Structural Color Patterns on Paper Fabricated by Inkjet Printer and Their Application in Anticounterfeiting

Dual-Mode Multicolor Display Based on Structural and Fluorescent Color CdS Photonic Crystal Hydrogel

In this study, we prepared a multicolor structural-fluorescent CdS-PEGDA photonic crystal hydrogel (SFC-CPH) with a dual display mode, which has two different optical states: structural color mode and fluorescent color mode. SFC-CPH displays structural color mode under visible light and fluorescent color mode under ultraviolet light. Initially, monodisperse CdS colloidal particles were synthesized via a hydrothermal method, leading to the self-assembly of a photonic crystal template. The high refractive index of CdS contributes to the photonic crystals' low-angle dependence and vivid structural colors. Then, a variety of fluorescent molecules were doped into poly(ethylene glycol) diacrylate (PEGDA) hydrogel and combined with photonic crystals with distinct structural colors to prepare three distinct colors of SFC-CPH. We also investigated the optical characteristics and surface properties of these photonic crystal hydrogels. Based on these dual-mode display characteristics, we designed several dual-mode display patterns and a method for information encoding. The unique property of this photonic crystal hydrogel material suggests its substantial potential for applications in information storage, security, and encoding, offering innovative avenues in the realm of information display.

Electrically triggered photonic crystal anti-counterfeiting tags with multi-level response fabricated by regioselective modification of ITO electrode surface

Anticounterfeiting materials based on the photonic crystal (PC) have attracted great interest due to their unique visual effects originating from the changeable structural colors under various external stimuli. However, there still are challenges to improving the anticounterfeiting performance by enhancing the complexity and diversity of the color changes. Here, we fabricated an electrically triggered anticounterfeiting tag by encapsulating the responsive PC with the surface-modified and patterned ITO electrode. The degree of Au deposition or chemical etching in different regions of the ITO was precisely controlled to achieve multi-level differentiated electrical responses, which made the invisible pattern of the tag at 0 V be "revealed in multicolor form" or "gradually revealed" under increasing voltages. The tag possessed two working modes, more diversified visual effects, good usability, and reversibility, which let it become a potentially useful material for anti-counterfeiting applications in the future.

Smart colloidal photonic crystal sensors

Smart colloidal photonic crystals (PCs) with stimuli-responsive periodic micro/nano-structures, photonic bandgaps, and structural colors have shown unique advantages (high sensitivity, visual readout, wireless characteristics, etc.) in sensing by outputting diverse structural colors and reflection signals. In this review, smart PC sensors are summarized according to their fabrications, structures, sensing mechanisms, and applications. The fabrications of colloidal PCs are mainly by self-assembling the well-defined nanoparticles into the periodical structure (supersaturation-, polymerization-, evaporation-, shear-, interaction-, and field-induced self-assembly process). Their structures can be divided into two groups: closely packed and non-closely packed nano-structures. The sensing mechanisms can be explained by Bragg's law, including the change in the effective refractive index, lattice constant, and the order degree. The sensing applications are detailly introduced according to the analytes of the target, including solvents, vapors, humidity, mechanical force, temperature, electrical field, magnetic field, pH, ions/molecules, and so on. Finally, the corresponding challenges and the future potential prospects of artificial smart colloidal PCs in the sensing field are discussed.

Versatile Double Bandgap Photonic Crystals of High Color Saturation

Double bandgap photonic crystals (PCs) exhibit significant potential for applications in various color display-related fields. However, they show low color saturation and inadequate color modulation capabilities. This study presents a viable approach to the fabrication of double bandgap photonic inks diffracting typical secondary colors and other composite colors by simply mixing two photonic nanochains (PNCs) of different primary colors as pigments in an appropriate percentage following the conventional RGB color matching method. In this approach, the PNCs are magnetically responsive and display three primary colors that can be synthesized by combining hydrogen bond-guided and magnetic field (H)-assisted template polymerization. The as-prepared double bandgap photonic inks present high color saturation due to the fixed and narrow full-width at half-maxima of the parent PNCs with a suitable chain length. Furthermore, they can be used to easily produce a flexible double bandgap PC film by embedding the PNCs into a gel, such as polyacrylamide, facilitating fast steady display performance without the requirement of an external magnetic field. This research not only presents the unique advantages of PNCs in constructing multi-bandgap PCs but also establishes the feasibility of utilizing PNCs in practical applications within the fields of anti-counterfeiting and flexible wearable devices.

Bio-Inspired Highly Brilliant Structural Colors and Derived Photonic Superstructures for Information Encryption and Fluorescence Enhancement

Inspired by the brilliant and tunable structural colors based on the large refractive index contrast (Δn) and non-close-packing structures of chameleon skins, ZnS-silica photonic crystals (PCs) with highly saturated and adjustable colors are fabricated. Due to the large Δn and non-close-packing structure, ZnS-silica PCs show 1) intense reflectance (maximal: 90%), wide photonic bandgaps, and large peak areas, 2.6-7.6, 1.6, and 4.0 times higher than those of silica PCs, respectively; 2) tunable colors by simply adjusting the volume fraction of particles with the same size, more convenient than the conventional way of altering particle sizes; and 3) a relatively low threshold of PC's thickness (57 µm) possessing maximal reflectance compared to that (>200 µm) of the silica PCs. Benefiting from the core-shell structure of the particles, various derived photonic superstructures are fabricated by co-assembling ZnS-silica and silica particles into PCs or by selectively etching silica or ZnS of ZnS-silica/silica and ZnS-silica PCs. A new information encryption technique is developed based on the unique reversible "disorder-order" switch of water-responsive photonic superstructures. Additionally, ZnS-silica PCs are ideal candidates for enhancing fluorescence (approximately tenfold), approximately six times higher than that of silica PC.

