Comment on "Molecular orientation determined by second-harmonic generation: Self-assembled monolayers"

A recent paper by Eisert et al. [Phys. Rev. B 58, 10 860 (1998)] describes a second-harmonic generation (SHG) technique for probing molecular orientation in self-assembled monolayers (SAMs). Their results have raised some questions about the frequently used description of the optical properties of thin films. In their paper, the SAM film is considered anisotropic for SHG, but is assumed to be isotropic in terms of its linear optical response. It is likely that these different treatments of the same system have contributed to the unusual perspectives developed in the above paper. The purpose of this communication is to attempt a clarification of these observations. Here, a phenomenological model is outlined that treats linear and nonlinear optics of the interface in a single framework and can be used to analyze SHG from SAMs. It is also shown, that, depending on the experimental system, conventional SHG measurements of phase and intensity may not be enough to determine the orientation of SAMs.