Anisotropic second-harmonic generation from chromophoric self-assembled monolayers upon oxide overlayers of Si(111) surfaces

The nonlinear optical response of stilbazolium-based organic monolayers self-assembled upon silicon(111) substrates, which have oxide layers 4–860 nm thick, is studied by reflection second-harmonic generation at an input wavelength of 1.053 µm. All samples, irrespective of oxide thickness, display clear threefold symmetry about the azimuth, which is shown to arise from interference between an isotropic component generated by the chromophoric monolayer, and an anisotropic component of sixfold symmetry, which originates from the Si(111) substrate. Maximum second-harmonic intensity and orientation relative to the silicon lattice vary with different oxide thicknesses as a consequence of multiple internal reflections within the oxide layer, which can be expressed as thickness-dependent local field corrections. Evaluation of the principal tensor components permits estimation of the molecular tilt angle from the surface normal as ∼25° for monolayers self-assembled upon thick oxide overlayers (>30 nm).