Drude–Smith ultrathin films for lightweight unpolarized broadband electromagnetic absorbers

Increasing the efficiency of thin film absorbers is critical for various applications, including photovoltaics and photodetection. Although high-performance thin film absorbers have been realized, most of them are based on interference effects, where the bandwidth is limited and strongly polarization dependent. Here, we theoretically reveal novel mechanisms for the lightweight unpolarized broadband electromagnetic absorber based on Drude–Smith ultrathin films (∼10 nm), which is particularly promising for the terahertz (THz) band. The impedance-matched condition for the theoretical maximum of free-space absorption is modified in a non-trivial way depending on the carrier localization parameter cDS. Notably, the absorption and its bandwidth deteriorate when the imaginary conductance cannot be neglected. Further results demonstrate that robust absorption is achieved under different incident angles and polarizations, which is strongly related to the complex reflection coefficients. Finally, calculations based on our previous experimental data in the THz range demonstrate the effectiveness of the above mechanisms. The results may be valuable for the development of lightweight unpolarized broadband electromagnetic absorbers.