Abstract
Photonic crystal structures can be created to manipulate electromagnetic waves so that many studies have focused on designing photonic band-gaps for various applications including sensors, LEDs, lasers, and optical fibers. Here, we show that mono-layered, self-assembled photonic crystals (SAPCs) fabricated by using an inkjet printer exhibit extremely weak structural colors and multiple colorful holograms so that they can be utilized in anti-counterfeit measures. We demonstrate that SAPC patterns on a white background are covert under daylight, such that pattern detection can be avoided, but they become overt in a simple manner under strong illumination with smartphone flash light and/or on a black background, showing remarkable potential for anti-counterfeit techniques. Besides, we demonstrate that SAPCs yield different RGB histograms that depend on viewing angles and pattern densities, thus enhancing their cryptographic capabilities. Hence, the structural colorations designed by inkjet printers would not only produce optical holograms for the simple authentication of many items and products but also enable a high-secure anti-counterfeit technique.
Highlights
Photonic crystals with 500 nm diameter in injected droplets are self-assembled by drying and their nanostructural shapes and sizes are mainly determined by the surface wettability, the number density of the photonic crystal suspension, and the chemical composition of the solvents; additional characterization is displayed in Fig. S1 (Supplementary Information)
An self-assembled photonic crystals (SAPCs) pattern is formed with a constant spacing distance of approximately 100 μm (ii) and under strong illumination, the pattern produces colors (iv)
It must be noted that the photonic crystal suspension solution was mixed with formamide (FA, 20% v/v) for the close-packed and mono-layered self-assembly of the photonic crystals, the detailed mechanisms of which have been intensively studied (Note, Supplementary Information)[30], and the number density of the photonic crystal suspension was calculated to fully fill the nanostructures and generate vivid structural colors (20% w/v, Fig. S2, Supplementary Information)
Summary
For this reason, we utilized a commercially available inkjet printer for the fabrication of SAPC patterns, which has the key advantage of easy fabrication and rapid production of various photonic prints[18]. Mono-layered SAPC patterns based on the diffraction mechanism may be a promising alternative for covert-overt transformation. Embossed patterns on a white background cannot be perceived by the naked eye under low-lighting conditions, thereby producing covert patterns. Such covert patterns can become overt by controlling the light intensity or changing the background. We describe inkjet-printing-based fabrication methods for mono-layered SAPC patterns on various substrates with different wettabilities and the engineering of an extremely low diffracted light to produce viewing-angle-dependent structural colors. We demonstrate that the unique encryption of mono-layered SAPC patterns can be designed by manipulating the pattern densities of the SAPCs and the subsequent decryption of the patterns can be performed by analyzing the red, green and blue (RGB) histograms, showing remarkable potential for a sophisticated anti-counterfeit system
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