Crystal growth and characterization of some nonlinear optical materials

Abstract

Single crystals of LAP were grown from aqueous solution under different growth conditions. The undesirable growth of microorganisms was avoided by protecting the surface of the solution by placing a thick layer of nhexane over it. The solution coloration could be avoided by carrying out the crystalgrowth run in the lowtemperature range and by protecting it from ambient light. The effect of pH on the solubility and habit of the grown crystals was studied in detail. Surface microtopography shows that growth on the (100) habit face occurs predominantly by a layergrowth mechanism, while the (010), (110), and (110) faces show hillocks. The grown crystals were characterized by transmission Xray topography. The Xray topographic results reveal that goodquality bulk crystals can be grown by using carefully selected selfnucleated crystals as seeds and employing the slowcooling technique. The symmetries of various molecular vibrations in LAP were determined using polarized Raman spectra. Assignment of the Raman and infrared bands to different molecular vibrations in LAP was carried out by considering molecular and crystalstructure data. The highly anisotropic thermal expansion of LAP is interpreted in terms of the crystal structure, which shows that expansion is maximum in a direction perpendicular to the chains of arginine molecules. The higher damage threshold observed in LAP is consistent with the thermalexpansion data. Despite being a complex organic crystal, the temperature variation of dielectric constant and resistivity of LAP shows behaviour similar to that of an ionic crystal. No anomaly was observed in any of these properties from liquidnitrogen temperature up to the decomposition temperature. This implies that LAP is quite stable until its decomposition point, making it suitable for device fabrication.

KTP crystals of optical quality were grown by the flux method. Interesting dendritic structures were obtained on the habit faces, mainly depending on the cooling rates. These dendritic structures form because of the rapid increase in supersaturation during the final stages of growth, i.e., when the flux approaches the solidification temperature. The surface structure also depends on crucible size. The (100) habit faces clearly show step patterns, which indicate a layergrowth mechanism. Growth by topseeding technique was also attempted. KTP crystals grown using potassiumphosphate flux are generally of high perfection. Flux inclusions and dendritic features could be minimized by optimizing the growth conditions. Growth hillocks were observed on {011} habit faces at the emergence points of dislocations, and their shapes appear to correlate with the inclination of the emerging dislocations. Etching and preliminary Xray topographic studies confirm that crystals with very low dislocation density can be grown using the flux method. Damagethreshold and SHG studies were carried out on these crystals. The observed threshold value matches the values reported in the literature. SHG studies on powder samples suggest that crystals grown in oxygen atmosphere exhibit higher SHG efficiency.

Highquality LiNbO crystals were grown by the Czochralski technique. Although crystals grown in air atmosphere were generally colourless, occasionally slightly brownish crystals were obtained; these could be bleached by oxygen annealing. Postgrowth poling was carried out to convert multidomain crystals into singledomain ones. Optical quality of the crystals was assessed using opticalabsorption measurements and conoscopic fringe patterns. These tests suggest that the grown crystals possess high optical homogeneity required for opticaldevice applications. DC conductivity was measured and activation energies were calculated. From indentation studies, the microhardness variation between +ve and ve domains was clearly established, with ve domains showing higher hardness values than the +ve domains.