Strategies for measuring third harmonic generation from ultrathin films

Optical third harmonic generation (THG) from ultrathin films supported on isotropic substrates is analyzed. Experiments in transmission and in reflection, and excitation with both linearly and circularly polarized light are considered. Circularly polarized excitation depresses the substrate THG, but also depresses the signal from the surface film, including complete cancellation at normal incidence. Working in reflection greatly reduces the interaction depth with the substrate without affecting the surface signal. Using a model system of polydiacetylene on glass we verify these predictions.