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

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.

A hybrid achromatic metalens

“…The lower part contains a square cross-section phase plate and the upper part contains a nanopillar-based metalens. A similar hybrid design has been used to shape spectral transmission and produce printed color filters 29 . In contrast, our hybrid metalens shapes the wavefront while maximizing broadband transmission.…”

Carbon Dots for Forensic Applications: A Critical Review

“…It is therefore crucial for modern societies to implement effective anti-counterfeit and brand protection policies. Mainstream strategies rely on integrated safety features and security graphics such as holograms, optically variable devices, watermarks, laser codes and chemical and biological taggants [53,54]. Unfortunately, even the most sophisticated designs are eventually expertly copied and reproduced.…”

Metasurfaces: Subwavelength nanostructure arrays for ultrathin flat optics and photonics

“…11,95 Research toward optical metasurfaces has been broadened to include multiplexing [96][97][98][99][100][101] and multifunctional devices. [102][103][104][105] Multiplexed metasurface holograms can reconstruct two distinct images under illumination by left and right circularly polarized light. 97 Multichannel holography can render different sets of images with a total of 12 polarization channels.…”

“…99 Multicolor holography can reconstruct three different images under red, green, and blue laser illumination in a single device. 100,101 A single metasurface that works simultaneously under coherent and incoherent light has been suggested; [102][103][104] this device generates structural colors under ambient light and reconstructs a hologram under a coherent laser source. By exploiting metasurface coloration, two independent images can be encoded, one hidden and one visible.…”

“…These features have obvious applications in optical steganography and cryptography, where they involve techniques that conceal/hide a message or information for security purposes. 87,102,103,105 Nonlinear and quantum metasurfaces Using metasurfaces with subwavelength control over polarization enables control of nonlinear optical processes, such as second-harmonic and third-harmonic generation 106 (as discussed in the Pertsch and Kivshar article in this issue 107 ) and four-wave mixing. 108 Symmetry selection rules in harmonic generation have been observed in nonlinear optical chirality using meta-atoms, or the artificial atoms constituting the metasurfaces.…”