Visible and infrared dual-band anti-counterfeiting with self-assembled photonic heterostructures

Self-assembled photonic structures have greatly expanded the paradigm of optical materials due to their ease of access, the richness of results offered and the strong interaction with light. Among them, photonic heterostructure shows unprecedent advances in exploring novel optical responses that only can be realized by interfaces or multiple components. In this work, we realize visible and infrared dual-band anti-counterfeiting using metamaterial (MM) - photonic crystal (PhC) heterostructures for the first time. Sedimentation of TiO₂ nanoparticles in horizontal mode and polystyrene (PS) microspheres in vertical mode self-assembles a van der Waals interface, connecting TiO₂ MM to PS PhC. Difference of characteristic length scales between two components support photonic bandgap engineering in the visible band, and creates a concrete interface at mid-infrared to prevent interference. Consequently, the encoded TiO₂ MM is hidden by structurally colored PS PhC and visualized either by adding refractive index matching liquid or by thermal imaging. The well-defined compatibility of optical modes and facility in interface treatments further paves the way for multifunctional photonic heterostructures.

Research Progress on Blue-Phase Liquid Crystals for Pattern Replication Applications

Blue-Phase Liquid Crystals (BPLCs) are considered to be excellent 3D photonic crystals and have attracted a great deal of attention due to their great potential for advanced applications in a wide range of fields including self-assembling tunable photonic crystals and fast-response displays. BPLCs exhibit promise in patterned applications due to their sub-millisecond response time, three-dimensional cubic structure, macroscopic optical isotropy and high contrast ratio. The diversity of patterned applications developed based on BPLCs has attracted much attention. This paper focuses on the latest advances in blue-phase (BP) materials, including applications in patterned microscopy, electric field driving, handwriting driving, optical writing and inkjet printing. The paper concludes with future challenges and opportunities for BP materials, providing important insights into the subsequent development of BP.

Refractory Metals and Oxides for High-Temperature Structural Color Filters

Refractory metals have recently garnered significant interest as options for photonic applications due to their superior high-temperature stability and versatile optical properties. However, most previous studies only consider their room-temperature optical properties when analyzing these materials' behavior as optical components. Here, we demonstrate structural color pixels based on three refractory metals (Ru, Ta, and W) for high-temperature applications. We quantify their optical behavior in an oxygenated environment and determine their dielectric functions after heating up to 600 °C. We use in situ oxidation, a fundamental chemical reaction, to form nanometer-scale metal oxide thin-film bilayers on each refractory metal. We fully characterize the behavior of the newly formed thin-film interference structures, which exhibit vibrant color changes upon high-temperature treatment. Finally, we present optical simulations showing the full range of hues achievable with a simple two-layer metal oxide/metal reflector structure. All of these materials have melting points >1100 °C, with the Ta-based structure offering high-temperature stability, and the Ru- and W-based options providing an alternative for reversible color filters, at high temperatures in inert or vacuum environments. Our approach is uniquely suitable for high-temperature photonics, where the oxides can be used as conformal coatings to produce a wide variety of colors across a large portion of the color gamut.

Effect of Solvents on the Color Recovery Responses of Swollen Structural-Color Epoxy Films Based on Inverse Opal Photonic Crystals

Photonic crystal (PC) films have been widely applied in color displays and the anticounterfeiting field due to their facile fabrication process and easily tunable properties. However, the method for improving the reusability of the color-changed swollen PC films is still a challenge. In this paper, we report the color recovery behavior of epoxy resin inverse opal photonic crystal (EP-IOPC) films, which show different responses after being infiltrated with ethanol, acetone, and dimethyl sulfoxide (DMSO) based on the swelling and deswelling process. DMSO achieved the best effect on the color recovery of the swollen EP-IOPC films compared to ethanol and acetone, and the reflection spectrum blue-shifted in a small range and finally stabilized at a 60 nm deviation from the original spectrum after 10 times recovery. This strategy of color recovery not only solved the problem that the swollen EP-IOPC film's color changes to a certain extent but also showed promising potential in the color display and anticounterfeiting field.

Security labeling and optical information encryption enabled by laser-printed silicon Mie resonators

Fighting against the falsification of valuable items remains a crucial social-threatening challenge stimulating a never-ending search for novel anti-counterfeiting strategies. The demanding security labels must simultaneously address multiple requirements (high density of the recorded information, high protection degree, etc.) and be realized via scalable and inexpensive technologies. Here, the direct reproducible femtosecond-laser patterning of thin glass-supported amorphous (α-)Si films is proposed for optical information encryption and the scalable and highly reproducible fabrication of security labels composed of Raman-active hemispherical Si nanoparticles (NPs). Laser printing conditions allow the precise control of the diameter of the formed NPs ensuring translation of their dipolar Mie resonance position within the entire visible spectral range. Two-temperature molecular dynamics simulations clarify the origin of α-Si NP formation by rupture of the molten Si layer driven by a negative GPa-range pressure near the liquid-solid interface. Arrangement of the laser-printed Mie-resonant NP allows the creation of hidden security labels offering several easy-to-realize information encryption strategies (for example, local laser-induced post-crystallization or mixing Mie-resonant and non-resonant NPs), additional protection modalities, facile Raman mapping readout and dense information recording (up to 60 000 dots per inch) close to the optical diffraction limit. The developed fabrication strategy is simple, inexpensive, and scalable and can be realized based on cheap Earth-abundant materials and commercially-available equipment justifying its practical applicability and attractiveness for anti-counterfeit and security applications.

Panther chameleon-inspired, continuously-regulated, high-saturation structural color of a reflective grating on the nano-patterned surface of a shape memory polymer

In this work, surface nano-stripes and a reflective grating have been fabricated on shape memory polymers (SMPs) to simulate the active color change of chameleons. The structural color resulting from the interference of reflected light exhibits high saturation and it can be regulated continuously based on the shape memory effect. In addition to the viewing angle, the attained color is sensitive to the deformation at the macroscale. Uniaxial tension along stripes at high temperature produces a remarkable blueshift of the resultant color (from red to green and blue) which can switch back to red after shape recovery upon heating. The evolution of structural color can be attributed to the lower and higher magnitudes of nano-structure periods in temporary (deformed) and permanent (recovery) states respectively. Based on the combination of angle and deformation dependences of structural color, a "colorful" product code has been fabricated. It exhibits enhanced ability to hide and display information which plays an important role in anti-counterfeiting.

