Direct Color Printing with an Electron Beam

Ultrafast Nanodrum-on-Chip Pixels

Environmentally friendly, ultrafast display pixels of micrometer sizes are fabricated with nanometer-thick gold films and Si/SiO2 wafers. The color displayed is due to both the plasmon response of the gold film and the optical interference from the Fabry-Peerot cavity formed by the underlying silicon substrate, the semitransparent gold film and the air gap between them. When an electric potential is applied to the gold film, the electrostatic force induces an attraction between the gold film and the silicon wafer. Due to the flexibility of the film, the size of the air gap changes, resulting in a changing color. By applying different driving signals, we have achieved cyan, magenta, and yellow reflected colors. The maximum switching rate of the pixel is primarily determined by the thickness dependence of the metal drum and its Young's modulus and is typically in the MHz regime.

High-purity and wide-angle reflective structural colors based on an all-dielectric Fabry-Pérot cavity structure

High-purity structural colors with low fabrication cost are in demand for their commercial applications. Here, we demonstrate an all-dielectric Fabry-Pérot cavity structure consisting of four-layer lossy and lossless dielectric films alternately stacked for producing high-purity and angle-invariant reflective colors. Multiple cavity resonances function together to significantly suppress the undesired reflection with the enhanced optical absorption, leading to a distinct and saturated color with a high efficiency of ∼70%. Besides, due to the high refractive indices of constituent materials, the color appearance of the designed structure can be maintained well at ±50° incident angle for two polarization states. The excellent color performance of the proposed device together with cost-effective manufacturing convenience opens up new avenues for their large-area applications in various areas.

Hydrogels for active photonics

Conventional photonic devices exhibit static optical properties that are design-dependent, including the material's refractive index and geometrical parameters. However, they still possess attractive optical responses for applications and are already exploited in devices across various fields. Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli. Over the past few years, various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties. Herein, we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels. In particular, fabrication techniques for hydrogel photonic devices are categorized into film growth, photolithography (PL), electron-beam lithography (EBL), and nanoimprint lithography (NIL). Furthermore, we provide insights into future directions and prospects for deformable hydrogel photonics, along with their potential practical applications.

In Situ Metal Deposition on Perhydropolysilazane-Derived Silica for Structural Color Surfaces with Antiviral Activity

Structural coloration has recently sparked considerable attention on the laboratory and industrial scale. Structural colors can create vivid, saturated, and long-lasting colors on metallic surfaces for optical filters, digital displays, and surface decoration. This study used an all-solution, low-cost method, free of a specific setup procedure, to fabricate structural colors of a multilayered metal-dielectric structure based on interference effects within a Fabry-Perot cavity. The insulating (dielectric) layer was produced from perhydropolysilazane, an inorganic silicon-containing polymer, from which hydrogen was liberated during conversion into silica and applied in situ to reduce metallic nanoparticles on the silica surface. This simple manufacturing technique contributes to the fabrication of large, high-quality surfaces, which could potentially be employed for surface decoration. The fabricated surfaces also exhibited excellent hydrophobic properties with contact angles up to 137°, endowing them with self-cleaning properties. In addition, the antiviral and antibacterial impact of the silver (Ag)/silica (SiO2)/stainless steel (SUS) film was also examined, as Ag has been reported to have antimicrobial and, recently, antiviral properties. According to three independently conducted antiviral assays, the fluorescence expression of virus-infected cells, PCR analysis, and modified tissue culture infectious dose assay, the film inhibited lentivirus by 75, 97, and 99% when exposed to the virus for 20 min, 1 h, and 20 min, respectively. Furthermore, the film had exceptional antibacterial activity with no colony growth observed for 24 and 12 h of inoculation. It is thus conceivable that these structural color-based films can be used to not only decorate metal surfaces with aesthetic colors but also limit virus and bacterium propagation successfully.

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.

Monolithic MOF-Based Metal-Insulator-Metal Resonator for Filtering and Sensing

Metal-insulator-metal (MIM) configurations based on Fabry-Pérot resonators have advanced the development of color filtering through interactions between light and matter. However, dynamic color changes without breaking the structure of the MIM resonator upon environmental stimuli are still challenging. Here, we report monolithic metal-organic framework (MOF)-based MIM resonators with tunable bandwidth that can boost both dynamic optical filtering and active chemical sensing by laser-processing microwell arrays on the top metal layer. Programmable tuning of the reflection color of the MOF-based MIM resonator is achieved by controlling the MOF layer thicknesses, which is demonstrated by simulation of light-matter interactions on subwavelength scales. Laser-processed microwell arrays are used to boost sensing performance by extending the pathway for diffusion of external chemicals into nanopores of the MOFs. Both experiments and molecular dynamics simulations demonstrate that tailoring the period and height of the microwell array on the MIM resonator can advance the high detection sensitivity of chemicals.

Humidity-Responsive RGB-Pixels via Swelling of 3D Nanoimprinted Polyvinyl Alcohol

Humidity-responsive structural coloration is actively investigated to realize real-time humidity sensors for applications in smart farming, food storage, and healthcare management. Here, humidity-tunable nano pixels are investigated with a 700 nm resolution that demonstrates full standard RGB (sRGB) gamut coverage with a millisecond-response time. The color pixels are designed as Fabry-Pérot (F-P) etalons which consist of an aluminum mirror substrate, humidity-responsive polyvinyl alcohol (PVA) spacer, and a top layer of disordered silver nanoparticles (NPs). The measured volume change of the PVA reaches up to 62.5% when the relative humidity (RH) is manipulated from 20 to 90%. The disordered silver NP layer permits the penetration of water molecules into the PVA layer, enhancing the speed of absorption and swelling down to the millisecond level. Based on the real-time response of the hydrogel-based F-P etalons with a high-throughput 3D nanoimprint technique, a high-resolution multicolored color print that can have potential applications in display technologies and optical encryption, is demonstrated.

