Molecular nonlinear optical materials third-order effects

To conclude this article, it is hoped that the discussion of relevant issues and opportunities for chemists presented here will sufficiently stimulate the interest of the chemical community. Their active participation is vital for building our understanding of optical nonlinearities in molecular systems as well as for the development of useful nonlinear optical materials. It is the time now to search for new avenues other than conjugation effects to enhance third-order optical nonlinearities. Therefore, we should broaden the scope of molecular materials to incorporate inorganic and organometallic structures, especially those involving highly polarizable atoms. 

This work provides a relatively comprehensive review of studies involving ruthenium coordination and organometallic complexes as nonlinear optical (NLO) compounds/materials, including both quadratic (second-order) and cubic (third-order) effects, as well as dipolar and octupolar chromophores. Such complexes can display very large molecular NLO responses, as characterised by hyperpolarizabilities, and bulk effects such as second harmonic generation have also been observed in some instances. The great diversity of ruthenium chemistry provides an unparalleled variety of chromophoric structures, and facile Ru" —> Ru" redox processes can allow reversible and very effective switching of both quadratic and cubic NLO effects... 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2o) = co + co + 0) in a quasi-third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect (co = co -I- 0), or the quadratic electrooptic (Kerr) effect ( = -I- 0 -I- 0). EFISH is discussed in this article, however, for the important role that it has played in the characterization of nonlinear optical materials for other applications. 

Third-Order Nonlinear Optical Effects in Molecular and Polymeric Materials... 

Azo-benzene molecules are widely recognized as attractive candidates for many nonlinear optical applications. A highly deformable distribution of the ic-electron gives rise to very lar molecular optical nonlinearitics, Phdto-isomerization of azo molecules allows linear and nonlinear macroscopic susceptibilities to be easily modified, giving an opportunity to optically control the nonlinear susceptibilities. In this chapter, we will discuss third-order nonlinear optical effects related to photoisornerization of azo-dye polymer optical materials. 

Cascading. In most cases, the distinction between second- and third-order nonlinearities is evident from the different phenomena each produce. That distinction blurs, however, when one considers the cascading of second-order effects to produce third-order nonlinear phenomena (51). In a cascaded process, the nonlinear optical field generated as a second-order response at one place combines anew with the incident field in a subsequent second-order process. Figure 2 shows a schematic of this effect at the molecular level where second-order effects in noncentrosymmetric molecules combine to yield a third-order response that may be difficult to separate from a pure third-order process. This form of cascading is complicated by the near-field relationships that appear in the interaction between molecules, but analysis of cascaded phenomena is of interest, because it provides a way to explore local fields and the correlations between orientations of dipoles in a centros5nnmetric material (52). 

In the case of third-order nonlinear phenomena the materials can be centrosymmetric in its molecular structure. One example is the optical Kerr effect reported for the first time by J. Kerr in 1877 and 1878 exposing a material to an electric field the refraction index of an optical medium changes, proportional to the square of the applied field. A double refraction can be generated with the difference between Kerr and Pockels effect being that in the latter case the double refraction is linearly proportional to the electric field. 

A number of optical effects arise out of both the first and second hyperpolarizibilities. However, only some among them have been systematically studied for practical applications. In this chapter, we will discuss the electric field-induced optical birefringence in second-order NLO materials and the light-induced optical nonlinearities including optical Kerr effect and two-photon absorption (TPA) in third-order NLO materials. Molecular design for... 

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