Perfect mid-infrared dual-band optical absorption realized by a simple lithography-free polar dielectric/metal double-layer nanostructure.

A perfect mid-infrared dual-band absorber based on a very simple lithography-free polar dielectric/metal double-layer nanostructure is demonstrated experimentally. Silicon dioxide (SiO) is chosen as the top polar dielectric, which is deposited through room-temperature plasma enhanced chemical vapor deposition to protect the bottom metal layer. A nearly 100% absorption is obtained at the wavelength of ∼ 10 µm due to the constructive interference resonance, which is related to the SiO thickness but insensitive to the light polarization or incident angle. Another enhanced absorption is observed experimentally at ∼ 8 µm under oblique incidence. Both numerical simulation and analytical calculation show that such absorption enhancement is induced by the excitation of the Berreman mode, where the refracted light propagates almost horizontally within the SiO layer. Different from the interference-induced absorption, the Berreman mode induced absorption exists even for a very thin absorber and is sensitive to the light polarization and incident angle.