Abstract
Potential solar energy applications of metamaterial absorbers require spectrally tunable resonance to ensure the overlap with intrinsic absorption profiles of active materials. Although those resonance peaks of metamaterial absorbers can be tuned precisely by lithography-fabricated nanopatterns with different lateral dimensions, they are too expensive for practical large-area applications. In this work, we will report another freedom to tune the spectral position of the super absorbing resonance, i.e. the spacer thickness. The structure was fabricated by evaporating an optically opaque metallic ground plate, a dielectric spacer layer, and a top metallic thin film followed by thermal annealing processes to form discrete nanoparticles. As the spacer thickness increases from 10-90 nm, two distinct shifts of the absorption peak can be observed [i.e. a blue-shift for thinner (10-30 nm) and a red-shift for thicker spacer layers (30-90 nm)]. To understand the physical mechanism, we characterized effective optical constants of top nanopattern layer and loaded them into numerical simulation models. A good agreement with experimental data was only observed in the thick spacer region (i.e. 30-90 nm). The optical behavior for thinner spacers cannot be explained by effective medium theory and interference mechanism. Therefore, a microscopic study has to be performed to reveal strongly coupled modes under metallic nanopatterns, which can be interpreted as separate antennas strongly coupled with the ground plate. Since the resonant position is sensitive to the spacer thickness, a tunable super absorbing metasurface is realizable by introducing spatial tunable materials like stretchable chemical/ biomolecules.