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KDP crystals were grown using rapid techniques and conventional temperature cooling methods. The nonlinear absorption of various growth sections in the crystals was explored using the z-scan technique. It was shown that the nonlinear absorption coefficient of the KDP crystals, grown using rapid techniques, is greater than that of conventional crystals. The largest nonlinear absorption response was discovered at the interface between the pyramid and prism sections; however, the nonlinear absorption of the prismatic sector was smaller than that of the pyramidal sector of the crystal. The nonlinear absorption of the crystals decreased with increase in supersaturation. The value of nonlinear absorption became larger when the conventional specimen was closer to the regenerated cap.

A series of KDP crystals at different rotation speeds ranging from 9 to 120 rpm were grown by rapid growth method. The results fully demonstrate that rotation speed as a key kinetic parameter has a great influence on the whole crystallization process. Moreover, an important characteristic has been proved that the growth rates along X and Z-direction increase with the increase of rotation speed lower than 100 rpm, while the rates of X-direction are always greater than Z-direction. Combined with numerical simulation method, the correlations of solution stability with kinetic factors including rotation speed, crystal size and growth platform are explored in detail. During the crystallization process, the thickness of boundary layer decrease and a higher flow rate is caused with the increase of rotation speed below 100 rpm, and the generation of inclusions at lower speed may be associated with the slower flow rate of growth solution. The stability of solution gradually decreases as the rotation speed ranges from 100 to 120 rpm. Furthermore, the computed results of Re imply that reducing rotation speed as increasing of crystal size may be an effective method to obtain high-quality and large-scale KDP crystal by rapid growth technique. Comparing to the growing platform with four columns, the platform with two circular connecting columns play an active role in influencing the growth rate of KDP crystal, especially in axial direction.

For KDP and 70%-DKDP crystals, knowledge of thermal-stress distribution induced by weak absorption effect is crucial for their applications, especially in high-power laser systems. Based on photo-thermal common-path interferometry (PCI) technique, the correlations of weak absorption effect with different crystal orientations were systematically evaluated at 1064 nm. An obviously anisotropic property is demonstrated, and a similar pattern of z > x > I > II is also represented for KDP and 70%-DKDP crystals. More importantly, crystal uniformity could be reflected by the weak absorption results, indicating that PCI method could be considered as a novel way to identify the crystal quality. Meanwhile, Thermal parameters including thermal expansion and thermal conductivity were obtained. Using the finite-element method (FEM), the temperature gradients and thermal-stresses along z and x-cut samples were distinctly explored, implying that the corresponding distributions are mainly attributed to crystal orientations. Importantly, another fundamental factor is confirmed that the variation of deuterium content could influence the behaviors of temperature gradients and thermal-stresses distributions. Thus, taking advantage of PCI and FEM as a new approach is more conducive to better understanding the mechanism of laser-matter interactions.

The cracking of KDP crystals during the growth & fabrication process is a serious problem that limits their application in the large-aperture high-power laser system; furthermore, the cracking is mainly attributed to their high brittleness. In order to explore the mechanical parameters related to KDP crystal cracking in detail, we measured the fracture toughness of KDP crystals as a function of temperature using the single-edge notched beam (SENB) method up to 170 °C. Meanwhile, the effects of crystallographic orientation, annealing and loading rate on the fracture toughness values were investigated in detail. Strong orientation dependence was observed on the fracture toughness of KDP crystals, and the relationship was concluded as follows: x-cut > type II > type I > z-cut. With the increase in temperature, the fracture toughness of crystals increased and reached its maximum value at 160 °C; then, it decreased, indicating that 160 °C is the temperature at which brittle-to-ductile transition occurs. Using an optical microscope, it was observed that the fractured surfaces of the samples become gradually rough with the increase in temperature. This may offer a viewpoint for understanding the dependence of fracture toughness on temperature. Furthermore, annealing and decrease in the loading rate were conducive to the significant increase in the fracture toughness of KDP crystals.

Large sized Fresnoite Ba 2 TiSi 2 O 8 single crystals were grown by the Czochralski method. Using coordinate transformation methods, the relative dielectric, piezoelectric and elastic constants of Fresnoite Ba 2 TiSi 2 O 8 as a function of orientation were investigated, and their two- and three-dimensional spatial distributions are presented. From them, the maximum values of the piezoelectric coefficients and the corresponding rotation angle can be obtained. The maximum values of coefficients d 22 ∗ , d 23 ∗ , d 24 , ∗ d 32 ∗ , d 33 ∗ and d 34 ∗ were achieved for rotation angles of 40°, 32°, 0°, 58°, 50° and 90°, respectively, and are on the order of 8.7, −4.7, 17.5, 4.7, 8.7 and 17.5 pC/N, respectively. Furthermore, the validity of the electromechanical properties investigation as a function of orientation was verified. The relative dielectric permittivity, elastic constant and piezoelectric coefficient of a ZXl50 o cut rod were calculated and found to be 14.1, 8.7 pC/N and 11.2 pm 2 /N, respectively, in good agreement with the experimental values of 12.7, 9.1 pC/N and 11.7 pm 2 /N. This investigation is important in providing direction for theoretical research and device design of piezoelectric Ba 2 TiSi 2 O 8 crystals.

