Direct near-field optical imaging of plasmonic interference fields for high-resolution photolithography

The plasmonic interference fields on the sub-wavelength gold gratings were directly investigated by using the collection-mode near-field scanning optical microscopy. For proper designs of structural size and periodicity of gratings, we found the plasmonic interference fields generated in the near filed on the gold strip surface. The light source was a visible laser, with a wavelength of 514 nm, illuminating in a normally incident direction to the sample. We demonstrated that the patterns of plasmonic interference fields can reach a narrow linewidth of λ/7, which is considerably beyond the diffraction limit. The examined widths of the near-field intensities achieved 75–90 nm on the gold strip surface and 110–120 nm on the slit. We also observed that the near-field intensities from both plasmonic interference fields and near-field light of the slits were spatially uniform at the surface, which indicated the capability of near-field intensity redistribution. The numerical simulations by using two-dimensional finite difference time domain (2D-FDTD) method supported the experimental results with excellent consistency. The simulation results also indicated the strong dependences between the plasmon interference fields and polarizations of the incident light. The experimental and simulation results provided the substantial insights for the working mechanism and advanced applications of plasmonic nanopatterns.