CHAPTER - I NONLINEAR OPTICAL MATERIALS

Transcription

1 CHAPTER - I NONLINEAR OPTICAL MATERIALS 1.1. INTRODUCTION Crystals are the pillars of modern technology like in electronic industry, photonic industry, fiber optic communications, which depend on materials or crystals such as semiconductors, superconductors, polarizers, transducers, radiation detectors, ultrasonic amplifiers, ferrites, magnetic garnets, solid state lasers, non-linear optics, piezo-electric, electro-optic, acousto-optic, photosensitive, refractory of different grades, crystalline films for microelectronics and computer industries. Crystal growth is an interdisciplinary subject covering physics, chemistry, material science, chemical engineering, metallurgy, crystallography and mineralogy (Brice 1986 and Nalwa et al., 1997). In the modern world,the development of science and technology in many areas has been achieved through the growth of single crystals. Single crystals are essential for device fabrication and efforts are taken to grow large single crystals in short durati on with less cost (Anca stanculescu et al., 2004, Zhoubin Lin et al., 2007). The search for efficient and new materials in which to carry out investigations on nonlinear optical processes has been very active since the discovery of second harmonic generation (SHG) in quartz crystal by Franken and his co workers in DEFINITION OF NONLINEAR OPTICS Nonlinear Optics deals with the interactions of applied electromagnetic fields in various materials to generate new electromagnetic field altered in phase, frequency, amplitude or other physical properties (Günter 2000). The field of NLO and photonics are rapidly emerging as the technology for the 21 st century. Photonics is the technology in which a photon instead of an electron is used to acquire, store, process and transmit information. A photonic circuit is equivalent to electronic circuit, in which photons are conducted through channels. Light can be switched from one channel to another at certain junction points. For optical switching at these junctions, one needs to use a material that allows the manipulation of an electric field or laser 24

2 pulse. The materials which manipulate the light at these junctions points are known as NLO materials and these are gaining importance in technologies such as optical communication, optical computing and dynamic image processing. Photonics has many distinct merits over electronics. The most important advantage is the gain in speed due the fact that a photon travels much faster than an electron. Other advantages are that there is no electrical and magnetic interference, there by the photonic circuits are fully compatible with the existing fiber optics networks. Nonlinear optics is a branch of optics which studies the nonlinear interactions of electromagnetic radiation and the media. The nonlinear interaction, which means the matter responds in a nonlinear manner to the incident radiation fields, endows the media a characterization to change the wavelength, or the frequency of the incident electromagnetic waves. Typically, this nonlinear interaction only observed at very high intensity (electric field) of incoming light. Fig. 1.1 shows the schematics of linear and nonlinear interactions of waves and the media (Boyd 1992). Fig. 1.1 Linear (left) and nonlinear (right) interactions of waves and the media. To explain the physics in Fig. 1.1, I would like first to show the energy diagram in Fig The molecular emitted light with the same energy as the input light under Fig. 1.2 Energy band diagram of linear (left) and nonlinear optics (right). 25

3 weak beam, and thus we obtain the same waves for linear optics. However, under high intensity of light, the excited photons in the matrix can be excited to higher energy level, and emitted light with higher energy than the input light energy (Bloembergen 1996). This is nonlinear optics, in which new waves have been created LINEAR OPTICAL EFFECTS When electromagnetic radiation is incident on matter, a variety of polarisations are induced depending on the frequency of the radiation. When the frequencies are relatively low (radiowave and microwave) three contributions to the total polarisation occur, orientation polarisation, vibrational polarisation and electronic polarisation. The first one involves reorientation of molecules in a field, whereas the second one is associated with the deformation of the nuclear framework of the molecule by the applied field. When the radiation has wavelengths in the optical range, only electronic polarisation occurs because of the higher frequencies involved. When the electric field associated with the electromagnetic radiation is small with ordinary light, the polarisation varies linearly with the electric field and the constant of proportionality is the familiar polarisability, in the case of molecules and the linear electric susceptibility χ (1) in the case of bulk solids (Butcher et al., 1991). Polarization = µ (ω) = α (ω) Ε (ω) P (ω) = χ ij (ω) Ε (ω) All materials that show this kind of response with light are termed linear optical materials. The movement of charge leads to the reemission of radiation without modifying the frequency of the incoming light. However, the induced polarization causes changes in the speed of the incoming light, generating optical phenomena related to variations in the refractive index such as refraction or birefringence (Lochran et al., 1997) NONLINEAR OPTICAL EFFECTS When very intense radiation (i.e. a laser) interacts with a molecule or a material, the electronic densities are polarized in such a way that the induced electrical polarization gives rise to many unusual and very attractive properties that are optically nonlinear because they depend on the asymmetric movement of the 26

