Subwavelength gratings for OVDs - From local interactions to using light-transport

TL;DR

This paper explores the optical interactions in subwavelength gratings used in security devices, demonstrating how light coupling and transport can be engineered for advanced optical effects and security features.

Contribution

It introduces new insights into light coupling in resonant waveguide gratings and proposes novel structures for enhanced security and multi-color light manipulation.

Findings

Leaky resonant modes significantly influence DIDs appearance.

Multi-frequency RWGs enable coupling of different colors and polarizations.

New optical couplers facilitate selective light transport in security substrates.

Abstract

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.