(a) Reflection contrast, ΔR, as a function of the amount of HCN within the holes (in ppm) at the spectral locations of ΔR, maxima. Inset: ΔR, as function of the frequency for different HCN concentrations. Reflectance as function of (b) the film thickness and (c)–(d) the long side of the rectangles. In (b) and (d) the HCN concentration is 2 × 10⁵ ppm, while in (c) the system is free of gas (bare structure). The geometry of (a) and (b) is similar that of Figure 2, where a_y = 120.9 μm. In (c) and (d) h = 1 mm. In all cases ε_III = ε_HCN.