Cracking enabled unclonability in colloidal crystal patterns authenticated with computer vision

Colloidal crystals with iridescent structural coloration have appealing applications in the fields of sensors, displays, anti-counterfeiting, etc. A serious issue accompanying the facile chemical self-assembly approach to colloidal crystals is the formation of uncontrolled and irregular cracks. In contrast to the previous efforts to avoid cracking, the unfavorable and random micro-cracks in colloidal crystals were utilized here as unclonable codes for tamper-proof anti-counterfeiting. The special structural and optical characteristics of the colloidal crystal patterns assembled with monodisperse poly(styrene-methyl methacrylate-acrylic acid) core-shell nanospheres enabled multi-anti-counterfeiting modes, including angle-dependent structural colors and polarization anisotropy, besides the physically unclonable functions (PUFs) of random micro-cracks. Moreover, by using the random cracks in the colloidal crystals as templates to guide fluorescent silica nanoparticle deposition, an fluorescent anti-counterfeiting mode with PUFs was introduced. To validate the PUFs of the fluorescent micro-cracks in the colloidal crystals, a novel edge-sensitive template matching approach based on a computer vision algorithm with an accuracy of ∼100% was developed, enabling ultimate security immune to forgery. The computer-vision verifiable physically unclonable colloidal crystals with multi-anti-counterfeiting modes are superior to conventional photonic crystal anti-counterfeiting materials that rely on angle-dependent or tunable structural colors, and the conventional PUF labels in the aspect of decorative functions, which will open a new avenue for advanced security materials with multi-functionality.

Low-Angle-Dependent Anticounterfeiting Label Decoded by Alcohol Tissue Wiping Based on a Multilayer Photonic Crystal Structure

The application of photonic crystals (PCs) as anticounterfeiting materials has been widely investigated because of their tunable photonic stop band and corresponding changeable structural colors. In this work, we designed a composite PC structure including an information CdS PC layer at the bottom and a polymer-based layer composed of an inverse opal PC (IOPC) layer and a disordered porous layer on the top, which can be decoded by an alcohol tissue. The high refractive index of the bottom patterned CdS PC layer provides the structure with a vivid low-angle-dependent structural color in the decoded mode, which ensures the stability of the information conveyed by this label. When the incident angle changed from 5 to 45°, the structural color of the patterned CdS layer changed slightly. In the hidden mode, the low transmittance shields the structural color of the CdS layer. When the structure was wiped with the alcohol tissue, the transmittance of the upper IOPC layer could be increased quickly due to the similar refractive indexes of the used polymer and alcohol, and the pattern of the CdS layer was decoded. Thus, the designed composite PC can act as an anticounterfeiting label, in which the encrypted pattern can be decoded by alcohol tissue wiping and shows a vivid low-angle-dependent structural color. To enhance the anticounterfeiting ability of the designed structure, a double-sided label with different encryption patterns on both sides was designed. Based on the simple reversible encryption and decryption process as well as the color stability, the label shows great application potential in the daily anticounterfeiting field.

Ultrafast and Irreversibly Thermochromic SiO2 -PC/PEG Double Layer for Green Thermal Printing

Traditional thermochromic photonic crystal (PC) usually has a slow and reversible thermal response, which limits its application in thermal printing. Here, the authors develop a thermochromic "SiO₂ -PC/PEG" double layer structure with a responding time of milliseconds for fast thermal printing. Controlled by the print-head, the polyethylene glycol (PEG) melts, infiltrates, and solidifies within the interparticle voids, which instantly and irreversibly changes the refractive index and produces the PC pattern. Multicolor printing can be realized by tuning the size and type of colloidal particles. Resolution as high as 300 DPI is achieved to print the high-resolution patterns and then the grayscale patterns based on the control of pixel densities. Different from fiber thermal paper, the "SiO₂ -PC/PEG" film has no toxic bisphenol A and possesses superior light stability for keeping the images longer. It is fully compatible with the commercial printer, which provides a mature solution for fast and convenient preparation of PC patterns.

Upconversion Nanoparticle-Integrated Bilayer Inverse Opal Photonic Crystal Film for the Triple Anticounterfeiting

Optical anticounterfeiting plays a vital role in information security because it can be recognized by the naked eye and is difficult to imitate. Herein, a hydrophilic modified upconversion nanoparticle (M-UCNP)-integrated bilayer inverse opal photonic crystal (IOPC) film was designed in which the luminescent M-UCNPs were deposited on the surface of the optimized bilayer structure with double photonic stop bands. The structure which can modulate light to produce structural colors can also enhance the upconversion luminescence (UCL) to improve the anticounterfeiting effect synergistically. On the one hand, the reflection colors from green to blue were observed in the specular angles on the front (540-layer) of the film. Meanwhile, the scattering colors under nonspecular angles from red to blue on the back (808-layer) appeared in the natural light. On the other hand, the bilayer structure in which the 808-layer functions as a "secondary excitation source" to improve the intensity of the excitation light on M-UCNPs and the 540-layer reflects the emission light of the M-UCNPs to enhance the UCL intensity endows the film with good night vision ability. Finally, the dual-mode structural colors and enhanced UCL of the patterned film work together to realize triple anticounterfeiting in banknotes.

