Ultra-broadband, polarization-independent, and wide-angle metamaterial absorber based on fabrication-friendly Ti and TiO₂ resonators

This work presents an ultra-broadband metamaterial absorber in the wavelength range of 250–4000 nm. The absorber consists of a Ti disk resonator and a stack of TiO₂/Ti square-shaped resonators, supported by TiO₂/Ti thin layers. This arrangement of the layers offers a metal–insulator-metal configuration, enhancing the absorptivity of the structure. The effects of geometrical parameters, including the thickness of the resonators, the radius of the disk resonators, and the width of the square resonator, on the absorption spectrum of the absorber are investigated. To attain the highest average absorption, the particle swarm optimization (PSO) algorithm is employed. The simulation results obtained by the finite-difference time-domain method indicate that the average absorption can reach a high value of 96.25% over the studied wavelength range. The over 90% absorption bandwidth is 3509 nm. Additionally, the solar absorption of the absorber is 94.89%. The absorption is more than 80% even for incident angles up to 50° for both TM and TE polarizations. The proposed absorber is a very promising option for solar energy harvesting, photo-thermal technology, photo-detection, and thermal-photovoltaics applications due to its high over 90% bandwidth, independence on the polarization and angle of incident light, and ease of fabrication.