The visual appearances of disordered optical metasurfaces

Semimetal/Substrate Cavities Enabling Industrial Materials for Structural Coloring

Color coatings are essential for the identification and safety of everyday objects as well as for the protection of surfaces from deterioration, in addition to their well-known use for enhancing their aesthetic appeal. However, conventional dyes and pigments are a major source of contamination and degrade easily over time. Structural coloring is a sustainable alternative capable of producing high-quality colors with nanometric structures. Nonetheless, many approaches to structural coloring rely on lithography or expensive back-reflectors made from noble metals. In this study, we approach surface coloring using lightweight, sustainable, and scalable optical coatings with subwavelength thickness. This method allows industrial surfaces to function as active elements in the color-generating structure, eliminating the need for metallic mirrors. The design is based on a semimetal/substrate cavity (SSC), directly deposited onto the surface to be colored. As a proof of concept, we designed and fabricated SSCs on silicon and stainless steel substrates, using ultrathin films of bismuth (Bi) and aluminum oxide (Al2O3) as the cavity components. These SSCs display vivid, well-defined colors with excellent angular stability for a cavity. Moreover, the SSC design can be adapted with other semimetal/dielectric combinations and offers an efficient, daylight-friendly, sustainable, and lightweight solution for functional coloration of everyday objects as well as components for industrial and technical applications.

Disordered gold nanoislands-dielectric-metal plasmon reflector for polarization-sensitive color display, humidity sensor and optical memory

The interaction between ultrafast, tightly focused lasers and materials has garnered significant interest owing to its distinctive properties. In this study, we present a versatile methodology for the fabrication of tunable plasmonic nanostructures by employing a disordered gold nanoisland-dielectric-metal configuration, achieved through femtosecond laser printing. By reshaping the gold nanoislands and reconfiguring them into nanograting-like structures, the orientation of these nanostructures is influenced by the polarization of the femtosecond laser light, leading to controllable plasmon resonance and polarization-sensitive color display. Furthermore, the system demonstrates a significant sensitivity to environmental humidity, as indicated by water adsorption, which leads to marked color changes. The hotspots generated through plasmonic coupling among disordered gold nanoislands significantly enhance polarization-multiplexed optical data storage, characterized by its high quality and low energy consumption. This experimental demonstration promotes the advancement of sophisticated optical devices for plasmonic color printing with tailored characteristics, thereby offering economical solutions for applications in optoelectronics and sensing.

Rapid fabrication of tunable structural color patterns by spray-coating

Structural color, a color generated based on physical principles, has broad applications such as displays, optical sensors, and anti-counterfeiting. Traditional methods for producing structural colors are often complex and time-consuming, whereas spray-coating colloidal self-assembly offers a simple and controllable alternative. However, due to the high-pressure atomization process, colloidal inks often form amorphous photonic structures (APSs), making it challenging to precisely control the assembly of colloidal particles on substrates to achieve ordered structures. By rationally designing the composition of colloidal mixed solutions, controlling particle concentration, and adjusting evaporation temperatures, it is possible to effectively regulate the assembly of colloidal particles and obtain angle-dependent iridescent colors. This work proposes a simple spray-coating process that enables the control of both ordered and disordered structures, with tunable optical properties, suitable for colloidal patterning on various substrates. This method not only simplifies the fabrication of photonic crystals (PCs) but also has broad potential, particularly in anti-counterfeiting, where it enables the creation of hard-to-replicate structured patterns with enhanced security.

Bioinspired Intelligent Ferrofluid: Old Magnetic Material with New Optical Properties

Fine-tuning of microstructures enables the modulation of optical properties at multiple scales from metasurfaces to geometric optics. However, a dynamic system with a significant deformation range and topology transformation remains challenging. Owing to its magnetic controllability, ferrofluid has proven to be fertile ground for a wide range of engineering and technological applications. Here, we demonstrate a series of intelligent optical surfaces based on ferrofluid, through which multiple optical functions inspired by nature can be realized. The tunability is based on the topological transition of the ferrofluid between the flat state and cone array upon magnetic actuation. In the visible band, a tunable visual appearance is realized. In the mid-infrared band, active manipulation of reflection is realized based on the gradient-index (GRIN) effect. This system also features low latency response and straightforward manufacturability, and it may open opportunities for novel technologies such as smart windows, color displays, infrared camouflage, and other infrared-related technologies.