4 electronic density. In such a case, a symmetrically varying electromagnetic radiation indicates sinusoidal wave with frequency ω can induce an unsymmetrically fluctuating polarization in a noncentrosymmetric material. Such a periodic, but unsymmetrical function can be decomposed into several symmetric functions with frequencies, 0, ω, 2ω and 3ω correspond to the phenomena of optical rectification, linear electro- optic effect, second harmonic generation and third harmonic generation (Zyss 1994 and Prasad et al., 1991). In this case both microscopic and macroscopic polarizations can be represented by equations respectively µ = µ o + α ij Ε + β ijk Ε Ε + γ ijkl Ε Ε Ε +... P = P ο + χ (1) ij Ε + χ (2) ijk Ε Ε + χ (3) ijkl Ε Ε Ε + where µ o is the original dipolar moment, α ij is the linear polarizability P ο is the original polarization of the material α, β and γ are the molecular polarisability, first hyperpolarisability and second hyperpolarisability and χ( n ) are the electric susceptibilities order of n (n=1,2,3). The magnitudes of these tensorial nonlinear coefficients are quite small and decrease with increasing order. It is due to this reason that the nonlinearities show up only at high electric fields. The quadratic term gives rise to phenomena such as second harmonic generation and the linear electro-optic effect. The cubic term leads to effects such as third harmonic generation and the quadratic electro-optic effect. For this reason these materials are called nonlinear optical materials (Marder et al., 1993). Nonlinear optical materials have long been known to interact with light, to produce a nonlinear response and the composition of these materials, generally falls into one of two classes, either inorganic or organic. The demonstration of harmonic generation in quartz (Franken et al., 1961) is useful in optical communications and signal processing in integrated optics, enriching the development of new nonlinear optical materials of inorganic, organic and semiorganic types. Thus the optical nonlinearities have been widely studied in view of their potential applications in photonic and electro-optic devices (Tian et al., 1997). 27

5 1.5. INORGANIC NONLINEAR OPTICAL CRYSTAL Nonlinear optical crystal will be the key elements for future photonic technologies based on the fact that photons are capable of processing information with the speed of light (Mukerji et al., 1999). The search for new and efficient materials in which to carry out nonlinear optical processes has been active in SHG of Inorganic NLO materials such as lithium niobate (LiNbO 3 ) or potassium dihydrogen phosphate (KH 2 PO 4 ) are known to exhibit second harmonic generation effect. Lithium niobate powders have attracted a great deal of attention due to their potential applications mainly because of its unique electro-optic, acousto-optic and nonlinear optical properties (Velikhov et al., 2000). Over the last 40 years, lasers using inorganic materials have been employed as optical materials and consequently these materials have dominated optical technology. The some other inorganic crystal examples are like ammonium dihydrogen phosphate, barium metaborate, cesium dihydrogen arsenate, potassium niobium oxide, lithium niobium oxide (Bordui et al., 1993) ORGANIC NONLINEAR OPTICAL MATERIALS Organic nonlinear optical materials have been investigated due to their potentially high nonlinearities and rapid response in electro-optic effect compared to inorganic NLO materials. These molecular organic compounds with one or more aromatic systems in conjucated positions, leading to charge transfer systems have been intensely studied for the past two decades (Marder et al., 1994). The conjugated π electron system provides a pathway for the entire length of conjugation under the perturbation of an external electric field. Fictionalization of both ends of the π conjugated system with appropriate electron donor and acceptor groups can increase the asymmetric electronic distribution in either or both the ground and excited states, thus leading to an increased optical non-linearity (Prasad et al., 1991). The strength of donor and acceptor groups and order of their stacking are important roles in determining the magnitude of nonlinear optical efficiency (Zyss 1994). Conjugated DA substituted organic molecules exhibit measurable NLO and electro-optical effects. Such materials can be used to double or triple the frequency of laser light and are of considerable interest for the high-speed processing of data, which is essential for numerous modern technologies like optical computing and optical telecommunication systems. Some major applications are optical data storage, optical information 28

