Linear optics in the second-order characterization of thin films

We study the importance of the linear optical properties of thin molecular films in their second-order nonlinear optical characterization. Two-beam second-harmonic generation is used to determine the susceptibility components of Langmuir-Blodgett films of terthiophene-vinylbenzoate and yields consistent results only when the linear properties of the films are included in the model. While this is expected for thick multilayer assemblies (∼140 nm), we show that the same holds also for a monomolecular layer (∼2.8 nm). In particular, we find that the linear response of the monolayer is well described by the refractive index of the thick films.