés et sont corrélés par leur localisation avec les perturbations optiques. Cette localisation, identifiée comme étant le secteur vicinal shallow, est également la partie du cristal subissant le plus fort échange isotopique H/D. De plus, il a été montré récemment que le crazing pouvait être provoqué par la présence d’impuretés silicate s’incorporant dans une partie spécifique du cristal, le secteur shallow. Afin de déterminer qui, de l’incorporation d’impureté ou de l’échange isotopique, est responsable des défauts observés, des mesures similaires ont été réalisées sur une lame de KDP. Des perturbations optiques et des défauts structuraux ont également été observés, traduisant le fait que le deutérium exacerbe certains défauts, tel que l’abaissement de surface, mais n’en est pas l’origine. L’incorporation d’impureté dans le secteur shallow est donc l’hypothèse retenue quant à l’origine des perturbations optiques. Afin d’améliorer la qualité des optiques, les paramètres de croissance tels que la température, la sursaturation et le pH ont été modifiés pour identifier des conditions optimales de croissance, sans défauts structuraux ni perturbations optiques. Les topographies aux RX des cristaux obtenues dans les différentes conditions testées, montrent qu’une croissance avec un pH acide présente des qualités optiques prometteuses. Parallèlement, des mesures en ICP-MS ont été réalisées afin d’identifier l’impureté s’incorporant dans le secteur shallow, révélant une corrélation possible entre concentration en calcium et contraste en topographie aux RX. / KDP (KH2PO4) and deuterated DKDP (K(H1-xDx)2PO4) crystals are widely used for their nonlinear optical properties as frequency converters or Pockels cells. The development of a system for rapid growth under stationary conditions led to obtain DKDP crystals in shorter time with homogeneous distribution of deuterium. However, the first uncoated optics cut from DKDP crystals grown with such system showed significant perturbations of the laser wave front (optical distortions) as well as crazing (formation of cracks on the surface due to the isotopic H/D exchange), which may render problematic the utilization of these crystals as optics for high energy lasers. Recently, it has been shown that crazing could be promoted by the presence of silicate impurities incorporated into specific parts of the crystal: the vicinal sectors, shallow. To obtain DKDP crystals with optimal optical properties it was thus of major importance to determine the origin of the optical distortions, their eventual correlation with crazing and identify the mechanism responsible: nature of impurity, its gradient between sectors and what part does the isotopic composition play. We have confirmed for KDP the direct correlations observed in DKDP between vicinal sectors, contrasts in X-ray topography and optical distortions. Complications specific to DKDP related to deuterium losses were observed: crazing and surface alteration takes place only in the vicinal sectors previously identified. So even if inhomogeneous deuterium losses in DKDP may cause optical distortion by itself and should thus be avoided in coated optics, other causes exist. Hence, gradients of traces impurities inside the crystal appear to be at the origin of several phenomena leading to optical distortions. To improve optical quality, growth parameters, as temperature, supersaturation and pH, are modified to identify optimal conditions. X-ray topography obtained for each parameters demonstrate that a crystal grown at acid pH havs a very low density of defects. Simultaneously, ICP-MS is used to identify impurity incorporating inside vicinal sector shallow, revealing a correlation between Ca2+ concentration and structural defects in X-ray topography.

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Present day advanced technologies heavily rely on one particular class of matter, i.e. the crystals. It is the periodic nature of the atoms and the properties arising due to the periodicity in crystals that is exploited to meet various technological feats. The technological revolutions in the semiconductor, optics and communication industries are the examples. The anisotropy in the crystals gives them enhanced properties as required in the field of non-linear optics. The field of non-linear optics became practically a reality after the invention of lasers. The coherent and monochromatic optical beams in the visible and ultraviolet ranges are in high demand due to their application in the fields like material processing, semiconductor lithography, laser micromachining, laser spectroscopy, photochemical synthesis, inertial confinement fusion and other basic scientific studies. In this thesis, work on the growth and characterization of two technologically important non-linear optical crystals has been carried out after developing the necessary instrumentation and some novel techniques for synthesis and growth. Also, studies on the glassy nature of one of the crystals have been carried out. This thesis consists of seven chapters. The first chapter gives a brief introduction to the nonlinear optical phenomenon, crystal growth and glassy state. Instrumentation is the backbone of crystal research technology. Without precision growth equipments large size crystals cannot be grown and without precision characterization instrumentation no conclusion regarding the quality and usefulness of the grown material can be drawn. The work reported in Chapter 2 describes the instrumentation developed for the growth, processing and characterization of crystals grown by solution and melt growth techniques. In low temperature solution growth, crystal growth workstations have been developed using tanks (made of acrylic), heating elements, and stirring propellers. Cooling coils have been inserted into the designed water bath to grow crystals below ambient also. This bath has an advantage to work over a wide range of temperatures, so that maximum retrieval of the material is possible. The growth of large crystals is usually hindered due to spurious nucleation precipitating during the growth process. A novel nucleation-trap crystallizer has been designed and developed that facilitates the continuation of the growth run in spite of extra nucleation precipitating after seeding. In this crystallizer, the spurious nuclei and any other particles generated after the filtration are forced into a well, and the growth of spurious nuclei