“…6 (a) [18, 26, 27, 28]. In the microwave, millimeter-wave and up to terahertz frequency region, TE cut-off metallic waveguides [55] provide negative effective permittivity below the cut-off frequency [29,30,31,32,33,34,35,36], in which polarization of the incident wave is parallel to the two metallic plates with the distance smaller than the half wavelength, as illustrated in Fig. 6 (b).…”