Full-Color-Tunable Nanophotonic Device Using Electrochromic Tungsten Trioxide Thin Film

Inorganic Electrochromic Metasurface in the Visible

Color printing based on metallic or dielectric nanostructures has revolutionized color science due to its unprecedented subwavelength resolution. Evidently, the evolution toward the active control of such structural colors with smart materials is in progress for real applications. Here we experimentally demonstrate a large color gamut with high intensity and purity, as well as switching on and off based solely on tungsten trioxide (WO3) cylindrical resonators. The strong resonances in the visible spectral range in these WO3 metasurfaces can be reversibly switched on and off due to its electrochromism by applying alternating voltages of +2.0 V and -0.3 V. Our approach opens up possibilities for the functional diversification of commercial smart windows, as well as the development of new display technologies in the future.

Super-Wide Color Tunability from a Single Electrochromic Device through In Situ Reconstruction of Optical Cavity

Electrochromic (EC) displays, as non-emissive (passive) displays with low energy consumption, have garnered significant attention from both industry and academia in recent years. Nevertheless, traditional EC technology faces challenges in achieving full-color displays within a single device due to its limited color gamut, even though full-color capability is highly desirable for eliminating the need for complex RGB subpixel mosaics. Herein, a new strategy is proposed utilizing in situ, electrically driven reconstruction of optical cavities on an electrochromic electrode surface to fabricate EC devices with super-wide color tunability. The device fabricated by this approach can create a wide variety of colors from yellow, orange, red, violet, blue, and cyan to green in a single EC device that almost spans the entire visible region (Δhue approaches 360°). Apart from the super-wide color tunability, the devices also have small working voltage window (0.2-1.8 V), outstanding bistability (>8 h), extremely low power consumption (≈2.3 mW cm-2) and good cycling ability (≈4.3% decay rate after 1,000 cycles). Moreover, the super-wide color tunability of these EC devices has been demonstrated in diverse applications, including shifting rainbow flower images, color palettes, and information displays.

Optical-Cavity-Incorporated Colorful All-Solid-State Electrochromic Devices for Dual Anti-Counterfeiting

The fusion of electrochromic technology with optical resonant cavities presents an intriguing innovation in the electrochromic field. However, this fusion is mainly achieved in liquid electrolyte-based or sol-gel electrolyte-based electrochromic devices, but not in all-solid-state electrochromic devices, which have broader industrial applications. Here, a new all-solid-state electrochromic device is demonstrated with a metal-dielectric-metal (MDM) resonant cavity, which can achieve strong thin-film interference effects through resonance, enabling the device to achieve unique structural colors that have rarely appeared in reported all-solid-state electrochromic devices, such as yellow green, purple, and light red. The color gamut of the device can be further expanded due to the adjustable optical constants of the electrochromic layer. What is more, this device exhibits remarkable cycling stability (maintaining 84% modulation capability after 7200 cycles), rapid switching time (coloration in 2.6 s and bleaching in 2.8 s), and excellent optical memory effect (only increasing by 13.8% after almost 36 000 s). In addition, this exquisite structural design has dual-responsive anti-counterfeiting effects based on voltage and angle, further demonstrating the powerful color modulation capability of this device.

Programmable directional color dynamics using plasmonics

Adaptive multicolor filters have emerged as key components for ensuring color accuracy and resolution in outdoor visual devices. However, the current state of this technology is still in its infancy and largely reliant on liquid crystal devices that require high voltage and bulky structural designs. Here, we present a multicolor nanofilter consisting of multilayered 'active' plasmonic nanocomposites, wherein metallic nanoparticles are embedded within a conductive polymer nanofilm. These nanocomposites are fabricated with a total thickness below 100 nm using a 'lithography-free' method at the wafer level, and they inherently exhibit three prominent optical modes, accompanying scattering phenomena that produce distinct dichroic reflection and transmission colors. Here, a pivotal achievement is that all these colors are electrically manipulated with an applied external voltage of less than 1 V with 3.5 s of switching speed, encompassing the entire visible spectrum. Furthermore, this electrically programmable multicolor function enables the effective and dynamic modulation of the color temperature of white light across the warm-to-cool spectrum (3250 K-6250 K). This transformative capability is exceptionally valuable for enhancing the performance of outdoor optical devices that are independent of factors such as the sun's elevation and prevailing weather conditions.

