We study the dielectric constant dependent diffraction phenomena of single slit apertures, both theoretically and
experimentally. We experimentally simulate perfect metal and real metal cases by investigating subwavelength
diffraction by a single slit, both in nano-optical and in terahertz regimes, keeping the slit-width/wavelength ratio
approximately the same for both of frequency regimes. The wave-front in optical regime separates itself into forward
propagating beam and surface-bound 90-degree diffracted wave, i.e., surface plasmon polaritons; while the separation of
modes is not observed in terahertz regime.