Chalcogenide-based, all-dielectric, ultrathin metamaterials with perfect, incidence-angle sensitive, mid-infrared absorption: inverse design, analysis, and applications.

The demand for miniature, low-cost, utmost efficient optical absorbers triggered ongoing research efforts to minimize the overall design thickness, particularly the photo-active layer, while still maintaining a high optical absorptance. In this study, we present all-dielectric nanophotonic metamaterials of optimized, fabrication compatible and tolerant, architecture for perfect mid-wave infrared absorptance. Overall sub-vacuum-wavelength thick designs are intended to couple and confine light inside an ultrathin 100 nm PbTe photo-absorbing film. Three application-oriented structures, with dimensions inversely designed to provide diverse requirements, are introduced: a two-dimensional metasurface embedded design for unpolarised wide-band absorption and two, one-dimensional metasurface embedded designs for s-polarised wide-band and non-polarised narrow-band absorption. A comprehensive study of the structures' spectral absorptance under normal- and oblique-incidence irradiation is performed. The conical-mounting absorptance analysis elucidates that the high absorption can be continuously spectrally tuned with the azimuthal component of the incidence angle. To the best of our knowledge, this property is discussed for the first time for all-dielectric metamaterials. Also, the ranges of geometrical tuning of the peak absorptance are investigated in detail, and usage of another prospective semiconductor absorber is explored. To unfold the mutual, and essentially different, physical mechanisms that fuel the perfect absorptance, an elaborated analysis is presented. The electromagnetic power transport, portrayed by the Poynting vector, displays three-dimensional singular flows around points, such as vorticity centers, saddles, sinks, and spirals. The potential mid-infrared applications which can benefit from the peculiar properties of the designed structures, such as spectroscopy, sensing, thermal radiation manipulations, and communication, are also discussed.