Ultra-compact metamaterial absorber for multiband light absorption at mid-infrared frequencies

We investigated a multiband metamaterial absorber composed of one-side open cavities. A metallic sidewall effectively prevents destructive coupling between the neighboring cavities. Thus, the number of subunits in one period, which determines the number of multibands or the width of broadband, can be greatly increased. A redshift effect of the resonances is observed theoretically and numerically when the distance between the subunits is reduced. Although the separation is very small and the coupling between the neighboring cavities is strong, the angular insensitivity of the absorption is well-maintained over a wide range. Our findings may enable applications of ultra-compact multiband sensors and thermal emitters.