Oriented crystallization of NLO organic materials

A new methodology to study the second order NLO coefficients of organic mat erials was developed. We have demonstrated that the combination of oriented crystal growth on structured surfaces, grazing incidence X-ray diffraction and second harmonic ellipsometry gives an insight of the relations between the molecular hyperpolarizability and the crystalline non-linear susceptib ility tensor. Thin organic layers of 2-methyl-4-nitroaniline (MNA), N-(4-ni trophenyl)-1-prolinol (NPP) and 2-(N,N-dimethylamino)-5-nitroacetanilide (D AN) were grown on poly(tetrafluoroethylene) (PTFE) substrates prepared by t he friction transfer method. From the observed crystallographic orientation s, a growth mechanism based on a "soft-epitaxy" was proposed. At low substr ate temperature, the overlayer-substrate interactions dominate and induce a two-dimensional epitaxial growth which corresponds to a specific physisorp tion of aromatic molecules on the polymer surface. At high substrate temper ature, the aromatic crystals nucleate via a grapho-epitaxy mechanism which occurs on topological defects of the surface. The formation of specific int eractions with the PTFE substrates is not a requisite to orient the crystal s in this model, Finally, we will show that the ratio between the non-vanis hing components of the first non-linear susceptibility tensor (chi (ijk) or d(ij)) can be estimated from second harmonic ellipsometry measurements. (C ) 2001 Elsevier Science B.V. All rights reserved.