Robust Broadband Optical Transmission Realized in a Dual-Metallic-Grating Structure

We present a new mechanism to realize broadband transmission (BT) at near infrared in a dual-layer metallic-silver-grating structure with a silicon nitride dielectric spacer. The physical mechanism is ascribed to the partial degeneracy between the first-order magnetic polariton (MP₁) mode supported in the thin dielectric spacer and the Fabry-Pérot-like resonant modes supported in the gratings´ slits. With parameters optimized, high transmittance up to 85% with full-width-at-half-maximum bandwidth of 64% near 1.9 μm can be realized for TM-polarized light. Moreover, the BT can be maintained in a large range of oblique incident angles from 0° to 30°. In another word, the BT is capable of not only collimated beams but also noncollimated incident lights. Moreover, the high-transmission window can be tuned in the IR spectrum region by modulating the metallic gratings´ thicknesses.