Optical nonlinearities, optimization

Expression (4) evidences the combined Influences of molecular and crystalline structure on the enhancement of the macroscopic optical nonlinearity. The molecular design must contain specific features so as to optimize while the crystalline structure... 

The material system is a Langmuir-Blodgett film of the S enantiomer of a chiral polymer deposited on a glass substrate. The polymer is a poly(isocyanide)30 functionalized with a nonlinear optical chromophore (see Figure 9.14). In this particular system the optical nonlinearity and chirality are present on two different levels of the molecular structure. The chirality of the polymer is located in the helical backbone whereas the nonlinearity is present in the attached chromophores. Hence, this opens the possibility to optimize both properties independently. 

In general, the optimization of organic molecules for third order nonlinear optical applications has enjoyed much less success than for second order optical nonlinearities. The major reason for this has been the questionable validity of the two-level model for y, and the difficult assessment of the contribution of two-photon states for the more acceptable three-level model. 

Reviewed in Zyss J, Chemla DS (1987) Quadratic nonlinear optics and optimization of the second-order nonlinear optical response of molecular crystals. In Chemla DS, Zyss J (eds) Nonlinear and optical properties of organic molecules and crystals. Academic Press, Orlando, p 23... 

J. Zyss and D.S. Chemla, Quadratic Nonlinear Optics and Optimization of Second-Order Nonlinear Optical Response of Molecular Crystals, Eds. D.S. Chemla and J. Zyss, Academic Press, Orlando, FL, 1987, Vol. 1, pp. 23-191. 

In the fabrication of practical E-O devices, all of the three critical materials issues (large E-O coefficients, high stability, and low optical loss) need to be simultaneously optimized. One of the major problems encountered in optimizing polymeric E-O materials is to efficiently translate the large P values of organic chromophores into large macroscopic electro-optic activity (r33). According to an ideal-gas model, macroscopic optical nonlinearity should scale as (M is the chromo-... 

Since in molecular materials the nonlinear effect occurs fundamentally at the molecular level, it is possible to assemble new molecules by substituting groups of donors or acceptors to optimize the nonlinear optical response. The optimization of optical nonlinearities at the molecular level constitutes a field that incorporates elements of chemistry, physics, polymer science and material science, known as organic molecular engineering. 

When designing chromophores, it is not sufficient to focus simply upon optimization of molecular optical nonlinearity (molecular first hyperpolarizability, )3), where /3 is defined by the expansion of molecular polarization in terms of various powers of applied fields E. 

First, let us turn our attention to the issue of optimizing chromophore hyperpolarizability. An obviously important and necessary aspect of the process is the characterization of hyperpolarizabilities. The two most popular methods for characterizing chromophore second-order molecular optical nonlinearity are electric field-induced second harmonic generation (EFISH) [3,10,21-24] and hyper-... 

Harper, A. W Systematic optimization of second-order optical nonlinearities in molecules and polymers, Ph.D. thesis. University of Southern California, Los Angeles, 1996. 

Polymers for second-order NLO applications have been extensively investigated during the last ten years. However, significant progress in material development efforts toward practical applications has been made only in recent years. This chapter discusses the current state of the art in the development of second-order NLO polymers. We start with a brief description of the NLO phenomena at both the molecular and bulk levels. The design considerations and property optimization of chromophores are associated with the molecular level optical nonlinearity. The induced noncentrosymmetric dipolar orientation of chromophores leads to the bulk second-order NLO properties, which are most commonly char-... 

Since the initiation of the investigation on poled polymers in the mid-1980s, most research has been focused on enhancing the optical nonlinearity and its temporal stability at elevated temperatures. So far, no particular system has been identified as the material of choice for use in the EO applications. With the rapid progress in research in recent years, polymeric materials exhibiting large and stable NLO properties for real applications may be anticipated in the near future. Other properties such as optical loss, optical power handling [109], pro-cessibility, and reproducibility are also of importance. Successful development of a material for NLO application will require optimization of properties in all aspects. 

Chemistry produces many materials, other than drugs, that have to be optimized in their properties and preparation. Chemoinformatics methods will be used more and more for the elucidation and modeling of the relationships between chemical structure, or chemical composition, and many physical and chemical properties, be they nonlinear optical properties, adhesive power, conversion of light into electrical energy, detergent properties, hair-coloring suitabHty, or whatever. 

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