Full-Color-Adjustable Nanophotonic Device Adopting Electrochromic Poly(3,4-ethylenedioxythiophene) Thin Films

Intercalation-based organic polymers that shift their colors during ion insertion and extraction provide a significant basis for existing electrochromic technology. Nevertheless, the complexity of modifying the structure in the skeleton or combining several diverse polymers to produce a full-color range has restricted the practical applications of electrochromic materials. Herein, we demonstrate two configurations of the poly(3,4-ethylenedioxythiophene) (PEDOT) Fabry-Perot (F-P) nanocavity-type electrochromic devices fabricated by spray coating lossless PEDOT on the F-P metasurfaces (Cr/ITO/Ag/Cr), which allows full-color response by simply controlling the thickness of dielectric layer indium tin oxide (ITO). However, the reflected light from the PEDOT F-P nanocavity-type electrode can be modulated by electrically controllable optical absorption of PEDOT. Besides, the subtle brightness regulation could be obtained in our F-P nanocavity electrochromic devices via altering the PEDOT thickness. Overall, our results offer a novel perspective for versatile color control of PEDOT.

Plasmonic-based electrochromic materials and devices

The development of electrochromic (EC) materials has paved the way for a wide range of devices, such as smart windows, color displays, optical filters, wearable camouflages, among others. However, the advancement of electrochromism faces a significant hurdle due to its poor stability and limited color options. This lack of stability is primarily attributed to the substantial alteration in the dielectric properties of EC materials during cycling. Consequently, the design of advanced plasmonic materials is a key strategy to achieve a stable EC device. In this review, we provide an overview of the current state-of-the-art designs of plasmonic-based EC materials and devices. We discuss their working principles, techniques for structure/morphology engineering, doping methods, and crystal phase design. Furthermore, we explore the integration of plasmonic materials with other EC materials to create advanced EC devices. Finally, we outline the challenges that need to be addressed and present an outlook on the development of high-performance EC devices.

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.

Dynamic Spectral Modulation Enabled by Conductive Polymer-Integrated Plasmonic Nanodisk-Hole Arrays

The electrically driven optical performance modulation of the plasmonic nanostructure by conductive polymers provides a prospective technology for miniaturized and integrated active optoelectronic devices. These features of wafer-scale and flexible preparation, a wide spectrum adjustment range, and excellent electric cycling stability are critical to the practical applications of dynamic plasmonic components. Herein, we have demonstrated a large-scale and flexible active plasmonic nanostructure constructed by electrochemically synthesizing nanometric-thickness conductive polymer onto spatially mismatched Au nanodisk-hole (AuND-H) array on the poly(ethylene terephthalate) (PET) substrate, offering low-power electrically driven switching of reflective light in a wide wavelength range of 550-850 nm. The composite structure of the polymer/AuND-H array supports multiple plasmonic resonance modes with strong near-field enhancement and confinement, which provides an excellent dynamic spectral modulation platform. As a result, the PPy/AuND-H array achieves 18.4% reversible switching of spectral intensity at 780 nm and speedy response time, as well as maintains a stable dynamic modulation range at two-potential cycling between -0.6 and 0.1 V after 200 modulation cycles. Compared to the case of the PPy/AuND-H array, the PANI/AuND-H array obtains a more extensive intensity modulation of 25.1% at 750 nm, which is attributed to the significant differences in the extinction coefficient between the oxidized and reduced states of PANI, but its modulation range degrades apparently after 20 cycles driven at applied voltages between -0.1 and 0.8 V. Additionally, the cycling stability could be further improved by reducing the modulation voltage range. Our proposed electromodulated composite structure provides a promising technological proposal for dynamically plasmonic reconfigurable devices.

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.

