Polarity/polarization total internal reflection systems

SHG is a coherent process and in principle the experimental system needed to observe the response is very simple. The fundamental radiation from a laser source incident at an interface generates the harmonic beam via non-linear polarization of the medium. Typically, this beam is observed in reflection, but many studies have been undertaken in total internal reflection and transmission geometries. As the harmonic beam is well separated from the fundamental in frequency, it can be detected the difficulties arise due to the inherent inefficiency of the harmonic generation and the low intensities that need to be detected. The sensitivity and selectivity of SHG to the interfacial species in the presence of the same species in the bulk phase provides the driving force to overcome these experimental difficulties. 

Figure 3.10 Definition of the coordinate system, a light ray that is totally internally reflected at a boundary between glass (above) and water (below). Electric (f), magnetic (B), and propagation (/r) vectors are shown for the case of p-polarized light. Subscripts I and r refer to incident and reflected light rays, respectively.

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