Fabrication of Bragg Mirrors by Multilayer Inkjet Printing

A 3D-Printed Bi-Material Bragg-Based Reflectarray Antenna

This paper presents a 3D-printed fully dielectric bi-material reflectarray with bandgap characteristics for multi-band applications. To achieve bandgap characteristics, a "1D Bragg reflector" unit cell is used. The latter is a layered structure characterized by a spatial distribution of refractive index that varies periodically along one dimension. By appropriately selecting the dimensions, the bandgap can be shifted to cover the desired frequency bands. To validate this bandgap characteristic, a (121.5 mm × 121.5 mm) with an f/D ratio of 0.5 reflectarray was fabricated. The measured gain at 27 GHz is 27.22 dBi, equivalent to an aperture efficiency of 35.05%, demonstrating good agreement between simulated and measured performances within the frequency range of 26-30 GHz. Additionally, the transparency of the reflectarray was verified by measuring the transmission coefficient, which exhibited a high level of transparency of 0.32 dB at 39 GHz. These features make the proposed reflectarray a good candidate for multi-band frequency applications.

Proximal High-Index Metamaterials based on a Superlattice of Gold Nanohexagons Targeting the Near-Infrared Band

Plasmonic nanoparticles can be assembled into a superlattice, to form optical metamaterials, particularly targeting precise control of optical properties such as refractive index (RI). The superlattices exhibit enhanced near-field, given the sufficiently narrow gap between nanoparticles supporting multiple plasmonic resonance modes only realized in proximal environments. Herein, the planar superlattice of plasmonic Au nanohexagons (AuNHs) with precisely controlled geometries such as size, shape, and edge-gaps is reported. The proximal AuNHs superlattice realized over a large area with selective edge-to-edge assembly exhibited the highest-ever-recorded RI values in the near-infrared (NIR) band, surpassing the upper limit of the RI of the natural intrinsic materials (up to 10.04 at λ = 1.5 µm). The exceptionally enhanced RI is derived from intensified in-plane surface plasmon coupling across the superlattices. Precise control of the edge-gap of neighboring AuNHs systematically tuned the RI as confirmed by numerical analysis based on the plasmonic percolation model. Furthermore, a 1D photonic crystal, composed of alternating layers of AuNHs superlattices and low-index polymers, is constructed to enhance the selectivity of the reflectivity operating in the NIR band. It is expected that the proximal AuNHs superlattices can be used as new optical metamaterials that can be extended to the NIR range.

On-Demand Picoliter-Level-Droplet Inkjet Printing for Micro Fabrication and Functional Applications

With the advent of Internet of Things (IoTs) and wearable devices, manufacturing requirements have shifted toward miniaturization, flexibility, environmentalization, and customization. Inkjet printing, as a non-contact picoliter-level droplet printing technology, can achieve material deposition at the microscopic level, helping to achieve high resolution and high precision patterned design. Meanwhile, inkjet printing has the advantages of simple process, high printing efficiency, mask-free digital printing, and direct pattern deposition, and is gradually emerging as a promising technology to meet such new requirements. However, there is a long way to go in constructing functional materials and emerging devices due to the uncommercialized ink materials, complicated film-forming process, and geometrically/functionally mismatched interface, limiting film quality and device applications. Herein, recent developments in working mechanisms, functional ink systems, droplet ejection and flight process, droplet drying process, as well as emerging multifunctional and intelligence applications including optics, electronics, sensors, and energy storage and conversion devices is reviewed. Finally, it is also highlight some of the critical challenges and research opportunities. The review is anticipated to provide a systematic comprehension and valuable insights for inkjet printing, thereby facilitating the advancement of their emerging applications.

