Full Color Generation Using Silver Tandem Nanodisks

All-dielectric structural coloration empowered by bound states in the continuum

The technological requirements of low-power and high-fidelity color displays have been instrumental in driving research into advanced coloration technologies. At the forefront of these developments is the implementation of dye-free approaches, which overcome previous constraints related to color resolution and fading. Resonant dielectric nanostructures have emerged as a promising paradigm, showing great potential for high efficiency, high color saturation, wide gamut palette, and image reproduction. However, they still face limitations related to color accuracy, purity, and simultaneous brightness tunability. Here, we demonstrate an all-dielectric metasurface empowered by photonic bound states in the continuum (BICs), which supports sharp resonances throughout the visible spectral range, ideally suited for producing a wide range of structural colors. The metasurface design consists of TiO2 ellipses with carefully controlled sizes and geometrical asymmetry, allowing versatile and on-demand variation of the brightness and hue of the output colors, respectively.

Tunable Plasmon Resonance in Silver Nanodisk-on-Mirror Structures and Scattering Enhancement by Annealing

In this study, we evaluated the surface plasmon characteristics of periodic silver nanodisk structures fabricated on a dielectric thin-film spacer layer on a Ag mirror substrate (NanoDisk on Mirror: NDoM) through finite difference time domain (FDTD) simulations and experiments involving actual sample fabrication. Through FDTD simulations, it was confirmed that the NDoM structure exhibits two sharp peaks in the visible range, and by adjusting the thickness of the spacer layer and the size of the nanodisk structure, sharp peaks can be obtained across the entire visible range. Additionally, we fabricated the NDoM structure using electron beam lithography (EBL) and experimentally confirmed that the obtained peaks matched the simulation results. Furthermore, we discovered that applying annealing at an appropriate temperature to the fabricated structure enables the adjustment of the resonance peak wavelength and enhances the scattering intensity by approximately five times. This enhancement is believed to result from changes in the shape and size of the nanodisk structure, as well as a reduction in grain boundaries in the metal crystal due to annealing. These results have the potential to contribute to technological advancements in various application fields, such as optical sensing and emission enhancement.

Bi-Directional Full-Color Generation and Tri-Channel Information Encoding Based on a Plasmonic Metasurface

Dynamic optical structural color is always desired in various display applications and usually involves active materials. Full-color generation, especially bi-directional full-color generation in both reflective and transmissive modes, without any active materials included, has rarely been investigated. Herein, we demonstrate a scheme of bi-directional full-color generation based on a plasmonic metasurface modulated by the rotation of the polarization angle of the incident light without varying the geometry and the optical properties of the materials and the environment where the metasurface resides. The metasurface unit cell consists of plasmonic modules aligning in three directions and is patterned in a square array. The metasurface structural color device is numerically confirmed to generate full colors in both reflection and transmission. Based on the proposed polarization-dependent structural color, the information encoding process is demonstrated for three multiplexed animal images and quick-responsive (QR) codes to verify the efficient information encoding and decoding of the proposed scheme. In the simulation, the animals can be seen under different polarization incidences, and the QR codes can be successfully decoded by the polarization rotation in transmission. The proposed bi-directional full-color generation metasurface has great potential in applications such as kaleidoscope generation, anti-counterfeiting, dynamic color display, and optical information encoding.

Structural color filters with compensated angle-dependent shifts

Structural color filters use nano-sized elements to selectively transmit incident light, offering a scalable, economical, and environmentally friendly alternative to traditional pigment- and dye-based color filters. However, their structural nature makes their optical response prone to spectral shifts whenever the angle of incidence varies. We address this issue by introducing a conformal VO2 layer onto bare aluminum structural color filters. The insulator-metal transition of VO2 compensated the spectral shift of the filter's transmission at a 15° tilt with 80% efficiency. Unlike solutions that require adjustment of the filter's geometry, this method is versatile and suitable also for existing structural filters. Our findings also establish tunable materials in general as a possible solution for angle-dependent spectral shifts.

