Designing Responsive Photonic Crystal Patterns by Using Laser Engraving

Prediction model for laser marking colors based on color mixing

The current techniques for coloring surfaces using lasers necessitate the identification of numerous laser marking parameters, which is a laborious process. Furthermore, the quantitative analysis of generating a wide variety of colors through fewer sets of laser marking parameters is a huge challenge. This work employs a nanosecond laser to generate mixed structural colors from micro-nano structures on the surface of stainless steel in order to address these issues. Additionally, the color mixing principle is investigated in relation to these micro-nano structures. On this basis, the spectral reflectance of the primary color is mapped to the linear mixed color space, and the linear mixed color space is constructed by minimizing the linear deviation function. In this space, a precise mathematical model for color prediction is developed, which effectively captures the correlation between the primary color and the resulting mixed color. Four primary colors are created using four sets of laser marking parameters. Mixing these primary colors in varying proportions can achieve more than 100 new tones with rich colors. The average color difference Δ E a b∗ and Δ E00∗ are 1.98 and 1.80, respectively. By utilizing this model to adjust the proportion of primary colors in each subgraph, an image with vibrant and rich colors is generated, thereby achieving the implementation of a structural color image based on mixed colors.

An investigation into multidimensional information encryption through structural color in electrically responsive subwavelength gratings

In the vanguard of safeguarding data integrity, optical encryption and anti-counterfeiting measures are indispensable. Structural color, with its inherent optical exclusivity, including tunable chromaticity and intricate high-resolution patterning, stands at the forefront of this domain. Despite its promise, the proliferation of structural color technologies in anti-counterfeiting applications is curtailed by the exorbitant production costs and the current limitations in information capacity and security. Addressing these constraints, our study delineates a novel encryption paradigm that interlaces color and digital data within a subwavelength grating matrix. This synergy is fortified by a tri-layered encryption schema, amalgamating electrical response signatures, inherent optical attributes, and the robust RSA algorithm, thereby elevating the information capacity exponentially to 10n and reinforcing multi-faceted security throughout transmission. Our approach heralds a new era in the realm of high density, secure information storage.

Self-regulating bioinspired supramolecular photonic hydrogels based on chemical reaction networks for monitoring activities of enzymes and biofuels

Inspired by the way many living organisms utilize chemical/biological reactions to regulate their skin and respond to stimuli in the external environment, we have developed a self-regulating hydrogel design by incorporating chemical reaction networks (CRNs) into biomimetic photonic crystal hydrogels. In this hydrogel system, we used host-guest supramolecular non-covalent bonds between beta-cyclodextrin (β-CD) and ferrocene (Fc) as partial crosslinkers and designed a CRN involving enzyme-fuel couples of horseradish peroxidase (HRP)/H₂O₂ and glucose oxidase (GOD)/d-glucose, by which the responsive hydrogel was transformed into a glucose-driven self-regulating hydrogel. Due to the biomimetic structural color in the hydrogel, the progress of the chemical reaction was accompanied by a change in the color of the hydrogel. Based on this principle, the designed supramolecular photonic hydrogel (SPH) can not only achieve naked-eye detection of H₂O₂ and glucose concentrations with the assistance of a smartphone but also monitor the reactions of HRP and GOD enzymes and determine their activity parameters. The sensitivity and stability of the sensor have been proven. In addition, due to the reversibility of the chemical reaction network, the sensor can be reused, thus having the potential to serve as a low-cost point-of-care sensor. The SPH was ultimately used to detect glucose in human plasma and H₂O₂ in liver tumor tissue. The results are comparable with commercial assay kits. By redesigning the chemical reaction network in the hydrogel, it is expected to be used for detecting other enzymes or fuels.

