SHG is a nonlinear optical second order process, the generation of frequency doubled light is the result of the interaction of an high power laser pulse with matter. It has been used for decades to extent the frequency range of laser light sources using non-centrosymmetric crystals. SHG as a surface specific tool, exploits the fact that there is no

generation of SHG light in centro-symmetric media. At the interface of two isotropic media, the centrosymmetry is broken and SHG light is generated within the transition region of both adjacent media.

In favorable cases, the analysis of polarization dependent SHG measurements allows the determination of the symmetry of the interface, the number density of the amphiphiles within the topmost layer and the orientation of the molecules in the interfacial layer. Depending on the hyperpolarizability of the adsorbed molecules, a sub-monolayer sensitivity can be achieved.

Unfortunately, only certain molecules give rise to an SHG signal and in order to fully exploit the potential offered by SHG it is necessary to design a suitable model system with a high hyperpolarizability. The SHG signal is then determined by the dipolar contribution and combinations of various components of the macroscopic susceptibility tensor are measured in reflection mode.