Wideband Absorbers in the Visible with Ultrathin Plasmonic-Phase Change Material Nanogratings

The narrowband surface plasmon resonance of metallic nanostructures was once thought to limit the bandwidth of absorptance, yet recent demonstrations show that it can be harnessed using mechanisms such as multiple resonances, impedance matching, and slow-light modes to create broadband absorptance. However, in the visible spectrum, realization of absorbers based on patterned plasmonic nanostructures is challenging due to strict fabrication tolerances. Here we experimentally compare two different candidates for visible light broadband high absorptance. The first candidate is planar thin film dual layers of Ge₂Sb₂Te₅ and aluminum (Al), while the second structure employs ultrathin Al grating/Ge₂Sb₂Te₅ dual layers. In both cases, the absorbers yield a measured absorptance greater than 78% in the visible. A remarkably high-absorptance bandwidth of 120 nm was measured and associated with the large imaginary part of Ge₂Sb₂Te₅ dielectric function. We find that the simple dual-layer planar structure is an effective absorber in the near-infrared, but its absorptance is less effective in the visible. However, for visible wavelengths the grating structure can blue-shift the absorptance peak to 422 nm. The simple geometries of the plasmonic absorbers facilitate fabrication over large areas. It has practical applications in light harvesting, sensing, and high-resolution color printing.