All-Dielectric Silicon Nanoring Metasurface for Full-Color Printing

Tunable Structural Coloration in Eccentric Water-in-Oil-in-Water Droplets

Structural colors show diverse advantages such as fade resistance, eco-friendliness, iridescence, and high saturation in comparison with chemical pigments. In this paper, we show tunable structural coloration in colorless water-in-oil-in-water double emulsion droplets via total internal reflection and interference at the microscale concave interfaces. Through experimental work and simulations, we demonstrate that the shell thickness and the eccentricity of the core-shell structures are key to the successful formation of iridescent structural colors. Only eccentric thin-shell water-in-oil-in-water droplets show structural colors. Importantly, structural colors based on water-oil interfaces are readily responsive to a variety of environmental stimuli, such as osmotic pressure, temperature, magnetic fields, and light composition. This work highlights an alternative structural coloration that expands the applications of droplet-based structural colors to aqueous systems.

Transfer-Printing Hydrogel-Based Platform for Moisture-Driven Dynamic Display and Optical Anti-Counterfeiting

Humidity-responsive materials offer a promising approach to achieving tunable metasurface systems due to their fast and reversible swelling responses to moisture, which enables many important applications, such as real-time humidity sensing, optical switches, dynamic displays, and optical information encryption. However, the humidity-responsive structural coloration generally cannot provide a high spatial resolution and requires a complex patterning process. Here, we present a scalable moisture-driven color-changing Fabry-Pérot (FP)-like cavity composed of a polyvinyl alcohol layer sandwiched between an upper gold nanoparticles assembly and a bottom gold mirror. Through nanoparticle contact printing, we pixelated these cavities with sub-micrometer sizes without crosstalk and achieved an ultrahigh display resolution of ∼400 nm. Meanwhile, these nanoparticle-based FP (NBFP) cavities exhibit more vibrant colors than those of conventional film-based ones due to broadband absorption of the disordered nanoparticle assembly. Moreover, the NBFP cavities exhibit a rapid response (<300 ms), benefiting from the membrane pores formed in the gaps between the adjacent nanoparticles. Finally, we demonstrated the applications of the NBFP cavities in optical anti-counterfeiting and dynamic multi-color printing. These results suggest that our approach will help to realize a colorful, fast, and ultrahigh-resolution dynamic display device in optical security and colorimetric sensing.

Structural color modulation by laser post-processing on metal-coated colloidal crystals

A method to use a pulsed solid-state laser to create structural color modulation on metal-coated colloidal crystal surfaces by changing the scanning speed has been proposed. Vivid colors as cyan, orange, yellow, and magenta are obtained with different predefined stringent geometrical and structural parameters. The effect of laser scanning speeds and polystyrene (PS) particle sizes on the optical properties is studied, and the angle-dependent property of the samples is also discussed. As a result, the reflectance peak is progressively red shifted along with increasing the scanning speed from 4 mm/s to 200 mm/s with 300 nm PS microspheres. Moreover, the influence of the microsphere particle sizes and incident angle are also experimentally investigated. For 420 and 600 nm PS colloidal crystals, along with a gradual decrease in the scanning speed of the laser pulse from 100 mm/s to 10 mm/s and an increase in the incident angle from 15° to 45°, there was a blue shift for two reflection peak positions. This research is a key, low-cost step toward applications in green printing, anti-counterfeiting, and other related fields.

High-speed laser writing of structural colors for full-color inkless printing

It is a formidable challenge to simultaneously achieve wide-gamut, high-resolution, high-speed while low-cost manufacturability, long-term stability, and viewing-angle independence in structural colors for practical applications. The conventional nanofabrication techniques fail to match the requirement in low-cost, large-scale and flexible manufacturing. Processing by pulsed lasers can achieve high throughput while suffering from a narrow gamut of ~15% sRGB or angle-dependent colors. Here, we demonstrate an all-in-one solution for ultrafast laser-produced structural colors on ultrathin hybrid films that comprise an absorbent dielectric TiAlN layer coating on a metallic TiN layer. Under laser irradiation, the absorption behaviours of the TiAlN-TiN hybrid films are tailored by photothermal-induced oxidation on the topmost TiAlN. The oxidized films exhibit double-resonance absorption, which is due to the non-trivial phase shifts both at the oxide-TiAlN interface, and at the TiAlN-TiN interface. By varying the accumulated laser fluence to modulate the oxidation depth, angle-robust structural colors with unprecedented large-gamut of ~90% sRGB are obtained. The highest printing speed reaches 10 cm²/s and the highest resolution exceeds 10000 dpi. The durability of the laser-printed colors is confirmed by fastness examination, including salt spray, double-85, light bleaching, and adhesion tests. These features render our technique to be competitive for industrial applications.

