Nonlinear Optical Properties of Ultrathin Polymer Films

Abstract

Third harmonic generation (THG) and degenerate four wave mixing (DFWM) experiments are used to study magnitude and response time of the third order nonlinear susceptibility of spin cast films of poly (p-phenylene vinylene) and poly (phenyl acetylene) derivatives with various substituents at the phenyl ring. The contribution of two and three photon resonances will be discussed. In the DFWM experiments we see picosecond response times for these polymers even under resonance conditions. DFWM experiments have been performed with phthalocyanine (PC) thin films. The relative distances of the PC rings were varied by dissolving or copo lymeri zing PC in polystyrene and by building up Langmuir-Blodgett films of PC monomers or polymers. The linear and nonlinear optical properties of the PC films depend strongly on the distance of the PC rings and therefore on the electronic coupling between them. Increased electronic coupling leads to a spectral broadening of the optical absorption bands and to a reduction of the response time to some picoseconds. Influences of energy migration and transfer to trap states are discussed.