is arrested by manipulating the temperature of this trap. Achieving adequate heat flow and mass flow profiles is of vital importance for growing good quality crystals. An optimized stirring procedure for the solution or melt is needed for ensuring the desired supply of growth units to the crystal-nutrient interface, and for transporting away any debris of the crystal-growth process. An ACRT set up has been designed and developed. For the growth of crystals by the flux technique and from direct melt, a crystal puller has been designed and developed. The crystal puller consists of a crystal rotation unit, slow and fast pulling mechanisms and a control unit. The pulling assembly is protected from damage caused by possible human errors through interlock mechanisms. The vibration at the shaft of the seed rotation assembly has been minimized by using a dc motor for rotation. A versatile triangular / square wave oscillator has been designed for developing a dc motor control. By implementing this control, the speed of the motor does not vary with supply-voltage variations. A quarter-step switching logic sequence is introduced for stepper motors, which is used for the slow UP/DOWN movement of the puller. This puller can be controlled locally by a control panel provided with the puller, or through a PC remotely by bypassing the local control. Additionally, for the processing and characterization of the grown DKDP crystals, a closed-loop thread-cutter, a ferroelectric loop tracer, and a computer-controlled system for measuring the half-wave voltage have been developed. A novel mercury encapsulant seeding technique that facilitates the processing of solution with immersed seed is invented and has been described in Chapter 3. This technique allows processing of solution with the seed inside the growth chamber, and still avoids contamination of the solution and formation of crystal clusters that are normally generated when seed is inserted after processing of the solution. DKDP and KAP crystal seeds have been used to check the dissolution of seeds, if any, when immersed in pure water for several hours and at high temperatures after introducing the seal. It has been observed that the mercury seal does not allow creeping of water into the seed holder, and there is no dissolution of the seed. This technique has been practically implemented for the growth of crystals from aqueous solution and its usefulness has been demonstrated by growing ammonium acid phthalate, potassium acid phthalate and potassium di-deuterium phosphate crystals. Nonlinear-optical crystals find major use in inertial-confinement fusion (ICF) experiments. For such applications, nonlinear crystals with very large damage-resistance are needed. Alternatively, crystals with moderate damage resistance but large size can be used for frequency-conversion for efficient plasma experiments. Potassium di-hydrogen phosphate, KH2PO4 (KDP) and its deuterated analog, K(DxH1-x)2PO4 (DKDP) are at present the only nonlinear optical crystals which can be grown to large sizes and are suitable for ICF studies. Also, solid-state light valves, light deflectors, and laser communication devices require large and perfect tetragonal DKDP crystals, with high deuterium concentration for easier operation. Chapter 4 describes the growth and characterization of DKDP crystals. DKDP crystals have been grown by all the three techniques i.e. conventional, platform and novel mercury encapsulant seeding techniques. Details about a new approach for the synthesis of DKDP solution have been given. A comparative study of the grown crystals by mercury-encapsulant technique and other techniques is described. Habit modification was observed due to the placement of seed crystals at an off-centre position and orientation in mercury encapsulant seeding technique and has been discussed. The grown crystals have been characterized for homogeneity, dislocations, transmission, DSC, rockng curve, etc. Due to the higher photon energies and the ability to be more tightly focused, coherent radiations of shorter wavelength (deep-UV) are in demand. The photon energies in this region are sufficient for bond-breaking processes in many materials, and find applications in fields like material processing, semiconductor lithography, laser micromachining, laser spectroscopy, photochemical synthesis, etc. Although excimer lasers (XeCl, KrF, ArF etc.) produce significant power in the deep-UV region, these laser systems involve corrosive gases, and are bulky, apart from requiring regular maintenance. A maintenance-free, compact, solid-state laser is preferable. But this, in turn, requires an efficient NLO crystal in that region. CLBO is one such crystal. Growth of CLBO crystals has been carried out by the flux-growth technique using B2O3-deficient flux, as well as from stoichiometric melt and has been discussed in Chapter 5. It was observed that the nucleation of material on platinum wire or spontaneous nucleation was difficult to achieve in spite of high supercooling. After forcing cracks into the mass deposited on platinum wire nucleation could be achieved. The growth of crystals was carried out on seeds with different orientations. Transmission studies, etch-pit studies and harmonic-generation experiments were performed on the grown crystals. The glass-forming tendency of CLBO has been studied and reported in Chapter 6. DTA experiments show that CLBO melt generally transforms to glass on cooling. Even at a cooling rate as low as 1°C/min, the material does not crystallize but transforms into glass. Ergodicity making and glass transition temperatures were determined for glassy CLBO. Since neither the crystallization peak nor the melting peak was observed in DTA experiments during the heating part of thermal cycle for glassy CLBO, a new approach of seeded crystallization was adopted in the calorimetric experiments to achieve crystallization. Since the size of added nuclei is already above the critical radius, the onset of crystallization peaks is independent of the critical-radius energy barrier. Kissenger method was applied to determine the activation energy of seeded- crystallization process. The transformation of glass CLBO to the crystalline phase is mediated by dendrites. Possibility of bulk crystal growth from the glassy state has been discussed, and a novel idea of surface crystallization is proposed. Chapter 7 summarizes the work carried out and projects the scope for future work.