The phenomenon of the cracking of KDP z-cut seed crystals with a series of sizes due to the temperature difference of the growth solution was explored in detail. Thermal stress generated in the KDP seed crystal at the moment of cracking was calculated using the finite-element method. Temperature differences leading to cracking for heated samples were nearly twice those of cooled specimens. For the cooled samples, the cracks occurred mainly along the (100), (110) and (001) crystal planes and expanded from the sample periphery to the interior spaces, and the opposite crack propagation direction was displayed in the heated samples, with cracks observed mainly along the (110) plane. More importantly, the temperature differences were observed to decrease with an increase in the sample size. In addition, the calculated results indicated that the field distribution of tensile stress is coincident with the points of crack initiation. The maximum stresses calculated were found to be greater than the fracture strength of 6.67 MPa for the KDP crystal along the [100] direction, and the direction of large stresses was found to be normal to the fracture surface. It is interesting that the maximum stress in fracture crystals decreases with an increase in the crystal size as cracks are generated.

By the process of etching and polishing, the dislocation-related etch pits were clearly created on the pyramidal faces for as-grown KDP crystals by rapid growth technique. Through a combination of 3D confocal microscope and optical microscope, the direction of dislocation lines were studied in pyramid section, and the corresponding morphologies of etch pits were also observed in detail. The results show that the dislocation lines are mainly located in the {0 1 0} faces, and the dislocation-related etch pits may be caused by screw dislocations. Moreover, the research indicates that the morphologies of dislocation-related etch pits are somewhat related to the direction of dislocation lines. For the inclined sectors of Iʹ and IIʹ on etch pits, the slope ratio of Iʹ/IIʹ decreases with the increases of β as an angle between the direction of dislocation line and the normal of {101} face. Therefore, the information of dislocation in crystal, especially the orientation of dislocation line, can be inferred by the spatial size characteristics of dislocation-related etch pits.

A large sized Nd-doped Ca2Ga2SiO7 single crystal was successfully grown by the Czochralski technique. The effective segregation coefficient of Nd3+ was measured to be 1.16. The thermal, spectroscopic and laser properties were systematically investigated. The thermal conductivity was found to increase with increasing temperature, exhibiting glass or alloy like behavior, and is inherently associated with the disordered structure of the Ca2Ga2SiO7 crystal. The polarized absorption and fluorescence spectra were measured at room temperature, and the spectrum parameters were calculated using the Judd-Ofelt theory. Furthermore, continuous wave laser operation at 1.06 μm along the crystallographic a- and c-axes was demonstrated. A laser output power of 430 mW with an optical conversion efficiency of 14.2% and a slope efficiency of 16.7% was achieved. These results show that Nd:Ca2Ga2SiO7 holds excellent promise for use as a disordered crystalline laser material.

A series of potassium dihydrogen phosphate (KDP) crystals and Fe3+ doped KDP crystals were grown at different temperatures and at the same supersaturation value of 0.02. The prismatic face growth rates of the KDP crystals were measured using a laser polarization interference system. The surface morphologies of the growth steps on the {100} face were comparatively studied using an atomic force microscope systematically. The results showed that the bunching of steps was related to the supersaturation interval in which it was located, and approached the supersaturation dead zone of σd in which the bunching and the slope of the steps were high. The supersaturation dead zone σd and linear zone σ* decreased with rising temperature. Varying the temperature caused considerable changes in the growth speed, step morphology and step motion speed. For the crystal grown in a doped solution, the bunching and slope of the steps reached a maximum value at around 45 °C. It was interesting that the crystal growth rate in the doped solution was higher than that in the undoped solution at around 65 °C and 75 °C, the bunching of the steps was reduced and the width of the steps widened significantly. At the same supersaturation, the growth rate increased with rising temperature and the steps became straight.

