Exploration For Laser-induced Damage Mechanism Of Crystals In L-arginine Family And Preparation Of Novel Crystals

Techniques used in superpower laser have attracted considerable attention because of their wide application in the inertial confinement fusion field. Recently, pulse laser with the maximum power intensity to 300 TW at 0.1 Hz repetition rate has been accomplished successfully. The upgrading and development of such high-power laser will strictly demand that optical materials should not only possess higher quality and larger size, but also have more outstanding resistance to laser-induced damage, which is one of the most important issues.L-Arginine phosphate monohydrate (abbreviated as LAP) crystal was firstly discovered by Shandong University as a promising ultraviolet nonlinear optical (NLO) material and has obtained a great deal of authorizations. The most attractive properties of LAP crystal, especially its deuterated analogue namely DLAP crystal, such as high conversion coefficient (maximum to 90%) and extremely high laser-induced damage threshold make it more outstanding than other NLO crystals. Research results achieved by Japanese scientists indicate that laser-induced damage threshold of DLAP crystal is 87 GW/cm2 with wavelength of 1053 nm and pulse-width at 1 ns, which is about 5 times higher than that of KDP crystal. Therefore, LAP and DLAP crystals are considered as extremely promising NLO materials in frequency conversion and wave-front shaping fields.Many experts all of the world express their interests in the reason that LAP crystal and its deuterated analogue have such high resistance to laser-induced damage. Many investigations have been carried out on this issue. However, it has not obtained very comprehensive achievements up to now. The innovation and exploration for the laser induced damage mechanism of LAP crystal has become the research topic.Our research group has concentratively devoted to exploration for the reason that LAP possesses high laser-induced damage threshold. Through sufficient literature investigations, we find that phosphate arginine (abbreviated as PA) works as the medium carrier for bioenergy transport and storage in invertebrates, whose particular structural features are responsible for the biological functions. In the whole process, arginine presents its conformational flexibilities and varieties. LAP crystal has the same structural units as PA in invertebrates. It is the question attracts our attention whether laser irradiation will arouse conformational variations of L-arginine molecule in LAP crystal. Thus, our exploration for the reason that LAP possesses high laser-induced damage threshold is established on the following issue:relationship between biological flexibilities and stimulated changes of arginine in crystals. Meanwhile, a series of novel crystals in L-arginine and amino acid derivate family have been prepared and characterized in order to develop NLO materials with excellent merits. The dissertation mainly comprises the following aspects:(Ⅰ) The dissertation has investigated and summarized the structural variations of PA molecule during bioenergy storage and transport process in invertebrates.PA is considered as the bioenergy carrier in invertebrates, which provides enough bioenergy for biological activities. It exhibits obvious flexibilities of conformations during bioenergy storage and conversion process. It is found that the bonding interactions of arginine with phosphate group make its conformation change from an extended state to a folded state, which will be reversible while the bioenergy transfers to others.(Ⅱ) The dissertation investigates structural diversifications induced by laser irradiation in LAP in combination with crystal structures of L-arginine derivates.The conformational features of L-arginine molecules in crystals have been investigated by the X-ray single-crystal diffraction. The plane equations resolved by Mercury indicate that dihedral angle between guanidyl group and carboxyl group is 29°in LAP crystal. In comparison with other crystals, the two above-mentioned planes are rotated against carbon chain due to the different electrovalent bonding interactions. L-Arginine molecular conformations are also influenced by crystal components, charge distribution and hydrogen bonds, etc.In LAP crystal, phosphate group has an ionic configuration with distorted tetrahedron. The asymmetric structure with two P=O bonds and two P-O bonds will be affected by laser irradiation. The Raman spectral lines at 929,950 and 973 cm-1 are assigned to symmetric stretching mode of O-P-O bonds. The characteristic lines at 413 and 394 cm-1 are caused by symmetric deformation of PO4. The asymmetric deformation bands of PO4 group are located at 518 and 530 cm-1. The varieties of phosphate groups in LAP crystal have been investigated by micro-zone Raman spectral technique. It is found that the characteristic lines at 973 and 413 cm-1 disappear. The relative intensities of 518 cm-1 and other vibrational bands have also been weaken, which are resulted from laser irradiation.