Perfect absorption of electromagnetic (EM) energy plays a vital role in solar cells, photonic/thermal imaging, sensors, radiative cooling and thermal emitters. However, absorbers made by natural materials suffer from the impedance-mismatch problem due to lacking magnetic responses, and thus cannot completely suppress light reflections, which degrade their light-absorbing abilities.

Recently, metamaterial-based electromagnetic absorbers attracts much attention for their high absorptivity, ultrathin thickness, scalable working wavelength, and/or flat configuration. However, narrow bandwidth, fixed working band, and time-consuming and high-cost fabrications hinder their greater practical applications; and ultrathin metamaterial absorbers with working frequencies freely tuned inside a broad range covering both visible and near-infrared domains are rarely seen.

The research team led by Professor ZHANG Long from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, together with Fudan University, has demonstrated a large-scale, broadband, polarization-insensitive and tunable optical metamaterial absorber with a low-cost, time-saving and lithography-free fabrication method. Their work was published in Nanoscale.

In their study, aluminum oxide (AAO) nanomasks were utilized for depositing aluminum (Al) nanoparticles (NPs) on an ultrathin Ge₂Sb₂Te₅ (GST) layer grown on a continuous metallic mirror, thus constituting an ultrathin MIM structure with a centimeter-scale total size.

The absorption spectra excited by the Fabry-Perot (FP) resonance of the GST film and the localized surface plasmon resonances (LSPR) of the Al NPs were designed to overlap with each other, and thus achieved a high absorptivity of light (>80%) within a broad frequency band covering both visible and near-infrared regimes. The two resonant modes were independent, which could be tailored by changing the size of Al NPs and the thickness of GST film, respectively.

Moreover, the bandwidths of such metamaterial absorbers could be dramatically tuned by varying the annealing temperature of the GST layer. The working band of the proposed design can also be continuously modulated by tuning the GST film at an arbitrary intermediate phase with different constitution ratio between the amorphous and crystalline molecule.

This work provides a resolution for dealing with the narrow bandwidth, nonadjustable and low throughout of traditional metamaterial absorbers via a simple configuration and may stimulate many potential applications in, for example, solar cells, energy harvesting, smart sensing/imaging, and color printing.

These works were supported by the National Natural Science Foundation of China.

Characterizations of the proposed metamaterial absorber (Image by SIOM)