6 processing, electro-optic switching and all-optic switching. Generally DA substituted compounds, where D and A are separated by an aromatic spacer group, represents the basis for all organic NLO compounds (Chemla et al., 1987). Recently organic compounds with delocalized conjugated p-electrons have gained much attention because of their large NLO properties and quick response (Wolff et al., 1999). Second order NLO materials have the ability to double the frequency of incident light and have important commercial applications (Zernike et al., 1973) SEMIORGANIC NONLINEAR OPTICAL CRYSTALS Crystalline compounds represent a new class of multifunctional materials with interesting simultaneous optical electronic, opto-electronic and photorefractive properties for a wide range of applications (Dmitriev et al., 1999). Molecular materials are considered now as a very important alternative for electronic components and electric active devices manufacturing. Energetic structure, electrical and optical properties are determined by their molecular configuration and the particularities of the crystalline construction. For future applications in electronics and optoelectronics it will be necessary to utilize compounds. Nonlinear optics has been recognized for several decades as a promising field with important application in the domain of opto-electronics and photonics. For future applications in electronics and optoelectronics it will be necessary to utilize compounds with good physical and chemical properties (Dorn et al., 1992). To attempt this aim are very important the crystal purity and quality. The purity requirements for the organic and inorganic materials are a difficult. In the context of nonlinear optics, organic materials have advantages such as large NLO coefficients and structural diversity or flexibility, compared to their inorganic counterparts (Long 1995). However their device applications have been impeded by their inadequate optical transmittance, poor optical quality, lack of robustness, low mechanical strength and low laser damage threshold and inability to be grown into large size crystals. The molecules in pure organic crystals are often bonded by weak van der Waals forces or hydrogen bonds, which result in poor mechanical robustness. To overcome these difficulties, the large nonlinearities of p-conjugated organics and the favourable crystal growth properties of ionic salts can be combined into a single NLO material called semiorganics (Dhanuskodi et al., 2003). As a result, world-wide attempts are on-way, to explore suitable semiorganic systems which overcome the 29

7 limitation on the maximum attainable. Semiorganics include organic inorganic salts and metal organic coordination compounds (Dhanuskodi et al., 2007). The interesting class of semiorganic crystals possesses high NLO coefficients, stable physicochemical properties, better mechanical and thermal behavior. Metal complexes also offer a wide range of metals with different oxidation states and ligands that can give raise to tunable electronic properties. The central ion not only offers a certain anisotropic field to keep NLO active chromophores involved in the NLO process. Hyperpolarizibility (β) value can be drastically varied with electronic configuration of metal ions (Krishnakumar et al., 2007). The non-centrosymmetric structures can also be achieved by coordination of different ligands to a metal centre. These kinds of semi-organic NLO crystals are characterized by a number of organic/inorganic ligands. Metal organic coordination compounds have attracted attention to fabricate the material for many device materials (Aggarwal et al., 1999). Since several decades, efforts have been made to grow amino acid (L-arginine, L-histidine, L-alanine, L-proline, L-valine, L-tyrosine) mixed inorganic complex crystals, in order to improve the mechanical and thermal stability. A series of semiorganic compounds such as L-histidine acetate (Madhavan et al., 2007), L-alanine cadmium chloride (Bright et al., 2010), L-arginine hydrofluoride (Tanusri Pal et al., 2002), L-valinium hydrochloride (Sweta Moitra et al., 2010), L-proline strontium chloride (Manoj K. Gupta et al., 2011) and L-tyrosine hydrobromide (Narayana Moolya et al., 2006) have been grown and reported to have improved the following advantages by the presence of inorganic compounds. Advantages of Semiorganic Materials The semiorganic materials found to be reputed materials for device fabrication technology owing to their High nonlinear coefficient Lower UV cutoff wavelength High damage threshold Wide transparency range Less deliquescence High mechanical stability High thermal stability 30

