Third-harmonic generation in polyphenylacetylene: Exact determination of nonlinear optical susceptibilities in ultrathin films

Third-harmonic generation measurements have been performed at λ0=1064 nm on ultrathin films of polyphenylacetylene. The nonlinear susceptibility χxxx x(3)(−3ω; ω, ω, ω) was determined to be (7.0±1.0)×10−13 esu with the phase of χ(3) equal to 152°±5° indicating that two- and three-photon resonances contribute to the observed nonlinearity. The influence of back-reflection effects was estimated experimentally by measurements in different optical configurations. We found the experimental data could only be explained by using an evaluation procedure including all bound waves in the layer system.