Photon Management Enabled by Opal and Inverse Opal Photonic Crystals: from Photocatalysis to Photoluminescence Regulation

Light is a promising renewable energy source and can be converted into heat, electricity, and chemical energy. However, the efficiency of light-energy conversion is largely hindered by limited light-absorption coefficients and the low quantum yield of current-generation materials. Photonic crystals (PCs) can adjust the propagation and distribution of photons because of their unique periodic structures, which offers a compelling platform for photon management. The periodicity of materials with an alternating refractive index can be used to manipulate the dispersion of photons to generate the photonic bandgap (PBG), in which light is reflected. The slow photon effect, i. e., photon propagation at a reduced group velocity near the edges of the PBG, is widely regarded as another valuable optical property for manipulating light. Furthermore, multiple light scattering can increase the optical path, which is a vital optical property for PCs. Recently, the light reflected by PBG, the slow photon effect, and multiple light scattering have been exploited to improve light utilization efficiency in photoelectrochemistry, materials chemistry, and biomedicine to enhance light-energy conversion efficiency. In this review, the fabrication of opal or inverse opal PCs and the theory for improving the light utilization efficiency of photocatalysis, solar cells, and photoluminescence regulation are discussed. We envision photon management of opal or inverse opal PCs may provide a promising avenue for light-assisted applications to improve light-energy-conversion efficiency.

Compact multispectral light field camera based on an inkjet-printed microlens array and color filter array

With emerging advanced optical sensing technologies and their wide-ranging applications, gathering comprehensive optical data from real-world scenes is becoming increasingly crucial for their accurate reconstruction and analysis. In order to capture both three-dimensional (3D) spatial and spectral information from a scene, multiple devices or time-intensive scanning processes are often involved. Here, we demonstrate a multispectral light field camera that allows for the simultaneous acquisition of 3D information and spectral data in a single snapshot. By utilizing inkjet printing as the fabrication technology, the miniaturized optical components in the camera were manufactured with high precision and can be integrated into a standard camera housing. Furthermore, the microlens arrays and the color filter arrays were fabricated on the same substrate, and a precise alignment between the two arrays was achieved. The compact multispectral camera opens the door to a multitude of possibilities for mobile applications, ranging from autonomous driving and consumer electronics such as smartphones to medical technology such as endoscopes.

Solution-Processed One-Dimensional Photonic Crystals Based on Hollow Silica Exhibiting High Refractive Index Contrast

Poor interfacial quality and low refractive index contrast (Δn) are critical challenges for the development of high-performance one-dimensional photonic crystals (1DPhCs) via solution methods that impede their optical efficiency. Herein, we introduce an innovative approach by hybridizing hollow SiO2 with poly(vinyl alcohol), referred to as PHS, followed by alternate assembly with TiO2 via spin-coating, achieving a 1DPhC with Δn = 0.76 at the wavelength of 550 nm. This method circumvents the need for high-temperature treatment and complex curing conditions, resulting in a 1DPhC with superior interfacial and optical characteristics. By adjusting the thickness of the PHS layers, we can finely tune the reflectance spectrum, attaining over 99% reflectance at the photonic band gap. Furthermore, 1DPhC demonstrates excellent adhesion to polycarbonate substrates and retains its optimal optical performance even after rigorous environmental testing, including hygrothermal cycles, exposure to hot water, friction, and solvent sonication. This research paves the way for the facile fabrication of high-performance 1DPhCs under mild conditions, offering new perspectives for photonic material processing.

Droplet-Pen Writing of Ultra-Uniform Graphene Pattern for Multi-Spectral Applications

Artificial optical patterns bring wide benefits in applications like structural color display, photonic camouflage, and electromagnetic cloak. Their scalable coating on large-scale objects will greatly enrich the multimodal-interactive society. Here, a droplet-pen writing (DPW) method to directly write multi-spectral patterns of thin-film graphene is reported. By amphiphilicity regulations of 2D graphene nanosheets, ultra-uniform and ultrathin films can spontaneously form on droplet caps and pave to the substrate, thus inducing optical interference. This allows the on-surface patterning by pen writing of droplets. Specifically, drop-on-demand thin films are achieved with millimeter lateral size and uniformity up to 97% in subwavelength thickness (<100 nm), corresponding to an aspect ratio of over 30 000. The pixelated thin-film patterns of disks and lines in an 8-inch wafer scale are demonstrated, which enable low-emittance structural color paintings. Furthermore, the applications of these patterns for dual-band camouflage and infrared-to-visible encryption are investigated. This study highlights the potential of 2D material self-assembly in the large-scale preparation and multi-spectral application of thin film-based optical patterns.