Hybridization between plasmonic and photonic modes in laser-induced self-organized quasi-random plasmonic metasurfaces

Plasmonic metasurfaces made of perfectly regular 2D lattices of metallic nanoparticles deposited on surfaces or close to waveguides can exhibit hybridized plasmonic and photonic modes. The latter arise from the excitation of surface or guided modes through the in-plane coherent scattering of periodic arrays. Recently, laser-induced self-organization of random plasmonic metasurfaces has been used to create nanoparticle gratings embedded in protective layers. Despite the broad size distribution and positional disorder of nanoparticles, the resulting nanostructures exhibit strong coupling between plasmonic and photonic modes in transverse electric polarization, leading to dichroism, which is well-reproduced from one laser printing to another. Here, we examine quantitatively the effect of inhomogeneities at the nanoscale on the hybridization between localized plasmonic modes and delocalized guided modes by considering realistic laser-induced self-organized nanoparticle arrays embedded in a two-layer system. By referring to regular samples, we describe the optical mechanisms involved in the hybridization process at characteristic wavelengths, based on far and near field simulations. Two kinds of real samples are considered, featuring different levels of coupling between the plasmonic and photonic modes. The results demonstrate that controlling the statistical properties of plasmonic metasurfaces, such as the nanoparticle size distribution and average position, over areas a few micrometers wide is enough to control in a reproducible manner the hybridization mechanisms and their resulting optical properties. Thus, this study shows that the inherent irregularities of laser-induced self-organized nanostructures are compatible with smart functionalities of nanophotonics, and confirms that laser processing has huge potential for real-world applications.

Plasmonic Metamaterial Ag Nanostructures on a Mirror for Colorimetric Sensing

In this study, we demonstrate the localized surface plasmon resonance (LSPR) in the visible range by using nanostructures on mirrors. The nanohemisphere-on-mirror (NHoM) structure is based on random nanoparticles that were obtained by heat-treating silver thin films and does not require any top-down nanofabrication processes. We were able to successfully tune over a wide wavelength range and obtain full colors using the NHoM structures, which realized full coverage of the Commission Internationale de l'Eclairage (CIE) standard RGB (sRGB) color space. Additionally, we fabricated the periodic nanodisk-on-glass (NDoG) structure using electron beam lithography and compared it with the NHoM structure. Our analysis of dark-field microscopic images observed by a hyperspectral camera showed that the NHoM structure had less variation in the resonant wavelength by observation points compared with the periodic NDoG structure. In other words, the NHoM structure achieved a high color quality that is comparable to the periodic structure. Finally, we proposed colorimetric sensing as an application of the NHoM structure. We confirmed the significant improvement in performance of colorimetric sensing using the NHoM structure and succeeded in colorimetric sensing using protein drops. The ability to fabricate large areas in full color easily and inexpensively with our proposed structures makes them suitable for industrial applications, such as displays, holograms, biosensing, and security applications.

Optical metasurfaces for multiplex high-performance grating-type structural colors

Optical metasurfaces provide a significant approach for the production of structural colors due to their excellent optical control abilities. Herein, we propose trapezoidal structural metasurfaces for achieving multiplex grating-type structural colors with high comprehensive performance originating from the anomalous reflection dispersion in the visible band. Single trapezoidal metasurfaces with different x-direction periods can tune the angular dispersion regularly from 0.036 rad/nm to 0.224 rad/nm to generate various structural colors, and composite trapezoidal metasurfaces with three kinds of combinations can achieve multiplex sets of structural colors. The brightness can be controlled by adjusting the distance between the trapezoids in a pair accurately. The designed structural colors have higher saturation than traditional pigmentary colors, whose excitation purity can reach 1.00. The gamut is about 158.1% of the Adobe RGB standard. This research has application potential in ultrafine displays, information encryption, optical storage, and anti-counterfeit tagging.

Multicolor detour phase holograms based on an Al plasmonic color filter

The remarkable advances in nanofabrication that have occurred over the last decade present opportunities for the realization of new types of holograms. In this work, for the first time to the best of our knowledge, a method for phase multicolor holograms based on nanohole arrays is described. The nanoholes are in an aluminum film that is interposed between the glass substrate and a silicon dioxide layer. The nanoholes serve as color filters for blue, green, and red wavelengths and provide the necessary phase distribution via the detour phase method. Our nanohole arrays are optimized to maximize the transmission efficiency of the red, green, and blue channels and to minimize the cross-talk between them. We design two multicolor holograms based on these filters and simulate their performance. The results show good fidelity to the desired holographic images. The proposed structure has the advantages of being very compact, of requiring only a simple fabrication method with one lithography step, and of employing materials (aluminum and silicon dioxide) that are compatible with standard CMOS technology.

