Recent Scientific Invention of Semiorganic NLO Crystals

Presently, inorganic and organic materials are being replaced by semi-organics. They share the properties of both organic and inorganic materials. Recent interest is concentrated on metal complexes of organic compounds owing to their large nonlinearity (Newman et al 1990). The approach of combining the high nonlinear optical coefficients of the organic molecules with the excellent physical properties of the inorganic has been found to be overwhelmingly successful in the recent past.

Hence, recent research concentrates on semiorganic materials due to their large nonlinearity, high resistance to laser induced damage, low angular sensitivity and good mechanical hardness (Xing et al 1987; Velsko 1990).

Metal co-ordination complexes of Urea and Urea analogs have been explored (Newman et al 1990). These complexes are zinc thioureasulfate(ZTS), zinc thiourea chloride (ZTC), copper thiourea chloride (CTC), cadmium thiourea acetate (CTA) and bis (thiourea) cadmium chloride (BTCC). These crystals have better nonlinear optical properties than KDP. Bis (thiourea) cadmium chloride (BTCC) is a promising semiorganic NLO material for second harmonic generation.

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BTCC crystals have the highest laser induced damage threshold among the solution grown NLO crystals (Ushasree et al 2000). In general, the crystals of the type M[tu]2X2[where M= Cd, Co, Hg, Pb, Ti and Zn, tu is thiourea and X is a halogen] have been found to exhibit good NLO properties (Xing et al 1987). The metal thiocyanates and their lewis-base adducts are one of the interesting themes of structural chemistry (Nakamato 1986). As a second order nonlinear optical (SONLO) materials, bimetallic thiocyanates: Zinc cadmium thiocyanate (ZCTC), Zinc mercury thiocyanate (ZMTC), Cadmium mercury thiocyanate (CMTC), Manganese mercury thiocyanate (MMTC) exhibit efficient SHG at short wavelengths.

Single crystals of cadmium thiourea acetate (CTA) were grown by the slow evaporation technique at ambient temperature. CTA has a good optical transmission in the entire visible region (Rajesh et al 2004). Kannan et al (2004) havegrownbisthiourea-zinc acetate (BTZA) by the slow evaporation technique and observed that BTZA has a better SHG efficiency than CTA.

The growth of bimetallic NLO material MnHg (SCN)4(C2H6OS)2 was reported by Wang et al (2001). Potassium hydrogen phthalate (KHP) is semi-organic NLO materials, widely used in the field of X-ray monochromators and X-ray analysers.

It has an interesting optical, piezoelectric and elastic property (Jones et al 1963). Single crystals of pure KHP and rare earth cerium ion doped nonlinear optical KHP crystals were subjected to the NLO test by Kasthuri et al (2010). Nonlinear optical single crystals of KHP and DKHP were grown by slow cooling method and grown crystals were characterized by etching and mechanical studies (Mohan Kumar et al 2002).

Scope of the thesis

In recent years, optical-quality single crystals had been great interest to develop the modern science and laser technology. The inorganic, organic and semi-organic materials are adapted too much interest to proven SHG effects in grown single crystals. The organic nonlinear optical supramolecular structure gives generally have larger second-order nonlinear optical coefficient; hence they are extensively used in many applications like optical communication, informational storage, optical switching etc. The organic supramolecules have ultra-fast response time, photo-stability and large first hyperpolarizability (?) values. In particular, the ??conjugated systems linking a donor and an acceptor show large NLO response and hence they have been well studied. The present thesis is aimed

1. To grow single crystals of BTPCA, BBTCA, DMAPAB and GCHBS by slow evaporation solution growth technique. The crystal structure, lattice parameters and crystalline nature of grown crystals are analysed by using single crystal and powder X-ray diffraction studies.
2. To identify the all the functional groups from the synthesised compound by using FTIR spectral analysis.
3. To investigate, the optical studies such as UV-Vis-NIR, photoluminescence, second and third harmonic generation of BTPCA, BBTCA, DMAPAB and GCHBS crystals.
4. To studied the thermal, dielectric and microhardness property of grown crystals. All the studies were made to find the correlations among the various properties to explain NLO behaviour of the material.