Iridescent coating of graphene oxide on various substrates

Two-dimensional nanomaterials have been incorporated into coating layers for exceptional properties in mechanic toughness, electronics, thermology and optics. Graphene oxide (GO), however, was greatly hindered by its strong adsorption within visible wavelength and hereby the intrinsic dark color at the solid state. Herein, we found a unique aqueous mixture of GO containing sodium dodecyl sulfate and l-ascorbic acid. It enabled to produce iridescent coating layers with tunable thickness of 0.3-50 μm on both hydrophilic and hydrophobic substrates (e.g., glass, aluminum foil, polytetrafluoroethylene), through brushing, liquid-casting, dipping and writing. Their iridescence could be further tuned by incorporating MXene nanosheets. And their mechanical properties could be enhanced by certain synthetic polymers (e.g., polyvinyl alcohol and polyethylene glycol). Their sensitivity to heat, laser and water also benefited to pattern the coating layers. Furthermore, by controlling laser intensity, the domain color could be changed (e.g., green to blue). Thus, this study may pave a new pathway of producing iridescent coatings of graphene oxide in a large scale for practical applications.

Chameleon-Inspired Brilliant and Sensitive Mechano-Chromic Photonic Skins for Self-Reporting the Strains of Earthworms

The skins of chameleons have attracted growing interest because they have sensitive mechano-chromic properties and bright colors due to the large surface-to-surface distances (D_s-s) between neighboring particles and contrast of the refractive index (Δn), respectively. Inspired by these, artificial mechano-chromic photonic skins (MPSs) mimicking those of chameleons were fabricated by the large Δn and D_s-s. The fabrication is considerably simple and efficient based on the self-assembly strategy using commercial chemicals and materials. The reflectance of MPSs depends on the value of Δn, which can be greatly increased to 70% with a Δn of 0.035, leading to their brilliant colors. Because of the large D_s-s, the MPSs possess outstanding mechano-chromic performances, including a large maximal (Δλ = 205 nm) and effective (Δλ_e = 184 nm) tuning range of the reflection wavelength, high sensitivity (368), fast responsiveness (2.2 nm/ms), good stabilities (>1 year), and reversibility (>100 times). Based on these advantages, MPSs have been used for self-reporting the strain of earthworms by outputting diverse colors during the peristaltic process, indicating the great potential of the MPSs as visual sensors and optical coatings.

Magnetic field responsive microspheres with tunable structural colors by controlled assembly of nanoparticles

Dynamic color tuning has many useful applications in nature for communication, camouflage, mood indication, etc. Structural colors have more advanced applications due to their ability to respond to external stimuli by dynamically changing color. In this work, we proposed an efficient method to prepare magneto-chromatic microspheres with tunable structural color. Through a microfluidic technique, the magneto-chromatic microspheres containing Fe₃O₄@C magnetic particles were continuously prepared. The size of the microspheres decreases with the increase of PVA solution phase to ETPTA phase flow rate ratio. Furthermore, the microspheres with larger sizes more easily form close packed structures. Microspheres can be constrained in PVA to form a free-standing film after the evaporation of water in PVA solution. The PVA film could display tunable brilliant structural colors when an external magnetic field is applied. Moreover, microspheres with fixed structural colors can also be acquired by polymerizing microspheres under UV light under an external magnetic field.

An efficient strategy was used for the preparation of magneto-chromatic microspheres with tunable structural color.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

Noniridescent structural color from enhanced electromagnetic resonances of particle aggregations and its applications for reconfigurable patterns

HYPOTHESIS

The conventional noniridescent structural colors refer to the coherent scattering of visible light by the short-range ordered structures assembled from the small colloids (100-250 nm). Our hypothesis is that noniridescent structural color can be generated by the random aggregations of large silica particles through the enhanced electromagnetic resonances.

EXPERIMENTS

The random aggregations of large silica particles (350-475 nm) were prepared through the infiltration of silica particles solution with the porous substrate. The mechanism of the structural color is investigated. Reconfigurable patterns are prepared.

FINDINGS

Dissimilar to the conventional noniridescent colors, the angle-independent colors of silica aggregations originate from the enhanced electromagnetic resonances due to the random aggregation of the particles. The colors (blue, green, and red) and corresponding reflection peak positions of the particle aggregations can be well controlled by simply altering the size of the silica particles. Compared to the traditional prints with permanent patterns, reconfigurable patterns with large-area and multicolor can be fabricated by the repeatedly selective spray of water on the substrate pre-coated with noniridescent colors. This work provides new insight and greenway for the fabrication of noniridescent structural colors and reconfigurable patterns, and will promote their applications in soft display, green printing, and anti-counterfeiting.

Core/shell colloidal nanoparticles based multifunctional and robust photonic paper via drop-casting self-assembly for reversible mechanochromic and writing

Photonic crystals film that possesses periodic dielectric structure have shown great prospect in developing environmentally friendly paper alternatives due to the unique properties of dye free and non-photobleaching, but their practical application is limited by the weak interaction between colloidal particles. Although some progress has been obtained, it is still a challenge to develop photonic paper with the desired mechanical and optical properties. Herein, multifunctional hard core/soft shell nanoparticles with controlled size are fabricated by semi-continuous seed emulsion polymerization method. Compared with convention colloidal particles, these core/shell nanoparticles can facile self-assemble into large-scale dense ordered structure film via dried at room temperature due to the relatively low glass transition temperature (Tg) of the shell layers. The facile fabrication route enables the continuous high-through put production of the photonic papers. The as-formed papers not only possess the capacity to solvent (water/ethanol) rewritable and multicolor painting, but also can rapidly reversible mechanochromic. Moreover, due to the good compatibility of core/shell interface, these photonic films possess excellent mechanical properties, demonstrating that this multifunctional film makes the fabrication of novel robust rewritable papers possible and enables visual monitoring of deformation degree.