Multicolor Conjugated Polymer Thin Films with Tunable Responsivity to Oxidative and Reductive Environments

Dynamic colors that respond to environmental changes are of great interest for diverse areas of science and technology ranging from chemical and biological sensors to smart information display. Here, we demonstrate a multitude of responsive colors from a conjugated polymer film arising from a thin-film interference. This mechanism provides an excellent control over the thin-film color by varying the film thickness, type of substrate, and degree of polaron population and is generally applicable to various conjugated polymers for further color variation. Furthermore, multiple sets of responsive colors are achieved from a single polymer layer by patterning the underlying substrate to spatially modify the interference conditions. Using this system, we demonstrate the reversible color changes induced by an oxidative or reductive environment with color responsivity controllable with the nature of the polaron state.

Multicolored inorganic electrochromic materials: status, challenge, and prospects

Against the backdrop of advocacy for green and low-carbon development, electrochromism has attracted academic and industrial attention as an intelligent and energy-saving applied technology due to its optical switching behavior and its special principles of operation. Inorganic electrochromic materials, represented by transition metal oxides, are considered candidates for the next generation of large-scale electrochromic applied technologies due to their excellent stability. However, the limited color diversity and low color purity of these materials greatly restrict their development. Starting from the multicolor properties of inorganic electrochromic materials, this review systematically elaborates on recent progress in the aspects of the intrinsic multicolor of electrochromic materials, and structural multicolor based on the interaction between light and microstructure. Finally, the challenges and opportunities of inorganic electrochromic technology in the field of multicolor are discussed.

Ultrathin Pyroelectric Photodetector with Integrated Polarization-Sensing Metasurface

An abundance of metallic metasurfaces have been realized with miniscule, intricate features capable of tailored scattering, reflection, and absorption; however, high losses through heat limit their use in optoelectronics. Here, codesign of a detector and a polarization-sensing metasurface overcomes this challenge by utilizing the heat generation for integrated pyroelectric detection of the incoming light polarization. Using a nanogap metasurface with asymmetric metallic elements, polarization-sensitive photodetection exhibits high extinction ratios up to 19 for orthogonally polarized light and allows extraction of Stokes parameters with <12% deviation from theoretical values. This polarization-sensitive photodetector is ultrathin, consisting of active layers of only 290 nm, and exhibits fast response times of ∼2 ns. The structure is fully integrated, requiring no external cameras, detectors, or power sources, and points toward the creation of layered, multifunctional devices that utilize exotic metasurface properties for novel and compact sensing and imaging.

Rejuvenation of Electrochromic Devices

Electrochromic devices (ECDs) are a hot pot due to their significant energy-saving effect in green buildings. However, the ECDs suffer from degradation induced by ion trapping during cycling, which restricts their further development. Here, it is demonstrated that the electrochromic performance of the degraded ECDs can be rejuvenated by heat treatment method The release mechanism of trapped ions in films is simulated and validated using three types of ECDs. The semi-solid-state ECD evinces a state of near-failure at low temperatures can regain its initial performance by heating. All-solid-state ECDs, including amorphous WO3 (a-WO3 ECD) and crystalline WO3 (c-WO3 ECD) as the electrochromic layer, can also release the trapped ions and regain the performance (97.3% and 95.5% of initial optical modulation) by annealing, regardless of the way of degradation. The research has extended the lifespan of multiple ECDs, providing significant practical value and promoting sustainable, and eco-friendly development.

Multicolor Electrochromic Display and Patterned Device Based on Hollow-SiO2-Supported WO3 Photonic Crystals

Electrochromic photonic crystals (PCs) have been intensively studied in the field of display, sensors, and intelligent materials due to their tunable brilliant structural colors. The mostly studied electrochromic PCs are based on the tunable lattice parameter after electrifying; namely, the electrochromic process is caused by the structural change of PCs. Besides the lattice parameter, the refractive index is another key factor to determine the structural color of PCs. Here, a kind of hollow-SiO2-supported WO3 (H-SiO2/WO3) PCs is designed, where the refractive index of the WO3 portion is changeable under charging. Benefiting from the support effect and tunable thickness of H-SiO2, large-area PC samples with good surface morphology and bright multicolor output are prepared. The reflection peaks of these composite PCs can shift by 30-90 nm, and their corresponding colors changed obviously after the voltage was applied. After being pixelated by laser-marking, the H-SiO2/WO3 PCs can dynamically display different numeric and alphabetic patterns in an electric-driven writing and erasing process. Not only does this composite PC structure broaden the color change range of WO3-based materials but also avoids the structural change in the electrochromic process. This work provides more possibilities for electrochromic PCs in the field of color-changing pattern displays.

