Second-order NLO polymers

Second-order NLO polymers have potential for technical applications (see Section 3.5 below), for example, in electro-optic modulation and switching or frequency doubling. A large body of compounds has hitherto been explored and, at present, relevant research is mainly focused on optimizing secondary properties such as thermal stability, adhesion, thermal expansion, etc. 

TABLE 49.1. The product fijS) obtained from EFISH measurements of several second-order NLO polymers. 

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-... 

Second-order NLO polymers for frequency conversion offer the possibility of obtaining a blue laser for... 

The electro-optic property of EO polymers comes from the NLO chromophores. When these chromophores are preferentially aligned to break the centrosymmetry of the material, the molecular level microscopic NLO effect of the molecules translates to the macroscopic second-order NLO effect of the polymer material. The poled material exhibits a strong macroscopic electro-optic effect. 

This account has summarized several of our approaches to the preparation of electric-field-aligned chromophoric polymers for second order NLO applications. Molecular design has been employed wherever possible to arrive at structures that probe particular aspects of the polar orientation issue. The rich variety of accessible organic structures has enabled us to consider the orientation problem from a variety of points of view, and to indicate by example the manner in which multifunctional organic synthesis may play a role in the fabrication of oriented materials. 

Finally, it should be mentioned that often solids are more desirable than liquids in typical applications of films. The prospects for obtaining polymer films with useful thermodynamically stable %(2) seems high given the recent demonstration that functional group orientation in FLC side chain polymers appears very similar to that observed for the low molecular weight materials (IQ). The fact that FLC polymers possess thermodynamically stable polar order in a non-crystalline solid film would appear to make this novel type of polymer glass uniquely suited for many second order NLO applications. 

In the view of the practical use, polymeric materials are expected to overcome the disadvantages of organic molecular crystals in mechanical properties and processability. Most polymers are usually in a symmetric structure as a whole, so they should be considered as third-order rather than second-order NLO materials, except for poled polymer systems. 

Second order, polyazomethine possesses both donor and acceptor groups at the molecular level, which when aligned gives a poled polymer. Tatsuura et al. fabricated an as-deposited poled polymer film by applying anin-situ electric field during CVD and reported on the second order nonlinear optical properties of the obtained film. The polymer film refractive index was reported to be about 1.68 at 632.8 nm. Second order NLO susceptability, p, was found to be... 

Disperse Red 1 (DRl) used for photoassisted poling in polymers, the back thermal reaction advantageously allows one to end up with materials containing the most stable fmws-isomers. In any case, the back reaction can be induced by light and plays an important role in both orientation and disorientation processes. Photoassisted poling does not necessarily require a stable photoisomer, as shown for azo compounds, which are very efficient. On the contrary, photoswitching of second-order NLO properties makes sense only when the photoisomer is thermally stable. 

This paper reviews experimental work done on the NLO properties of photochromes other than azo derivatives, which have been described in the preceding chapters of this book. In the first part, the molecular second-order NLO polarizabilities will be given the NLO properties of the materials will indeed depend on these values. In the second part, photoassisted poling of different photochromes in polymer matrices will be described. The photo-switching of second-order NLO properties of poled polymers or crystals will then be described in the third section. Both second and third parts will end with some applications of these optical phenomena. We will conclude with the prospectives of these materials in NLO. 

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