Transient numerical calculations were carried out to predict the evolution of temperature and thermal stress in traditionally grown large-size KDP crystals during the removal process, considering two methods that are used to accomplish the crystal extraction. The influence of the crystal size and the difference of temperature between the crystal and environment on the stresses inside the KDP crystals were also investigated in detail. Results indicate that, in both processes of isolating crystals, the highest stress transfers from the crystal periphery to the internal part from the early to the later time stage. In the case of extracting the crystal from solution directly after crystallization and exposing to air, the maximum stress at the crystal periphery is larger than that inside the crystal, and the probability of failure from the outside surface of crystals is large. In the case of retaining the solution for a time after crystallization, the maximum stress in the crystal internal region is larger than that of the crystal surface, leading to a large possibility to originate cracks in the inner region. Both increased crystal sizes and increased temperature differences between the crystals and the environment at the end of crystal growth are factors which aggravate crystal cracking. The maximum stress in crystals in the case of retaining the solution is less than that in the case of extracting the solution, which brings about a decreased likelihood of cracking. Thus, retaining solution for a period of time after the growth is completed, such as 96 h, is suggested to be adopted to accomplish successful crystal extraction.

A series of trace Fe3+-doped KH2PO4 single crystals were grown using the conventional temperature cooling method. The Fe3+ ion content in the as-grown crystals and the corresponding transmission spectrum were measured, respectively. With an increase of Fe3+ ion concentration, the transmittance of the crystal gradually decreases within the wavelength range of 200 nm to 370 nm. Under picosecond pulse laser irradiation, third-order nonlinear optical characteristics were systematically measured at λ = 532 nm. The existence of nonlinear absorption and nonlinear refraction was demonstrated. The results show that the nonlinear refractive index n2 is positive, indicating the presence of a self-focusing effect for the as-grown crystals. Specially, nonlinear absorption and refraction have a heavy dependence on the Fe3+ ion content and crystal directions (II and z). With increasing Fe3+ ion concentration, a significant variation has been observed with respect to the nonlinear absorption coefficient β, nonlinear refractive index n2 and third-order nonlinear susceptibility χ(3). A similar pattern for II < z has been presented for nonlinear absorption and refraction. The investigations suggest that nonlinear absorption and refraction caused by distortion of the electron cloud might be associated with the H2PO4− and (FeO4)2− groups.

For rapidly grown KDP crystal, the whole growth zone is composed of prismatic- and pyramidal-sector. In order to further understand the distinctions between them, KDP crystals doped with Fe3+ in trace amount were selected to explore the influences of metal cation with high-valence on growth characteristics, Optical quality and nonlinear refraction. The growth characteristics for as-grown crystals indicate that the growth rate of prismatic sector can be obviously inhibited by Fe3+ ions with less than 15 ppm due to the stronger electronegativity of (100) face. With the increase of Fe3+, the crystal and optical qualities show significantly lower trends, especially for the prismatic sector. The influence of Fe3+ ions on nonlinear refraction was observed by Z-scan technique, which shows that self-focusing effect could be caused in high-intensity laser system because of the nonlinear refractive index obtained with a positive sign. Specially, nonlinear refraction effects along II-type and z-cut in prismatic area both show a significant variation with increase of Fe3+, while a negligible effect on nonlinear refraction is proved for pyramidal sector. Additionally, the generation of nonlinear refraction is mainly attributed to the electron-cloud distortion of H2PO4− groups, and the changes in prismatic sector may be due to the existence of (FeO4)2- groups.

Potassium dihydrogen phosphate crystals were grown around 65 °C. The growth rate of KDP (KH2PO4) crystal prismatic faces was measured by laser polarization interference system. The surface micro morphology of KDP crystals grown at various supersaturation were comparatively observed by atomic force microscope systematically. The results show that the growth rate increased with the rise of supersaturation, the aspect ratio reduced. The steps bunching height occurred at σ = 0.03 and 0.04. At this point, the steps height reached maximum value about 11.3~24.4 nm. The step bunching was almost constant at higher supersaturation. A few pits and holes appeared on the crystal surface at high supersaturation. In our opinion, the forming of these pits and holes might be related with the step movement and 2D nuclei mechanism.

Nd3+:Ba3Gd2(BO3)4 single crystal of sized ∅20 mm × 20 mm was grown by the Czochralski technique. The effective segregation coefficient of Nd3+ in Ba3Gd2(BO3)4 crystals was measured to be 1.147. A series of thermal properties, including specific heat, the average linear thermal expansion coefficient, thermal diffusivity and thermal conductivity, were systematically determined as a function of temperature. The thermal conductivity along a, b and c-axes increases with increasing temperature. The room temperature polarized absorption spectrum was obtained over the range of 330–1000 nm, together with the room temperature fluorescence spectrum. The spectral parameters of the crystal were calculated by using the Judd–Ofelt (J–O) theory. The fluorescence lifetime is 80.58 μs and the fluorescence quantum efficiency is ηc = 43.8%. A large FWHM value, moderate absorption cross-section and high thermal conductivity show that Nd3+:Ba3Gd2(BO3)4 crystal holds promise as a useful solid state laser material.

