Nonlinear Optics

Polymers with extended p-electron delocalization can exhibit nonlinear optical effects at large light intensities. Second-order nonlinearities like second-harmonic generation or electrooptical effects appear in materials which do not possess a center of inversion. Third-order nonlinearities lead to intensity-dependencies of refractive index or absorption coeffient. They appear in every material and do not have the symmetry constraints like the second-order effects. Investigations of the nonlinear optical materials coefficients c(2) or c(3) over a large spectral range can be used as nonlinear optical spectroscopy. The development of sensitive laser techniques provides detailed information on the photophysics of p-conjugated materials such as 2-photon-, multiphoton- or excited state-absorptions and relaxations that are not accessible to linear optical spectroscopy. The latter is sensitive to dipole allowed transitions only. Third-order effects like the intensity dependencies of the refractive index and the absorption coefficient offer exciting perspectives to control the propagation of light beams by purely optical means. The availability of appropriate materials would allow all-optical signal processing which could have strong impact on the information technology. Polymers with a highly delocalized p-electron system have extraordinary large c(3)-values comparable to or even larger than those of semiconductors. A major goal is to establish relations between chemical structure and nonlinear optical properties of polymers, e.g., to understand the dominant role of the p-electron delocalization length. Several techniques have been applied and further developed to characterize nonlinear optical processes in solutions or thin films of polymers such as: Third-harmonic generation, degenerate four wave mixing, and intensity-dependent prism coupling of nonlinear waveguides.

Schematic view of a polymer waveguide which shows partial transmission and reflection at a grating or photonic crystal structure which is Bragg reflection at distinct combinations of wavelength, refractive index and grating periodicity. By means of the intensity dependence of the refractive index, the transmission ratio can be controlled.