Supermolecules, nonlinear optical

Polymers and supermolecules modified using electron push-pull chro-mophores are also of particular interest for nonlinear optics (NLO) [10-15]. NLO material has attracted much interest over the past 20 years and has been widely applied in various field (telecommunications, optical data storage, information processing, microfabrication, etc.). Chemists have developed ways to introduce NLO chromophores into many type of polymers, such as Hnear polymers, cross-linked polymers, and branched polymers, and have demonstrated their performance in NLO appHcations. 

Nonlinear optical (NLO) properties are usually considered to depend on the intrinsic features of the molecule and on the arrangement of a material. An intermediate level of complexity should also be taken into account, that of the formation of well-defined supermolecules, resulting from the association of two or more complementary components held together by a specific array of intermolecular interactions (1). Such intermolecular bonding may yield more or less pronounced NLO effects in a variety of supramolecular species (2). Thus, three levels of nonlinear optical properties may be distinguished the molecule, the supermolecule and the material. The molecular and supramolecular levels involve respectively - intramolecular effects and structures, -... 

In tune with the above introductory remarks, we have arranged this review in the following way Section II deals with the oriented gas model that employs simple local field factors to relate the microscopic to the macroscopic nonlinear optical responses. The supermolecule and cluster methods are presented in Section III as a means of incorporating the various types of specific interactions between the entities forming the crystals. The field-induced and permanent mutual (hyper)polarization of the different entities then account for the differences between the macroscopic and local fields as well as for part of the effects of the surroundings. Other methods for their inclusion into the nonlinear susceptibility calculations are reviewed in Section IV. In Section V, the specifics of successive generations of crystal orbital approaches for determining the nonlinear responses of periodic infinite systems are presented. Finally,... 

Selected References Using the Supermolecule Approach to Determine the Macroscopic Nonlinear Optical Responses in Molecular Crystals... 

Several methods have been developed in order to determine the macroscopic optical properties [63], of which the simplest is the oriented gas model due to Chemla et al. [64, 65] In that method, the hnear and nonlinear susceptibilities (Eq. (8.2) are calculated from simple tensor sums of the (hyper)polarizabihties of the molecules constituting the elementary unit cell. Corrective factors can subsequently be added to account for the effects of local electric fields. The relevance of this method is ensured provided the intermolecular interactions are weak, while the macroscopic responses are strongly dependent on the values of local field factors. More sophisticated schemes take into account the intermolecular interactions. They include the supermolecule model [66-69], where an aggregate of... 

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