8 High resistance to LASER induced damage Low angular sensitivity Higher environmental stability 1.8. AMINO ACID CHOOSEN FOR NONLINEAR OPTICS Amino acids display specific features of interest such as (i) molecular chirality, which secures acentric crystallographic structures, (ii) absence of strongly conjugated bonds, leading to wide transparency ranges in the visible and UV spectral regions, (iii) Zwitterionic nature of the molecule, which favours crystal hardness. Further they can be used as a basis for synthesizing organic inorganic compounds and derivatives. Amino acids contain a proton donor carboxyl (COO) and the proton acceptor (NH 2 ). This dipolar nature exhibits peculiar physical and chemical properties in amino acid which makes them an ideal candidate for NLO application (Anbuchezhiyan et al., 2010). Besides, compounds composed of amino acids present some particular features such as molecular chirality, wide transparency range within the visible and UV regions, and zwitterionic nature of the molecule, which favors crystal hardness. Since several decades, efforts have been made to grow amino acid mixed inorganic complex crystals, in order to improve the mechanical and thermal stability (Mary Linet et al., 2011). A series of amino acid based complex semiorganic compounds such as L-arginine phosphate (Arjunan et al., 2010), L-histidine bromide (Vijayan et al., 2009), L-arginine hydrochloride monohydrate (Kalaiselvi et al., 2008), L-arginine trifluoroacetate (Liu et al., 2007) and glycinium phosphate (Perumal et al., 2010), have been grown and reported to have improved mechanical and thermal stabilities by the presence of inorganic compounds APPLICATIONS OF NONLINEAR OPTICS Frequency conversion Light modulation Optical switching Optical memory storage High density data storage High-resolution printing Telecommunication 31

9 Optical computing Frequency mixing Electro-optical modulation Photonics Colour displays Photolithography Under water communications Medical diagnostics Optical parametric oscillation Optical bistability Optical information processing Electro-optic phase modulators High resolution spectroscopy REVIEW OF LITERATURE Mohan Kumar et al., (2005) has reported a new nonlinear optical material, L-alanine acetate (L-AlAc) has been synthesized. Single crystals of L-AlAc have been grown by the slow cooling method. Bulk crystals have been grown with a solution ph of 3.0. The grown crystals were subjected to various characterization studies such as chemical etching, dielectric measurement, FTIR analysis, UV-visible spectrum and powder SHG test. The structure of the crystal was confirmed by single-crystal X-ray diffraction analysis. L-AlAc crystals exhibit better thermal and mechanical stability with improved optical properties. Justin Raj et al., (2008) reported bulk single crystals of L-alanine formate of 10 mm diameter and 50 mm length have been grown with an aid of modified Sankaranarayanan Ramasamy (SR) uniaxial crystal growth method within a period of ten days. The optical properties of the grown crystal were calculated from UV transmission spectral analysis. The second harmonic generation efficiency of the grown crystal was confirmed by Kurtz powder test. In order to determine the mechanical strength of the crystal, Vickers microhardness test was carried along the growth plane (001). Dielectric studies reveal that both dielectric constant and dielectric loss decreases with increase in frequency. Photoconductivity study confirms the negative photoconducting nature of the crystal. 32

