We show that the electronic part of the nonlinear susceptibility ${\mathbf{\chi }}^3$ of thin films can be easily measured by third harmonic microscopy. The phenomenon of third harmonic generation (THG) is excited by a femtosecond laser beam focused at the interface between the thin film and a reference layer. The value of ${\mathbf{\chi }}^3$ is deduced from the THG intensity measurements with the help of a classical model. The validity of this simple and alternative method is established by testing reference liquid films.

• Received 10 May 2002

DOI:https://doi.org/10.1103/PhysRevE.66.067602