Terahertz antireflection properties of sub-wavelength metallic double wire grid structures

Summary form only given. Metallic wire grids (WG) are popular terahertz (THz) polarizing elements. For freestanding structures, the near perfect properties of metals provide high transmission efficiency and extinction ratio over a wide frequency range. For substrate-supported structures however, these nearly ideal properties are lost as the transmission efficiency is dictated by the refractive index of the substrate. Recently, double layer WG (DWG) structures have been proposed for their exceptional polarizing properties [1]. With the help of a semi-analytical model, we show here that such structures can act as excellent anti-reflection coatings for transverse magnetic (TM) waves. Experimental verifications on selected examples are provided.The considered structure, consisting of a sub-wavelength gold-coated dielectric grating, is shown in Fig. 1a. In the dielectric core, the transmission is mediated by one single propagating mode with effective index n. Therefore, the transmission properties of the structure are accurately represented by a Fabry-Pérot cavity (FPC) model. The mode is reflected at the top (bottom) interface with complex amplitude rueIψυ (rdeiφd), and the transmission is maximized when Φu + Φd + 4π λ nCtd = 2mπ, m E 7L and ru = rd. The complex reflection coefficients and the mode effective index are numerically computed with rigorous coupled wave analysis (RCWA) [2]. Comparisons of the FPC model with rigorous computations show excellent agreement.