Far-infrared bands in plasmonic metal-insulator-metal absorbers optimized for long-wave infrared

Abstract

Metal-insulator-metal (MIM) resonant absorbers comprise a conducting ground plane, a dielectric of thickness t, and thin separated metal top-surface structures of dimension l. The fundamental resonance wavelength is predicted by an analytic standing-wave model based on t, l, and the dielectric refractive index spectrum. For the dielectrics SiO₂, AlN, and TiO₂, values for l of a few microns give fundamental resonances in the 8–12 µm long-wave infrared (LWIR) wavelength region. Agreement with theory is better for t/l exceeding 0.1. Harmonics at shorter wavelengths were already known, but we show that there are additional resonances in the far-infrared 20–50 µm wavelength range in MIM structures designed to have LWIR fundamental resonances. These new resonances are consistent with the model if far-IR dispersion features in the index spectrum are considered. LWIR fundamental absorptions are experimentally shown to be optimized for a ratio t/l of 0.1 to 0.3 for SiO₂- and AlN-based MIM absorbers, respectively, with TiO₂-based MIM optimized at an intermediate ratio.