  In the past 30 years, subwavelength gratings have been developed and produced
as highly secured Diffractive Optical Variable Image Devices (DOVIDs). They
allowed new distinct optical effects and dramatically lowered DOVIDs
counterfeiting. In particular, subwalength gratings coated with a high
refractive index dielectric are well-known and mass-produced to secure
documents, such as the Diffractive Identification Devices (DIDs). These
submicronic gratings are called Zero Order Devices or Filters (ZOD, ZOF) or
diffractive microstructures designed for Zero-order read-out. Similar
structures are called Resonant Waveguide Gratings (RWG) or Resonant Leaky Mode
Waveguides, when optimized for different purposes. A study using time-resolved
optical simulations can demonstrate and quantify how light is coupled and
propagation in DIDs structure when observed across the gratings (in collinear
incidence). The leaky resonant modes of the RWG are playing a significant role
in the appearance of DIDs in collinear incidence, which has practical impacts
on the engineering of such security elements. From the learning of such
non-local light interactions in DIDs, new optically variable effects can be
designed and produced by using light transport in RWG. As an example,
multi-frequency RWGs can be designed to couple light for different colors and
polarizations and to outcouple them away from the specular configuration
typical to the Zero Order Devices. As a further step, we present new structures
able to couple light in the substrates or in selected layers of security
documents or labels, embling waveguiding light in these layers. These optical
couplers enable selective light transport at a macroscopic scale using total
internal reflection in the volume of the substrates, while being compatible
with current roll-to-roll methods.

Optics