Polarization tunable transmitted full-color display enabling switchable bright and dark states

Although structural colors based on nanostructures have attracted many researchers' attentions due to their superior durability and high resolution, most previous reports focused on the static and dynamic structural colors in reflection mode and few researchers focus on the static and dynamic transmission colors for high-saturation RGB models. Here, the hybrid Al-Si₃N₄ nanogratings with the top SiO₂ capping layer and the bottom MgF₂ layer that can switch full-hue and high-saturation transmitted structural colors on and off completely by changing the polarization state are theoretically demonstrated. Meanwhile, the hybrid Al-Si₃N₄ nanogratings with the top capping layer and the bottom layer also achieve the transmittance spectra with the full width at half maximum of ∼58 nm and the transmittance efficiency of over 70% in the on state. The added top capping layer and bottom layer can suppress the sideband of transmittance spectra in the on state and maintain the near-zero transmittance in the off state, thus improving the switching performance between bright and dark states. The realizable high-saturation colors in the on state can take up 125% sRGB space and 80% Adobe sRGB space. More interestingly, with the incident angle varying from 0° to 50°, full-hue color can be also realized in the on state and nearly black color can be also maintained in the off state. The strategy will provide potential applications in advanced color encryption and multichannel imaging.

Versatile full-colour nanopainting enabled by a pixelated plasmonic metasurface

The growing interest to develop modern digital displays and colour printing has driven the advancement of colouration technologies with remarkable speed. In particular, metasurface-based structural colouration shows a remarkable high colour saturation, wide gamut palette, chiaroscuro presentation and polarization tunability. However, previous approaches cannot simultaneously achieve all these features. Here, we design and experimentally demonstrate a surface-relief plasmonic metasurface consisting of shallow nanoapertures that enable the independent manipulation of colour hue, saturation and brightness by individually varying the geometric dimensions and orientation of the nanoapertures. We fabricate microscale artworks using a reusable template-stripping technique that features photorealistic and stereoscopic impressions. In addition, through the meticulous arrangement of differently oriented nanoapertures, kaleidoscopic information states can be decrypted by particular combinations of incident and reflected polarized light.

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.

Lattice relaxation effects on the collective resonance spectra of a finite dipole array

Applying lattice parameter relaxation on a finite photonic crystal can adjust the smoothness of its surface lattice resonance spectral peak.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

All-dielectric high saturation structural colors enhanced by multipolar modulated metasurfaces

A visible light depth modulation based on a metasurface consisting of TiO₂ nanorings and SiO₂ substrate is proposed to significantly enhance the saturation and structural colors' gamut. Compared with the nanostructure of the TiO₂ nanodisks, the developed TiO₂ nanorings can enhance monochromatic excitation by inhibiting the multipole mode, particularly electric quadrupole (EQ) mode at a shorter wavelength. Furthermore, when TiO₂ nanorings are combined with a refractive index matching layer - water, reflection bandwidth, and background reflection are reduced, and the brightness and color purity are significantly enhanced. The novel and unique nanostructures developed can generate a significant gamut of 140% sRGB and 103% Adobe RGB. Additionally, the color structure based on the TiO₂ nanoring metasurface is sensitive to the surrounding medium's refractive index and can be employed in sensor display and other fields, as well as to amplify color information in high resolution display and imaging applications.

Robustness Analysis of Metasurfaces: Perfect Structures Are Not Always the Best

Optical metasurfaces rely on subwavelength scale nanostructures, which puts significant constraints on nanofabrication accuracies. These constraints are becoming increasingly important, as metasurfaces are maturing toward real applications that require the fabrication of very large area samples. Here, we focus on beam steering gradient metasurfaces and show that perfect nanofabrication does not necessarily equate with best performances: metasurfaces with missing elements can actually be more efficient than intact metasurfaces. Both plasmonic metasurfaces in reflection and dielectric metasurfaces in transmission are investigated. These findings are substantiated by experiments on purposely misfabricated metasurfaces and full-wave calculations. A very efficient quasi-analytical model is also introduced for the design and simulations of metasurfaces; it agrees very well with full-wave calculations. Our findings indicate that the substrate properties play a key role in the robustness of a metasurface and the smoothness of the approximated phase gradient controls the device efficiency.

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.

Active electrochemical high-contrast gratings as on/off switchable and color tunable pixels

To be viable for display applications, active structural colors must be electrically tunable, on/off switchable, and reversible. Independently controlling the first two functions, however, is difficult because of causality that ties the real and imaginary parts of the optical constants or changing overlap of fields during structural variations. Here, we demonstrate an active reflective color pixel that encompasses separate mechanisms to achieve both functions reversibly by electrochemically depositing and dissolving Cu inside the dielectric grating slits on a Pt electrode with ΔV < 3 V. Varying the modal interference via Cu occupancy in the slits changes the CIE space coverage by up to ~72% under cross-polarized imaging. In the same pixel, depolarization and absorption by the dissolving porous Cu switches the color off with a maximum contrast of ~97%. Exploiting these results, we demonstrate an active color-switching display and individually addressable on/off pixel matrix that highlights their potential in reflective display applications.

