Bulk crystal growth and nonlinear optical characterization of a stilbazolium derivative crystal: 4-[2-(3,4-dimethoxyphenyl)ethenyl]-l methylpyridinium tetraphenylborate (DSTPB) for NLO device fabrication

Abstract

The search for organic crystals with highly efficient nonlinear optical (NLO) properties with a high surface laser damage threshold (LDT) has become more of a demand in the frontier areas of optical switching and communications applications. A single crystal of 4-[2-(3,4-dimethoxyphenyl)ethenyl]-l methylpyridinium tetraphenylborate (DSTPB), which is an organic material has been successfully synthesized in the pure phase. Bulk single crystals were grown with dimensions of 28 × 11 × 6 mm³ using a slow evaporation method and reported for the first time. The structure of the title crystal and its lattice parameter were confirmed by single-crystal X-ray diffraction studies and it was found that it crystallizes in the non-centrosymmetric space group Cc with a monoclinic crystal system. The calculated HOMO and LUMO energies show that charge transfer takes place within the molecular structure, and they also indicate the NLO activity of the title crystal. A Kurtz powder test showed large second harmonic generation (SHG) about 4.53 times that of KDP crystal and make it a suitable candidate for electro-optic applications. It also exhibits good transparency (371 nm to 1100 mm) in the visible and near infra-red spectral (NIR) ranges and its thermal stability was found to be up to 260 °C. It shows a high laser-induced damage threshold of up to 4.3 GW cm⁻², which is greater than that of some known organic and inorganic NLO materials. Third-order nonlinear optical properties of the title crystal were studied by the single beam Z-scan technique at 632.8 nm using a He–Ne laser. It was found that it exhibits saturable absorption (SA) and a self-focusing nature with large second-order hyperpolarizability (γ) 3.15 × 10⁻³³ esu, which are mainly associated with electronic processes. The results indicate that the crystals exhibit large third-order optical susceptibility compared with some reported NLO crystals. The enhanced second-order and third-order optical nonlinearity of DSTPB makes it a promising candidate in the field of nonlinear optical devices.