Solar energy, combined with light-trapping technology, has enormous potential to replace fossil fuels as a clean, secure, and sustainable energy source1. Broadband optical absorbers for energy harvesting applications have attracted great research interest in a variety of applications, including photothermal, solar thermophotovoltaics (STPV), photo-chemical, solar steam production, and catalytic reactions. By employing plasmonic structures, electromagnetic energy can be localized into a very small volume, and the effective conversion between photons and plasmons may be monitored on a subwavelength scale. Due to their strong optical performance from nanoparticles to nanostructures, plasmonic absorbers have recently been constructed to achieve broadband absorption2, 3. Light may be controlled and focused in the active layers by carefully constructing these metallic-dielectric nanostructures4, 5. Broadband plasmonic absorbers with wide-angle incidence, polarization independence, and full absorption across a broad wavelength range are desirable. A substantial amount of research has been conducted to enhance the performance of absorbers 6.
Plasmonic Infrared (IR) absorbers can be utilized in various fields of study. However, achieving a light absorber that has several resonances over a broad spectrum of wavelengths is a complex task. Using structures with multiple resonators to couple various absorption peaks is one approach; however, these structures have a large influence due to the incorporation of multiple components7. In addition, other structures with high absorption have been proposed based on metallic plasmonics8, Silicon nanowires9, and porous silicon10; however, they are unstable, costly, and thick.
Implementing fractal metamaterials is one of the approaches used to create broadband absorbers. Metamaterials are periodic structures composed of micro/nanostructural elements of sub-wavelength scale. These structures increase the diffraction limit of conventional optical materials, resulting in optical phenomena such as negative refraction; consequently, metamaterials can be regarded as ideal lenses that improve the interaction between matter and light11. In addition, metamaterials are utilized for chiral imaging12, cloaking13, and concentrators14. The fractal metamaterial technology builds metamaterials using fractal structure. Fractal structures are influenced by natural architectures such as corals, tree branches, and snowflakes; their forming cells and the entire structure exhibit a similar geometry. Multiple resonances enhance the wideband absorption of plasmonic fractal antennas compared to conventional antennas11. Fractal plasmonics may involve various configurations, including dendritic15, triangular16, 17, and circular structures18, for achieving absorbance in the Terahertz, visible, and infrared regions.
Due to their dependence on metals such as silver or gold, the majority of fractal plasmonics operate in the near-infrared or visible spectrum. Gold and silver are common plasmonic materials in the visible and near-infrared spectral ranges. To take advantage of plasmonic enhancement in the mid-and long-wave infrared regions, materials with plasma frequencies in this range are essential. Plasma frequencies of common metals exist in the visible and near-infrared regions. On the other hand, the plasma frequency of semiconductors can be modulated by varying the concentration of charge carriers (doping) or by applying potential gating. Consequently, highly doped semiconductors emerge as advantageous alternatives to metals in plasmonics, particularly in the infrared region. Several theoretical and experimental investigations have assessed the efficacy of various semiconductor materials. Due to silicon-incorporated photonic devices such as sub-wavelength interconnects, modulators, and emission sources, silicon has attracted considerable interest among these semiconductors19, 20. Silicon also enables chip-scale integration in the mid-infrared (MID-IR) band, which has numerous potential applications, such as chemical and biological sensors, imagers, light sources, and spectroscopy. In order to increase absorption in the mid-IR region, doped silicon (D-Si) can be used as an alternative for metals21, 22. In addition, D-Si will make the fabrication of absorbers compatible with CMOS and cost-effective.
Without the need to design a complex grating structure, we study the absorption in the mid-IR wavelength range using the fractal absorber. In this research, we propose a D-Si-based fractal Sierpinski carpet structure. Previously, this structure was introduced by utilizing metals for visible-range absorption. However, the incorporation of D-Si into our fractal structure increased broadband absorption and produced a plasmonic effect in the mid-IR range of 3 to 9µm.