Second-harmonic near-field optical microscopy of periodic nanoholes in metal films

摘要

Using a second-harmonic (SH) scanning near-field optical microscope (SNOM) working in transmission with an uncoated fiber tip, we obtain highly detailed fundamental har monic (FH) and SH images of a ～50-nm-thick gold film de posited on a glass substrate and containing a ～830-nm-period array of slightly elongated holes (diameters in perpendicular di rections are ～170 and 260 nm). The FH and SH images for three excitation wavelengths, (730, 800, and 870 rim) are presented for; different polarization combinations of the FH-incident and FH- and SH-detected radiations, it is seen that the symmetry of the FH images is primarily determined by the symmetry of surface plas- mon polaritons (SPPs) excited (at the EH wavelength) at the gold- air interface, whereas the symmetry of the SH images, is mainly governed by the symmetry of holes. We notice large differences in the strength of SH signals originating at different (nominally- identical) holes and relate this feature to interference in the de veloped muliple scattering of SPPs.