Colorful Electrochromic Displays with High Visual Quality Based on Porous Metamaterials

The introduction of metamaterials into electrochromic (EC) displays has recently inspired a great breakthrough in the EC field, as this can offer a variety of new attractive features, from a very wide gamut of colors to very fast switching times. However, such metamaterial-based EC displays still face significant constraints when developing from single electrodes to full devices, because other supportive components in devices, such as counter electrodes and electrolytes, significantly affect light propagation and the subsequent perceived color quality in metamaterial-based EC devices. Herein, a new, cost-effective device design structured around a new type of porous metamaterial is reported to circumvent the critical problem in metamaterial-based EC displays. Owing to its unique design, the metamaterial-based EC device achieves good color quality with no drop in brightness or shift in color chromaticity when compared with a single electrode. Moreover, the porous-metamaterial-based EC device can exhibit non-iridescence and be viewed from a wide range of angles (5°-85°) and has fast switching response (2.4 and 2.5 s for coloration and bleaching, respectively), excellent cycling performance (at least 2000 cycles), and extremely low power consumption (4.0 mW cm^-2 ).

Novel ultrasensitive Raman assay method based on enzyme mimetics for ultra trace of H2O2

Enzyme mimetics have been widely applied on H₂O₂ assay, but it is still challenging and interesting to realize the sensitive detection for ultra-trace H₂O₂. Here, an ultrasensitive Raman assay method based on novel WO₃@IP6-Fe³⁺ enzyme mimetics with peroxidase-like activity was established. WO₃ microspheres (MSs) were found to have weak peroxidase-like activity, and the combination of IP6-Fe³⁺ and WO₃ can produce stronger activity. WO₃@IP6-Fe³⁺ MSs showed polyhedron-like structure, uniform size, and smooth surface. Although WO₃@IP6-Fe³⁺ enzyme mimetics have low catalytic efficiency and high absorbance background, the proposed Raman method can bypass the above problems. In Raman method, high concentration of WO₃@IP6-Fe³⁺ can be used to overcome low catalytic efficiency without high absorbance background. Moreover, 3,3',5,5'-tetramethylbenzidine oxide has prominent characteristic Raman peak at 1608 cm^-1, greatly improving the sensitivity and eliminating interference of impurities. Due to the high sensitivity and low background, Raman assay showed the ultra-low limit of detection (5.49 × 10^-15 M), which was 4-7 orders of magnitude lower than other detection methods. The ultrasensitive Raman assay not only provided the possibility for the enzyme mimetics-based detection of ultra-trace H₂O₂, but also enable the enzyme mimetics with low activity to be applied.

Multicolor Tunable Electrochromic Materials Based on the Burstein-Moss Effect

Inorganic electrochromic (EC) materials, which can reversibly switch their optical properties by current or potential, are at the forefront of commercialization of displays and smart windows. However, most inorganic EC materials have challenges in achieving multicolor tunability. Here, we propose that the Burstein-Moss (BM) effect, which could widen the optical gap by carrier density, could be a potential mechanism to realize the multicolor tunable EC phenomenon. Degenerated semiconductors with suitable fundament band gaps and effective carrier masses could be potential candidates for multicolor tunable EC materials based on the BM effect. We select bulk Y₂CF₂ as an example to illustrate multicolor tunability based on the BM effect. In addition to multicolor tunability, the BM effect also could endow EC devices with the ability to selectively modulate the absorption for near infrared and visible light, but with a simpler device structure. Thus, we believe that this mechanism could be applied to design novel EC smart windows with unprecedented functions.

