Poled chromophore-functionalized polymers

These results illustrate the diversity of synthetic and processing approaches that can be taken in the synthesis of thin-film frequency doubling materials. Specifically, we have demonstrated that it is possible to assemble chromophore-functionalized polymers with greater than one chromophore substitutent per monomer subunit, with d33 values as high as 65 x 10 esu, with Tg values as high as 173°C, with improved temporal stability, and with good transparency characteristics at A 0.633 fim. We have also shown that known chromophore-functionalized polymers can be simultaneously poled and cross-linked... 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

NLO chromophore-functionalized polyimides have attracted a lot of interest thanks to their high T., and excellent temporal stability. Verbiest et al. [72,73] and Miller et al. [74] reported several highly stable aromatic polyimides (Fig. 4.3), including the synthesis of the PI-1 polymer, which is the first reported example of a processable donor-embedded side-chain polyimide having a very high Tg of 350°C, and a chemical stability at temperatures as high as 350°C. Poled samples of polymer PI-1 have an EO coefficient of 4-7 pm/V (at 1.3 im) and a much higher orientational... 

Several types of functional decay forms have been used to describe time-dependent processes in poled polymer systems. The Kohlrausch-Williams-Watts (KWW) [72] stretched exponential function has often been used to fit the orientation decay of chromophores in polymers ... 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

Fig. 19. Dynamical thermal stability of electric field poling-induced electro-optic activity for two samples. The data were obtained as described in [121] by slowly increasing temperature while monitoring second harmonic generation. The chromophore is a DEC chromophore described in [138]. Uncrosslinked refers to the precursor polymer where only one end of the DEC chromophore is attached to the polymer lattice. Crosslinked refers to the situation where both ends of the DEC chromophore have been reacted to achieve covalent coupling to the polymer lattice. The ends of this DEC chromophore are asymmetrically functionalized so that attachment reactions can be carried out independently...

The hb-PAEs of hb-P13 and hb-P15 contain NLO-active azo-functionalities, which are soluble, film-forming, and morphologically stable (Tg > 180 °C). Their poled films exhibited high SHG coefficients ( 33 up to 177pm/V), thanks to the chromophore-separation and site-isolation effects of the hyperbranched structures of the polymers in the three-dimensional space (Table 5) [28]. The optical nonlinearities of the poled films of the polymers are thermally stable with no drop in d33 observable when heated to 152 °C (Fig. 8), due to the facile cross-linking of the multiple acetylenic triple bonds in the hb-PAEs at moderate temperatures (e.g., 88 °C). 

Fig. 13. Temporal stability of the poled/crosslinked NLO dendrimer 19 and guest/host polymer system (NLO chromophore 16/PQ-lOO) at 85 °C in nitrogen. Normalized rjj as a function of baking time...

The superscriptsp and r in Eq. (28) refer to polymer thin film and reference harmonic intensities. The reference, with simultaneous measurements of SHG intensity from a poled, stabilized thin film, was used to correct for eventual laser intensity fluctuations. The fits of the measured temporal growth of SHG intensity with mono-, bi-, and triexponential functions are shown in Fig. 31. The best agreement is obtained when using a triexponential function (Eq. (28)). The time constants T2, and T3 depend on the polymer, temperature, and the chromophore itself. 

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