Structural Contributions to Second-Order Optical Nonlinearities in Oriented Interfacial Multilayers

We report on the synthesis and characterization of Zr-phosphate/phosphonate (ZP) self-assembled multilayer structures using surface second harmonic generation measurements. We use two structurally complementary χ (2) -active chromophores that can be deposited with orientational control relative to the substrate. These chromophores produce multilayer χ (2) responses identical in magnitude but of opposite sign. We form opposing bilayers with these chromophores to produce two different structural motifs, each with a local center of inversion about the ZP interlayer bonding plane. The χ (2) responses of these two bilayer systems are different and reveal the extent to which the dipole approximation is valid in these layered assemblies. Our data elucidate the role that quadrupolar terms play in the χ (2) responses of thin interfacial films. This work illustrates the complex and subtle structural issues associated with the design and construction of layered interfaces and provides a means to evaluate the vacancy and orientational defect density in layered materials.