Co-Self-Assembly of Amphiphiles into Nanocomposite Hydrogels with Tailored Morphological and Mechanical Properties

As one of the most amazing aspects of life, all living organisms are formed by self-assembly, a fundamental biological design process in which ordered nanostructures are assembled from small parts. For example, most of the biological tissues contain structurally soft and hard parts that are usually hierarchically organized at nano or micro levels to achieve specific functions. Hydrogels are one of the most promising soft materials owing to their potential applications in building of biological tissues and stretchable sensors. In this work, a series of hydrogels are synthesized through the co-self-assembly of two types of amphiphiles in their aqueous solution prior to polymerization. Soft and hard parts with nanostructures of different order parameters are incorporated into the hydrogels. The hydrophilic segment (as soft phases) of the polymer network provides water absorption, fluid flow, and softness, whereas the hydrophobic segment (as hard phases) provides strength and tearing and fracture resistance. Appropriate soft/hard nanostructures and their interfaces allow for the tailoring of the desired morphological and mechanical properties, including a different wetting ability, toughness, energy dissipation, self-recovery, and fracture resistance arising from their nanostructures. This work provides insights into the design of nanostructured anisotropic hydrogels with controlled morphological and mechanical properties.

Solvent-Responsive Invisible Photonic Patterns with High Contrast for Fluorescence Emission Regulation and Anti-Counterfeiting

Invisible photonic patterns (IPPs) are photonic materials that can display hidden patterns under external stimulation and are attractive in anti-counterfeiting devices and information storage. In this work, we report a solvent-responsive invisible photonic pattern (SRIPP) with high contrast by polymerizing two monomers of acrylamide (AAm) and poly(ethylene glycol) methacrylate (PEGMA) with different solubility parameters in different regions of poly(hydroxyethyl methacrylate) photonic gels. The two regions with different solvent responsiveness can shrink and swell in the same environment, thus causing the colors of different regions of photonic gel to shift in opposite directions from the initial state. As a result, the contrast of photonic patterns is significantly improved, increasing naked-eye visualization. In addition, by introducing fluorescent substances into the photonic gel and adjusting the photonic band gap (PBG) of photonic gels, we realize the regulation of fluorescence emission and display of fluorescence patterns by utilizing different PBGs on the SRIPP. Dynamic solvent responsiveness patterns and fluorescence patterns are integrated into a photonic gel, showing great potential in information storage and multiple-mode anti-counterfeiting applications.

Supramolecular DNA Photonic Hydrogels for On-Demand Control of Coloration with High Spatial and Temporal Resolution

Hydrogels embedded with periodic arrays of nanoparticles display a striking photonic crystal coloration that may be useful for applications such as camouflage, anticounterfeiting, and chemical sensing. Dynamically generating color patterns requires control of nanoparticle organization within a polymer network on-demand, which is challenging. We solve this problem by creating a DNA hydrogel system that shows a 50 000-fold decrease in modulus upon heating by ∼10 °C. Magnetic nanoparticles entrapped within these DNA gels generate a structural color only when the gel is heated and a magnetic field is applied. A spatially controlled photonic crystal coloration was achieved by photopatterning with a near-infrared illumination. Color was "erased" by illuminating or heating the gel in the absence of an external magnetic field. The on-demand assembly technology demonstrated here may be beneficial for the development of a new generation of smart materials with potential applications in erasable lithography, encryption, and sensing.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

H2O- and ethanol concentration-responsive polymer/gel inverse opal photonic crystal

Responsive photonic crystals have attracted much attention due to their strong capability to manipulate the propagation of light in the visible region, but it is still a big challenge to invisibility and mechanical stability. Here, the novel Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals, which have high mechanical stability and can release visible patterns after wetting with water, are discussed. The Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals are also responsive to the concentration of ethanol, and the structural color response times increase with increasing ethanol concentration. This design uses the selective infiltration, hydrogen bonding and capillary action of solvent to realize the spectral diversity of reflectance. Owing to the high polarity and hydrogen bonding ability of carboxyl groups, water molecules are adsorbed easily by the poly(acrylic acid) gel. Subsequently, the encrypted information is decrypted due to the redshift of the structural color. Because of its lower polarity and hydrogen bonding ability relative to water, ethanol can impede the absorption of solvent by gel. Therefore, the ethanol concentration can be identified based on the structural color response time. Furthermore, reliable information decryption methods make Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals potentially uesful as trusted encryption devices.

