Broadband polarizer or band-pass filter based on a dielectric grating with top and bottom metallic coverings

Abstract

In this paper we evidenced a broadband transmission from middle infrared to radio spectrum and a band-pass transmission in far infrared based on a sandwich grating structure, which consists a dielectric grating and two metallic covering layers on the top and bottom side of it. It indicates that broadband polarizer or band-pass filter can be inspired from the structure. As a polarizer, the extinction ratio is about 50dB in middle infrared which increases as the wavelength increases; over 90% transmittance for TM polarized light can be maintained from middle infrared to radio spectrum, which doesn’t degrade for incident angle from 0° to 60° . As a band-pass filter working in far infrared region, the resonant transmission peak can be tuned by either varying the thicknesses of the dielectric grating layer or of the metallic coverings. When the resonant transmission peak is tuned from shorter to longer wavelength by increasing the thickness of dielectric grating, the peak transmittance is increased from 61% to 93%, with full width at half maximum (FWHM) bandwidth increases from 13% to 20%. Besides, in order to achieve high peak transmission, the thickness of metallic coverings should be optimized. In consideration of physical mechanism, the step-boosted characteristic of broadband transmission is contributed by the first-order Fabry-Perot (FP)-like cavity resonance supported in the dielectric grating layer along the horizontal direction, while the band-pass transmission is a hybridization resonant mode of the first order FP-like cavity resonance along the horizontal direction and the first-order FP-like cavity resonance along the vertical direction supported in the dielectric grating layer.