Sub-wavenumber linewidth mid-infrared notch filter enabled by a dual-period plasmonic structure

A transmission notch filter based on surface-plasmon resonance is proposed and numerically studied in the mid-infrared region. The metal structure is a dual-period grating constructed by using smaller-period gold arrays as the duty portions of a larger-period grating. The plasmonic filter shows high transmission of >85% over broad mid-infrared range, and a single notch at ∼4.073μm with 24 dB extinction ratio. The stop bandwidth of the filter is ∼0.75 nm (∼0.45 cm−1), corresponding to the quality factor as high as 5400, which is improved by at least one order of magnitude. The rejection wavelength and bandwidth of the filter can be designed by adjusting the structure parameters. Additionally, the angle and polarization of the incidence and refractive index of the surrounding medium affect the performance of the filter.