Laser‐Empowered Random Metasurfaces for White Light Printed Image Multiplexing

Abstract

Printed image multiplexing based on the design of metasurfaces has attracted much interest in the past decade. Optical switching between different images displayed directly on the metasurface is performed by altering parameters of the incident light such as polarization, wavelength or incidence angle. When using white light, only two-image multiplexing is implemented with polarization switching. Such metasurfaces are made of nanostructures perfectly controlled individually, which provide high-resolution pixels but small images and involve long fabrication processes. Here, we demonstrate that laser processing of nanocomposites offers a versatile low-cost, high-speed method with large area processing capabilities for controlling the statistical properties of random metasurfaces, allowing up to three-image multiplexing under white light illumination. By controlling independently absorption and interference effects, colors in reflection and transmission can be varied independently yielding two-image multiplexing under white light. Using anisotropy of plasmonic nanoparticles a third image can be multiplexed and revealed through polarization changes. The design strategy, the fundamental properties and the versatility of implementation of these laser-empowered random metasurfaces are discussed. The technique, applied on flexible substrate, can find applications in information encryption or functional switchable optical devices, and offers many advantages for visual security and anticounterfeiting.

Anti‐Counterfeiting White Light Printed Image Multiplexing by Fast Nanosecond Laser Processing

“…All this process has already been used in references. [37,53] Nanosecond laser processing: An industrial fibered laser (IPG Photonics) of wavelength 532 nm with a typical pulse duration of 1.3 ns is scanned over the sample with a galvanometric mirror scanner head (Sunny Technology). The laser is linearly polarized using a Brewster angle polarizer placed before the scanner head (Figure S1, Supporting Information).…”

“…As the period value is lower than the incident wavelengths (white light), the -1 st diffracted order only appears for an incidence angle larger than 10° for the blue (400 nm) part of the spectrum and is measured in the backscattering configuration described in Figure 2. When comparing these metasurfaces with the ones produced by femtosecond lasers, [37,40,41,45] the embedded nanoparticles can have higher sizes at low speed and the surface grating-like structures are parallel to the laser polarization while they were perpendicular to the laser polarization with the femtosecond laser processing. 2, obtained at various scan speeds and laser fluences.…”

“…Here the number of multiplexed images is limited to two or three only, but this multiplexing strategy is the only one compatible with the production of overt security features observable with the naked eye in ambient light. However, all the outcomes published so far use polarizers to switch between the two or three multiplexed images, [35][36][37][38] which prevents their use as overt security features.…”

“…Except our recent work, [37] plasmonic metasurfaces used for image multiplexing involve electron or ion beam lithography. [35,36] These technologies are difficult to transfer to an industry where the cost must be affordable and the fabrication processes are completely customized on a large scale.…”

Nanosecond Laser Switching of Phase‐Change Random Metasurfaces with Tunable ON‐State

“…We use the term random metasurface to make reference to metasurfaces in which the nanostructured resonant constituent elements are randomly placed with certain size, density, and orientation distributions, rather than composed of well-ordered elements with a single size, density, and orientation as those, which can be fabricated using lithography techniques. [39][40][41][42] They are also called lithography-free metasurfaces. [43,44] The random Bi-Al 2 O 3 metasurfaces considered in this work have been intentionally designed to achieve a broad plasmonic response to light in the ultraviolet-visible region of the spectrum [45] and optical switching upon heating.…”