Figure 2: SHG from a single pentacene/C₆₀ interface.

(a) Experimental geometry used to measure transmitted second harmonic generation (SHG) as a function of the pump polarization (ϕ) and incidence (θ) angles; the fundamental wavelength is λ_ω=800 nm. The inset shows the sample structure, which consists of a pentacene/C₆₀ bilayer deposited on a sapphire substrate. (b) Quadratic dependence (denoted by the red line) of SH power on fundamental power that is typical of all samples studied. The error bars reflect s.d.’s calculated from multiple measurements on the same sample structure. (c) Tilt scan results of p-polarized SHG from bilayers with fixed C₆₀ (20 nm) and varying pentacene thickness (blue squares) as indicated in the legend; the individual pentacene (red circles) and C₆₀ films (black triangles) are included for reference. (d) The same measurement carried out on bilayers with fixed pentacene (18 nm) and varying C₆₀ thickness. The strong increase in bilayer SHG independent of constituent film thickness points to a dominant contribution from the DA interface. (e,f) Polar plots show the dependence of p- and s-polarized SHG on pump polarization angle, consistent with a nonlinear susceptibility exhibiting C_∞v symmetry. Solid magenta lines in c–f are the result of global fits to all of the bilayer sample data associated with each respective panel using the surface SHG model described in Supplementary Note 1. The left-hand axes in e and f denote the radial scale.