Nonlinear Optical Properties of Polythiophene and Thiophene Oligomers

6 Nonlinear optical properties of polythiophene and thiophene oligomers 

Nonlinear optical properties of conjugated polymers have been extensively studied in the last decade [165-167]. Experimentally it has been shown that third-order non-linearities of conjugated polymers are very fast and reach very high magnitudes opening the perspective of the possible development of organic based all-optical switches and modulators. 

While third-harmonic generation probes the intrinsic ultra-fast electronic response of the system, degenerate four-wave mixing, by measuring the nonlinear diffraction efficiency of the medium, is monitoring not only the instantaneous electronic response but also the nonlinear response of the photo-induced excitations (intra and intermolecular vibrations, acoustic phonons etc.) in which the system (decays in time once the excited states have been prepared (i.e. in resonance). The 

The rather weak response of 5T and 6T [174] is due to the unfavorable orientation of the long molecular axis with respect to the normal of the textured thin films. Since the long molecular axis makes an angle of 32° with respect to the normal [12,133] (section 7.4.3.1), the lowest Bu state contributes accidentally very little to the actual three-photon resonance enhancement (vide infra). A marked increase of the three-photon resonance would be observed if the long axis would lie in the substrate plane. 

We should point out that the off-resonance response of polythiophene (about 10 e.s.u.) is of the same order of magnitude of other conjugated polymers indicating that the nonlinear optical response of a conjugated chain does not depend 

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