In this work, we investigate transmission enhancement through ridged-apertures of nanometer size in a metal film in the optical frequency range. It is demonstrated that the fundamental propagation TE10 mode concentrated in the gap between the two ridges of the aperture provides transmission efficiency higher than unity, and the size of the gap between the two ridges determines the sub-wavelength resolution. Fabry-Perot-like resonance with respect to the thickness of the aperture and the red-shift phenomena with respect to the wavelength of the incident light are observed. As a comparison, transmission through regular apertures is also computed, and is found much lower. Localized surface plasmon (LSP) is excited on the edges of the aperture in a silver film but plays a negative role with respect to the field concentration and signal contrast. With optimized geometries, the ridged apertures are capable of achieving sub-wavelength resolution in the near field with transmission efficiency above unity and high contrast.