Two key display operations, color tuning and on/off switching, are achieved with reflective structural colors by changing the modal interference conditions and absorption via electrochemical control of Cu occupancy inside dielectric grating slits.

Low Cost and Easy Validation Anticounterfeiting Plasmonic Tags Based on Thin Films of Metal and Dielectric

Multilevel anticounterfeiting Physical Unclonable Function (PUF) tags based on thin film of silver (Ag), Zinc Oxide (ZnO) and PolyVinylPyrrolidone (PVP), are experimentally demonstrated and validated. We exploit the low adhesion of silver to glass and consequent degradation during ZnO deposition to induce morphological randomness. Several photographs of the tag surfaces have been collected with different illumination conditions and using two smartphones of diverse brand. The photos were analyzed using an image recognition algorithm revealing low common minutiae for different tags. Moreover, the optical response reveals peculiar spectra due to labels of plasmonic nature. The proposed systems can be easily fabricated on large areas and represent a cost-effective solution for practical protection of objects.

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.

Large-Scale and Wide-Gamut Coloration at the Diffraction Limit in Flexible, Self-Assembled Hierarchical Nanomaterials

Unveiling physical phenomena that generate controllable structural coloration is at the center of significant research efforts due to the platform potential for the next generation of printing, sensing, displays, wearable optoelectronics components, and smart fabrics. Colors based on e-beam facilities possess high resolutions above 100k dots per inch (DPI), but limit manufacturing scales up to 4.37 cm² , while requiring rigid substrates that are not flexible. State-of-art scalable techniques, on the contrary, provide either narrow gamuts or small resolutions. A common issue of current methods is also a heterogeneous resolution, which typically changes with the color printed. Here, a structural coloration platform with broad gamuts exceeding the red, green, and blue (RGB) spectrum in inexpensive, thermally resistant, flexible, and metallic-free structures at constant 101 600 DPI (at the diffraction limit), obtained via mass-production manufacturing is demonstrated. This platform exploits a previously unexplored physical mechanism, which leverages the interplay between strong scattering modes and optical resonances excited in fully 3D dielectric nanostructures with suitably engineered longitudinal profiles. The colors obtained with this technology are scalable to any area, demonstrated up to the single wafer (4 in.). These results open real-world applications of inexpensive, high-resolution, large-scale structural colors with broad chromatic spectra.

Tunable circular dichroism through absorption in coupled optical modes of twisted triskelia nanostructures

We present a system consisting of two stacked chiral plasmonic nanoelements, so-called triskelia, that exhibits a high degree of circular dichroism. The optical modes arising from the interactions between the two elements are the main responsible for the dichroic signal. Their excitation in the absorption cross section is favored when the circular polarization of the light is opposite to the helicity of the system, so that an intense near-field distribution with 3D character is excited between the two triskelia, which in turn causes the dichroic response. Therefore, the stacking, in itself, provides a simple way to tune both the value of the circular dichroism, up to 60%, and its spectral distribution in the visible and near infrared range. We show how these interaction-driven modes can be controlled by finely tuning the distance and the relative twist angle between the triskelia, yielding maximum values of the dichroism at 20° and 100° for left- and right-handed circularly polarized light, respectively. Despite the three-fold symmetry of the elements, these two situations are not completely equivalent since the interplay between the handedness of the stack and the chirality of each single element breaks the symmetry between clockwise and anticlockwise rotation angles around 0°. This reveals the occurrence of clear helicity-dependent resonances. The proposed structure can be thus finely tuned to tailor the dichroic signal for applications at will, such as highly efficient helicity-sensitive surface spectroscopies or single-photon polarization detectors, among others.

Bidirectional nanoprinting based on bilayer metasurfaces

Bidirectional nanoprinting, has received significant attention in image display and on-chip integration, due to its superior advantages. By manipulating the amplitude in a narrow- or broad-band wavelength range of forward and backward incident light, different spatially varied intensities or color distributions can be generated on the structure plane. However, the current scheme cannot fully decouple the bidirectional light intensity due to the limitation of design degree of freedom, and it would hinder the development of asymmetric photonic devices. In this paper, we propose and demonstrate bidirectional nanoprinting based on an all-dielectric bilayer metasurface, which can independently control the intensity of forward and backward incident light, resulting in two different continuous grayscale meta-image displaying in the visible region. This asymmetric but still bidirectional optical response is introduced by stacking two layers of nanostructures with different functionality in space, in which the first- and second-layer nanostructures act as a half-wave plate and a polarizer, respectively. Interestingly, these bidirectional nanoprinting metasurfaces have flexible working modes and may bring great convenience for practical applications. Specifically, two different meta-images generated by a bidirectional nanoprinting metasurface can be displayed not only on two sides of the metasurface (working mode in transmission or reflection), but on the same side due to the forward transmitted light and backward reflected light also having asymmetric optical properties. Similar phenomena also exist for forward reflected light and backward transmitted light. Our work extremely expands the design freedom for metasurface devices and may play a significant role in the field of optical display, information multiplexing, etc.

