Imaging contrast under aperture tip–nanoantenna array interaction

Periodic arrays of paired and single gold nanorods were imaged in the near field using reflection and transmission modes of a near-field scanning optical microscope at various wavelengths and polarizations of light in the visible range. The paired nanorods act like nanoantenna, and an array of them was initially designed as a negative-index material for the near infrared. Reverse contrast in reflection and transmission images is observed under illumination from the small aperture of a metal-coated fiber probe. By changing the relative orientation of the rods to the polarization, the reverse contrast switches to the normal contrast of near-field imaging. Coupling between the aperture and the nanorod array makes the contrast higher. Transmission through the aperture is enhanced if the aperture probe is positioned between the nanorods. The average near-field transmission exhibits an opposite sign of anisotropy relative to the far-field case. Aperture probes with larger diameters always show normal imaging contrast. The results demonstrate that the broad angular spectra of small-aperture sources play a crucial role in near-field interactions with nanorod arrays. The results also show that angular redistributions of these spectra after transmission or reflection from the nanorod array are likely due to excitation of localized and propagating plasmons.