Photonic/Optical Materials

NLO Materials Tutorial in Material Science Research

  Introduction
Crystal Grade Materials
Plastic Optical Fibers
References

Organic NLO Materials

The field of organic nonlinear optics has come a long way since the its triggering development in 1970. In that year, Davydov et al. reported a strong second harmonic generation (SHG) in organic molecules having electron donor and acceptor groups bridged by a benzene ring.8 In general, second-order nonlinearity originates from an organic molecule as shown in Figure 1 and having an acentric structure. These push-pull chromophores, specially tailored for large optical nonlinearity, are being applied in emerging electronic and photonic technologies. They are replacing the existing technology that is based on inorganic single crystals, which are expensive, difficult to grow in high quality, and are not easy to incorporate into electronic devices.9

Figure 1. Schematic illustration of a nonlinear optical (NLO), push-pull chromophore.

Organic NLO molecules possess donor-acceptor groups attached to an aromatic ring system that increases charge transfer through p-electron delocalization.10 Such dye molecules are characterized by intramolecular charge transfers that give rise to large ground and excited state dipole moments, and second-order molecular hyperpolarizability.

The organic materials are superior to inorganics both in the speed of response and in the magnitude of the third-order effect.4,5,11 The largest third-order susceptibilities have been observed in macromolecules with p-conjugation along an extended backbone. The p electrons, distributed along the backbone, react quickly when other molecules, an electric field, or light changes the electrical environment. This quick response as well as the extensive spreading of the p cloud is the source of a large, fast, third-order response in these quasi, one-dimensional systems.

The last decade has seen an increasing trend towards the use of organic polymers as photonic components because of their ease of processing and fabrication; compatibility with metals, ceramics, semiconductors, and glasses; good mechanical strength; and flexibility to tailor nonlinear optical properties.12 A primary requirement for a material to exhibit nonlinear optical (NLO) activity is that it should be noncentrosymmetric. In polymer-based NLO materials, the chromophore can be incorporated into a polymer matrix in a number of ways. Early efforts focused on guest–host systems. Alternatively, the chromophores were covalently attached to the polymer backbone as side chains, or made part of the polymer backbone itself, markedly improving long–term stability, and permitting their use in practical devices.12,13

More recently, the electrostatic self-assembled monolayer (ESAM) technique for fabricating noncentrosymmetric structures resulting in a large second order NLO response is being increasingly researched.14-16 The advantages of this technique include long-term stability of χ(2) in contrast to electric field poling of a glassy polymer, thicker (tens of microns thick) films than by the Langmuir-Blodgett technique, and easier fabrication than by covalent self-assembly methods. By ESAM processing, a multilayer film is formed by alternately immersing the substrate in aqueous solutions of a polyanion and a polycation. Either the polycation, the polyanion, or both may contain polarizable chromophores, i.e., the active polyelectrolyte. In concert with the other variables, the choice of polyelectrolyte can have a marked effect on reinforcing or disrupting the orientation of chromophores in successive layers and, hence, on the second order NLO response.

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