Fast generation of spectrally shaped disorder

Media with correlated disorder display unexpected transport properties, but it is still a challenge to design structures with desired spectral features at scale. In this work, we introduce an optimal formulation of this inverse problem by means of the nonuniform fast Fourier transform, thus arriving at an algorithm capable of generating systems with arbitrary spectral properties, with a computational cost that scales O(NlogN) with system size. The method is extended to accommodate arbitrary real-space interactions, such as short-range repulsion, to simultaneously control short- and long-range correlations. We thus generate the largest-ever stealthy hyperuniform configurations in 2d (N=10^{9}) and 3d (N>10^{7}) and demonstrate the flexibility of the approach by generating structures with designed spectral features at scale. By an Ewald sphere construction we link the spectral and optical properties at the single-scattering level and show that stealthy hyperuniform structures generically display transmission gaps, providing a concrete example of fine-tuning of a physical property. We also show that large 3d power-law hyperuniformity in particle packings leads to single-scattering properties nearly identical to those of simple hard spheres. Finally, we demonstrate generalizations of the approach to impose features in either continuous or discrete real space, using constraints in either continuous or discrete reciprocal space. In particular, enforcing large spectral power at peaks with the right symmetry leads to the nondeterministic generation of quasicrystalline structures in 2d and 3d. This technique should become an essential tool to embed, and understand the role of, long-range correlations in disordered metamaterials.

Control over the spatial correlation of perforations in silica thin films as a function of solution conditions

A perforated silica layer with structural correlation is engineered using sol-gel chemistry, applied to large-scale flat and curved surfaces. The anion(s) used in the preparation give tailored spatial correlation, and control over perforation size and density. Surface structuration is rapidly and reproducibly created using water and salts as inexpensive and ecofriendly reagents.

Ultra-broadband microwave absorber based on disordered metamaterials

Metamaterial absorption technology plays an increasingly important role in military and civilian sectors, serving crucial functions in communication, radar technology, and electromagnetic cloaking. However, traditional metamaterial absorbers are predominantly composed of periodic structures, thus limiting their absorption bandwidth, polarization, and angular flexibility. This study employs disordered structures, utilizing their randomness and diversity, to optimize and enhance the performance of periodic structure metamaterial absorbers. Building upon a well-designed periodic perfect absorption structure, a uniform distribution function is introduced to analyze the effects of positional and size disorder on the absorptive properties of the metamaterial. The mechanisms of the disorder are further investigated through simulation analysis. Subsequently, an innovative approach based on disorder engineering for broadband enhancement of metamaterial absorbers is proposed. Numerical simulation results and experimental validations demonstrate that absorbers constructed using this method significantly broaden the absorption bandwidth while maintaining excellent angular and polarization stability. This research not only offers a new method for the design and performance optimization of metamaterial absorbers but also provides a theoretical foundation for the development of metamaterial self-assembly techniques.

Meta-Structured Covalent Organic Framework Nanocoatings with Active and Angle-Independent Structural Coloration

High-performance multifunctional nanocoatings not only protect and enhance substrate materials but also offer additional functionalities. This demands a sophisticated coordination of the coating's inherent properties and microstructural features. Here, a multifunctional active nanocoating via meta-structural engineering of covalent organic framework (COF) deposition materials is presented. This COF nanocoating, characterized by well-defined micropores (1-2 nm), meta-structured textures (30-300 nm), tailored thickness (100-300 nm), and good uniformness, showcases a unique combination of angle-independent structural coloration and ultrafast responsiveness to gaseous stimuli. Remarkably, it demonstrates good compatibility with a wide range of inert substrate materials, from rigid ones like glass and metal to flexible elastomers and nanomaterial films of various shapes and sizes. This versatility enables the facile development of devices that can optically report information about their environments. Examples include chemically active coatings with ultrafast (≈10 ms) color-changing behaviors and programmable actuation behaviors upon exposure to gaseous stimuli, and mechanically active coatings that can detect substrate strain up to 50% yet maintain structural robustness and consistent coloration hue. It is believed that meta-structural engineering of COF nanocoatings on inert substrates can enable them to respond to environmental stimuli, potentially indicating a new trend in developing multifunctional materials and smart devices.