Orthogonal Trichromatic Upconversion with High Color Purity in Core-Shell Nanoparticles for a Full-Color Display

Materials with tunable emission colors has attracted increasing interest in both fundamental research and applications. As a key member of light-emitting materials family, lanthanide doped upconversion nanoparticles (UCNPs) have been intensively demonstrated to emit light in any color upon near-infrared excitation. However, realizing the trichromatic emission in UCNPs with a fixed composition remains a great challenge. Here, without excitation pulsed modulation and three different near-infrared pumping, we report an experimental design to fine-control emission in the full color gamut from core-shell-structured UCNPs by manipulating the energy migration through dual-channel pump scheme. We also demonstrate their potential application in full-color display. These findings may benefit the future development of convenient and versatile optical methos for multicolor tuning and open up the possibility of constructing full-color volumetric display systems with high spatiotemporal resolution.

Advances in electrochromic device technology through the exploitation of nanophotonic and nanoplasmonic effects

Research regarding electrochromic (EC) materials, such materials that change their color upon application of an electrochemical stimulus, has been conducted for centuries. However, most recently, increasing efforts have been put into developing novel solutions to utilize these on-off switching materials in advanced nanoplasmonic and nanophotonic devices. Due to the significant change in dielectric properties of oxides such as WO3, NiO, Mn2O3 and conducting polymers like PEDOT:PSS and PANI, EC materials have transcended beyond simple smart window applications and are now found in plasmonic devices for full-color displays and enhanced modulation transmission and photonic devices with ultra-high on-off ratios and sensing abilities. Advancements in nanophotonic ECDs have further decreased EC switching speed by several orders of magnitude, allowing integration in real-time measurement and lab-on-chip applications. The EC nature of such nanoscale devices promises low energy consumption with low operating voltages paired with bistability and long lifetimes. We summarize these novel approaches to EC device design, lay out the current short comings and draw a path forward for future utilization.

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.

Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Organic electrochromic energy storage materials and device design

While not affecting electrochemical performance of energy storage devices, integrating multi-functional properties such as electrochromic functions into energy storage devices can effectively promote the development of multifunctional devices. Compared with inorganic electrochromic materials, organic materials possess the significant advantages of facile preparation, low cost, and large color contrast. Specifically, most polymer materials show excellent electrochemical properties, which can be widely used in the design and development of energy storage devices. In this article, we focus on the application of organic electrochromic materials in energy storage devices. The working mechanisms, electrochemical performance of different types of organics as well as the shortcomings of organic electrochromic materials in related devices are discussed in detail.

Active and tunable nanophotonic metamaterials

Metamaterials enable subwavelength tailoring of light-matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

Temporal and spatial tuning of optical constants in praseodymium doped ceria by electrochemical means

Temporal and spatial tuning of the refractive index of optical thin films is desired for flat optics applications. The redistribution of mobile ions in mixed ionic-electronic conductors (MIEC) has been demonstrated to serve as a viable means for achieving optical tuning down to the nanoscale. Here we studied the dynamic range of the optical tuning achievable in the refractive index, in the MIEC oxide - Pr x Ce1-x O2-δ (PCO), for x = 0.1, 0.2 and 0.4, at 500 °C, by in-situ spectrophotometry. Significant increases in the modulation of both the imaginary and real optical constants in the visible and the adjacent spectra were obtained for increased doping levels. Device employing an electrochemical titration method was implemented to modulate the oxygen concentration, and thereby the optical transmission of PCO. Incorporation of a patterned top electrode allowed for the demonstration of spatial control of PCO thin film properties by in-situ video imaging of the optical switching process. The electrochemically induced optical state is shown to remain non-volatile upon quenching the device to room temperature under applied bias.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Liquid crystal-powered Mie resonators for electrically tunable photorealistic color gradients and dark blacks

Taking inspiration from beautiful colors in nature, structural colors produced from nanostructured metasurfaces have shown great promise as a platform for bright, highly saturated, and high-resolution colors. Both plasmonic and dielectric materials have been employed to produce static colors that fulfil the required criteria for high-performance color printing, however, for practical applications in dynamic situations, a form of tunability is desirable. Combinations of the additive color palette of red, green, and blue enable the expression of further colors beyond the three primary colors, while the simultaneous intensity modulation allows access to the full color gamut. Here, we demonstrate an electrically tunable metasurface that can represent saturated red, green, and blue pixels that can be dynamically and continuously controlled between on and off states using liquid crystals. We use this to experimentally realize ultrahigh-resolution color printing, active multicolor cryptographic applications, and tunable pixels toward high-performance full-color reflective displays.

