Theory of Non-linear Optical Response in Molecular Layers

Nonlinear optics has provided a useful testing ground for the concept of molecular engineering. One seeks to design and synthesize molecules of high nonlinear optical response, and then to fabricate a molecular material in which this response is effectively expressed. In many such materials the active region is limited in thickness. Obvious examples are Langmuir-Blodgett films1 and polymer films. 2 Less obvious are molecular crystalline thin films or crystalline cored fibres, 3 or active species diffused into the surface of an optical polymer. 4 Finally, centrosymmetric materials lacking any bulk quadratic nonlinearity may display surface nonlinearity5 or pyroelectricity. 6 Molecular theories of nonlinear optical response in thin layers are not well developed. As in theories of bulk nonlinear response, a common approach has been to approximate the local electric field by the Lorentz expression. The Lorentz approximation is known to be poor for crystals of elongated molecules such as p.-terphenyl7 and the polydiacetylenes, 8 so that its reliability is questionable in Langmuir-Blodgett films, at least