Lotus Seedpod Inspiration: Particle-Nested Double-Inverse Opal Films with Fast and Reversible Structural Color Switching for Information Security

The integration of novel structures into colloidal crystals provides the possibility of constructing stimuli-responsive photonic materials. However, in most opal and inverse opal structures, replacing the interior air with an infiltrated liquid will cause partial refractive index matching, resulting in the reduction or even disappearance of the photonic band gap. Herein, inspired by the lotus seedpod, an innovative particle-nested double-inverse opal film with fast and reversible structural color switching (≈1 s) is first fabricated by introducing polystyrene (PS) spheres into an inverted opal backbone. Importantly, refractive index matching can be effectively avoided due to the existence of internal PS spheres, and optical switching from diffusive to photonic behavior is achieved by a liquid with low surface tension for the response. Furthermore, a reversible ethanol stimuli-response bilayer double-inverse opal film with multistate switching for information encryption is proposed by combining optical scattering and diffraction. The scattered light from the top layer caused by the randomly distributed and weakly scattering PS spheres within the pores makes the pattern at the bottom invisible. Simultaneously, the display and discoloration of the pattern can be realized instantaneously by ethanol response. Thus, this new preparation strategy exhibits great potential in the security fields.

Simple and efficient fabrication of multi-stage color-changeable photonic prints as anti-counterfeit labels

Color changeable photonic prints (CCPPs) show their potential applications in high-level information storage and anti-counterfeiting, but usually suffer from the complex fabrication process and limited color variation. Here, a simple and efficient method is developed to generate CCPPs with multilevel tunable color contrasts by packing the solvent responsive photonic crystals with diverse cross-linking degrees and desired way. The key to the successful fabrication is to create and control over the optical response of each part of the CCPPs through altering the cross-linking degree of PCs and thus the affinity between the CCPPs and solvents. A CCPPs based anti-fake label with the encrypted information functionality which originates from reversible color change between dried state and swelling with the mixture of acetic acid and ethanol is investigated. Compared with conventional CCPPs, the as-prepared CCPPs can reveal multistage information depending on the volume fraction of ethanol. This work provides a new insight for the simple fabrication of CCPPs and will facilitate their applications in the information protection and high-level anti-counterfeiting.

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λ_em) of film A and the central selective reflection wavelength (λ_m) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Highly Efficient Detection of Homologues and Isomers by the Dynamic Swelling Reflection Spectrum

Precise and efficient detection of solvents with similar refractive index is highly desired but remains a big challenge for the conventional opal because the shift of its reflection wavelength only depends on the refractive index of the solvent to be detected. Here, homologues (alcohols, acids, alkalis, esters, and aromatic hydrocarbons), isomers, and other solvents with similar refractive index and structures were precisely distinguished through the dynamic swelling reflection spectrum (DSRS) pattern based on the different swelling behavior of swellable photonic paper in solvents. The one reflection signal of photonic paper will split into two reflection peaks, which then tend to merge together during the swelling process. The variation of the reflection signals and merging time are highly sensitive to the polarity and refractive index of the solvent, and the differences can be significantly amplified in DSRS, resulting in the distinction of the solvent from its unique geometric pattern. Moreover, the variation tendency of the reflectance provides an additional parameter in recognition of the solvent, which can be explained by calculation and comparison of the practical volume ratio of the solvent swelled into the photonic paper and the corresponding critical volume ratio of the solvent determined by its refractive index.

Controllable Structural Colored Screen for Real-Time Display via Near-Infrared Light

Patterned colloidal crystals with stimuli-responsive materials provide sensitive and versatile means for investigating the varying ambiance of heat, light, electricity, magnetism, and stress. However, it remains a challenge to integrate stimuli-responsive materials with colloidal crystals by a simple and efficient method, thus restricting them from being used in general applications. Inspired from chameleons, we present a facile yet high-quality approach for the fabrication of the assembly of colloidal nanoparticles based on the hydrophilic-modified thermosensitive films. Various kinds of integral thermosensitive structural colored (TSSC) films are simply prepared in a high-quality screen on a large scale, with low cost, angle independence, and excellent flexibility. Simply turning on the near-infrared (NIR) laser brings heat to the irradiated region to increase the temperature. Integration of the multi-colored photonic bandgap (PBG) of the thermal-sensitive colloidal crystal and flexible anti-counterfeit labels into the NIR light exciting screens can change the intensity of PBG obviously. This advanced technology not only provides an efficient strategy for the preparation of colloidal crystal but also demonstrates a highly thermosensitive structural colored screen that has great prospect for information storage, anticounterfeiting, and real-time display materials.

Optical Nanomaterials and Enabling Technologies for High-Security-Level Anticounterfeiting

Optical nanomaterials have been widely used in anticounterfeiting applications. There have been significant developments powered by recent advances in material science, printing technologies, and the availability of smartphone-based decoding technology. Recent progress in this field is surveyed, including the availability of optical reflection, absorption, scattering, and luminescent nanoparticles. It is demonstrated that advances in the design and synthesis of lanthanide-doped upconversion nanoparticles will lead to the next generation of anticounterfeiting technologies. Their tunable optical properties and optical responses to a range of external stimuli allow high-security level information encoding. Challenges in the scale-up synthesis of nanomaterials, engineering of assessorial devices for smart-phone-based decryption, and alignment to the potential markets which will lead to new directions for research, are discussed.

Printing a Wide Gamut of Saturated Structural Colors Using Binary Mixtures, With Applications in Anticounterfeiting

Use of colloidal suspensions to generate structural colors has the potential to reduce the use of toxic metals or organic pigments in inkjet printing, coatings, cosmetics, and other applications, and is a promising avenue to create large-scale nanostructures that produce long-lasting colors. However, expanded use of structural colors requires a reduction in coffee-ring effects during printing, which currently requires intricately patterned substrates or high particle concentrations, and diversification of colors to compete with conventional printing inks. Here, we treat substrate surfaces with cold plasma to facilitate spontaneous assembly of particles into colloidal nanostructures, reducing the need for highly concentrated particle suspensions. Moreover, by employing binary mixtures, we can tune the lightness of the hue produced or change the hue itself, allowing us to cover wider regions of color space. We demonstrate the use of this cold-plasma approach on a variety of substrates, favoring substrate diversity on which printing can be performed. This methodology enables creation of high-resolution, complex designs and opens a path for extending the limits of anticounterfeiting applications by using binary mixtures.