(Ⅲ) The relationship has been discussed between biological flexibilities of PA and stimulated changes of arginine in LAP crystal. The influences on laser-induced damage have also been discussed.Absence of these splitted peaks implies that the distorted phosphate tetrahedron trends to become more regular and homogenous. Thus, molecular conformation of L-arginine changes due to weak effect of phosphate group. Energy transport in LAP crystal leads to varieties of its component and molecular conformation. In bioenergy transport process, the zwitterionic conformation of L-arginine changes from an extended state to the folded state, which has been proved by many methods. However, the similar variety induced by laser irradiation is quite difficult to observe directly by experiments. Using ab initio total-energy calculations and density functional theory (DFT), we mainly discuss the influences caused by the different molecular conformations on total density of states and band structures. It is found that conformational varieties do not change band structures of LAP crystal, which will not obviously affect the laser-induced damage threshold. However, the band structures will be influenced because of intrinsical defects induced by conformational varieties.(Ⅳ) Thermal properties of NLO crystals in L-arginine family, such as specific heat, thermal expansion and thermal conductivity, are systematically discussed as important influencing factors on their laser-induced damage thresholds under different laser conditions.Larger specific heat of L-arginine crystals is responsible for higher resistance to laser-induced damage. The respective specific heat values of LAP, LATF. and LABTF are 158.99,340.1 and 560.8 J·mol-1·K-1, which are much larger than inorganic materials, plus higher laser-induced damage thresholds under laser irradiation with low repetition rates. Properties of thermal expansion are also contributive to their resistance to laser-induced damage. The measured thermal expansion coefficients of LATF along three principal axes are 51.4×10-6,7.5×10-6 and 16.4×10-6 K-1, while the corresponding values of LABTF are 98.7×10-6,-8.6×10-6 and 70.4×10-6K-1. In contrast with LATF, LABTF crystal has more anisotropic coefficients and the corresponding properties are quite abnormal along b-axis. Thus, LABTF is easily cracked under laser irradiation, accordingly exhibiting low laser-induced damage threshold.Thermal conductivity coefficient of LAP crystal along two-fold axis is 0.59 W·m-1·K-1. The measured thermal conductivity coefficient of LATF along b-axis is 1.264 W·m-1·K-1, which is inferior to those of inorganic materials. Thus, LAP and LATF become easily destroyed under laser with high repetition rates exhibiting weak resistance to laser-induced damage.(V) Several novel NLO crystals in L-arginine and other amino-acid derivates have been prepared and characterized in the dissertation.LATF and LABTF, two organic crystals exhibiting excellent laser-induced damage thresholds, have been discovered and prepared from L-arginine plus trifluoroacetic acid system. Bulk LATF crystals with maximum size up to 35×17×9.5 mm3 and high optical qualities have been grown by the temperature lowering method. LABTF single-crystals with sizes of 32×21×4 mm3 and 20x29x5 mm3 have also been obtained by temperature lowering and solvent evaporation methods, respectively. Crystal structures of LATF and LABTF are considered to reveal differences of their growth habits. Defects of stripes, inclusions, cracks and tapering, etc in LATF crystals are investigated and analyzed, which inhibit crystal qualities and laser-induced damage threshold. Corresponding measurements have also been adopted to improve crystal qualities.The properties of LATF and LABTF crystals have been investigated, and it is found that the respective ultraviolet cutoff wavelengths for LATF and LABTF are 232 and 237 nm with excellent transparent efficiencies in Uv-vis-NIR region. Their powder SHG efficiencies are equally estimated to be 2.5 times higher than that of KDP. The experimental figures reveal that LATF possesses much higher thermal stability and more outstanding resistance to laser-induced damage than that of LABTF crystal. All the outcomes indicate that LATF is a promising organic NLO crystal with high laser-induced damage threshold.Crystals of other amino-acid derivates, named L-lysine trifluoroacetate (LYTF) and L-lysine acetate (LYAC) with respective sizes of 28×12×6 mm3 and 17×10×4 mm3, have been prepared by the solvent evaporation and temperature lowering method. The powder SHG efficiency of LYTF is about 1 time higher than that of KDP, while LYAC is estimated to be 0.5 times higher than that of KDP. Their structural, thermal and optical properties have also been investigated.All the outcomes reveal that variations of molecular conformations induced by laser irradiation are similar to those of PA in bioenergy process, which provides an important foundation for laser-induced damage mechanism of LAP crystal. The experimental results suggest that LATF and LABTF crystals are promising NLO materials with high laser-induced damage threshold.