Numerical analysis of transmission through a sub-wavelength metallic aperture or grating at visible and Terahertz wavelengths

We analyse the coupling of THz radiation to a GaAs waveguide through a metallic grating with sub-wavelength slits. This result is compared to the coupling through a sub-wavelength-sized aperture in a chromium mask to a diffraction-free layered superlens at visible wavelengths. We demonstrate efficient coupling of a plane-wave to the multilayer. The mode resembles a ray - a term usually reserved for geometric-optics, and unknown to nano-optics. However, the width of the mode is sub-wavelength and the propagation direction is determined by the normal direction to the layer boundaries.