Conjugated polymers, third-order

Up to now, many conjugated polymers have been found to possess large and very rapid third-order nonlinear optical response, which originates from the one-dimensionally delocalized ji-conjugation system along the polymer chain. Their application to the all optical signal processing devices has been expected. 

Polyarylenevinylene (PAV) expressed by the chemical formula of [-Ar-CH=CH-]n, where Ar is an arylene ring, is an attractive n-conjugated polymer family because of the following features (i) by the thermal conversion from polyelectrolyte or organic-solvent-soluble precursors, one can obtain the PAV films which have large third-order susceptibility and excellent optical quality, and (ii) the band gap can be adjusted by suitable selection of the arylene rings. 

Here, we demonstrate that oriented PAV films with well-developed -conjugated system can be fabricated through the regulation of orientation of precursor polymer chains by use of the Langmuir-Blodgett technique, and that large and anisotropic third-order optical nonlinearity was observed in the oriented PAV films. 

Another chemical approach to improve our microscopic understanding of optical nonlinearities is a study of nonlinear optical behavior of sequentially built and systematically derivatized structures. Most past work for third-order nonlinearities have focused on conjugated polymers. This ad hoc approach is not helpful in identifying functionalities necessary to enhance optical nonlinearities. A systematic study and correlation of Y values of systematically varied structure is an important approach for material development. 

A New Molecular Crystal DEANST. Another example of a low molecular weight compound with a large third-order NLO property is a styrene derivative. By selecting an appropriate -conjugation length and chemical structures of donor and acceptor, several new organic molecular crystals such as stilbene and azo benzene derivatives which exhibit %(3) larger than 10 12 esu can be developed. The %(3) of 4-(N,N-diethylamino)-4 -nitrostilbene (DEANS), an intramolecular CT compound, has been reported in a solution, in a polymer matrix and in a crystalline state(27.29.30). 

It is also revealed that the intramolecular charge transfer derived from substituted donor and acceptor contributes to the increase in the third-order optical nonlinearity of dye-attached polymer systems. values larger than 10 10 esu can be achieved even in relatively short Tt-electron conjugated dye-attached polymers. 

In the mid 1980s, conjugated polymers were a hot topic in nonlinear optics. Many people promoted third-order nonlinear optics based on conjugated polymers and predicted their incorporation in future all-optical applications. However, the disillusionment came during the beginning of nineties when it was realized that it will not be so easy to have all required parameters under control. The initial hopes did not fulfill on the short time scale and quite some problems were left to be tackled. 

A full quantum mechanical description of the third order non-linearity requires forty-eight terms, which are more complex than those describing second order effects, see for example Boyd (2003). The simple model of molecular non-linearity considered above (Fig. 3.22) indicates that the delocalised 7r-electron systems of conjugated polymers will have large third order non-linearity. Other factors governing the third order non-linearity of conjugated polymers will be discussed in Chapter 9, Section 9.4.2. 

---