The recent development of promising organic photonic materials and special device architectures such as photonic bandgap lattices suggests that organic materials will play an important role in next generation electro-optical (EO) switching devices. When compared to lithium-niobate based devices, chromophore-doped polymeric EO devices hold the promise of larger bandwidths and lower operational voltages. For example, polymer-based materials with electro-optic coefficients in excess of 100 pm/V and switching frequencies greater than 100 GHz have been demonstrated. However, fulfilling the promise of these materials is dependent on understanding the molecular details that currently limit their overall efficiency.
Perhaps the central issue surrounding the development of wide-bandwidth photonic electro-optic modulators is the translation of molecular systems with large hyperpolarizabilties into macroscopic assemblies having a correspondingly large electro-optical activity. This issue is nicely illustrated in the following figure where the macroscopic electro-optic coefficient (r) versus chromophore loading (or number density, N) for the chromophore CLD dispersed in PMMA is depicted. The figure illustrates that at low loadings r increases proportionally with N; however, a maximum is eventually reached with continued loading resulting in a reduction in electro-optical activity. The position of the maximum depends on molecular properties such as shape and dipole moment. This observation suggests that the chromophore properties in combination with the polymer define the maximum achievable EO activity. If the molecular details behind the reduction in EO activity at high loading density can be understood, then a rational path towards the development of more efficient EO materials can be defined.
Our research in this area involves the characterization of non-linear optical materials from single molecules up to molecular assemblies. Chromophore non-linear optical activity is characterized using frequency-agile hyper-Rayleigh spectroscopy. The intensity of hyper-Rayleigh scattering can be used to determine the non-linear optical performance of individual chromophores. This research is performed in close collaboration with other scientists paticipating in the Science and Technology Center for Materials and Devices for Information Technology Research (CMDITR) supported by the NSF (Click here to link to the CMDITR website). The information gained in these studies is critical in identifying the most promising compounds for use in device applications. Characterization of chromophore-polymer composite materials is performed using standard confocal and non-linear optical microscopy. These techniques combines 3-dimensional spatial resolution with single-molecule sensitivity allowing for detailed studies of chromophore-doped systems. We use the linear-dichroism of the chromophore emission to measure molecular alignment as a function of number density and poling voltage, and directly compare this information to predictions of molecular order derived from Monte-Carlo simulations. Through such comparisons, accurate theoretical methods will be developed thereby increasing the predictive ability of these techniques. For example, the following figures present fluorescence images of the laser dye DCM dispersed in the polymer polymethyl acrylate (PMA). The images correspond to polarization components of the emission that are parallel and perpendicular to an applied electric field. One can measure these components as a function of time to determine the orientation of the molecule, and to follow the rotational dynamics. By measuring the perturbation of the rotational dynamics with the application of the electric field, we are learning about one of the fundamental steps in device construction, electric-field poling to induce molecular alignment.
Another approach to inducing molecular alignment is to employ host systems inherently provide for alignment. Specifically, the Kahr Group at the University of Washington has demonstrated that when crystals of potassium hydrogen phthalate (KAP) are grown from aqueous solution in the presence of chromophores, the chromophores are incorporated into specific growth sectors of the crystal with the optical properties of the resulting crystal demonstrating that the chromophores are aligned. In collaboration with the Kahr group, we have performed single-molecule orientational studies of DCM and Violamine R in KAP. By measuring fluorescence dichroism, the orientation of each molecule can be determined as demonstrated in the following figure. Ultimately, comparison with polymers at the single molecule level will firmly establish the differences in alignment that are a consequence of two distinct mechanisms: poling of polymers and growth anisotropy in crystals.
