Imaging heterogeneous nanostructures with a plasmonic resonant ridge aperture

We introduce a plasmonic resonance ridge aperture capable of sensing changes in refractive index and absorption with nanoscale resolution. Using this aperture, we devised a plasmonic near-field scanning nanoscope (PNSN) to record images of heterogeneous nanostructures. Compared to a conventional near-field scanning optical microscope (NSOM) that measures light scattered by the sample, the PNSN directly measures the change in a beam reflected from the aperture to detect buried objects. Using the PNSN we recorded images of nanoscale rectangular groove arrays on a SiO₂ substrate with patterns typical of a dynamic random access memory circuit. By comparing the experimental and calculated image of the nanostructure, we estimate the resolution of PNSN to be ∼20 nm, which is ∼50% smaller than the near-field spot generated by the aperture. Also, we theoretically analyzed the feasibility of the PNSN detecting an object underneath a metal film.