New Second-Order Nonlinear Optical Organic Crystals
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NEW SECOND-ORDER NONLINEAR OPTICAL ORGANIC CRYSTALS D.S. DONALD, L.-T. CHENG, G. DESIRAJU, G.R. MEREDITH AND F.C. ZUMSTEG E.I. du Pont de Nemours & Co., Central Research and Development Department, Experimental Station, Wilmington, DE 19880-0328 ABSTRACT The design of molecular crystals with specific optical properties, which are thought to arise from constituent molecules' polarizability properties, is a desirable but currently unachievable goal. One can partially achieve this goal by choosing compounds with specific molecular attributes and empirically determining the manner in which these are translated into crystal properties. Besides the fact that there are no certain rules for prediction of crystal packing arrangements, there is also a problem in specifying molecular properties from what are today incomplete polarizability structure-property relationships. We have, realizing these limitations, identified new molecular crystals by a nonlinear optical (powder-SHG) scouting-screening program from lists of compounds chosen because of desirable molecular properties. Examination of successful materials has revealed interesting, new alignment motifs. Some of these materials, a set of halogen and cyano derivatives of aromatic compounds, are described relating properties and structures of molecules and crystals. In particular, the orientation directing influence of intermolecular halogen-cyano interactions and the use of heterocyclic compounds to improve transparency in the near infrared and in the blue and near ultraviolet spectral regions are demonstrated. INTRODUCTION Recent activity in the design and selection of organic materials for second-order nonlinear-optical (NLO) applications, such as second harmonic generation (SHG), has produced a variety of techniques for generating the, by now, well known requisite asymmetrically aligned ensembles [1]. An oriented-gas model is often used to relate microscopic-molecular and macroscopic-crystalline dielectric properties [1, 2]. In this model molecules are thought to have some molecule-specific polarizability attributes (which may be more realistically considered to be those of molecules in the condensed phase, as opposed to molecules in vacuum), and, in addition to being polarized by the Maxwellian electric fields, are mutually polarized by fields arising from polarization of neighbors. Disregarding anisotropy of local-field corrections f, the second-order NLO susceptibility tensor is approximated as the volume-normalized, local-field corrected, orientational average of second-order hyperpolarizability tensor X(2)
= (N/V) orientation
f
(1)
Straightforward geometric considerations of a crystal's sublattices allow an understanding of the gross features of X(1) and X(2). For example, if two molecular sublattices exist whose sites are related by an inversion center, < 3 > and the nonlinearity vanish. In other cases interferences intrinsic to orientational averaging of polar odd-rank tensors reduce the degree of projection of the molecular nonlinearity to the macroscopic
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