10 Lydia Caroline et al., (2008) are reported the L-alanine alaninium nitrate (LAAN), an organic nonlinear optical material was grown by slow evaporation technique at room temperature from its aqueous solution at ph value of 2.5. As grown single crystals were characterized for its spectral, thermal, linear and second order nonlinear optical properties. The Unit cell parameter was determined by Single crystal X-ray diffraction. The mode of vibrations of different molecular groups present in LAAN was identified by FTIR studies. The thermal behaviour of the crystals has been investigated using thermogravimetry (TG) and differential thermal analysis (DTA), which indicates that the material does not decompose before melting. Transmittance spectrum reveals that the crystal has a low UV cut-off of 320 nm and has a good transmittance in the entire visible region. NLO property of the crystal was confirmed by Kurtz SHG powder technique. Madhavan et al., (2006) studied the single crystals of L-histidine hydrofluoride dihydrate (LHHF), a semiorganic nonlinear optical material have been successfully grown by slow evaporation technique. The crystals were characterized by X-ray diffractometry (XRD), Fourier transform infrared (FTIR), UV-vis-NIR, TGA, DTA, microhardness and solubility studies. The SHG efficiency of LHHF was found to be higher than that of KDP. The dielectric constant and dielectric loss of the crystal was studied as a function of frequency and the results are discussed. Photoconductivity studies carried out on the sample revealed its positive photoconductivity nature. Reena Ittyachan et al., (2003) reported new nonlinear optical single crystal of L-histidine diphosphate was grown by slow evaporation technique from aqueous solution. Its dimension was found to be mm 3. The crystal was characterized by X-ray diffractometry (XRD), Fourier transform infra-red (FTIR), DRS-UV and solubility measurements. XRD analysis showed L-histidine: phosphoric acid stoichiometry equal to 1:2. In the FTIR spectrum, the characteristic peaks due to C O H in-plane and out-of-plane bends clearly demonstrate protonation of COO group of histidine. Since the heterocyclic ring nitrogen is more basic than COO, protonation of the former prior to the latter is indirectly evident. The DRS-UV spectrum of the crystal shows transparency in the entire visible region. The SHG efficiency was measured by Kurtz s powder technique using Nd-YAG laser. 33

11 Madhavan et al., (2007) reported the bulk single crystals of L-histidine acetate dihydrate (LHA), a new organic nonlinear material was successfully grown from aqueous solution for the first time by slow evaporation method. The solubility of LHA was determined in water and good optical quality single crystal of dimensions up to mm 3 was obtained. The grown crystals were characterized by X-ray diffraction, FTIR, UV vis NIR, microhardness and DTA/TGA studies. The SHG efficiency is found to be higher than that of KDP crystal. The dielectric constant and dielectric loss of the crystal were studied as function of frequency and the results are discussed. Photoconductivity studies of LHA reveal its positive photoconducting nature. Sweta Moitra et al., (2010), are reported a new nonlinear optical material L-valine L-valinium hydrochloride (VHCl) has been synthesized and identified by CHN test, FT-IR spectroscopy and XRD analysis. Formation of new crystal has been confirmed by single crystal X-ray diffraction analysis. VHCl crystallizes in monoclinic system with cell parameters. Thermal stability of the crystal was investigated using the thermogravimetric and differential thermal analysis studies. The suitability of this material for NLO application was studied by optical transmission studies and second harmonic generation efficiency measurement by powder method. SHG efficiency of VHCl is found to be comparable to KDP. Sweta Moitra et al., (2008) reported synthesis and single crystal growth of a new nonlinear optical crystal L-valine hydrobromide (VHBr) is reported here. Lowtemperature solution growth method was employed to grow bulk single crystal of VHBr. The grown crystals were characterized by single-crystal X- ray diffraction analysis, Fourier transform infrared, UV Vis NIR spectral studies and dielectric measurement. Thermal stability of VHBr has been studied by DTA TG and DSC analysis. DSC studies reveal the possibility of ferro para phase transition in VHBr at 118 C, which is further confirmed by dielectric measurement with respect to temperature. Kurtz powder second harmonic generation measurement confirms the nonlinear optical property of the grown crystal and SHG value is comparable to urea. Morphology of the grown crystal is also determined by using crystallographic data. Ramachandra Raja et al., (2009) reported nonlinear optical single crystals of L-valine and L-valinium succinate were grown by slow evaporation solution growth 34

