Efficient Second-Harmonic Generation In Single Crystal Of N-Methoxymethy1-4-Nitroaniline

Abstract

In recent years, there has been a growth of interest in organic materials for applications in the field of nonlinear optics!'2 Materials possessing both large second-order nonlinear susceptibilities and bulk crystallinity are important for highly efficient frequency conversion. Organic molecular compounds are quite promising for achieving such high conversions because their second-order nonlinearity can be enhanced by extended conjugation and by the presence of spectroscopic charge-transfer arising from electron-donating and electron-accepting substituents on an aromatic ring or a similar conjugated system. Crystallographically, for quadratic nonlinearity the crystal unit cell must lack a center of symmetry. 4-Nitroaniline is one of the simplest organic compounds having an electron donor group and an electron acceptor group at the para positions. It has large molecular hyperpolarizabilityβ, but does not show second-order nonlinear optical effects because of the centrosymmetry of the crystal. Therefore, in order to achieve noncentrosymmetric structure, large nonlinear susceptibility and transparency, we have modified 4-nitroaniline system. A new organic nonlinear optical material N-methoxymethy1-4-nitroaniline(MMNA) was synthesized. Second-harmonic generation(SHG) measurements performed in powder form shows high second-order optical nonlinearity. This has also been confirmed by the observation of efficient phase-matched SHG from the single crystal. In this article we report the details of synthesis, crystal growth, crystal structure, phase-matching behavior, and effective bulk nonlinearity of MMNA crystal.