Third-order NLO materials

It must be noted that the values reported in the literature vary over broad ranges. Therefore, the values listed here reflect only the general behavior of several classes of compounds. It can be seen in Table 3.5 that trans-polyacetylenes (PAs) and polydiacetylenes (PDAs) exhibit the largest third-order NLO susceptibilities. The x value of cis-PA (not shown) is more than an order of magnitude smaller than that of trans-PA. Derivatives of poly-p-phenylene, poly(phenyl-ene vinylene), and polythiophene also exhibit NLO activity, but to a much lesser extent than PAs and PDAs. As pointed out above, polysilanes also possess quite large x values. This is explained by the cr-conjugation of the silicon chain, which implies a pronounced delocalization of cr-electrons. A very large x value 

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Some quinones, having the ability to form intra- and/or intermolecular hydrogen bonds, exhibit high molecular hyperpolarizability and are third-order nonlinear optical (NLO) materials. Compound 39 has a %(3) of 5 x 10 11 esu at 1.9 pm, and is a third-order NLO material.23 The optoelectric properties of quinoid compounds correlate with their structures in crystals or on thin films.23... 

The structure/property relationships that govern third-order NLO polarization are not well understood. Like second-order effects, third-order effects seem to scale with the linear polarizability. As a result, most research to date has been on highly polarizable molecules and materials such as polyacetylene, polythiophene and various semiconductors. To optimize third- order NLO response, a quartic, anharmonic term must be introduced into the electronic potential of the material. However, an understanding of the relationship between chemical structure and quartic anharmonicity must also be developed. Tutorials by P. Prasad and D. Eaton discuss some of the issues relating to third-order NLO materials. 

Third order NLO materials are often molecules in which the electrons are delocalized as with conjugated molecules. It may be shown that as the size of the region of delocalization increases, y increases more rapidly than a. 

Phthalocyanines are macrocycles characterized by an extensive 2D planar and centrosymmetric 18 7i-electron system. For this reason they have been widely investigated as third-order NLO materials and in particular as potential materials for OL some recent reviews have appeared in this specific field [163, 185, 186]. Only in the last decade have the second-order NLO properties been investigated and some of the reviews reported above describe the first significant results [164, 185]. Hereafter, we will highlight the must relevant results achieved since 2003 in the field of phthalocyanines showing second-order NLO properties. 

Another important class of photonic materials is polysilanes (Fig. 49.15) [217-222]. In polysilanes the delocalization of (T orbitals of the Si atom along the polymer backbone plays a significant role in their NLO properties. Their physical and chemical properties can be tailored by the choice of an appropriate substituent, which also determines the conformation of the polymer and its solubility. What is of more interest is their transparency in their NLO properties, values of a series of polysilanes are given in Table 49.9. Tables 49.10-49.13 contain y and values of some other third-order materials, namely metallophthalocyanine, bis-metallophthalocyanine, metallonaphthalocyanines, fuller-enes, and j8-carotenes. Although these molecules are not considered as polymers, they are macromolecules that have drawn a considerable amount of attention as third-order NLO materials. For example, the highly stable icosa-... 

Based on Eq. (2), all materials. Including gases, liquids, and solids, can show third-order NLO effects. In contrast to the third-order NLO materials, second-order NLO materials require a noncentrosymmetric alignment of NLO molecules. In the case of SHG, the relationship between the molecular and bulk NLO properties can usually be described as [ 1IJ... 

Although a guideline of molecular design for third-order NLO materials is not as yet thoroughly established, compared with second-order systems, investigations on... 

Third order NLO materials have potential applications such as optical switches, modulations and other nonlinear optical devices [189], Polydiacetylenes [190], polyacetylenes [191], poly(phenylene-vinylene)s [192] and polythiophenes [193] have large third-order nonlinear susceptibility % ranging from lO to 10 esu. 

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... 

Pr n Prabhakaran is currently a PhD student in the Department of Advanced Materials at Hannam University Korea. He received his BS degree (2000) in Pure Chemistry from the University of Kerala in India and his MS degree (2003) in Applied Chemistry from Cochin University of Science and Technology, India. His current research deals with third-order NLO materials and two-photon direct writing of miaostructures. 

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