12 method using water as the solvent. Purity of crystals was increased by the method of recrystallization. The grown crystals were analyzed by the single crystals X-ray diffraction pattern which proved that LVS belongs to orthorhombic crystals system. The presence of functional groups and modes of vibrations were identified by FT-IR spectroscopy. The chemical structure of crystals was established by FT-NMR technique. The transmission range of grown crystals was identified by UV vis NIR. The transmission region of LVS was more effective than LV which could be used for NLO applications. The nonlinear optical property of the grown crystals was confirmed by second harmonic generation test which shows the suitability of NLO applications. Ramachandran et al., (2009) studied the L-valine, an essential amino acid of monoclinic space group P2 1, Z = 4 and its lattice parameters and of size: 6.0 mm across and 0.5 mm thick was crystallized in silica gel under suitable ph conditions by reduction of solubility method. Density measurement and single crystal X-ray diffraction were used to characterize the grown crystals. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopic (SEM) studies were made and crystal packing also discussed. Sweta Moitra et al., (2010) reported the growth of a new nonlinear optical material L-valine by solvent evaporation method is reported here. To grow good quality crystals ph value of growth solution has been optimized and solubility of L-valine in different solvents and different ph values was determined. The grown crystals were characterized by IR, single crystal XRD, DTA and TGA, optical transmission and second harmonic generation efficiency measurement. SHG efficiency of L-valine was found equivalent to KDP and its transmission is 75-80% from ultraviolet to near IR region. Kirubavathi et al., (2008), studied the single crystals of a new semiorganic nonlinear optical material, L-valine hydrochloride (LVHCl), having dimensions up to 20mm 6mm 4mm have been grown by slow evaporation solution growth technique. Single crystal X-ray diffraction studies confirm that the grown crystal belongs to the monoclinic system. The functional groups presented in the crystal were confirmed by Fourier transform infrared (FTIR) technique. Optical transmission spectrum shows very low absorption in the entire visible region. Differential thermal and 35

13 thermogravimetric analyses confirmed that the crystal is stable up to 211 C. The powder second harmonic generation efficiency of LVHCl is 1.7 times efficient as potassium dihydrogen phosphate (KDP). Redrothu Hanumantharao et al., (2012) reported the efficient, novel, organic nonlinear optical crystal of L-threonine formate (LTF), with dimensions 15 mm 9 mm 3 mm was grown by slow evaporation technique at ambient temperature. Solubility and metastable zone-width measurements have been carried out for LTF sample. The metastable zone-width studies have been carried out for different cooling rates. The grown crystals were subjected to single crystal X-ray diffraction to determine the unit cell dimensions and morphology. The formation of the new crystal has been confirmed by NMR and mass spectroscopic analysis. The dielectric constant and dielectric loss measurements of the grown crystal at different temperatures and frequencies of the applied field are measured and reported. The existence of SHG conversion efficiency was measured using an Nd:YAG laser (1064 nm). Ramesh Kumar et al., (2005) reported single crystals of L-threonine, a nonlinear optical material, have been grown from solution by the temperature lowering technique. L-threonine was found to have extremely high solubility in water. Hence it was feasible to grow bulk crystals of L-threonine in aqueous solution. The cell parameters were estimated by powder X-ray diffraction analysis. The vibrational structure of the molecule is elucidated from FT-IR and FT-Raman spectra. Thermal analysis was performed to study the thermal stability of the grown crystals. The transmission spectra of L-threonine show that the lower UV cutoff wavelength lies at 220 nm. Kurtz powder SHG measurements confirm the NLO property of the grown crystal. Microhardness studies have also been carried out on L-threonine crystals. Suresh Kumar et al., (2006) reported, L-threonine, an organic material with interesting non-linear optical properties, has been grown in solution and characterized by X-ray diffraction, FT infrared spectral, Raman spectral, UV VIS spectral and thermal techniques. The structural and spectral characteristics show that strong hydrogen bonds appearing in L-threonine have an influence on the stabilization of the crystal structure. The main contributions to the second harmonic generation in threonine results from the hydrogen bond and vibrational part due to very intense IR 36

