Hyper-Rayleigh scattering (HRS) experiments performed on nanometer-scale SiO2/water interfaces (aqueous colloidal suspensions) display finite, measurable signals. HRS is not constrained by the orientational, size, and/or charge restrictions inherent to conventional interfacial second harmonic generation (SHG) or electric-field-induced SHG measurements. Thus, apparently for the first time, the second-order nonlinear optical (NLO) response of ultrasmall interfaces has been measured. pH- and electrolyte-dependent experiments show the response to be sensitive to changes in chemical composition at the silicon dioxide/solution interface. HRS provides detailed information about the surface of these technologically interesting and important particles in their native solution-phase environment.

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The sensitivity of optical molecular orientation measurements to assumptions regarding thin film refractive index was investigated. Specifically, the influence of the interfacial refractive index on second harmonic generation (SHG) and linear dichroism measurements made in a total internal reflection (TIR) geometry was probed for five distinct molecular systems. The five molecular thin films ranged from weakly adsorbed species in equilibrium with solution to covalently bound molecules. Polarization data from the two techniques were fit using a range of assumed interfacial refractive indices. Surprisingly, a linear relationship between the difference in calculated apparent orientation angle and the difference in solvent-prism refractive index was observed. The trend indicates that for a TIR geometry, the error introduced by the thin film refractive index is negligible when the difference in solvent and prism refractive indices is less than approximately 0.08. However, there are clearly cases, such as a glass/air interface, in which assumptions regarding the thin film refractive index can result in significant error in the extracted orientation angle.

The film structure of a nonlinear optical azo dye (Disperse Red-1, DR-1) was probed by a combination of spectroscopic and microscopic techniques. Second harmonic generation (SHG) and linear dichroism provided evidence for the spontaneous formation of DR-1 multilayers at a 2-propanol/fused-silica interface under equilibrium conditions. Specifically, the adsorption isotherm measured by SHG exhibited two distinct adsorption regimes; the second regime (>10-3 M) being indicative of multilayer formation in which some degree of polar order was retained. The trend in apparent tilt angle measured by linear dichroism was consistent with multilayer formation at solution-phase concentrations above 10-3 M. Multilayer formation was directly observed, on a dip-coated substrate, by atomic force microscopy and further confirmed by ellipsometry. The significance of spontaneous multilayer formation with z-axis polar order in higher layers is the potential for growth of molecular multilayer systems without the need for covalent or sequential deposition.