A series of potassium dihydrogen phosphate (KDP) crystals were grown by “point seed” rapid growth technique from solutions with different concentrations of cyclohexane diamine tetraacetic acid (CDTA). Effects of CDTA on rapid growth and optical performances of KDP crystals were investigated. The measurement result of laser polarization interference showed that, with increasing the concentration of CDTA in solution, the growth critical supersaturation for the (100) face of KDP crystals decreased continuously, while the growth rate for the (100) face of KDP crystals first increased and then decreased, passing through a maximum. The addition of an appropriate amount of CDTA enhanced the solution stability, increased the UV transmittance and the laser damage threshold, and mitigated the light scattering of these as-grown KDP crystals. However, excess addition of CDTA had adverse effects. The effects of CDTA on the rapid growth and optical properties of KDP crystals are discussed in terms of the chelation of CDTA with impurity metal ions and adsorption of CDTA molecules onto the faces of KDP crystals.

Potassium dihydrogen phosphate (KDP) crystals were grown in the presence of a series of chromate (CrO\(_{4}^{2-}\)) additive concentrations via rapid growth method. CrO\(_{4}^{2-}\) made KDP crystals were coloured by yellow-green, suggesting CrO\(_{4}^{2-}\) had entered into the crystal lattice. The elemental analysis indicated that Cr element in KDP crystal was at ppm level. High resolution X-ray diffraction data revealed that the crystalline perfection of these as-grown KDP crystals was destroyed after CrO\(_{4}^{2-}\) entered into crystal lattice, embedded in the full width at half maximum was broadened and satellite peaks appeared. Additionally, the extinction ratio was decreased with rise of CrO\(_{4}^{2-}\) concentration. CrO\(_{4}^{2-}\) introduced two absorption peaks centred at 360 and 280 nm and enhanced the intrinsic absorption near 220 nm, which were at the same band positions compared with the CrO\(_{4}^{2-}\) or HCrO\(_{4}^{-}\) transmittance spectra. Additionally, CrO\(_{4}^{2-}\) could increase the size of light scattering, which was attributed to the point defects and microscopic defects by the replacement by CrO\(_{4}^{2-}\) at PO\(_{4}^{3-}\) position.

In this work, using large-sized single crystals of high optical quality, the optical properties of Ba2TiSi2O8 were systematically investigated, including transmission spectra, refractive indices and nonlinear absorption properties. The crystal exhibits a high transmittance (>84%) over a wide wavelength range from 340 to 2500 nm. The refractive indices in the range from 0.31256 to 1.01398 μm were measured, and Sellmeier’s equations were fitted by the least squares method. The nonlinear absorption properties were studied by using the open-aperture Z-scan technique, with a nonlinear absorption coefficient measured to be on the order of 0.257 cm/GW at the peak power density of 16.4 GW/cm2. Such high transmittance and wide transparency indicate that optical devices using the Ba2TiSi2O8crystal can be applied over a wide wavelength range. Furthermore, the small nonlinear absorption observed in Ba2TiSi2O8 will effectively increase the optical conversion efficiency, decreasing the generation of laser damage of the optical device.

High-quality KDP and DKDP crystals with 12%, 70% and 80%-deuterium content were grown by the conventional temperature cooling method. The nonlinear optical characteristics of K(DxH1-x)2PO4 crystals, including nonlinear absorption and refraction, were systematically investigated through the Z-scan method at λ = 532 nm. The results demonstrate the existence of nonlinear absorption and refraction, and a nonlinear refractive index with a positive sign indicates the generation of self-focusing effect. Moreover, the results indicate that nonlinear absorption and refraction were associated with crystal direction and structure, especially the distribution of H2PO4 - or D2PO4 - groups. For K(DxH1-x)2PO4 crystals, a similar rule of β was z > I > II, while n2 (or χR(3)) was z > II ≈I. These results will evoke more attention to KDP and DKDP applications, especially frequency conversion and electro-optic switch.

Large Ba2TiSi2O8 crystals were successfully grown by the Czochralski technique. The stimulated Raman scattering properties of the Ba2TiSi2O8 crystals were investigated for the first time, and the first (558 nm) and second (587 nm) Stokes lines were generated in an ultracavity single-pass configuration excited by 532 nm picosecond pulses. The output energy and conversion efficiency of the scattering process as functions of pump energy were studied, and the highest conversion efficiency was found to be 31.8%. The experiments indicate that Ba2TiSi2O8 is a novel active crystal suitable for solid-state Raman laser applications.