ABSTRACT

The development of organic materials for use in non-linear optical devices is of interest because their optical non-linearities are orders of magnitude larger than those of conventional materials. Moreover organic materials offer flexibility of molecular design, virtually an unlimited number of crystalline structures and a high damage resistance to optical radiation. In these molecules, the non-linear optical properties are believed to arise from an optically intense electronic state, in which electron density is transferred from the electron donor to electron acceptor traditionally referred to as charge transfer state. The main interest in nitro anilines comes from their recognized second harmonic generation capability and their large microscopic hyperpolarizabilities. Intermolecular hydrogen bonds are an effective tool for organizing organic molecules, very well typified by nitro aniline compounds, which associate via intermolecular H-bonds between the amino and nitro groups.

The growth of single crystals of the title compound has been undertaken from methanol solution by the slow evaporation technique from aqueous solution at room temperature. Transparent optical quality yellow colored crystals were harvested after the growth period of a week.

The hyperpolarizability calculations were carried out for the investigated compound by the Density Functional triply parameter hybrid model DFT/B3LYP using GAUSSIAN 98W, The HF/3-21 G (d, p) basis set has been employed. The calculated first hyperpolarizability of m-NA is 1.347006x10^-30 esu, which is nearly seven times that of Urea. The experimental relative SHG efficiency was found to be 2.5 times that of urea.

The vibrational spectral studies also provide evidence for the charge transfer interaction between the donor and acceptor groups through p- electron movement. This p- electron cloud makes the molecule highly polarized and the intermolecular charge transfer interaction is highly responsible for the NLO properties of the title compound.

Therefore, the presence of intermolecular hydrogen bonding is responsible for high b value and this mechanism plays an important role in NLO activity

Optical pulses with a time width of 200 fs, repetition rate of 42 MHz and centre of wavelength 800 nm from a mode locked Ti: Sapphire laser were used for Teraherz studies of the title crystal. According to the geometrical considerations for the molecular arrangements in the crystal, the b-tensor components can be explicitly described using the first-order molecular hyperpolarizabilities (β). The largest b tensor component is found for b_xxx (137.5), which is equal to b_xxx in the title crystal. This is the indication of the non-linear polarization induced by the component and this particular tensor is responsible for THz generation in the crystal. The presence of phonon modes is consistent with the THz radiation of these crystals. In this direction a strong phonon modes observed from 0.9 - 2.3 THz for the title NLO crystal is concurrent with the values obtained from the FFT amplitude spectrum. Thus the spread of terahertz bandwidth of the investigated compound is of moderate range. This material can be recommended for terahertz imaging or the other applications where a large bandwidth is not needed. Further studies are in progress.

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1. Investigations on crystal growth and characterization ofK[CS(NH₂) ₂]₄Br- a semi organic non-linear optical crystal