Photoexcitations and nonlinear optical properties in thiophene-based conjugated systems

Abstract

The nonlinear optical properties of a series of thiophene based polymers have been investigated by studying the nonlinear optical excitations generated upon photoinjection of electron-hole pairs as well as the cubic susceptibility by degenerate four wave mixing (DFWM). The polymers have a similar electronic structure and slightly different molecular structures thus allowing to directly address the relation between the molecular structure and the nonlinear optical properties. Photoexcitation spectra show that all the systems give rise to bipolarons characterized by lattice distortions associated with intragap optical excitations. The DFWM study is performed at various laser frequencies encompassing the low energy transparent spectral range as well as the rising slops of the optical absorption due to the lowest (pi)-(pi) electronic transition. By doing so we measure both the nonresonant and resonant optical responses. The typical values of nonresonance (chi) (3)(-(omega); (omega), -(omega), (omega)) are in the range of 10-11 e.s.u. while in resonance are two orders of magnitude higher. The highest value of 1.13 X 10--8 e.s.u. is observed in polydithieno(3,2-b;2',3'-d)thiophene (PDTT); this value is among the highest of those observed in conjugated polymers. The most severe limitation to the use of these materials is in general due to the fact that the large optical nonlinear coefficients are accompanied by increasingly large optical absorption losses. The effect of photogenerated excited states on the nonlinear optical properties is investigated by a pump and probe DFWM experiment on polythiophene. The transient population of intrachin polarons gives rise to a substantial enhancement of the (chi) (3)(-(omega); (omega), -(omega), (omega)) indicating that the additional nonlinear processes which involve transient photoexcited states have a net positive contribution on the overall (chi) (3)(-(omega); (omega), -(omega), (omega)) process.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.