Wideband mechanism of superwavelength gratings with a low blaze angle

We explain the wideband mechanism of superwavelength gratings (SWGs) with low blaze angles based on electromagnetic field theory. The study shows that the grating scale corresponding to the blaze wavelength of a triangular-grooved metallic grating shifts to the superwavelength direction as the blaze angle decreases. At the blaze wavelength, the SWG with low blaze angle suppresses the polarization effect and achieves near-perfect blazing of TE- and TM-polarised light. On the other hand, it eliminates the higher-order Rayleigh anomalies through the compensation effect, thereby achieving a wideband output with polarization insensitivity. Based on the obtained mechanism, an Al SWG with a blaze angle of 3° is designed, which has an unpolarized efficiency (-1^st order) of more than 40% (70%) in the 900-2500 nm (1030-1900 nm) band, with a maximum efficiency of 94.59%. In addition, the comparison of angular selectivity and wavelength selectivity demonstrates that the Al SWG is superior to the Al near-wavelength grating in terms of bandwidth, diffraction efficiency, and polarization effect suppression. These results can be used to guide the design and fabrication of wideband metallic gratings in different wavelength bands.