Enhancement of second-harmonic generation from self-assembled monolayers on gold by excitation with a radially polarized beam

We have studied the enhancement of second-harmonic generation (SHG) from self-assembled monolayers on Au surfaces excited by radially polarized beams. The electric field at the metal surface was enhanced by constructive interference between the incident and the reflected beams due to a longitudinal field, which is the field parallel to the optical axis, generated around the focus by the radially polarized beam. Since even-order nonlinear phenomena are surface sensitive, the combination of SHG and a radially polarized beam has the potential to be a powerful new imaging tool for characterization of metal surfaces. The SHG signal excited by the radially polarized beam was about 3 times higher than that excited by a linearly polarized beam; in addition, the SHG from a 7-(dimethylamino)-4-methylcoumarin-3-isothiocyanate monolayer was about 1.3 times higher than that from a bare Au substrate.