Symmetry-reduced double layer metallic grating structure for dual-wavelength spectral filtering.

In this paper, we propose a metal-dielectric dual-wavelength spectral filtering structure based on symmetry-reduced double layer metallic gratings (SRDMG) coupled to a guided-mode dielectric resonator. The grating symmetry is reduced by alternatively shifting metal nanowires of the top layer metallic grating. Compared to a symmetric double layer metallic grating that usually provides one resonance dip, this SRDMG structure generates two remarkable narrow band transmission dips with a transmission peak in-between at normal incidence. The appearance of the two narrowband resonance dips is attributed to the excitation of different current modes in the metallic grating, leading to different guided mode resonances in the dielectric layer, which is induced by the structural symmetry breaking. Moreover, these two guided modes do not split under oblique incidence and a flat dispersion band over a small angular range can be obtained. The positions of two dips and the frequency gap between them can be controlled by adjusting the thickness of metallic grating without the need to modify the structure period and width, which is an easy method to tune resonance position and bandwidth, and make the fabrication of some filters more convenient. This work can be used to develop subwavelength metallic-grating-based multi-wavelength and narrow-band spectral filters.