  We demonstrate thin, flexible, metamaterial films with a strong, narrowband,
polarization- and angle-insensitive absorption designed for wavelengths near
one millimeter. These structures, fabricated by photolithography on a
commercially available, copper-backed polyimide substrate, are nearly
indistinguishable to the unaided human eye but can be easily observed by
imaging at the resonance frequency of the film. We demonstrate that these
patterns can be used to mark or barcode objects for secure identification with
a terahertz imaging system.