Structural Color Materials for Optical Anticounterfeiting

The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long-term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli-responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

Nanosphere-Aggregation-Induced Reflection and Its Application in Large-Area and High-Precision Panchromatic Inkjet Printing

Artificial structural colors have attracted more and more attention due to their high photostability, low toxicity, and brilliant colors. Inkjet printing of photonic crystals or amorphous photonic structures can realize large-scale structural color patterns, while plasma printing of metals can achieve high-precision color images. However, still no method is available to fabricate structural color patterns on both a large scale and with high precision. Here, nanosphere-aggregation-induced reflection (NAIR) is first theoretically and experimentally demonstrated and vivid full-spectrum structural color can be generated based on NAIR. Dramatically different from photonic crystals, the accumulation of only a few monodisperse dielectric spheres with an appropriate refractive index and diameter can produce bright structural colors, which makes high resolution possible. By introducing commercial inkjet printers, this aggregate structure can be constructed at high speed in a large scale. Importantly, the color mixing is easily performed by simultaneously applying spheres with different sizes, which allow us to sophisticatedly control the generated color. The demonstrated NAIR printing paves the way toward a full-spectrum, large-scale, and high-precision structural color, offering great potential for daily commercial utilization.

Bilayer Heterostructure Photonic Crystal Composed of Hollow Silica and Silica Sphere Arrays for Information Encryption

Utilizing photonic crystals to fabricate information encryption materials has attracted widespread interest due to their tunable optical properties and responsiveness to external stimuli. In most of the previously reported systems, the information is hidden at a specific angle and the angle-dependent invisibility is a limitation. Meanwhile, poor structural stability is still a key issue that needs to be solved for potential applications. In this paper, a bilayer heterostructure photonic crystal containing ordered hollow silica inverse opal arrays, amorphous silica opal arrays, and poly(vinyl alcohol) (adhesive) is successfully constructed. It makes the information highly invisible at any angle and also achieves information encryption. With this strategy, the information can be hidden by the noniridescent structural color derived from the strong scattering effect of light from the top layer of amorphous silica sphere arrays. After wiping with ethanol or a refractive-index-matching solvent, the scattering effect vanishes and the amorphous silica sphere arrays become transparent. The reflected light of the bottom layer caused by the increasing refractive index contrast between the inside and outside of the hollow silica spheres could rapidly reveal the hidden information. The bilayer photonic crystal exhibits robust structural stability, and the hiding/revealing process is completely reversible, which shows great potential applications in steganography and information encryption.

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.

Highly Invisible Photonic Crystal Patterns Encrypted in an Inverse Opaline Macroporous Polyurethane Film for Anti-Counterfeiting Applications

Invisible photonic crystal (PC) pattern with encrypted and discoverable information is potentially useful for anti-counterfeiting labels, but it is still a big challenge to realize strict invisibility, fast response, and convenient triggering. Here, a new kind of soaking-revealed invisible PC pattern is fabricated by the regional coating of "ethylene glycol-ethanol" ink on a collapsed inverse opaline macroporous polyurethane (IOM-PU) film, followed by a quick thermal treatment. During the above process, wet heating retains the collapsed but recoverable IOM structure, but dry heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the "pattern" and the "background" with different optical responses to the solvent. In the dry state, the pattern was invisible because both the collapsed IOM-PU film and the adhesive PU film are colorless and transparent. Once the sample is soaked in ethanol-water mixtures, the invisible pattern appears immediately because only the "wet-heated" region recovers the ordered macroporous structure and shows color, which forms a significant contrast in color to the "dry-heated" region. Compared to the previously invisible PC pattern, the current material has many superior properties, such as high invisibility, large color contrast in showing, excellent recyclability, and good toughness in bending and stretching.

Patterned SiO2/Polyurethane Acrylate Inverse Opal Photonic Crystals with High Color Saturation and Tough Mechanical Strength

Patterned structural color photonic crystals (PCs) based on periodic photonic nanostructures have attracted great interest in developing high-performance sensors and other smart optical materials as well as tunable structurally colored fashion textiles. However, previously reported patterned PCs with both high color saturation and tough mechanical strength were difficult to achieve, which restricts their practical applications. Herein, arbitrarily patterned silica/polyurethane acrylate (SiO₂/PUA) inverse opal photonic crystals (IOPCs) with high color saturation and tough mechanical strength were innovatively designed and fabricated by writing with photopolymerizable PUA "ink" on a self-assembled hollow SiO₂ PC template. The high color saturation of the prepared SiO₂/PUA IOPCs originated from the high refractive index contrast between the encapsulated air-filled core and the SiO₂/PUA composite skeleton. The cross-linked flexible PUA matrix tightly warped the self-assembled hollow SiO₂ nanospheres together, endowing the obtained SiO₂/PUA IOPCs a structural color pattern with tough mechanical strength. The structural colors of SiO₂/PUA IOPCs could be finely tuned by regulating their basic parameters, and a redshift in the resultant structural color was observed due to an increase in the lattice constant when increasing the core size and/or shell thickness of the hollow SiO₂ nanospheres.

Inkjet Printing of Patterned, Multispectral, and Biocompatible Photonic Crystals

Patterning of photonic crystals to generate rationally designed color-responsive materials has drawn considerable interest because of promising applications in optical storage, encryption, display, and sensing. Here, an inkjet-printing based strategy is presented for noncontact, rapid, and direct approaches to generate arbitrarily patterned photonic crystals. The strategy is based on the use of water-soluble biopolymer-based opal structures that can be reformed with high resolution through precise deposition of fluids on the photonic crystal lattice. The resulting digitally designed photonic lattice formats simultaneously exploit structural color and material transience opening avenues for information encoding and combining functions of optics, biomaterials, and environmental interfaces in a single device.