Scalable Hydrogel-Based Nanocavities for Switchable Meta-Holography with Dynamic Color Printing

Devices used for meta-optics display are currently undergoing a revolutionary transition from static to dynamic. Despite various tuning strategy demonstrations such as mechanical, electrical, optical, and thermal tunings, a longstanding challenge for their practical application has been the achievement of a conveniently accessible real-life tuning scheme for realizing versatile functionality dynamics outside the laboratory. In this study, we demonstrate a practical tuning strategy to realize a dynamic color printing with a switchable meta-holography exhibition based on hydrogel-based nanocavities. On the basis of the inflation sensitivity of a hydrogel to humidity alteration, its transmissive color was notably tuned from 450 to 750 nm. More intriguingly, by controlling the sample dry/immersed states in real time, we successfully enabled dual-channel switchable meta-holography. With the advantages of facile architecture, daily stimulus with large-area modulation, and high chromaticity, our proposed hydrogel-based nanocavities provide a promising path toward tunable display/encryption, optical sensors, and next-generation display technology.

Mie Resonances Enabled Subtractive Structural Colors with Low-Index-Contrast Silicon Metasurfaces

All-dielectric structural colors are attracting increasing attention due to their great potential for various applications in display devices, imaging security certification, optical data storage, and so on. However, it remains a great challenge to achieve vivid structural colors with low-aspect-ratio silicon nanostructures directly on a silicon substrate, which is highly desirable for future integrated optoelectronic devices. The main obstacle comes from the difficulty in achieving strong Mie resonances by Si nanostructures on low-index-contrast substrates. Here, we demonstrate a generic principle to create vivid bright field structural colors by using silicon nanopillars directly on top of the silicon substrate. Complementary colors across the full visible spectrum are achieved as a result of the enhanced absorption due to Mie resonances. It is shown that the color saturation increases with the increasing of the nanopillar height. Remarkably, blue and black colors are generated by trapezoid nanopillar arrays as a result of the absorption at long wavelengths or all visible wavelengths. Our strategy provides a powerful scheme for accelerating the integrated optoelectronic applications in nanoscale color printing, imaging, and displays.

Pitfalls in the spectral measurements of polarization-altering metasurfaces

The optical characterization of metasurfaces and nanostructures that alter the polarization of light is tricky and can lead to unphysical results, such as reflectance beyond unity. We track the origin of such pitfalls to the response of some typical optical components used in a commercial microscope or a custom-made setup. In particular, the beam splitter and some mirrors have different responses for both polarizations and can produce wrong results. A simple procedure is described to correct these erroneous results, based on the optical characterization of the different components in the optical setup. With this procedure, the experimental results match the numerical simulations perfectly. The methodology described here is simple and will enable the accurate spectral measurements of nanostructures and metasurfaces that alter the polarization of the incoming light.

Dual-channel anticounterfeiting color-nanoprinting with a single-size nanostructured metasurface

Metasurface-based structural-colors are usually implemented by changing the dimensions of nanostructures to produce different spectral responses. Therefore, a single-size nanostructured metasurface usually cannot display structural-colors since it has only one design degree of freedom (DOF), i.e., the orientation angles of nanostructures. Here, we show structural-color nanoprinting images can be generated with a single-size nanostructured metasurface, enabled by designing the anisotropic nanostructure with different spectral responses along its long- and short-axis directions, respectively. More interestingly, the concept of orientation degeneracy of nanostructures can be applied in the metasurface design, which shows two spectral modulations can be implemented under different polarization directions of output light, thus extending the color-nanoprinting from single-channel to dual-channel. The proposed dual-channel metasurface used for anticounterfeiting color-nanoprinting has presented the advantages of ultra-compactness, high information capacity, and vivid colors, which can develop broad applications in fields such as high-end anticounterfeiting, high-density information storage, optical encryption, etc.