14 bands of the hydrogen bond vibrations. The OH and NH involved in strong hydrogen bonds are responsible for the higher thermal stability of L-threonine compared to other organic non-linear optical materials APPROACH OF THE THESIS WORK Nonlinear optical materials capable of generating second-harmonic frequency play an important role in the domain of optoelectronics and photonics. Nonlinear optical crystals with high conversion efficiencies for second harmonic generation and transparent in the visible and ultraviolet regions are required for numerous device applications. Thus the search for new nonlinear optical materials has been the subject of intense research. Materials with large second-order nonlinearities, large optical transmission window and stable physicochemical performance are needed for various applications. A strong need continues to exist for lower cost, more efficient and higher average power materials. Therefore this attempt became necessary. Organic crystals can have very large nonlinear susceptibilities relative to inorganic crystals but exhibit low damage threshold, poor optical quality, inadequate transparency and poor processibility. In contrast, purely inorganic NLO materials typically have excellent mechanical and thermal properties but often possess relatively modest optical nonlinearities owing to their lack of extended Π electron delocalization. In order to overcome these difficulties, the metal-organic complexes combine the high optical nonlinearity and chemical flexibility of organics with the physical ruggedness of inorganics, high laser damage threshold, good mechanical and thermal stability. In this connection, the solid state amino acids (organic compounds) contain a deprotonated carboxylic acid group (COO ) and protonated amino group (NH + 3 ). This dipolar nature exhibits peculiar physical and chemical properties in amino acids, thus making them ideal candidates for NLO application. In recent years, efforts have been made to synthesize amino acid mixed organic inorganic complex crystals in order to improve the chemical stability, laser damage threshold and linear and non-linear optical properties. Particular amino acids, which possess some peculiar properties, were selected. L-alanine is an efficient organic compound under the amino acid category, because the carboxyl group is present as a carboxylate ion and amino group as an ammonium ion. The structural arrangement (head to tail hydrogen bond 37

15 sequence) and the occurrence of the π π* transition in the carboxylic group are accounted for the nonlinearity in this material. L-valine is a branched chain amino acid, which has aliphatic non-polar side chain. Other than side chain all amino acids have both a primary amino group and a primary carboxyl group. The carboxylate acid group donates its proton to the amino group. So in solid state, amino acid exists as zwitterion. Till now several hundreds of donar and acceptor substituted delocalized π electron system have been reported which show NLO properties. L-Histidine salts can display higher NLO properties due to the presence of imidazole group in addition to amino-carboxylate. L-threonine is an important polar uncharged R amino acid. The system crystallizes in the noncentrosymmetric P space group with four zwitterionic molecules in the unit cell, linked by a three dimensional network of four hydrogen bonds of unequal strength, which shows higher SHG efficiency. The present work, which focused on the use of amino acids namely (L-alanine, L-valine, L-histidine and L-threonine) combined with inorganic salts (cadmium bromide, cadmium chloride and cadmium acetate) based semiorganic NLO crystals were grown by slow evaporation technique at room temperature. As for the metallic part focus is on the group II B metal (Cadmium), as this compound usually has high transparency in the UV region, because of its closed d 10 shell (M. Jiang and Q. Fang 1999). Also metals with d 10 configuration readily combine with organic salts resulting in stable compounds with optical nonlinearity, good physiochemical behavior and hence applications in optoelectronics and photonics AIM AND SCOPE OF THE WORK Nonlinear optics is a new frontier of science and technology playing a major role in the emerging era of photonics and opto-electronics. The search is focused on the high efficient nonlinear optical single crystal for visible and ultraviolet regions are extremely important for laser and material processing. A thorough literature survey of the semiorganic NLO crystals indicates that very little research work has been done on the physical and chemical properties by using slow evaporation method at room temperature with low cost. In the present investigation, it was aimed at the grown crystals having higher second harmonic generation efficiency with good transparency and possessing good thermal and hardness properties leading to laser technology, 38

16 opto-electronics, optical communication and optical data storage. The single crystal of semiorganic nonlinear optical crystals, namely L-alanine cadmium bromide (LACB) L-valine cadmium acetate (LVCA) L-histidine cadmium chloride monohydrate (LHCCM) L-valine cadmium bromide (LVCB) L-threonine cadmium acetate (LTCA) These single crystals were grown by the slow evaporation growth techniques at room temperature and were characterized through elemental analysis, Powder X- ray diffraction, Single crystal X-ray diffraction analysis, Fourier transform infrared spectroscopy, UV-Vis-spectroscopy, Fluorescence study, thermogravimetric analysis, Differential thermal analysis, Differential scanning calorimetric analysis, microhardness, Dielectric study and Photoconductivity study. The SHG efficiency was estimated by the modified Kurtz-Perry powder technique. aaaaa 39