Large-Area, Lithography-Free Super Absorbers and Color Filters at Visible Frequencies Using Ultrathin Metallic Films

Nanostructured photonic materials enable control and manipulation of light at subwavelength scales and exhibit unique optical functionalities. In particular, plasmonic materials and metamaterials have been widely utilized to achieve spectral transmission, reflection, and absorption filters based on localized or delocalized resonances arising from the interaction of photons with nanostructured materials. Realization of visible-frequency, high-performance, large-area, optical filters based on nanoplasmonic materials is rather challenging due to nanofabrication related problems (cost, fabrication imperfection, surface roughness) and optical losses of metals. Here, we propose and demonstrate large-area perfect absorbers and transmission filters that overcome difficulties associated with the nanofabrication using a lithography-free approach. We also utilize and benefit from the optical losses in metals in our optical filter designs. Our resonant optical filter design is based on a modified, asymmetric metal−insulator−metal (MIM) based Fabry−Perot cavity with plasmonic, lossy ultrathin (∼30 nm) metallic films used as the top metallic layer. We demonstrated a narrow bandwidth (∼17 nm) super absorber with 97% maximum absorption with a performance comparable to nanostructure/nanoparticle-based super absorbers. We also investigated transmission (color) filters using ultrathin metallic films, in which different colors can be obtained by controlling the dielectric spacer thickness. With performance parameters of transmittance peak intensity reaching 60% and a narrow-band of ∼40 nm, our color filters exceed the performance of widely studied plasmonic nanohole array based color filters. Proposed asymmetric Fabry−Perot cavities using ultrathin metallic films could find applications in spectrally selective optical (color and absorber) filters, optoelectronic devices with controlled bandwidth such as narrow-band photodetectors, and light-emitting devices.