A water-soluble label for food products prevents packaging waste and counterfeiting

Sustainability, humidity sensing and product origin are important features of food packaging. While waste generated from labelling and packaging causes environmental destruction, humidity can result in food spoilage during delivery and counterfeit-prone labelling undermines consumer trust. Here we introduce a food label based on a water-soluble nanocomposite ink with a high refractive index that addresses these issues. By patterning the nanocomposite ink using nanoimprint lithography, the resultant metasurface shows bright and vivid structural colours. This method makes it possible to quickly and inexpensively create patterns on large surfaces. A QR code is also developed that can provide up-to-date information on food products. Microprinting hidden in the QR code protects against counterfeiting, cannot be physically detached or replicated and may be used as a humidity indicator. Our proposed food label can reduce waste while ensuring customers receive accurate product information.

Matte surfaces with broadband transparency enabled by highly asymmetric diffusion of white light

The long-standing paradox between matte appearance and transparency has deprived traditional matte materials of optical transparency. Here, we present a solution to this centuries-old optical conundrum by harnessing the potential of disordered optical metasurfaces. Through the construction of a random array of meta-atoms tailored in asymmetric backgrounds, we have created transparent matte surfaces that maintain clear transparency regardless of the strength of disordered light scattering or their matte appearances. This remarkable property originates in the achievement of highly asymmetric light diffusion, exhibiting substantial diffusion in reflection and negligible diffusion in transmission across the entire visible spectrum. By fabricating macroscopic samples of such metasurfaces through industrial lithography, we have experimentally demonstrated transparent windows camouflaged as traditional matte materials, as well as transparent displays with high clarity, full color, and one-way visibility. Our work introduces an unprecedented frontier of transparent matte materials in optics, offering unprecedented opportunities and applications.

Brochosome-inspired binary metastructures for pixel-by-pixel thermal signature control

Microscale thermal signature control using incoherent heat sources remains challenging, despite recent advancements in plasmonic materials and phase-change materials. Inspired by leafhopper-generated brochosomes, we design binary metastructures functioning as pixel twins to achieve pixelated thermal signature control at the microscale. In the infrared range, the pixel twins exhibit distinct emissivities, creating thermal counterparts of "0-1" binary states for storing and displaying information. In the visible range, the engineered surface morphology of the pixel twins ensures similar scattering behaviors. This renders them visually indistinguishable, thereby concealing the stored information. The brochosome-like pixel twins are self-emitting when thermally excited. Their structure-enabled functions do not rely on the permittivities of specific materials, which distinguishes them from the conventional laser-illuminated plasmonic holographic metasurfaces. The unique combination of visible camouflage and infrared display offers a systemic solution to microscale spatial control of thermal signatures and has substantial implications for optical security, anticounterfeiting, and data encryption.

Experimental Investigation of the Thermal Emission Cross Section of Nanoresonators Using Hierarchical Poisson-Disk Distributions

Effective cross sections of nano-objects are fundamental properties that determine their ability to interact with light. However, measuring them for individual resonators directly and quantitatively remains challenging, particularly because of the very low signals involved. Here, we experimentally measure the thermal emission cross section of metal-insulator-metal nanoresonators using a stealthy hyperuniform distribution based on a hierarchical Poisson-disk algorithm. In such distributions, there are no long-range interactions between antennas, and we show that the light emitted by such metasurfaces behaves as the sum of cross sections of independent nanoantennas, enabling direct retrieval of the single resonator contribution. The emission cross section at resonance is found to be on the order of λ_{0}^{2}/3, a value that is nearly 3 times larger than the theoretical maximal absorption cross section of a single particle, but remains smaller than the maximal extinction cross section. This measurement technique can be generalized to any single resonator cross section, and we also apply it to a lossy dielectric layer.

