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The wavelength-selective infrared absorptance of a single-layered aluminum subwavelength structure (SWS) is optimized using a hybrid numerical scheme comprising the rigorous coupled-wave analysis method and a genetic algorithm. The results show that the optimized SWS yields a strong absorptance peak (0.99) and a full-width-at-half-maximum (FWHM) of 1.42 μm. In addition, it is shown that the absorptance spectrum of the SWS is insensitive to the angle of incidence of the incoming light and the grating period, but shifts toward a longer (shorter) wavelength as the grating thickness or grating ridge width is increased (decreased). The enhanced absorptance is examined by computing the governing equations of the excitations of Rayleigh-Wood anomaly, surface plasmon polaritons, cavity resonance, and magnetic polaritons. The magnetic field patterns and Poynting vector distribution within the grating structure are also analyzed to support the physical mechanism using the finite-difference time-domain (FDTD) method. The results indicate that the absorptance peak of the SWS is the result of cavity resonance. Also, for a double-layered SWS comprising an aluminum grating and a dielectric layer, a widening of the absorptance spectrum occurs. Overall, the results presented in this study show that SWS gratings which can be easily manufactured using microfabrication technology provide a simple and versatile solution for such applications in tailoring the spectral absorptance used for infrared detection, energy harvesting, and so on.