Ultra-broadband long-wavelength infrared metamaterial absorber based on a double-layer metasurface structure

In this paper, we report a metamaterial absorber design that achieves a broad absorption band encompassing the whole long-wavelength infrared (LWIR) region. The structure consists of two parallel metasurfaces buried in an amorphous silicon dielectric layer, where the minimum size for all possible planar details does not go below 1 μm, making the use of standard optical lithography possible for fabrication. The dielectric layer of the structure is placed over a metallic ground plane that inhibits the transmission of incident waves. A substrate underneath the ground plane may also be needed for the purpose of mechanical support. This structure achieves a minimum absorptivity of about 90% in almost the full LWIR band in the case of normal incidence in a polarization-insensitive manner due to the four-fold symmetry of the structural geometry. The absorber also shows reduced sensitivity to off-normal incidence angles, satisfying a minimum absorption level of approximately 80% up to the incidence angle of 45 deg. This broadband metamaterial absorber design is anticipated to find applications in thermal emitters/coolers and in thermal infrared sensors.