Massively Scalable Self-Assembly of Nano and Microparticle Monolayers via Aerosol Assisted Deposition

An extremely rapid process for self-assembling well-ordered, nano, and microparticle monolayers via a novel aerosolized method is presented. The novel technique can reach monolayer self-assembly rates as high as 268 cm2 min-1 from a single aerosolizing source and methods to reach faster monolayer self-assembly rates are outlined. A new physical mechanism describing the self-assembly process is presented and new insights enabling high-efficiency nanoparticle monolayer self-assembly are developed. In addition, well-ordered monolayer arrays from particles of various sizes, surface functionality, and materials are fabricated. This new technique enables a 93× increase in monolayer self-assembly rates compared to the current state of the art and has the potential to provide an extremely low-cost option for submicron nanomanufacturing.

Thermal-Assisted Multiscale Patterning of Nonplanar Colloidal Nanostructures for Multi-Modal Anti-Counterfeiting

Nanotransfer printing of colloidal nanoparticles is a promising technique for the fabrication of functional materials and devices. However, patterning nonplanar nanostructures pose a challenge due to weak adhesion from the extremely small nanostructure-substrate contact area. Here, the study proposes a thermal-assisted nonplanar nanostructure transfer printing (NP-NTP) strategy for multiscale patterning of polystyrene (PS) nanospheres. The printing efficiency is significantly improved from ≈3.1% at low temperatures to ≈97.2% under the glass transition temperature of PS. Additionally, the arrangement of PS nanospheres transitioned from disorder to long-range order. The mechanism of printing efficiency enhancement is the drastic drop of Young's modulus of nanospheres, giving rise to an increased contact area, self-adhesive effect, and inter-particle necking. To demonstrate the versatility of the NP-NTP strategy, it is combined with the intaglio transfer printing technique, and multiple patterns are created at both micro and macro scales at a 4-inch scale with a resolution of ≈2757 pixels per inch (PPI). Furthermore, a multi-modal anti-counterfeiting concept based on structural patterns at hierarchical length scales is proposed, providing a new paradigm of imparting multiscale nanostructure patterning into macroscale functional devices.

Topological Learning for the Classification of Disorder: An Application to the Design of Metasurfaces

Structural disorder can improve the optical properties of metasurfaces, whether it is emerging from some large-scale fabrication methods or explicitly designed and built lithographically. For example, correlated disorder, induced by a minimum inter-nanostructure distance or by hyperuniformity properties, is particularly beneficial for light extraction. Inspired by topology, we introduce numerical descriptors to provide quantitative measures of disorder with universal properties, suitable to treat both uncorrelated and correlated disorder at all length scales. The accuracy of these topological descriptors is illustrated both theoretically and experimentally by using them to design plasmonic metasurfaces with controlled disorder that we then correlate to the strength of their surface lattice resonances. These descriptors are an example of topological tools that can be used for the fast and accurate design of disordered structures or as aid in improving their fabrication methods.

Resonant-Cavity-Enhanced Electrochromic Materials and Devices

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Revisiting the Design Strategies for Metasurfaces: Fundamental Physics, Optimization, and Beyond

Over the last two decades, the capabilities of metasurfaces in light modulation with subwavelength thickness have been proven, and metasurfaces are expected to miniaturize conventional optical components and add various functionalities. Herein, various metasurface design strategies are reviewed thoroughly. First, the scalar diffraction theory is revisited to provide the basic principle of light propagation. Then, widely used design methods based on the unit-cell approach are discussed. The methods include a set of simplified steps, including the phase-map retrieval and meta-atom unit-cell design. Then, recently emerging metasurfaces that may not be accurately designed using unit-cell approach are introduced. Unconventional metasurfaces are examined where the conventional design methods fail and finally potential design methods for such metasurfaces are discussed.

