Holographic colour prints for enhanced optical security by combined phase and amplitude control

Kevin T P Lim,Joel K W Yang,Yejing Liu,Hailong Liu

doi:10.1038/s41467-018-07808-4

Abstract

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. For instance, microscopic colour prints modulate the amplitude, whereas holograms typically modulate the phase of light. However, their relatively simple structure and behaviour is easily imitated. We designed a pixel that overlays a structural colour element onto a phase plate to control both the phase and amplitude of light, and arrayed these pixels into monolithic prints that exhibit complex behaviour. Our fabricated prints appear as colour images under white light, while projecting up to three different holograms under red, green, or blue laser illumination. These holographic colour prints are readily verified but challenging to emulate, and can provide enhanced security in anti-counterfeiting applications. As the prints encode information only in the surface relief of a single polymeric material, nanoscale 3D printing of customised masters may enable their mass-manufacture by nanoimprint lithography.

Highlights

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature
With coherent monochromatic illumination, the incident light is filtered such that only the relevant phase plates with colour filters that closely match the illumination wavelength are selected for a given holographic projection, whereas the other phase plates do not form a projection as their colour filters are mismatched and reject the incident light
Using the freedom to divide the space into regions of arbitrary shapes and sizes, the individual hologram areas can be strategically allocated such that the arrangement of their colour filters encodes a chosen colour image

Summary

Introduction

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. Traditional phase elements consisting of dielectric structures of different thicknesses (phase plates) enable holographic projection to be achieved with higher transmission efficiency, simpler illumination methods (e.g. a handheld laser pointer), and little restriction on the polarisation or incidence angle of the light. They are usually easier and cheaper to manufacture than metal nanostructures as their larger dimensions are within the resolution limit of photolithography. By encryption we refer to the fact that the holographic information cannot be read except by illumination with coherent monochromatic light of a suitable wavelength, noting that the introduction of one or more additional phase masks as security keys[19] could in principle enable even more secure encryption

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Conclusion