Ultra-broadband absorber based on metamaterial resonators utilizing particle swarm optimization algorithm

In this paper, we propose an ultra-broadband metamaterial absorber optimized by the particle swarm optimization (PSO) algorithm. A unit cell of the absorber is made of TiN/TiO2/TiN square disks, offering a metalinsulator-metal (MIM) configuration, surrounded by a TiN square ring resonator which all are located on a thin stack of TiO2/TiN films. The optimized structure shows high average absorption of 91.63% over the wavelength range of 200-4500 nm. The over 90% absorption bandwidth is 1590 nm, extended from 200 nm to 1790 nm. Furthermore, the absorber absorbs more than 80% of the incident light with wavelengths from 200 nm to 4480 nm, which covers the ultraviolet, visible, and near-infrared regions. The absorber indicates high absorptivity of over 75% under an oblique incidence up to 60 degrees for both TM and TE polarizations. The effect of the presence of the square ring resonators as well as each layer of the MIM on the absorption of the absorber is also studied. It is shown that the use of square ring resonators combined with square disks significantly enhances the absorption of the absorber at wavelengths longer than 1100 nm. The structure has high thermal and chemical stability due to the use of TiN and TiO2. Owing to the outstanding features of the proposed absorber, it can be used in different fields such as imaging, thermal emitting, and solar applications.