In this study, a fractal absorber was designed to enhance light absorptivity and improve the efficiency of converting solar energy into electricity for a range of solar energy technologies. The absorber consisted of multiple layers arranged from bottom to top, and the bottom layer was made of Ti metal, followed by a thin layer of MgF₂ atop it. Above the two layers, a structure comprising square pillars formed by three layers of Ti/MgF₂/Ti was formed. This pillar was encompassed by a square hollow with cylindrical structures made of Ti material on the exterior. The software utilized for this study was COMSOL Multiphysics^® (version 6.0). This study contains an absorption spectrum analysis of the various components of the designed absorber system, confirming the notion that achieving ultra-wideband and perfect absorption resulted from the combination of the various components. A comprehensive analysis was also conducted on the width of the central square pillar, and the analysis results demonstrate the presence of several remarkable optical phenomena within the investigated structure, including propagating surface plasmon resonance, localized surface plasmon resonance, Fabry–Perot cavity resonance, and symmetric coupling plasma modes. The optimal model determined through this software demonstrated that broadband absorption in the range of 276 to 2668 nm, which was in the range of UV-B to near-infrared, exceeded 90.0%. The average absorption rate in the range of 276~2668 nm reached 0.965, with the highest achieving a perfect absorptivity of 99.9%. A comparison between absorption with and without outer cylindrical structures revealed that the resonance effects significantly enhanced absorption efficiency, as evidenced by a comparison of electric field distributions.

This study focused on designing an ultra-wideband metamaterial absorber, consisting of layers of Mn (manganese) and MoO₃ (molybdenum trioxide) arranged in a planar interleaving pattern, with a matrix square-shaped Ti (titanium) on the top MoO₃ layer. Key features of this research included the novel use of Mn and MoO₃ in a planar interleaving configuration for designing an ultra-wideband absorber, which was rarely explored in previous studies. MoO₃ thin film served as the fundamental material, leveraging its favorable optical properties and absorption capabilities in the infrared spectrum. Alternating layers of Mn and MoO₃ were adjusted in thickness and order to optimize absorptivity across desired wavelength ranges. Another feature is that the Mn and MoO₃ materials in the investigated absorber had a planar structure, which simplified the manufacturing of the absorber. Furthermore, the topmost layer of square-shaped Ti was strategically placed to enhance the absorber’s bandwidth and efficiency. When the investigated absorber lacked a Ti layer, its absorptivity and bandwidth significantly decreased. This structural design leveraged the optical properties of Mn, MoO₃, and Ti to significantly expand the absorption range across an ultra-wideband spectrum. When the Ti height was 280 nm, the investigated absorber exhibited a bandwidth with absorptivity greater than 0.9, spanning from the near-infrared (0.80 μm) to the mid-infrared (9.07 μm). The average absorptivity in this range was 0.950 with a maximum absorptivity of 0.989. Additionally, three absorption peaks were observed at 1010, 2510, and 6580 nm. This broad absorption capability makes it suitable for a variety of optical applications, ranging from near-infrared to mid-infrared wavelengths, including thermal imaging and optical sensing.

In this study, a simple pyramid-like ultra-wideband absorber was designed to explore high absorptivity across a wide bandwidth. The absorber consisted of eight layers organized into four groups, and each group comprised a metal layer followed by an oxide layer, both of which were square with equal side lengths. Specifically, the chosen oxides, arranged from bottom to top, included SiO₂ (t7 layer), Al₂O₃ (t5 layer), SiO₂ (t3 layer), and Al₂O₃ (t1 layer). In the initial design phase, the thickness of the t8 Ti layer was set to 50 nm and assigned initial values to the thicknesses of the t7-t1 layers, and the widths of the four groups w4, w3, w2, and w1, decreased successively from bottom to top, creating a structure reminiscent of a pyramid. Comsol (version 6.0) was utilized to simulate and systematically vary one parameter at a time, ranging from the thicknesses of the t7-t1 layers to the widths of w4-w1, in order to identify the most suitable structural parameters. Our analyses demonstrated that multimode resonance arose due to the emergence of absorption peaks at lower wavelengths between larger and smaller areas. Additionally, surface plasmon resonance and interference effects between various layers and materials were attributed to the alternating arrangement of metal and oxide layers. The enhancements in the electric field observed at different resonance peak wavelengths illustrated the Fabry–Perot cavity effect, while the impedance matching effect was observed through variations in the real and imaginary parts of the optical impedance with respect to the wave vector. After simulating using these optimally found thicknesses and widths, the aforementioned effects manifested in the pyramid-like ultra-wideband absorber we designed, with its absorptivity surpassing 0.900 across the spectrum from ultraviolet A (335 nm) to middle infrared (4865 nm).