Resonant-optical-second-harmonic generation from thinC60films

We studied the optical-second-harmonic generation from thin ${\mathrm{C}}_{60}$ films, using a combination of frequency-, rotational-, angular-, and film-thickness-dependent measurements. We present a method to resolve the phase of ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ by exploiting the interference between the ${\mathrm{C}}_{60}$ overlayer contribution and the anisotropic Si(111) substrate contributions. We find a strong and sharp resonance at 2ħω=3.60 eV. The linewidth is found to be surprisingly small as compared to linear ellipsometry and third-harmonic-generation experiments. The intensity as a function of film thickness agrees with a model considering interference effects between equal second-harmonic contributions from the inner ${\mathrm{C}}_{60}$/Si interface and the outer ${\mathrm{C}}_{60}$ surface. The possibility of nonlocal bulk contributions is discussed.