Studies On Growth And Physical Properties Of Certain Nonlinear Optical And Ferroelectric Crystals
Nonlinear optics and ferroelectrics have been recognized for several decades as promising fields with important applications in the area of opto-electronics, photonics, memory devices, etc. High performance electro-optical switching elements for telecommunications and optical information processing are based on the material properties. Hence, there is always a continuous search for new and better materials. In this thesis we have investigated the growth and physical properties of four crystals viz. two NLO and two ferroelectric crystals. This thesis consists of eight chapters. The first chapter gives an overview of historical perspectives of nonlinear optical phenomenon, ferroelectricity and materials developed therein. The second chapter gives a brief description of the underlying theories of crystal growth, nonlinear optics and ferroelectricity. A major portion of this chapter consists of gist of the earlier work carried out on compounds of our interest viz. urea L-malic acid, sodium p-nitrophenolate dihydrate, glycine phosphite and lithium niobate. Synthesis, growth, crystal structure details and some physical properties of these materials are briefed. The third chapter describes the experimental techniques needed to grow as well as characterize these crystals. The experiments are performed on single crystals grown in the laboratory using the solution growth setup and Czochralski crystal puller. These growth units are described in detail. Preliminary characterization techniques like powder Xray diffraction, optical transmission, scanning electron microscopy, Vickers and Knoop hardness are described briefly. Various experimental methods viz. dielectric, polarization reversal, photoacoustic spectroscopy and laser induced damage for characterizing the grown crystals are explained. Urea L-malic acid (ULMA) is a new NLO organic material which is reported to exhibit second harmonic efficiency three times that of the widely used inorganic crystal, KDP. Hence, this material is selected for detailed investigation and the results obtained are discussed in chapter 4. This chapter contains details of single crystal growth and characterization of ULMA. The crystals are grown by slow cooling technique. The complete morphology of the crystal is evaluated using optical goniometry. The grown crystals are characterized for their optical and thermal properties. The defect content in the grown crystal is evaluated by chemical etching. As the surface damage of the crystal by high power lasers limits its performance in NLO applications, a detailed laser induced damage studies are performed on ULMA. Both single shot and multiple shot damage threshold values for 1064 nm and 532 nm laser radiation are determined and correlated with the mechanical hardness. In addition, the thermal diffusivity and thermal conductivity of ULMA along various crystallographic orientations are evaluated using laser induced photoacoustic spectroscopy and the results are interpreted in terms of crystal bonding environment. Another NLO crystal taken up for study is sodium p-nitrophenolate dihydrate (NPNa 2H2O), a semiorganic material. This crystal is a very efficient NLO material and has the advantages of both organics and inorganics. Earlier investigations on growth of NPNa.2H2O in various solvents have shown methanol as the most suitable solvent for growth. Growth from aqueous solution was discarded as it did not yield crystals which are stable. In the present investigation, stable, NLO active NPNa.2H2O crystals are obtained using aqueous solution itself by varying the crystallization conditions and exploring the suitable temperature range. The details of growth and characterization form the subject of fifth chapter. The grown crystals are characterized using optical transmission, XRD and thermo gravimetric analysis. Later, laser induced damage threshold is evaluated for both 1064 nm and 532 nm laser radiation and compared wit the methanol grown ones. A possible mechanism of damage is given. The sixth and seventh chapters deal with growth and characterization of ferroelectric materials namely glycine phosphite and lithium niobate respectively. Glycine phosphite is a low temperature ferroelectric crystal which is well studied in terms of its dielectric and ferroelectric properties. But very few radiation damage studies are reported. The effect of ionizing radiation on ferroelectrics is of considerable interest as it significantly modifies the physical properties of these materials. In the present investigation, effects of X-ray irradiation (_ = 1.5418 °A) on the lattice parameters, dielectric constant, loss tangent, polarization switching characteristics and domain dynamics of glycine phosphite are investigated. X-ray irradiation is performed in the non-polar phase of the sample. The effect as a function of duration of exposure is studied. X-ray irradiation in GPI has resulted in drastic reduction in _ values and shift in transition temperature towards lower temperatures. X-ray irradiation on polarization switching properties of the crystal are also investigated. The activation energy and threshold field of switching increase with the irradiation time. The behaviour of domain wall mobility is quite different from that exhibited by other well known ferroelectrics. These results are discussed in chapter 6 and a possible explanation for the unusual behaviour of domain wall mobility is given. The defect generated is identified as PO32− radical by electron paramagnetic measurement. Lithium niobate (LiNbO3) is an extensively studied material in terms of its NLO and ferroelectric properties. This material has high piezoelectric coupling coefficients along certain directions which makes it suitable for wide band surface acoustic wave applications. Hence there is a demand for good quality, single domain YZ-LiNbO3 substrates. Chapter 7 describes the growth of Z-pulled congruent LiNbO3 using Czochralski technique. Large single crystals of diameter 30 mm and length 80 mm are grown from congruent composition employing Czochralski technique. The grown crystals are multidomain and hence electric field poling is performed to get single domain crystals. Their subsequent characterization for SAW devices upto 200 MHz was performed and compared with the imported substrate. The general conclusions are given in chapter 8 along with possible future work that could be performed on these crystals.
- Physics (PHY) 
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