In this paper, we present a THz 3-dB evanescent wave directional coupler, operating at 300 GHz based on the silicon micromachining waveguides surrounded by effective-medium with subwavelength perforations. This technique offers very low losses due to the absence of metals, allows fabrication in a single etching step and provides a method for engineering the permittivity of the cladding, thus allowing wave manipulation in all-dielectric platforms, extending the concepts of photonics to the THz region.