Lithography-free wide-angle polarization-independent ultra-broadband absorber based on anti-reflection effect.

We propose a lithography-free wide-angle polarization-insensitive ultra-broadband absorber by using three pairs of tungsten (W) and calcium fluoride (CaF) films. The simulation results show that the absorptivity is larger than 0.9 with normal incidence in the wavelength range from 400 nm to 1529 nm. By adding a pair of CaF-W films, we can get a broader absorption bandwidth with absorptivity larger than 0.9 over the wavelength of 400-1639 nm. In addition, the absorption performance is insensitive to the polarization and angle of incidence. The electric field distributions at the absorption peaks show that the absorption is originated from the destructive interference between the reflection waves from the top and bottom interfaces of the multilayer CaF-W films. Furthermore, the ultra-broad bandwidth is attributed to the anti-reflection effect from the increased effective refractive index from top to down of the proposed absorber. Such physical mechanism of broadening bandwidth based on anti-reflection effect provides a new idea for the design of broadband absorber. Meanwhile, this broadband absorber is a good candidate for potential applications such as detection and energy harvesting.