Plasmonic metamaterials go dark with interference-based absorption

Plasmons, localized electromagnetic waves that propagate along metallic surfaces, can be nimble and versatile tools for developing optical device features at the smallest scales. A new nanoscale patterned metamaterial described in the Journal of Applied Physics features a novel use of dark modes formed by plasmon interference to create perfectly absorbing bands of different wavelengths.

The new absorber design consists of a periodic sub-wavelength array of silver nanogrooves etched in alignment to neighboring silicon ring resonators. Researchers developed the theoretical model based on plasmonic interference, as opposed to the layered “frequency stacking” approaches to creating absorption bands that typical metamaterial designs adopt.

Among some of the other advantages of the near-field surface excitations is their ability to couple to far-fields, namely, incident light. The most common plasmon resonances are dipolar in nature and radiate (back) into the far-field. But clever nanoarchitectures of plasmonic metals have also revealed plasmonic dark modes, hybridized modes that are purely near-field.

“The realization of multispectral absorbers only depends on the introducing of dark modes with identical resonant frequencies,” said co-author Hong-Ju Li. Here, the authors introduced a dark mode nanoring into a narrow-band perfect absorber consisting of a periodic nanogroove array, thereby leading to dual-band, near-unity absorption.

The dark modes cannot interact simultaneously with both the incident light and the splitting created in the absorption peaks that result from the coupling between bright and dark modes. “The dark-mode-induced absorbers hence are angularly independent,” said Li. “The strong destructive interference between bright and dark modes also gives rise to slow light effects, which is unavailable in traditional metamaterial absorbers based on the frequency-stacking method.”

Source: “Multispectral perfect absorbers using plasmonically induced interference,” by Hong-Ju Li, Yong-Ze Ren, Meng Qin, and Ling-Ling Wang, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5017022 .