Millimeter Wave Absorber for Secure Identification
  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. 
Scott A. Skirlo, Jonathan T. Richard, Magued Nasr, Martin S. Heimbeck,   John D. Joannopoulos, Marin Soljacic, Henry O. Ever...
physics.optics