Nonlinear chromophore

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). 

Figure 8. Schematic representation of a noncentrosymmetric Lang-muir-Blodgett film. The nonlinear chromophore is incorporated into alternate layers, those represented by the squares, for example.

The applicable fundamental concepts of nonlinear integrated optics for SHG were outlined decades ago and can be found in a number of review papers [6-8]. The basic theory as applied to organic materials and polymers is of course unchanged from that for dielectric materials and these papers are still very useful. Some twenty plus years ago, nonlinear integrated optical experiments started to be conducted, but mostly on inorganics and crystals. The specific field of amorphous and semi-ordered organics came when the chemical engineering of nonlinear chromophores was developed. 

A composition comprising a Ceo fullerene-terminated poly(A-[(propylphenyl]-AA, A -triphenyl-(l,l - biphenyl)-4,4 -diamine)methacrylate has been prepared by living radical polymerization. When blended with a plasticizer and a nonlinear chromophore, photorefractive efficiencies were improved by up to 9% at abiased voltage of 60 V/pm. 

NLO polymers are designed by incorporating nonlinear chromophores into a polymer matrix. The simplest approach is the use of polymer solutions, so-called guest-host systems, in which the nonlinear chromophore is dissolved in a compatible polymer matrix. Unfortunately, the solubility of guest molecules in a polymer matrix is usually low, which limits the magnitude of the NLO response. This problem is solved by attaching the chromophores covalently to the polymer backbone (side chain polymers) or by incorporating them into the backbone of the polymer (main chain polymers). 

The concept of a multiphase nanostructured composite can be used to prepare a wide variety of optical materials. We have been able to dope two (or more) different optically responsive materials, each of which can be in different phases of the matrix ( e silica phase, the PMMA phase and the interfacial phase), to make multifimctional bulk materials for photonics. For example, we have doped in addition to the fiillerene, which is adsorbed in the interfacial phase, a fluorescent and optically nonlinear chromophore bisbenzothiazole 3,4-didecyloxy thiophene (BBTODT) in the PMMA phase. This nonlinear chromophore was developed by B. Reinhardt and co-workers at the Polymer Branch of U.S. Air Force Wright Laboratory 14). 

It was pointed out that the performance of photorefractive polymer composites is too large to be explained by the simple electrooptic photorefractive effect alone. A theoretical model was offered where both the birefringence and electrooptic coefficient are periodically modulated by the space-charge field due to the orientational mobility of the nonlinear chromophores at ambient temperatures. 

For instance, one material was prepared from photoconducting poly(N-vinyl carbazole) that was doped with a blue-shifting optically nonlinear chromophore, 3-fluoro-4-N,N-diethylamino-p-nitrostyrene. This material was sensitized for charge generation with 2,3,7-trinitro-9-fluorenone. ... 

Table 6.1. gives examples of some nonlinear chromophores. These nonlinear opties ehromophores are intended for incorporation into polyimides, polyurethanes, polyureas, and polyamides. Among them are some promising chromophores that were reported by Jen and coworkers. ... 

On the other hand, formation of photorefi active silicone composite with good performance was reported A carbazole-substituted polysiloxane that was sensitized by 2,4,7-trinitro-9-fluorenone was used as a photoconducting medium and l-[4-(2-nilrovinyl)phenylpiperidine was added as an optically nonlinear chromophore. The photorefractive property of polymer was determined by diffraction efficiency using a 100 pm-thick film. The maximum diffraction efficiency (rj max) of 71% was obtained at the electric field of 70 V/pm. 

The second term on the right side of equation 19 ultimately determines the usefulness of the molecule when incorporated into a polymer for second-order applications (96). A DC field aligns the dipole moment fx so that the first hyperpolarizability, /3, can contribute to the bulk response. A similar expression is applied to poled polymers (24). The assumption that y is negligible compared to p is usually valid for nonlinear chromophores, but not for extended -electron donor-acceptor systems because y increases with conjugation length faster than p (97). 

These practical considerations have led investigators away from simple guest-host polymer systems, perceived to have less intrinsic orientation stability, toward systems in which the nonlinear chromophore mobility is hindered. This can be achieved by chemically bounding the active molecules, in side- or in main-chain, or alternatively promoting cross-linking after orientation [17]. 

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