  Fabrication, characterization, and analysis of an ultra-broadband
lithography-free absorber is presented. An over 94% average absorption is
experimentally achieved in the wavelength range of 450-1400 nm. This
ultra-broadband absorption is obtained by a simple annealed tri-layer
metal-insulator-metal (MIM) configuration. The metal used in the structure is
Manganese (Mn), which also makes the structure cost-effective. It is shown that
the structure retains its high absorption for TM polarization, up to 70
degrees, and, for TE polarization, up to 50 degrees. Moreover, the physical
mechanism behind this broadband absorption is explained. Being both
lithography-free and cost-effective, the structure is a perfect candidate for
large-area and mass production purposes.
