Fully Planarized Perfect Metamaterial Absorbers With No Photonic Nanostructures

In the past, perfect metamaterial absorbers (PMAs) have required nanolithography patterning to boost broadband absoprtion. Tapered structures, in particular, are shown to achieve close-to-unity absorption over broadband using adiabatic light coupling. A nontapered PMA is desirable due to the fact that it is easier to fabricate using regular lithography techniques. This facilitates the scalability to large-area photonic applications such as thermophotovoltaics. In this work, we propose a fully planarized design with ultrathin metallic films for broadband PMAs. The design provides close-to-unity absorbance over a wide spectral range and is wavelength scalable from middle ultraviolet to long wavelength infrared. The planarized design is extremely easy to fabricate, and it requires no lithography nor etching. The design can be used with different moderate-extinction metals such as tungsten, titanium, tantalum, and nickel. The physics is that the thin layer of the moderate-extinction metal allows photons to penetrate through itself. The insertion of the dielectric between thin metal layers is necessary to spatially separate the ultrathin metallic thin film to boost the effect of thin-film absorption. As far as the bandwidth normalized to center wavelength is concerned, we believe that the experimental result demonstrated here shows the broadest bandwidth to date.