Nonlinear Optical Properties of Thiophene Based Conjugated Polymers

Abstract

Investigation of the nonlinear optical (NLO) properties of organic systems possessing delocalized π electrons has attracted considerable interest because of the large and fast NLO response [1–4]. The most studied systems have been polyacetylene [5–10] and polythiophene [11–14]. Even though the NLO response in these macromolecules derives from the high hyperpolarizability of π delocalized electrons in the carbon polymer backbone, the origin of their large optical nonlinearity is not yet clear and therefore it is not yet possible to take full advantage of the chemical tailoring of new systems in order to make more advanced NLO materials. One way to contribute to an understanding of the origin of the NLO response is to study the electronic cubic susceptibility x ⁽³⁾ as a function of frequency for different conjugated polymers with different electronic structures. In fact in the absence of a spectrum it is difficult to distinguish between the resonant and nonresonant values of x ⁽³⁾ because in principle, resonance may occur not only in spectral regions where the material is absorbing but also in transparent regions by resonance with forbidden electronic states which do not contribute to the absorption. Up to now, only a limited set of data have been available on the electronic x ⁽³⁾ and many of these are limited to a single fundamental frequency. Wavelength dispersed x ⁽³⁾ _THG by third harmonic generation (THG) has been studied in polyacetylene [15] and in two thiophene based polyconjugated systems [16].