Growth And Physical Properties Of Nonlinear Optical Crystals CsH(C4H4O5)H2O, CsLiB6O10 And Near-Stoichiometric LiNbO3
NLO materials have been researched for nearly five decades from the point of view of understanding the basic mechanisms and also in the pursuit of new materials possessing improved properties. Materials satisfying a set of physico-chemical properties such as wide transmission range, good mechanical hardness, high po-larizablity, noncentric crystal structure, good chemical stability, etc are the ones which are sought after. Several organic and inorganic molecules have been synthe-sized with the hope of finding materials that possess the desired NLO properties. Most of the organic materials are known to possess high figure of merit. However, their poor mechanical strength and needle like growth habit are hindrances to practical applications. Inorganic NLO materials have better mechanical properties but many of them possess small deff and laser induced damage threshold. Semi-organic NLO materials are intended to have the merits of both organic and inorganic counterparts. In this investigation, three important NLO crystals, viz cesium hydrogen L-malate monohydrate (CsLM, CsH[C4H4O5]H2O), cesium lithium borate(CLBO,CsLiB6O10)and near stoichiometric lithium niobate codoped with Nd and Zn (Nd:Zn:LiNbO3)are chosen for detailed study. The thesis is organized into 6 chapters. First chapter contains the theoretical background of the physical and chemical phenomena including a review of nonlin-ear optics, second harmonic generation, multiphoton absorption & refraction con-cepts, single crystal growth, principles of ferroelectricity and the scope of the work involved. For better connectivity, a brief review of the earlier work carried out on the chosen materials is given in the beginning of each chapter. The second chapter discusses the methodology of work and experimental details used in the present study. The third chapter deals with studies on CsLM, the new organometallic NLO crystal. Its structure, electro-optical properties and dielectric properties in FIR region are reported very recently. This material is also reported to show a phase transition at 50 0C though not much is understood about its nature. Further details of crystal growth conditions, nonlinear optical properties and laser damage thresholds are little known on this newly discovered NLO material. In this chapter detailed crystal growth studies and investigations of physical properties are presented. During growth, CsLM crystals manifest in platy and prismatic morphologies depending on level of super saturation invoked. The maximum dimensions of the grown crystal are 20 x15 x35 mm3. It has very good transmission in the range, 250-1300 nm and dislocation density of ≈104/cm2 . The dc conductivity measurements place this crystal between an ionic conductor and a dielectric. Dielectric properties show considerable frequency dispersion and axial anisotropy with є′ being the highest along the polar b axis. Maker fringes experiment reveals that CsLM possesses good second harmonic generation efficiency, an order of magnitude higher than KDP. It also has high laser damage threshold for fundamental and second harmonic wavelengths of Nd:YAG pulsed laser. TGA/DTA experiments are performed on the crystals grown below and above the reported transition temperature(labelled as CsLM and Anhydrous-CsLM respectively). The present investigations on Anhydrous-CsLM show that it crystallizes in a structure different from that of CsLM and the nature of the reported ′phase transition ′is driven by sluggish hydration and dehydration processes. Recently UV transparent nonlinear optical materials were evaluated for the fab-rication of all solid state UVlasers and CLBO is one such NLO material. It crystal-lizes in non-centrosymmetric tetragonal space group, I42d. It is highly transparent in the wide range of wavelengths from 180 to 2500 nm and has good deff. It pos-sesses very good angular and spectral bandwidth tolerances compared to its contemporaries such as lithium triborate(LBO) and beta barium borate(β-BBO). There is, however, a problem associated with this material that it cracks when exposed to atmosphere due to its hygroscopic nature. This chapter details the fabrication of the required instrumentation to grow single crystals of this material and the study of possible solutions to avoid cracking problem besides its new nonlinear properties. Since the melts of borate materials are known to be highly viscous, the crystal growth apparatus should have the options for tuning the parameters like seed and crucible rotation rates, temperature isotherms, slow pulling rate, etc. Keeping the above in mind, a high temperature top seeded solution growth unit was designed and fabricated inhouse. Highly transparent single crystals of CLBO were grown using the above unit which were characterized for the defects/dislocations using X-ray topography. The average dislocation density estimated is ≈103/cm2. The nonlinear optical absorption(NLA) and refraction(NLR) properties are studied. Z-scan experiments reveal that five photon absorption(5PA )is responsible for nonlinear absorption when the wavelength and pulse width are 800 nm and 110 fs respectively. For 532 nm and 6 ns pulses, dielectric breakdown occurs before NLA could occur due to high pulse influence. CLBO is found to show negative nonlinear refraction under high intensities. Hygroscopicity of CLBO is attributed to the entry of water through the channels that are present along a and b axes, which in turn, cause cracking. Doping, is expected to modify the size of the channels. Since certain dopants are found to improve the stability of CLBO, substitution of Cs site with Zn and Gd is carried out to reduce the size of channels. As there was no significant improvement with doping experiments, an alternate approach is attempted by coating with SiO2 thin films on the optical elements to prevent the water molecules from entering lattice through the channels. The results and discussion of the above studies are presented in the chapter 4. Lithium niobate is the most widely used single crystal for fabricating optical modulators, waveguides, SAW devices and optical parametric oscillators. Although single crystals of this ferroelectric material were grown way back in 1965 by Ball-man and Fedulov independently, most of the work till the beginning of 90’s was concentrated on crystals with congruent composition(CLN) because there were no suitable methods available for growing homogeneous single crystals of stoichiomet-ric lithium niobate(SLN). Recently, Double Crucible Czochralski method with au-tomatic powder feeding technique and top seeded solution growth technique with Li2O and K2O fluxes are shown to produce SLN crystals. In this work, top seeded solution growth technique with58.6 mol% Li2O composition(self flux) is adopted to grow SLN crystals and the details of the growth and investigations are presented in chapter 5. Initially, crystal growth of SLN, and Zn & Nd codoped SLN are de-scribed. The maximum dimesions of the SLN crystals are 20 mm diameter and 35 mm length. CLN crystals(30mm diameter and 70 mm length) are also grown for comparison. The growth rate for SLN crystals is approximately 25 times lower than that for CLN. The maximum amount of Zn added to the melt is 2.5 mol%. For Nd codoping, four concentrations (0.2, 0.5, 0.9, 1.5 mol%) have been chosen with Zn concentration in the melt ﬁxed at 2.5 mol%. Addition of Zn is to enhance the pho-torefractive damage threshold and Nd to use SLN as laser host. Structural studies on the grown crystals using powder X-ray diffraction show no additional phases. The domain structure analysis by chemical etching studies reveal that it is sensitive to doping and temperature gradient above the melt surface. The grown crystals possess good transmission in the UV-Vis-NIR region. Apparent increase in the dielectric constant found in doped crystals is attributed to space charge effect. In Nd:Zn codoped SLN, the parameters corresponding to lasing (Judd-Ofelt parameters, radiative transition probabilities, branching ratios) have been evaluated and found to be better than those obtained for codoped CLN. Surface laser damage and photorefractive damage thresholds are enhanced by 2 and 4 orders of magnitude respectively for the crystals grown with 2.5 mol% Zn in the melt. Nonlinear absorption and refraction studies using femtosecond Z-scan experiments reveal a correlation between the nonstoichiometric defects and nonlinear absorption & refraction coefficients. Polarization switching studies carried out on pure and Zn doped samples indicate an enhancement in switching rate at elevated temperatures. In the sixth and final chapter, a comprehensive summary of the present work and the scope for further investigations related to this work are given.
- Physics (PHY) 
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