AIT-based gas sensor: reflected THz radiation carries information about whether or not a given target substance (here hydrogen cyanide, HCN) is present inside the holes of a holey metal film. The bare holey film (a) has been designed to be opaque at the spectral range of interest (one of the absorption lines of the molecules, Figure 1), as shown in (b). The presence of the gas (c) boosts a strong drop of reflectivity, as shown in (d). The reflection spectra were calculated with the CMM method (solid lines) and with the FDTD method (dashed line). The results for two different configurations are represented: rectangular holes with a_x = 50.0 μm, a_y = 120.9 μm (black solid line) and a_x = 50.0 μm, a_y = 120.0 μm (blue solid line and dashed line), being ε_III = ε_HCN and ε_III = 1 respectively. Both HAs are arranged in a rectangular lattice (p_x = 80.0 μm, p_y = 150.0 μm) and drilled in a 1.0 mm thick metal film, everything on top of a transparent PTFE (Teflon) substrate, ε_PTFE = 2.06. The THz source is aligned so the system is illuminated at normal incidence from the substrate side (from bottom to top). The electric field points along the x-direction.