Heterogeneous Component Au (Outer)-Pt (Middle)-Au (Inner) Nanorings: Synthesis and Vibrational Characterization on Middle Pt Nanorings with Surface-Enhanced Raman Scattering

We report a synthetic approach for heterometallic (Au-Pt-Au) nanorings with intertwined triple rings (NITs), wherein three differently sized metal circular nanorings concentrically overlap in a single entity. The synthetic method allows one to control the component of core nanorings (Au or Pt) with a tunable gap distance. The narrow circular nanogaps between inner and outer Au rings strongly enhance the electromagnetic near-field via intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level. Importantly, when the component of the middle ring is Pt, in situ SERS measurement for electrochemical reactions on Pt domains could be monitored with electrochemical potential variations due to the near-field focusing that is assisted by plasmonically active inner and outer Au nanorings, which is not feasible with pure Pt nanoparticle systems. The resulting NIT systems are robust and may benefit the synthesis of complicated nanostructures, giving myriad applications.

High-Efficiency, Mass-Producible, and Colored Solar Photovoltaics Enabled by Self-Assembled Photonic Glass

Building-integrated photovoltaics is a crucial technology for developing zero-energy buildings and sustainable cities, while great efforts are required to make photovoltaic (PV) panels aesthetically pleasing. This places an urgent demand on PV colorization technology that has a low impact on power conversion efficiency (PCE) and is simultaneously mass-producible at a low cost. To address this challenge, this study contributes a colorization strategy for solar PVs based on short-range correlated dielectric microspheres, i.e., photonic glass. Through theoretical studies, first we demonstrate that the photonic glass self-assembled by high-index microspheres could enable both colored solar cells and modules, with easily variable colors and negligible parasitic absorption. By a fast spray coating process of colloidal monodisperse ZnS microspheres, we show the photonic glass layer could be easily deposited on silicon solar cells, enabling them to have structural colors. Through varying microsphere sizes, solar cells with different colors are achieved, showing low PCE loss compared to normal black cells. These colored solar cells are also encapsulated with a general lamination process to produce PV modules with various colors and patterns at a stunning PCE approaching 21%. Moreover, the long-term stability is subsequently verified by aging tests including an outdoor exposure for 10 days and a damp-heat test for 1000 h, and the mass producibility is demonstrated by presenting a colored PV panel with an output power over 108 W. These results confirm photonic glass as a promising strategy for colored PVs possessing high efficiency and practical applicability.

Machine learning-assisted design of polarization-controlled dynamically switchable full-color metasurfaces

Dynamic color tuning has significant application prospects in the fields of color display, steganography, and information encryption. However, most methods for color switching require external stimuli, which increases the structural complexity and hinders the applicability of front-end dynamic display technology. In this study, we propose polarization-controlled hybrid metal-dielectric metasurfaces to realize full-color display and dynamic color tuning by altering the polarization angle of incident light without changing the structure and properties of the material. A bidirectional neural network is trained to predict the colors of mixed metasurfaces and inversely design the geometric parameters for the desired colors, which is less dependent on design experience and reduces the computational cost. According to the color recognition ability of human eyes, the accuracy of color prediction realized in our study is 93.18% and that of inverse parameter design is 92.37%. This study presents a simple method for dynamic structural color tuning and accelerating the design of full-color metasurfaces, which can offer further insight into the design of color filters and promote photonics research.

Recent Progress in Responsive Structural Color

Structural color has been regarded as an ideal alternative to pigments because of the advantages of environmental friendliness, resistance to fading, and dynamic regulation. Responsive structural color can give real-time visible feedback to external stimuli and thus has great prospects in many applications, such as displays, sensing, anticounterfeiting, information storage, and healthcare monitoring. In this Perspective, we elucidate basic concepts, controllable fabrications, and promising applications of responsive structural colors. In particular, we systematically summarize the general regulation mode of all kinds of responsive structural color systems. First, we introduce the basic chromogenic structures as well as the regulation modes of responsive structural color. Second, we present the fabrication methods of patterned structural color. Then, the promising applications of responsive structural color systems are highlighted in detail. Finally, we present the existing challenges and future perspectives on responsive structural colors.

