Finitte-difference time-domain studies on optical transmission through planar nano-apertures in a metal film

The finite-difference time-domain (FDTD) method is employed to numerically study the transmission characteristics of an H-shaped nano-aperture in a metal film in the optical frequency range. It is demonstrated that the fundamental TE10 mode concentrated in the gap between the two ridges of the H-shaped aperture provides a high transmission efficiency above unity and the size of the gap determines the sub-wavelength resolution. Fabry-Perot-like resonance is observed. Localized surface plasmon (LSP) is excited on the edges of the aperture in a silver film but has a negative effect on the signal contrast and field concentration, while aluminum acts similar to an ideal conductor if the film thickness is several times larger than the finite skin depth. In addition, it is shown that two other ridged apertures, C-shaped and bowtie-shaped apertures, can also be used to achieve a sub-wavelength resolution in the near field with a transmission efficiency above unity and a high contrast.