Off-resonant nonlinear optical properties of conjugated organic polymers - Origins and nature

Results of a series of nonlinear optical studies of organic conjugated systems are reported and discussed with a view towards deriving an understanding of the origins and nature of the near infrared nonlinear optical properties of organic conjugated polymers. Measurements were made in solution by the method of self-diffraction from laser induced gratings at a wavelength of 1.064nm. In a series of enyne oligomers, concentration dependences of the diffraction efficiencies show large contributions from imaginary components of the nonlinear susceptibilities. The origins of these imaginary components are linked to the nature of the response of one dimensional conjugated systems to a perturbation of the electronic configuration in the form of electron-lattice interaction. The role of the influence of such interactions in limiting the nonlinear optical response of polymers is discussed. In a range of polymeric systems, intensity dependences of the diffraction efficiencies reveal the strong influence of the multiphoton resonant enhancement of the nonlinearity. In three polymers, exhibiting strong similarities in linear optical properties, a striking variation of the order of the nonlinear optical susceptibilities at the wavelength studied is observed, originating in differing distributions of multiphoton states. The positioning of these states is largely determined by electron correlation interactions and the strength of these interactions are discussed in terms of monomelic structure. Finally the inclusion of electron rich species in the polymeric backbone is investigated and studies of metal containing organic species are reported.