Inkjet-printed optical interference filters

Optical interference filters (OIFs) are vital components for a wide range of optical and photonic systems. They are pivotal in controlling spectral transmission and reflection upon demand. OIFs rely on optical interference of the incident wave at multilayers, which are fabricated with nanometer precision. Here, we demonstrate that these requirements can be fulfilled by inkjet printing. This versatile technology offers a high degree of freedom in manufacturing, as well as cost-affordable and rapid-prototyping features from the micron to the meter scale. In this work, via rational ink design and formulation, OIFs were fully inkjet printed in ambient conditions. Longpass, shortpass, bandpass, and dichroic OIFs were fabricated, and precise control of the spectral response in OIFs was realized. Subsequently, customized lateral patterning of OIFs by inkjet printing was achieved. Furthermore, upscaling of the printed OIFs to A4 size (29.7 × 21.0 cm²) was demonstrated.

Fully inkjet-printed Ag2Se flexible thermoelectric devices for sustainable power generation

Flexible thermoelectric devices show great promise as sustainable power units for the exponentially increasing self-powered wearable electronics and ultra-widely distributed wireless sensor networks. While exciting proof-of-concept demonstrations have been reported, their large-scale implementation is impeded by unsatisfactory device performance and costly device fabrication techniques. Here, we develop Ag2Se-based thermoelectric films and flexible devices via inkjet printing. Large-area patterned arrays with microscale resolution are obtained in a dimensionally controlled manner by manipulating ink formulations and tuning printing parameters. Printed Ag2Se-based films exhibit (00 l)-textured feature, and an exceptional power factor (1097 μWm-1K-2 at 377 K) is obtained by engineering the film composition and microstructure. Benefiting from high-resolution device integration, fully inkjet-printed Ag2Se-based flexible devices achieve a record-high normalized power (2 µWK-2cm-2) and superior flexibility. Diverse application scenarios are offered by inkjet-printed devices, such as continuous power generation by harvesting thermal energy from the environment or human bodies. Our strategy demonstrates the potential to revolutionize the design and manufacture of multi-scale and complex flexible thermoelectric devices while reducing costs, enabling them to be integrated into emerging electronic systems as sustainable power sources.

Physiochemical Coupled Dynamic Nanosphere Lithography Enabling Multiple Metastructures from Single Mask

Metastructures are widely used in photonic devices, energy conversion, and biomedical applications. However, to fabricate multiple patterns continuously in single etching protocol with highly tunable photonic properties is challenging. Here, a simple and robust dynamic nanosphere lithography is proposed by inserting a spacer between the nanosphere assembly and the wafer. The nanosphere diameter decrease and uneven penetration of the spacer during etching lead to a dynamic masking process. Coupled anisotropic physical ion sputtering and ricocheting with isotropic chemical radical etching achieve highly tunable structures with various 3D patterns continuously forming through a single etching process. Specifically, the nanosphere diameters define the periodicity, the etched spacer forms the upper parts, and the wafer forms the lower parts. Each part of the structure is highly tunable through changing nanosphere diameter, spacer thickness, and etch conditions. Using this protocol, numerous structures of varying sizes including nanomushrooms, nanocones, nanopencils, and nanoneedles with diverse shapes are realized as proof of concepts. The broadband antireflection ability of the nanostructures and their use in surface-enhanced Raman spectroscopy are also demonstrated for practical application. This method substantially simplifies the fabrication procedure of various metastructures, paving the way for its application in multiple disciplines especially in photonic 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.

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.

Developing Solution-Processed Distributed Bragg Reflectors for Microcavity Polariton Applications

Improving the performance of organic optoelectronics has been under vigorous research for decades. Recently, polaritonics has been introduced as a technology that has the potential to improve the optical, electrical, and chemical properties of materials and devices. However, polaritons have been mainly studied in optical microcavities that are made by vacuum deposition processes, which are costly, unavailable to many, and incompatible with printed optoelectronics methods. Efforts toward the fabrication of polariton microcavities with solution-processed techniques have been utterly absent. Herein, we demonstrate for the first time strong light-matter coupling and polariton photoluminescence in an organic microcavity consisting of an aluminum mirror and a distributed Bragg reflector (DBR) made by sequential dip coating of titanium hydroxide/poly(vinyl alcohol) (TiOH/PVA) and Nafion films. To fabricate and develop the solution-processed DBRs and microcavities, we automatized a dip-coating device that allowed us to produce sub-100 nm films consistently over many dip-coating cycles. Owning to the solution-based nature of our DBRs, our results pave the way to the realization of polariton optoelectronic devices beyond physical deposition methods.

