An ultra-high resolution mid-infrared dual-band notch filter based on an asymmetric grating structure

An asymmetric grating structure was investigated for realizing mid-infrared (mid-IR) ultra-high resolution dual-band notch filters. The structure is constructed by an asymmetric gold grating on the stack of an Al2O3 layer and a CaF2 substrate. The grating consists of three gold strips in each unit cell, and the center-to-center distances between two adjacent gold strips are different. The filter provides two high-extinction transmission dips over a broad spectral region. One is at a wavelength of ∼3.8381μm with a full-width at half-maximum (FWHM) of ∼0.7 nm, and the other is at a wavelength of ∼3.8435μm with a FWHM of ∼1.7 nm. The quality-factor (Q-factor) of the filter is >2200. It was found that the two transmission dips result from the excited (+1) and (-1) resonance modes, respectively. Thus, there is an interesting feature that either of the two transmission dips does not split at oblique incidence.