A polarization-sensitive diffractive optical device for document security

Devices for document security consisting of a thin metallic film the boundaries of which are undulated with a relief of a rainbow hologram, with the lower boundary facing to a lossless medium and the upper one facing (i) to a lossless medium with the same refractive index or (ii) to a thin dielectric film with a higher refractive index, are proposed. When viewed through in collimated and monochromatic light, they exhibit unique angular variation of appearance - a marked succession of dark and bright areas if the incident beam is polarized in the plane of recording of the hologram (the s-polarization). The devices are modelled as single-film and double-film structures that are composed either of a 30 nm thick silver film, which is sandwiched between two photoresist layers, or of a 30 nm thick silver film, which is deposited on a photoresist layer, and a 60 nm thick overlay film of titanium dioxide. Sensitivity to the polarization and the unique angular variation for the s-polarization follow from theoretical calculations, assuming a sinusoidal modulation of the surface relief, and they are demonstrated experimentally by capturing intensity distributions of light transmitted through realized samples in the zero diffraction order.