Sub-wavelength periodic structures have demonstrated wavelength- or polarization-selective transmittance, which is critical for many optical devices and thermal applications. For example, linear polarizers composed of metallic wire grids on a transparent substrate have been used for a long time [1]. Near-field microscopy and photon extraction also much benefited from two-dimensional hole arrays [2]. Specifically, nanolithography and energy conversion devices recently made significant progresses with the help of one-dimensional periodic metallic slit arrays [3]. In fact, the unique transmittance spectrum through slit arrays was attributed to several physical mechanisms, including Wood’s anomaly, cavity resonance, surface plasmon polaritons, and effective medium behavior [4,5]. Either only one or the interplay of multiple mechanisms can tailor the spectrum depending strongly on slit features and the wavelength. Though correlations between tailoring mechanisms and unique radiative properties attracted abundant research interests, exactly rectangular cross-section is usually assumed for gratings in most numerical investigations with feature variation in slit width, slit depth, or period [4–7]. Other types of feature variation include the combination of two rectangular gratings made of different materials [8] or the rectangular gratings themselves filled with both metal and dielectrics [9].

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