Recent Advances in Colloidal Photonic Crystal-Based Anti-Counterfeiting Materials

Colloidal photonic crystal (PC)-based anti-counterfeiting materials have been widely studied due to their inimitable structural colors and tunable photonic band gaps (PBGs) as well as their convenient identification methods. In this review, we summarize recent developments of colloidal PCs in the field of anti-counterfeiting from aspects of security strategies, design, and fabrication principles, and identification means. Firstly, an overview of the strategies for constructing PC anti-counterfeiting materials composed of variable color PC patterns, invisible PC prints, and several other PC anti-counterfeiting materials is presented. Then, the synthesis methods, working principles, security level, and specific identification means of these three types of PC materials are discussed in detail. Finally, the summary of strengths and challenges, as well as development prospects in the attractive research field, are presented.

Different Structural Colors or Patterns on the Front and Back Sides of a Multilayer Photonic Structure

The application of photonic crystals in the field of color display and anticounterfeiting has been widely studied because of their brilliant and angle-dependent structural colors. Most of the research is focused on structural colors on the front side of photonic crystals, and both sides of the crystals usually display the same or similar optical properties. Here, multilayer photonic crystals with different structural colors or different patterns on the front and back sides were designed. In a trilayer photonic structure, an amorphous SiO₂ layer with a thickness of about 10 μm was inserted into two layers of highly ordered photonic crystals with band gaps of 625 and 470 nm. The amorphous SiO₂ layer acts as a gate to prohibit light transmission, and thereby, the structural colors of the two photonic crystals were separated. Hence, the trilayer structure shows red and blue colors on each side. Then, a light window was opened in the disordered layer using a patterned mask; thus, a pattern with a mixed color of both ordered layers was observed on each side in the window field, which was obviously different from the background color. Finally, completely different patterns on each side were also realized by building a multilayer structure. The different structural colors or patterns on each side of the photonic structures provide them with enriched color range and enhanced display or anticounterfeiting ability.

Self-assembled colloidal arrays for structural color

Structural color materials that are colloidally assembled as inspired by nature are attracting increased interest in a wide range of research fields. The assembly of colloidal particles provides a facile and cost-effective strategy for fabricating three-dimensional structural color materials. In this review, the generation mechanisms of structural colors from colloidally assembled photonic crystalline structures (PCSs) and photonic amorphous structures (PASs) are first presented, followed by the state-of-the-art and detailed technologies for their fabrication. The variable optical properties of PASs and PCSs are then discussed, focusing on their spatial long- and short-order structures and surface topography, followed by a detailed description of the modulation of structural color by refractive index and lattice distance. Finally, the current applications of structural color materials colloidally assembled in various fields including biomaterials, microfluidic chips, sensors, displays, and anticounterfeiting are reviewed, together with future applications and tasks to be accomplished.

Designing Structural-Color Patterns Composed of Colloidal Arrays

Structural coloration provides a great potential for various applications due to unique optical properties distinguished from conventional pigment colors. Structural colors are nonfading, iridescent, and tunable, which is difficult to achieve with pigments. In addition, structural color is potentially less toxic than pigments. However, it is challenging to develop structural colors because elaborate nanostructures are a prerequisite for the coloration. Furthermore, it is highly suggested the nanostructures be patterned at various length scales on a large area to provide practical formats. There have been intensive studies to develop pragmatic methods for producing structural-color patterns in a controlled manner using either colloidal crystals or glasses. This article reviews the current state of the art in the structural-color patterning based on the colloidal arrays. We first discuss common and different features between colloidal crystals and glasses. We then categorize colloidal arrays into six distinct structures of 3D opals, inverse opals, non-close-packed arrays, 2D colloidal crystals, 1D colloidal strings, and 3D amorphous arrays and study various methods to make them patterned from recent key contributions. Finally, we outline the current challenges and future perspectives of the structural-color patterns.

Enlarged Color Gamut Representation Enabled by Transferable Silicon Nanowire Arrays on Metal-Insulator-Metal Films

Artificial structural colors arising from nanosized materials have drawn much attention because of ultrahigh resolution, durability, and versatile utilizations compared to conventional pigments and dyes. However, the limited color range with current approaches has interrupted the supply for upcoming structural colorimetric applications. Here, we suggest a strategy for the widening of the color gamut by linear combination of two different resonance modes originating from silicon nanowire arrays (Si NWAs) and metal-insulator-metal nanoresonators. The enlarged color gamut representations are simply demonstrated by transferring Si NWAs embedded in a flexible polymer layer without additional treatment/fabrication. Optical simulation is used to verify the additive creation of a new resonance dip, without disturbing the original mode, and provides "predictable" color reproduction. Furthermore, we prove that the proposed structures are applicable to well-known semiconductor materials for various flexible optical devices and other colorant applications.

Structural Color Circulation in a Bilayer Photonic Crystal by Increasing the Incident Angle

Photonic crystals (PCs) have been widely applied in the anticounterfeiting field according to their easily tunable and angle-dependent structural colors. However, most studies are now focused on single-layer PCs assembled from monodisperse colloidal spheres, which have only one bandgap. Here, we prepared bilayer photonic crystal films by choosing 250 and 330 nm silica spheres as the bottom and top layer, respectively. The effect of the incident angle on the bandgap of PCs was investigated and the results showed that the bandgap of the bilayer PCs was incident angle dependent-the structure exhibited two strong bandgaps within small incident angles, while as the incident angle increases, both the bandgaps blue-shifted and more importantly, the bandgap of the bottom layer disappeared with a further increase in the incident angle. Furthermore, with the delicate design of the thickness of the top layer, this bilayer structure selectively displayed the structural colors of the bottom layer, overlap colors of both the top and the bottom layer, and the color of only the top layer, respectively. By changing the incident angle, the color circulation from green to magenta, orange, yellow, and green again was realized. The realization of the controllable color tunability further motived us towards the patterning of the bilayer PCs, which showed promising potential in the anticounterfeiting field.

