Versatile Control of Spectral Emissivity in Mid-Infrared Metasurfaces via Silicon Microstructuring

Three classes of mid-infrared metasurfaces operating in the 12–24 µm spectral range are produced solely through the structuration of a single-crystal silicon surface. By controlling surface topography, we achieve tailorable spectral responses, including resonant, broadband, and anti-resonant absorptance. Two metasurfaces use smooth gratings, while the third employs a hierarchical topography involving further nanostructuration, leading to a rough grating. Simulations and experiments reveal distinct spectral behaviors: (i) narrow-band absorptance with mid-quality factor and angle-tunable resonant wavelength, also enabling precise control over the photon lifetime, (ii) broadband absorptance, and (iii) broadband absorptance with narrow-band anti-resonant absorption dip. Compared to flat silicon surfaces, these metasurfaces enable significantly enhanced light-matter interactions, achieving a two-fold increase in the absorptance with only 10% increase in effective surface area. The observed effects are attributed to different expressions of Wood anomalies, driven by the high carrier concentration in heavily doped silicon, which critically shape the metasurfaces’ optical behavior.