THz Bragg structures fabricated with additive manufacturing

The advancement of THz science and technology is desirable to facilitate the application of THz technologies in many sectors. Specialized THz photonic elements for these applications require desirable absorption and refractive characteristics in the THz regime. THz photonic elements can be created with additive manufacturing, and specifically 3D printing, forgoing the need for complex fabrication procedures and methodologies. Such THz photonic elements include periodic Bragg structures, which are capable of filtering specific THz frequencies. The authors present a THz Bragg structure fabricated with 3D printing via fused filament fabrication. The THz Bragg structure is made from high-impact polystyrene filament material, which is characterized in this paper with THz time-domain spectroscopy. The geometry and theoretical operation of the THz Bragg structure is investigated with finite-difference time-domain electromagnetic simulations. The THz Bragg structure is evaluated using a THz experimental test bed. There is agreement between the theoretical and the experimental filtering placement within the frequency domain for the THz Bragg structure. The capability of tunable frequency filtering of the presented THz Bragg structure, fabricated with 3D printing, is established and facilitates future advancements in applications of THz science and technology.

Nonpolar Solvent Modulated Inkjet Printing of Nanoparticle Self-Assembly Morphologies

Patterning of luminescent nanomaterials is critical in the fields of display and information encryption, and inkjet printing technology have shown remarkable significance with the advantage of fast, large-scalable and integrative. However, inkjet printing nanoparticle deposits with high-resolution and well controlled morphology from nonpolar solvent droplets is still challenging. Herein, a facile approach of nonpolar solvent modulated inkjet printing of nanoparticles self-assembly patterns driven by the shrinkage of the droplet and inner solutal convection is proposed. Through regulating the solvent composition and nanoparticle concentration, multicolor light-emissive upconversion nanoparticle self-assembly microarrays with tunable morphologies are achieved, showing the integration of designable microscale morphologies and photoluminescences for multimodal anti-counterfeit. Furthermore, inkjet printing of nanoparticles self-assembled continuous lines with adjustable morphologies by controlling the coalescence and drying of the ink droplets is achieved. The high resolution of inkjet printing microarrays and continuous lines' width < 5 and 10 µm is realized, respectively. This nonpolar solvent-modulated inkjet printing of nanoparticle deposits approach facilitates the patterning and integration of different nanomaterials, and is expected to provide a versatile platform for fabricating advanced devices applied in photonics integration, micro-LED, and near-field display.

Bioinspired MXene-Based Soft Actuators Exhibiting Angle-Independent Structural Color

In nature, many living organisms exhibiting unique structural coloration and soft-bodied actuation have inspired scientists to develop advanced structural colored soft actuators toward biomimetic soft robots. However, it is challenging to simultaneously biomimic the angle-independent structural color and shape-morphing capabilities found in the plum-throated cotinga flying bird. Herein, we report biomimetic MXene-based soft actuators with angle-independent structural color that are fabricated through controlled self-assembly of colloidal SiO2 nanoparticles onto highly aligned MXene films followed by vacuum-assisted infiltration of polyvinylidene fluoride into the interstices. The resulting soft actuators are found to exhibit brilliant, angle-independent structural color, as well as ultrafast actuation and recovery speeds (a maximum curvature of 0.52 mm-1 can be achieved within 1.16 s, and a recovery time of ~ 0.24 s) in response to acetone vapor. As proof-of-concept illustrations, structural colored soft actuators are applied to demonstrate a blue gripper-like bird's claw that can capture the target, artificial green tendrils that can twine around tree branches, and an artificial multicolored butterfly that can flutter its wings upon cyclic exposure to acetone vapor. The strategy is expected to offer new insights into the development of biomimetic multifunctional soft actuators for somatosensory soft robotics and next-generation intelligent machines.