Enhanced optical third harmonic generation in thick organic films

In order to enhance optical third-harmonic generation (THG) in polymer films, we investigated linear and nonlinear optical properties of polymer films containing ethyl-violet with varying thickness. The THG intensity increased monotonically with thickness, as opposed to the theory for non-phase-matched THG in films thicker than the coherence length. By measuring the refractive indices, we found that the difference between the indices of the fundamental and the third-harmonic was smaller in thicker films. Thus, the coherence lengths for thicker films were evaluated to be larger than those for the thin films, explaining the monotonic increase of the THG signal with thickness.