Computational visualization of semi-transparent metallic thin films with roughness

We model the visual appearance of thin, semi-transparent metallic films coated on arbitrary three-dimensional substrates, incorporating effects including nanoscale film roughness, microscale substrate roughness, and source of light. Film reflectance is modeled by combining electrodynamic simulations with the Schlick approximation, which is adapted and validated to describe the color appearance of thin semi-transparent metallic films with nanoscale, subwavelength roughness. Diffuse scattering originating from microscale roughness of the substrate and partial reflectance is described by a microfacet model. Photorealistic rendered images generated by our approach are qualitatively compared to photographs of fabricated thin-film samples under similar lighting conditions. We render images of semi-transparent metallic films as a function of film thickness, multilayer composition, substrate type, nanoscale film roughness, microscale substrate roughness, and environmental lighting, yielding physically plausible results consistent with previously reported observations.

Tailoring Iridescent Visual Appearance with Disordered Resonant Metasurfaces

The nanostructures of natural species offer beautiful visual appearances with saturated and iridescent colors, and the question arises whether we can reproduce or even create unique appearances with man-made metasurfaces. However, harnessing the specular and diffuse light scattered by disordered metasurfaces to create attractive and prescribed visual effects is currently inaccessible. Here, we present an interpretive, intuitive, and accurate modal-based tool that unveils the main physical mechanisms and features defining the appearance of colloidal disordered monolayers of resonant meta-atoms deposited on a reflective substrate. The model shows that the combination of plasmonic and Fabry-Perot resonances offers uncommon iridescent visual appearances, differing from those classically observed with natural nanostructures or thin-film interferences. We highlight an unusual visual effect exhibiting only two distinct colors and theoretically investigate its origin. The approach can be useful in the design of visual appearance with easy-to-make and universal building blocks having a large resilience to fabrication imperfections and potential for innovative coatings and fine-art applications.

VO2 particle-based intelligent metasurface with perfect infrared emission for the spacecraft thermal control

Thermochromism film can automatically adjust its emittance without additional energy consumption, which shows great prospect in the application of spacecraft thermal control. However, it is still challenging to achieve a large infrared emittance at a high temperature and emittance tunability of the thermochromism film. In this work, we propose a V O ₂ particle-based intelligent metasurface for spacecraft thermal control, which consists of a square lattice array of hollow spheroidal V O ₂ particles on Au substrate. The metasurface with a V O ₂ particle having a large aspect ratio (∼10) displays perfect emission throughout the entire mid-infrared spectral range. The emittance tunability can exceed 0.63 with total normal emittance of 0.85. The underlying mechanisms involved in the metasurface are attributed to particle-dependent scattering, by which the infrared emittance is dramatically enhanced for the metallic state and restricted for the dielectric state. In addition, the infrared emittance at a high temperature and emittance tunability of the metasurface remain large for incident angles up to 60°. To the best of our knowledge, this work proposes the first thermochromism film structure with perfect infrared emission, which could accelerate the development and practical application of the thermochromic film in the field of spacecraft.

Photo-induced stress relaxation in reconfigurable disulfide-crosslinked supramolecular films visualized by dynamic wrinkling

Stress relaxation in reconfigurable supramolecular polymer networks is strongly related to intermolecular behavior. However, the relationship between molecular motion and macroscopic mechanics is usually vague, and the visualization of internal stress reflecting precise regulation of molecules remains challenging. Here, we present a strategy for visualizing photo-driven stress relaxation induced by infinitesimal perturbations in the intermolecular exchange reaction via reprogrammable wrinkle patterns. The supramolecular films exhibit visible changes in microscopic wrinkle topography through ultraviolet (UV)-induced dynamic disulfide exchange reaction. In accordance with the trans-scale theoretical models, which quantitatively evaluate the chemical-dependent mechanical stresses in the supramolecular network, the unexposed disordered wrinkles evolved into highly oriented patterns and underwent subsequent mutations after thermal treatment. The stress-sensitive wrinkle macro-patterns can be repetitively written/erased through network topology rearrangement using different stimuli. This strategy provides an approach for visualizing and understanding the molecular behavior from dynamic chemistry to mechanical changes, and directly programming wrinkle patterns with regulated structures.