Spectral separation of surface-plasmon-polariton modes to achieve ultranarrow dual-band absorbers

We designed and studied a narrowband absorber with two absorption peaks for mid-infrared spectroscopy and multispectral detection applications. The absorber is composed of a silicon grating loaded on a continuous gold film. The grating has two silicon strips in each unit cell, with the same height but different widths. Numerical results indicate that the absorption peaks under normal incidence locate at wavelengths of ∼3.863 and ∼4.004  μm, with bandwidths of ∼28 and ∼33  nm, respectively. Both peaks exhibit high absorptivities of >0.996. We found that the excited surface-plasmon-polariton modes traveling in different directions in the structure are spectrally separated and result in the two absorption peaks.