Responsive photonic nanopixels with hybrid scatterers

Metallic and dielectric nanoscatterers are optical pigments that offer rich resonating coloration in the subwavelength regime with prolonged material consistency. Recent advances in responsive materials, whose mechanical shapes and optical properties can change in response to stimuli, expand the scope of scattering-based colorations from static to active. Thus, active color-changing pixels are achieved with extremely high spatial resolution, in conjunction with various responsive polymers and phase-change materials. This review discusses recent progress in developing such responsive photonic nanopixels, ranging from electrochromic to other color-changing concepts. We describe what parameters permit modulation of the scattering colors and highlight superior functional devices. Potential fields of application focusing on imaging devices, including active full-color printing and flexible displays, information encryption, anticounterfeiting, and active holograms, are also discussed.

Dynamic Color Tuning with Electrochemically Actuated TiO2 Metasurfaces

Dynamic tuning of metamaterials is a critical step toward advanced functionality and improved bandwidth. In the visible spectrum, full spectral color tuning is inhibited by the large absorption that accompanies index changes, particularly at blue wavelengths. Here, we show that the electrochemical lithiation of anatase TiO₂ to Li_0.5TiO₂ (LTO) results in an index change of 0.65 at 649 nm with absorption coefficient less than 0.1 at blue wavelengths, making this material well-suited for dynamic visible color tuning. Dynamic tunability of TiO₂ is leveraged in a Fabry-Perot cavity and a gap plasmon metasurface. In the Fabry-Perot configuration, the device exhibits a shift in reflectance of over 100 nm when subjected to only 2 V bias while the gap plasmon metasurface achieves enhanced switching speed. The dynamic range, speed, and cyclability indicate that the TiO₂/LTO system is competitive with established actuators like WO₃, with the additional advantage of reduced absorption at high frequencies.

Multilevel Absorbers via the Integration of Undoped and Tungsten-Doped Multilayered Vanadium Dioxide Thin Films

Reconfigurable light absorbers have attracted much attention by providing additional optical responses and expanding the number of degrees of freedom in security applications. Fabry-Pèrot absorbers based on phase change materials with tunable properties can be implemented over large scales without the need for additional steps such as lithography, while exhibiting reconfigurable optical responses. However, a fundamental limitation of widely used phase change materials such as vanadium dioxide and germanium-antimony-tellurium-based chalcogenide glasses is that they have only two distinct phases; therefore, only two different states of optical properties are available. Here, we experimentally demonstrate active multilevel absorbers that are tuned by controlling the external temperature. This is produced by creating large-scale lithography-free multilayer structures with both undoped and tungsten-doped solution-processed monoclinic-phase vanadium dioxide thin films. The doping of vanadium dioxide with tungsten allows for the modulation of the phase-transition temperature, which results in an extra degree of freedom and therefore an additional step for the tunable properties. The proposed multilevel absorber is designed and characterized both numerically and experimentally. Such large-scale multilevel tunable absorbers realized with nanoparticle-based solution fabrication techniques are expected to open the way for advanced thermo-optical cryptographic devices based on tunable reflective coloration and near-infrared absorption.

A Reversible Moisture-Responsive Plasmonic Color-Raman and Transmittance Modulation Device by Dispersing Hyaluronan-Functionalized Ag into Nanofibers

Plasmonic physical color generation, which mostly depends on selective absorption, creates unique colors by light transmission and scattering. Based on this, regulating plasmon and transparency with external stimulation is a promising approach for fabricating optical devices with enhanced visual displays; however, few studies have addressed the implementation of dual-optical modulation. In addition, developing a color response to environmental stimuli through the highly shape-sensitive plasmon depth modulation has long remained a significant challenge once the nanostructure is determined. Some stimulations also require high amounts of electricity, which can be costly. In this study, strategically designed hyaluronan-functionalized triangular silver nanoparticles (AgNPs) were embedded in polyvinyl alcohol-polyethylene nanofiber films to achieve a breakthrough in the moisture-responsive dual-optical modulation of the plasmonic color-Raman and transparency. Switchable colors that are reversible were induced in plasmon-resonance-modulation AgNPs via moisture stimulation, adjusting the expansion-tunable dielectric constant of hyaluronan-functionalized AgNPs and varying the electron density due to electron transfer. Furthermore, a moisture gradient was used to decrease the Raman scattering and increase the photoluminescence, which is a significant demonstration of smart-plasmonic evolution. This effect occurred due to the gradual transition from plasmon-driven photoluminescence quenching to photoluminescence enhancement as the interval of the Ag and hyaluronic acid molecules was increased. The transparency of the composite film was also dynamically regulated by turning moisture on/off. This occurred because of the significant difference in hygroscopic expansion between hyaluronan and the nanofibers, which generated a large variation in the total refractive index and caused changes in the surface roughness.