Laser-Induced Patterned Photonic Crystal Heterostructure for Multimetal Ion Recognition

In this work, a new method of direct laser writing patterned photonic crystal heterostructure on a glass surface is proposed. A multi-heterostructure photonic crystal (MHPC) is predeposited on the glass surface and then the laser spot is focused on it and moves according to the set program, leading to the formation of patterned MHPC. Scanning electron microscopy (SEM) and finite element simulation show that the patterning is caused by the local thermal annealing of the polymer colloidal spheres through the thermal conduction effect of the substrate on the laser energy. The patterned area presents a function of the water confinement effect and can be used as a high-performance droplet analysis chip. By integrating the patterned MHPC array and seven fluorescent dyes, nine metal ions can be successfully recognized and discriminated. This approach is quite facile and fast for designing colloidal photonic crystals with controllable patterns. Moreover, it is of considerable significance for the practical application of photonic crystal heterostructure in the detection, sensing, anti-counterfeiting, and display fields.

A two-dimensional photonic crystal hydrogel biosensor for colorimetric detection of penicillin G and penicillinase inhibitors

A simple penicillinase functionalized two-dimensional photonic crystal hydrogel (2DPPCH) biosensor was developed for colorimetric detection of penicillin G and penicillinase inhibitors. The penicillinase can specifically recognize penicillin G and catalyze it to produce penicilloic acid, which decreases the pH of the hydrogel microenvironment and shrinks the pH-sensitive hydrogel. The particle spacing decrease of the 2D photonic crystal array induced by the hydrogel shrinkage further causes a blue-shift in the diffraction wavelength. While the hydrolysis reaction is repressed upon treatment with clavulanate potassium (a kind of penicillinase inhibitor), no significant change in the diffraction wavelength is found. The detection of targets can be achieved by measuring the Debye diffraction ring diameter or observing the structural color change in the visible region. The lowest detectable concentrations for penicillin G and clavulanate potassium are 1 μM and 0.1 μM, respectively. Moreover, the 2DPPCH is proved to exhibit high selectivity and an excellent regeneration property, and it shows satisfactory performance for penicillin G analysis in real water samples.

Ultrafast Yb-Doped Fiber Laser Using Few Layers of PdS2 Saturable Absorber

Two-dimensional (2D) transition metal dichalcogenide (TMD) materials have exceptional optoelectronic and structural properties, which allow them to be utilized in several significant applications in energy, catalyst, and high-performance optoelectronic devices. Among other properties, the nonlinear optical properties are gaining much attention in the research field. In this work, a unique pentagonal TMD material, palladium disulfide (PdS2), is employed as a saturable absorber (SA) in an ytterbium-doped fiber (YDF) laser cavity and mode-locked laser pulse is generated. At first, liquid phase exfoliation is performed to prepare PdS2 nanoflakes. Afterward, the PdS2-nanoflakes solution was incorporated in the side-polished fiber (SPF) to form SPF-based PdS2-SA. By utilizing this SA, a highly stable mode-locked laser pulse is realized at pump power of 160 mW, which has a center wavelength of 1033 nm and a 3-dB spectral bandwidth of 3.7 nm. Moreover, the pulse duration, maximum power output and corresponding single-pulse energy were determined as 375 ps, 15.7 mW and 0.64 nJ, respectively. During the experiment, the mode-locked pulse remained stable till the pump power reached a value of 400 mW and, for the regulation of power, the slope efficiency is calculated at about 4.99%. These results indicate that PdS2 material is a promising nonlinear optical material for ultrafast optical applications in the near-infrared (NIR) region.

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.

Gold clay from self-assembly of 2D microscale nanosheets

Nature has always demonstrated incredible ability to create amazing materials such as soft clay which are built from nanoplatelet packing structures. It is challenging to produce artificial clays owing to the difficulty in obtaining large volume fractions of nanoplatelets and the lack of structural control in layer-by-layer packing. Here, single-crystalline Au nanosheets are synthesized by controlled growth in the bilayer membranes of succinic acid surfactants. Then, a self-assembly strategy is used to make {111}-oriented gold nanostructures at the liquid-liquid interface. The stiffness of the nanosheet assemblies are six orders of magnitude softer than bulk gold. The Au nanosheet aggregates show high plasticity and deformable into macroscale free-standing metallic architectures. They show a stress/strain-dependent conductivity owing to morphological changes. Our study provides valuable insights on the chemical synthesis of 2D nanostructures as well as for the self-assembly strategy on fabrication of mouldable metals for producing free-standing metallic architectures with microscale resolutions.

Ratiometric sensors with selective fluorescence enhancement effects based on photonic crystals for the determination of acetylcholinesterase and its inhibitor

Ratiometric fluorescent sensors are powerful tools for quantitative analyses.