Solution-Processable Nanocrystal-Based Broadband Fabry-Perot Absorber for Reflective Vivid Color Generation

Structural reflective colors based on Fabry-Perot (F-P) cavity resonances have attracted tremendous interest for diverse applications, such as color decoration and printing, display, and imaging devices. However, the asymmetric F-P cavity-based reflective colors proposed to date have low color purity and have difficulty to realize a desired vivid color because of a narrow absorption band characteristic in the visible light region. Here, a solution-processed, F-P ultra-broadband light absorber is newly proposed using a high lossy nanoporous material for vivid color generation. An asymmetric metal-insulator-metal structure consists of a high lossy nanoporous metallic film with coupled silver nanocrystals (Ag NCs) as the top layer. The absorbers not only increase the maximum absorption intensity up to ∼98% but also widen the bandwidth by 300 nm, resulting in high color purity in micrometer-scale pixels. Furthermore, the solution-based absorber shows potential to realize a high-resolution display pixel and anticounterfeiting devices having mechanical flexibility using the inkjet printing technology.

Facile Fabrication of Amorphous Photonic Structures with Non-Iridescent and Highly-Stable Structural Color on Textile Substrates

Amorphous photonic structures with non-iridescent and highly-stable structural color were fabricated via a simple one-step spray-coating technique. With this strategy, the obtained films on textile substrates presented short-ordered and amorphous photonic structures (APSs) similar to the amorphous nanostructures of avian feathers. The structural color presented the same hue when viewed at different angles and could be well controlled by varying the diameters of the SiO₂ nanospheres. The prepared fabrics with structural color exhibited high color stability due to stability in both the assembled physical structure and the refractive index. The high stability of the assembled physical structure was attributed to the cementing effect of Poly(methylmethacrylate-butylacrylate) P(MMA-BA) existing between textile substrate and SiO₂ nanospheres and among SiO₂ nanospheres, while the high stability in the refractive index was contributed by the liquid-resistance achieved by both the surface roughness and the low-surface-energy of the as-sprayed APSs. With the resistances to external forces and liquid invasion, the non-iridescent brilliant structural color of the as-prepared fabrics could be kept steady. In this study, an approach of fabricating APSs with non-iridescent and stable structural color was established to enhance its potential application in structural coloration of textiles, and other color-related smart textiles.

High-Performance and Multifunctional Colorimetric Humidity Sensors Based on Mesoporous Photonic Crystals and Nanogels

Colorimetric sensors, as a key branch of the application of photonic crystals (PCs), brings enthusiasm to scientists to do research. Here, simple mesoporous and structurally colored one-dimensional photonic crystals (1DPCs) constructed by alternating assembly of poly(acrylamide- N,N'-methylene bis(acrylamide)) (P(AM-MBA)) nanogels and TiO₂ nanoparticles are reported as high-performance colorimetric humidity sensors. The sensors with bright colors display rapid response to relative humidity (RH) change and reach sensing balance in 0.5 s. By varying RH from 47.0% to 89.3%, stopband of a sensor changes from 426 to 668 nm, almost spanning the whole visible range. Meanwhile, visual sensing of RH possesses good reversibility and repeatability. Moreover, the sensors with delicate patterns are facilely fabricated by partial UV photodegradation of the polymer layers with nano TiO₂ as catalyst. The delicate patterns and backgrounds show different colors and change color simultaneously and quickly by varying the ambient humidity. Accurate QR code pattern is also realized on the PC sensor; it is found successful reading of the data is only achieved by increasing RH to realize high color contrast between the code and background. Given their excellent properties, the porous hybrid PCs are promising as high-performance humidity sensors with potential display, decoration, information-storage, and encryption functions.

Structurally colored polymer films with narrow stop band, high angle-dependence and good mechanical robustness for trademark anti-counterfeiting

The photonic stop bands of colloidal crystals appear as structural colors, which are potentially useful for display devices, colorimetric sensors, optical filters, paints, and photonic papers. However, low durability and pale colors caused by the undesired scattering of light seriously limit their practical applications. In this article, a polydimethylsiloxane (PDMS)/photonic crystal (PC)/PDMS sandwich structure was designed as a free standing structural colored film with good durability and brilliant color. The monodispersed polystyrene (PS) spheres were self-assembled on the hydrophobic PDMS surface to facilitate the integrity of the assembled structure and then, PDMS with a refractive index of 1.41 was filled in the gaps between the PS spheres (nPS = 1.59), replacing air (nair = 1). The surface was finally covered with a thin layer of PS PCs, forming a continuous and free standing PC film. The continuous feature of the composite PC can greatly improve their mechanical properties. At the same time, the lower index contrast results in narrow reflection peaks for the composite films, which indicates that higher color purity and brightness could be achieved. Clearly distinguished, vivid structural colors can be observed between red to green or green to blue by tuning the viewing angle from 5° to 50° for films composed of PS spheres with diameters of 247 nm or 209 nm, respectively. They can also be easily patterned by spraying methods and embedded as a trademark on clothes. Patterns with different structural colors at different angle can be clearly be observed under sunlight, which makes them potentially useful as security materials.

Bioinspired Stimuli-Responsive Color-Changing Systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect.

Inkjet-Printed Multiwavelength Thermoplasmonic Images for Anticounterfeiting Applications

Inkjet printing of thermoplasmonic nanoparticles enables instantaneous, large-area heat pattern generation upon light illumination from distance. By printing multiple metal nanoparticles of different shapes overlaid, we can fabricate multiwavelength thermoplasmonic images, which generate different heat patterns from a single printed image depending on the wavelength choice of light. In this work, we propose a novel multiwavelength thermoplasmonic image printing process that can be used for anticounterfeit technology. With this technology, "printed thermoplasmonic labels" allow fully secured anticounterfeit inspection procedure. Input stimulus of near-infrared or infrared light illumination and output signal reading of thermal patterns can be both completely invisible. Wavelength selective photothermal effect also enables the encryption of the contained information, which adds more complexity and thus higher security.