Asymmetric nanocavities with wide reflection color gamut for color printing

Symmetric metal-dielectric-metal (MDM) nanocavities based on Fabry-Perot resonance play a crucial role in transmission colors. However, their reflection color gamuts are generally limited owing to the narrow dip of resonance spectrum. In this work, we propose and fabricate symmetric titanium-indium tin oxide-silver (Ti/ITO/Ag) nanocavities to realize the reflection colors. The experimental and simulation results show that reflection color gamut of the asymmetric nanocavity is wider than that of symmetric MDM nanocavity due to the generation of broader resonance spectral dip. Moreover, a grayscale focused ion beam (FIB) etching method is employed to fabricate the thickness-controlled microstructures, and the etching depth satisfies a linear relationship with the gray value. The reflection color image can be observed by fabricating the ITO layer in the asymmetric MDM nanocavity with grayscale FIB etching method, which is more vivid than the image from fabricated symmetric MDM nanocavities. This work will provide a new way for color printing, color display, and ultra-small anti-counterfeiting technology.

Self-assembled pagoda-like nanostructure-induced vertically stacked split-ring resonators for polarization-sensitive dichroic responses

Stacked split-ring resonators (SSRR) arrays exhibiting polarization-sensitive dichroic responses in both visible and near-infrared wavelengths are realized over a centimeter-scale large area. The SSRR arrays are derived from pagoda-like nanorods fabricated from the self-assembly of a lamellae-forming polystyrene-b-poly (methyl methacrylate) copolymer (PS-b-PMMA) confined in cylindrical pores of anodized aluminum oxide (AAO) template. Along the nanorod direction, PS and PMMA nanodomains were alternately stacked with the same distance. Silver crescents and semi-hemispherical covers, which are essential for SSRR with the polarization sensitivity, were obliquely deposited on the single side of the nanorod after removing the AAO template and reactive-ion etching treatment. These sophisticated nanoscale architectures made by bottom-up fabrication can be applied to structural color, optical anti-counterfeiting, and commercial optical components in a large area.

Development of a Photonic Switch via Electro-Capillarity-Induced Water Penetration Across a 10-nm Gap

With narrow and dense nanoarchitectures increasingly adopted to improve optical functionality, achieving the complete wetting of photonic devices is required when aiming at underwater molecule detection over the water-repellent optical materials. Despite continuous advances in photonic applications, real-time monitoring of nanoscale wetting transitions across nanostructures with 10-nm gaps, the distance at which photonic performance is maximized, remains a chronic hurdle when attempting to quantify the water influx and molecules therein. For this reason, the present study develops a photonic switch that transforms the wetting transition into perceivable color changes using a liquid-permeable Fabry-Perot resonator. Electro-capillary-induced Cassie-to-Wenzel transitions produce an optical memory effect in the photonic switch, as confirmed by surface-energy analysis, simulations, and an experimental demonstration. The results show that controlling the wetting behavior using the proposed photonic switch is a promising strategy for the integration of aqueous media with photonic hotspots in plasmonic nanostructures such as biochemical sensors.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Direct laser writing of vapour-responsive photonic arrays

Using direct laser writing, arrays of optically responsive ionogel structures were fabricated. To demonstrate their responsive nature, visible colour changes in the presence of different solvent vapours were investigated. This represents a new departure for photonic structural colouration, in which the fabricating structure shows a programmable, controllable, and dynamic stimuli response.

Tunable Structural Color Images by UV-Patterned Conducting Polymer Nanofilms on Metal Surfaces

Precise manipulation of light-matter interactions has enabled a wide variety of approaches to create bright and vivid structural colors. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive-index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability of these approaches impede their further development toward flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Here, a novel method is presented to generate structural color images based on monochromic conducting polymer films prepared on metallic surfaces via vapor phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose enables synergistic control of both nanoscale film thickness and polymer permittivity, which generates controllable structural colors from violet to red. Together with grayscale photomasks this enables facile fabrication of high-resolution structural color images. Dynamic tuning of colored surfaces and images via electrochemical modulation of the polymer redox state is further demonstrated. The simple structure, facile fabrication, wide color gamut, and dynamic color tuning make this concept competitive for applications like multifunctional displays.

Generation of additive colors with near unity amplitude using a multilayer tandem Fabry-Perot cavity

In this paper, the generation of additive red-green-blue (RGB) colors in reflectance mode with near unity amplitude is demonstrated. For this purpose, a multilayer structure made of metal-insulator-metal-semiconductor-insulator stacks is designed to achieve >0.9 reflection peaks with full-width-at-half-maximum values <0.3λ_peak. The proposed design also shows near zero reflection in off-resonance spectral ranges, and this, in turn, leads to high color purity. The optimized designs are fabricated, and simulation and theoretical results are verified with characterization findings. This work demonstrates the potential of multilayer tandem cavity designs in the realization of lithography-free large-scale compatible functional optical coatings.