Optical document security represents an important field of application of analogue and synthetic diffractive structures. Most of the security elements are based on visual effects formed by diffraction on a structure with the details in the order of hundreds of nanometers. However, to improve the anti-counterfeiting properties of these structures, various types of hidden features are included within the area of the security elements. They are not visible under normal lighting but it is possible to easily reveal the hidden information under specially-defined geometry and/or type of illumination.
In this paper, theory and application of a novel type of hidden diffractive security element are presented. It combines standard visual properties of synthetic holograms with waveguide effects. The hidden information is recorded using a special grating, which is not visible under normal observation geometry. The encoded image can be reconstructed only when the proper guided mode appears in a substrate. During the reconstruction, light is coupled into a waveguide (holographic foil) using a grating coupler and after traveling through the substrate in a chosen direction it is selectively out-coupled within the areas containing the hidden information. Several elements with different properties have been designed, fabricated and compared with theory. Principles of diffraction and waveguide effects, realization technology and properties of the realized test samples are presented.
The advantage of the combination of diffractive and waveguide effects is that the resulting hidden effect is sophisticated but easily readable with no additional tools.