Dynamically Tuneable Reflective Structural Coloration with Electroactive Conducting Polymer Nanocavities

Dynamic control of structural colors across the visible spectrum with high brightness has proven to be a difficult challenge. Here, this is addressed with a tuneable reflective nano-optical cavity that uses an electroactive conducting polymer (poly(thieno[3,4-b]thiophene)) as spacer layer. Electrochemical doping and dedoping of the polymer spacer layer provides reversible tuning of the cavity's structural color throughout the entire visible range and beyond. Furthermore, the cavity provides high peak reflectance that varies only slightly between the reduced and oxidized states of the polymer. The results indicate that the polymer undergoes large reversible thickness changes upon redox tuning, aided by changes in optical properties and low visible absorption. The electroactive cavity concept may find particular use in reflective displays, by opening for tuneable monopixels that eliminate limitations in brightness of traditional subpixel-based systems.

Manipulating Nanowire Assemblies toward Multicolor Transparent Electrochromic Device

Assembling various nanowires together, enabling the assemblies with tailored optical, electrical, and multifunctional properties, represents a promising technology for next generation multifunctional electronics. Here we demonstrate a novel multicolor electrochromic device by coassembling W₁₈O₄₉ and V₂O₅ nanowires using solution-based Langmuir-Blodgett technique. The transparent W₁₈O₄₉ nanowire film became orange with the increasing addition of V₂O₅ nanowires and the film underwent a dynamic color change (orange, green, and gray) on application of different electrochemical biases of 2, 0, and -0.5 V (vs Ag/AgCl). Both the transmittance and color of the device can be easily controlled by manipulating the layers of coassembled nanowires and the ratios between the two nanowires. On the basis of this approach, different patterns can be easily fabricated with the addition of corresponding masks, and the solid electrochromic device is assembled, suggesting its significant potentials in smart windows and multicolor electrochromic displays.

Reversible Zn2+ Insertion in Tungsten Ion-Activated Titanium Dioxide Nanocrystals for Electrochromic Windows

Zinc-anode-based electrochromic devices (ZECDs) are emerging as the next-generation energy-efficient transparent electronics. We report anatase W-doped TiO₂ nanocrystals (NCs) as a Zn²⁺ active electrochromic material. It demonstrates that the W doping in TiO₂ highly reduces the Zn²⁺ intercalation energy, thus triggering the electrochromism. The prototype ZECDs based on W-doped TiO₂ NCs deliver a high optical modulation (66% at 550 nm), fast spectral response times (9/2.7 s at 550 nm for coloration/bleaching), and good electrochemical stability (8.2% optical modulation loss after 1000 cycles).

Electrochromic Inorganic Nanostructures with High Chromaticity and Superior Brightness

The possibility of actively controlling structural colors has recently attracted a lot of attention, in particular for new types of reflective displays (electronic paper). However, it has proven challenging to achieve good image quality in such devices, mainly because many subpixels are necessary and the semitransparent counter electrodes lower the total reflectance. Here we present an inorganic electrochromic nanostructure based on tungsten trioxide, gold, and a thin platinum mirror. The platinum reflector provides a wide color range and makes it possible to "reverse" the device design so that electrolyte and counter electrode can be placed behind the nanostructures with respect to the viewer. Importantly, this makes it possible to maintain high reflectance regardless of how the electrochemical cell is constructed. We show that our nanostructures clearly outperform the latest commercial color e-reader in terms of both color range and brightness.