Optical Janus Effect in Large Area Multilayer Plasmonic Films

Abstract

Plasmonic and other nanoparticles have attracted considerable interest for their role in structural coloration. The optical “Janus” effect, where the color of light reflected from a partially transmitting film depends on whether the device is viewed from the substrate or the coating side, is observed using a variety of nanostructured films. Herein, the optical Janus effect produced by homogeneous thin‐film structures comprising only four layers of three different materials with a total thickness less than 300 nm is demonstrated. An asymmetric Fabry–Perot (FP) nanocavity is formed with a dielectric film bounded by two different metal films of nanoscale thickness. The semitransparent device has a transmitted color that is independent of the viewing direction. A broad color palette is available through the selection of various thicknesses and film materials. In addition to the directional optical effect, the device possesses iridescence properties and can generate images by selective removal of regions of one of the metallic films using simple photolithography. From a manufacturing perspective, this device is scalable and holds significant promise for applications in architecture, producing decorative features, and the development of overt and covert security features.

Recent progress on structural coloration

“…In general, structural colors can be realized by nanophotonics systems ranging from multilayered thin films with engineered thicknesses to dielectric and/or metallic nanoparticles and the array they form. One-dimensional (1D) photonic crystals, formed by stacking multilayers of thin-film materials, realize the structural colors by prompting constructive and destructive interferences for photons with different wavelengths via the thin-film interference effect [22–27] , where the limited degrees of freedom of the multilayer film system constrains the variety of colors that can be realized [28] . Additionally, changes in the angle of incidence of light can cause iridescence in multilayer pairs, which are often considered as undesirable display artifacts for common applications.…”