Growth and characterization of organic and semi-organic nonlinear optical single crystals

Abstract

Crystals of Ammonium Dihydrogen Phosphate (ADP), Potassium Dihydrogen Phosphate (KDP), Potassium Acid Phthalate (KAP) and Amino Acid L-alanine (LA) are the best nonlinear optical (NLO) materials increasingly being used for second harmonic generation, frequency doubling of Neodium : Yttrium-aluminium garnet (Nd-YAG) laser and also in electro-optical applications. The typical functions of these materials are to modulate carrier waves, to amplify and to rectify signals and acts as very fast switch. Though KDP and ADP have all essential qualities but these suffer from relatively low electro-optic coefficients. The electro-optic coefficient of these crystals mainly originates from their P-O bonds. The electro-optic properties of these crystals can be improved by the replacement of K+ by NH3+ or COO- ion of the LA materials. LA has a Zwitterionic nature of the molecule (+NH3 – C2H4 – COO-), which favors the way for possessing high electro-optic parameters and good mechanical and thermal stability of the crystals. The KAP is a noncentrosymmetric molecular ionic crystal. It is widely used in the field of X-ray spectroscopy as monochromator and also as analyzer. So it would be a good approach for improving the electro-optic properties of the LA doped ADP, KDP and KAP crystals. The growth of pure LA and LA doped ADP, KDP and KAP crystals with different concentrations (such as 0.3, 0.5, 0.7 and 1.0 mol% of LA) by slow evaporation method at room temperature are discussed. The solubility of pure LA, pure and LA doped ADP, KDP and KAP was determined at different fixed temperatures (viz. 30, 35, 40, 45 and 50 0C) in a constant temperature bath maintained with an accuracy of ±0.01 0C. It is found that the solubility is increased with temperature but it is decreased with doping concentration. The metastable zone width of pure LA first decreased and then increased with increasing temperature. A colorless large size crystal with dimensions 17 x 5 x 3 mm3 of pure LA was harvested in 30 days. Large size (such as 25 x 10 x 5 mm3 ) transparent pure and doped ADP, KDP and KAP crystals have been grown in 35 days by a slow solvent evaporation method. The grown crystals were subjected to characterize structurally, optically, thermally, mechanically and electrically by various experimental studies. The structural investigations were performed by Fourier Transform Infrared (FTIR) Spectroscopy, Energy Dispersive X-ray (EDX) Spectroscopy and powder X-ray Diffraction (XRD). Functional groups of pure and doped crystals were confirmed by FTIR analysis. The presence of the elements as atomic percent in the pure and doped crystals was confirmed by EDX analysis. The XRD studies confirmed the structure of all grown crystals. The presence of LA doping has marginally altered the lattice parameters without affecting the basic structure of crystals. UV-Visible studies for pure and doped crystals have sufficient transmission in the entire visible region. The transmittance of doped crystals was increased with doping concentration of LA. The good transmission of the crystals in the entire visible region suggests the suitability for second harmonic generation devices. Optical quality of the crystals was studied by UV-Visible spectroscopy. The values of optical parameters were calculated from transmittance data. The thermal behavior of the grown crystals was investigated by Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA). DTA and TGA results reveal that the substitution of amino acid dopants slightly increases the decomposition temperature of all the selected crystals. The thermal studies also reveal that the LA doped crystals are thermally stable compared to the pure crystals. Different thermodynamic parameters of pure LA were calculated from the TGA data. DC electrical conductivity of the crystals was measured at various temperatures ranging from 35 to 140 0C by conventional two probe method. AC electrical conductivity and dielectric properties of pure and doped KDP crystals were measured by LCR meter at various temperatures ranging from 35 to 140 0C at constant frequency 1 kHz. DC electrical conductivity of pure LA, pure and doped ADP and KAP increases with temperature and also with doping concentration. AC electrical conductivity of pure and doped KDP crystals increases with temperature and decreases with doping concentration. Dielectric constant and dielectric loss of pure and doped KDP crystals were decreased with temperature and also with doping concentration. Mechanical microhardness of the grown crystals was studied by the Vicker’s microhardness indentation measurements. Microhardness measurements were made using a Leitz microhardness tester fitted with a diamond pyramidal indenter. The applied load was varied from 25 to 100 g for a constant indentation period of 7 s for all indentations. It is observed that the Vicker’s Hardness number of the crystals decreases with increasing load and the pure crystals are harder than doped crystals. The surface morphology such as crystal symmetry and lattice defects of the pure and doped crystals was studied by etching 10 s in water. When a surface is etched, well defined etch patterns were produced at the dislocation sites. Etching studies revealed the growth mechanism and to assess the perfection of the grown crystals of pure and doped ADP, KDP, KAP and pure LA crystals.