Sum-frequency generation at interfaces: A Fresnel story. I. Designing high contrast in two interface systems.

When a homogeneous film of finite thickness is optically probed, interference due to multiple reflections modulates in amplitude the electric field of light. For optical processes located only at the interfaces between two media, as is common for Sum-Frequency Generation (SFG), interference also modulates the contrast between signals generated at the entrance and exit planes of the film. We introduce a universal formalism for Fresnel factors, which bear all the information about interference, valid at any point in a three-layer system and for the three beams involved in the SFG process. Their analysis provides general rules for obtaining a high contrast. In particular, we define four configurations leading to the cancellation of the SFG response from the entrance side of the film when its thickness or the angles of incidence are tuned. Cancellation conditions depend on the polarization of light and follow simple analytical rules, leading to a straightforward experimental implementation. Such a selective cancellation makes it possible to independently measure single components of the nonlinear susceptibility, for example, in a ppp experiment and to separate surface from bulk response by combining a few measurements.