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability, eco-friendly manufacturing, and ultra-high resolution. However, most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching. Here, we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition (PPD), a room temperature, ambient, and additive manufacturing process without requiring heating, vacuum deposition or etching. The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount (estimated <20% volume) of polymers, producing a smooth surface (roughness 2.5 nm) even better than vacuum-deposited silver films (roughness 2.8 nm) at ~4 nm thickness. Further, the printed composite films have a much larger effective refractive index n (~1.90) and a smaller extinction coefficient k (~0.92) than PVD ones in the visible wavelength range (400 to 800 nm), therefore modulating the surface reflection and the phase accumulation. The capability of PPD in printing both ultra-thin (~5 nm) composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry-Perot (FP) cavity structures to exhibit vivid and saturated colors. We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of ~6.5 μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses (75 to 200 nm). Finally, PPD-based color picture printing is demonstrated on a wide range of substrates, including glass, PDMS, and plastic, proving its broad potential in future applications from security labeling to color displays.

Reversible Mechanochromisms via Manipulating Surface Wrinkling

Mechanochromic structural-colored materials have promising applications in various domains. In this Letter, we report three types of reversible mechanochromisms in simple material systems by harnessing mechano-responsive wrinkling dynamics including (i) brightness mechanochromism (BM), (ii) hue change mechanochromism (HCM), and (iii) viewable angle mechanochromism (VAM). Upon stretching, the BM device exhibits almost a constant hue but reduces light brightness due to the postbuckling mechanics-controlled deformation, while the HCM device can change the hue from blue to red with almost constant intensity because of the linear elastic mechanics-controlled deformation. The VAM device shows a constant hue because of the thin film interference effect. However, the viewable angles decrease with increasing applied strain owing to the light scattering of wrinkles. All of the mechanochromic behaviors exhibit good reversibility and durability. We clearly elucidated the underlying mechanisms for different mechanochromisms and demonstrated their potential applications in smart displays, stretchable strain sensors, and antipeeping/anticounterfeiting devices.

Generation of Airy beam arrays in real and K spaces based on a dielectric metasurface

Airy beams are widely used in various optical devices and optical experiments owing to their unique characteristics such as self-acceleration, self-recovery, and non-diffraction. Here we designed and demonstrated a metasurface capable of encoding two phase distributions independently in dual circular polarization channels. We experimentally observed the generated Airy beam arrays loaded on the metasurface in the real and K spaces. Compared with the traditional method, such method provides a more efficient solution to generate large capacity Airy beam arrays with switchable working modes in the broadband spectrum. The results may pave the way for the integration and miniaturization of micro-nano devices and provide a platform for information processing, particle manipulation, space-time optical wave packets, and Airy lasers.

Single-celled multifunctional metasurfaces merging structural-color nanoprinting and holography

Nanostructured metasurfaces applied in structural-color nanoprinting and holography have been extensively investigated in the past several years. Recently, merging them together is becoming an emerging approach to improve the information capacity and functionality of metasurfaces. However, current approaches, e.g., segmenting, interleaving and stacking schemes for function merging, suffer from crosstalk, low information density, design and fabrication difficulties. Herein, we employ a single-celled approach to design and experimentally demonstrate a high-density multifunctional metasurface merging nanoprinting and holography, i.e., each nanostructure in the metasurface can simultaneously manipulate the spectra (enabled with varied dimensions of nanostructures) and geometric phase (enabled with varied orientation angles of nanostructures) of incident light. Hence, with different decoding strategies, a structural-color nanoprinting image emerges right at the metasurface plane under white light illumination, while a holographic image is reconstructed in the Fraunhofer diffraction zone under circularly polarized laser light incidence. And the two images have no crosstalk since they are independently designed and presented at different distances. Our proposal suggests a space-multiplexing scheme to develop advanced metasurfaces and one can find their markets in high-density information storage, optical information encryption, multi-channel image display, etc.