An annulus-shaped nanostructure having a three-layer structure consisting of an annulus aluminum pattern, Al2O3 spacer layer, and Al substrate is presented. High absorption peaks can be easily obtained between 2.5 μm and 12.5 μm in the case of a uniform plane wave at normal incidence. A unilateral annulus structure is designed to obtain an absorption peak of almost 100% by optimal geometry parameters. Moreover, a double annulus consisting of two unilateral annuli is proposed to compare their performances. The results indicate that the double annulus inherit the single one's absorption characteristics, and that the outer loop coupling with the inner one enhances the absorption peak of the inner loop. This structure provides a great potential for application in designing selective thermal emitters, biosensing, etc.