Polarization and phase characteristics of nonresonant sum-frequency generation response from a silicon (111) surface

Silicon (111) [Si(111)] surfaces both with and without a thin film of polystyrene are investigated with sum-frequency generation (SFG) spectroscopy with s-polarized visible and p-polarized infrared inputs. On uncoated Si, the nonresonant polarization changes from s to p, with the maximum signal in each polarization component repeating every 60° of sample rotation. With polystyrene on Si(111), the resonant features go in and out of phase with the nonresonant signal every 120°. The resonant response is only s polarized, as expected; however, the nonresonant response again switches between s and p polarizations. Implications for proper collection and interpretation of SFG spectra from crystalline substrates are discussed.