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.

Laser nanoprinting of floating three-dimensional plasmonic color in pH-responsive hydrogel

Recent demonstrations of metasurfaces present their great potential to implement flat and multifunctional optical elements, which are accomplished with the designs of planar optics and micro-/nano- fabrications. Integrating metasurfaces in three dimensions has manifested drastically increasing advantages in manipulating light fields by extending design freedom. However, fabricating three-dimensional metasurfaces remain a tough challenge due to the lack of stereo printing protocols. Herein, we demonstrate laser nanoprinting of floated silver nanoparticle array in transparent hydrogel films for 3D metasurface to achieve color patterning. It is found that spatially resolved nanoparticles can be produced through laser induced photoreduction of silver ions and robustly anchored to the gel backbones by a focused femtosecond laser beam within a pH-responsive smart hydrogel matrix. With the aid of expansion properties of the pH-responsive hydrogel, repetitive coloration of the patterned plasmonic nanoparticle array over a wide spectrum range is achieved via reversible regulation of nanoparticle spacing from 550 to 350 nm and vice versa. This approach allows broadband 3D color-regulation in nanoscale for applications in active spectral filtering, information encryption, security tagging and biological colorimetric sensing, etc.

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.

Surface plasmon induced spot and line formation at interfaces of ITO coated LiNbO3 slabs and gigantic nonlinearity

Remarkable spots and lines were clearly observed at the two interfaces of indium-tin-oxide coated Z-cut Fe-doped lithium noibate plates under illumination by milliwatt continuous-wave laser light; this occurred because of the visible surface plasmons (SPs) supported by the promising non-metal plasmonic system. The intriguing observations are here explained via the SP-strengthened nonlinear effect, through consideration of the electrostatic field (which is comparable to the atomic field) and its large gradient; this hints at a promising, highly sensitive plasmonic system. The gigantic nonlinear effect discussed in this paper should be ubiquitously existed in many oxide ferroelectric/semiconductor combinations and is promising for visible plasmonic applications.

Single pixel wide gamut dynamic color modulation based on a graphene micromechanical system

Dynamic color modulation in the composite structure of a graphene microelectromechanical system (MEMS)-photonic crystal microcavity is investigated in this work. The designed photonic crystal microcavity has three resonant standing wave modes corresponding to the three primary colors of red (R), green (G) and blue (B), forming strong localization of light in three modes at different positions of the microcavity. Once graphene is added, it can govern the transmittance of three modes. When graphene is located in the antinode of the standing wave, it has strong light absorption and therefore the structure's transmittance is lower, and when graphene is located in the node of the standing wave, it has weak light absorption and therefore the structure's transmittance is higher. Therefore, the graphene absorption of different colors of light can be regulated dynamically by applying voltages to tune the equilibrium position of the graphene MEMS in the microcavity, consequently realizing the output of vivid monochromatic light or multiple mixed colors of light within a single pixel, thus greatly improving the resolution. Our work provides a route to dynamic color modulation with graphene and provides guidance for the design and manufacture of high resolution, fast modulation and wide color gamut interferometric modulator displays.

Surface plasmonic resonance tunable nanocomposite thin films applicable to color filters, heat mirrors, semi-transparent electrodes, and electromagnetic-shields

This study proposes a plasmonic resonance-tunable nanocomposite thin film, which applies to a color filter, heat mirror, semi-transparent color electrode, and electromagnetic shield, given that the size and structure of nanoclusters can be controlled by a sputtering power density. The structural and functional properties of silver/plasma-polymer-fluorocarbon (Ag/PPFC) nanocomposite thin films, which were sputtered by ternary composite targets, were investigated with various compositions and sputtering power densities. The growth of Ag nanoclusters of the thin film was suppressed as the sputtering power density increased due to the rich functional group of -CF_x- fluorine. As a result, a continuous color change from blue to yellow could be expressed on films given the precise control of the surface plasmonic resonance phenomenon. Grazing-incidence small-angle scattering (GISAXS) analysis indicated that the sputtering power density had a significant effect on the size, distribution, and orientation of the Ag nanoclusters in the thin film. For low sputtering power densities, Ag nanoclusters were forming aggregations along the out-of-plane direction, but as the sputtering power density increased, the nanoclusters showed random distribution instead of large aggregates. We also demonstrated applications of Ag/PPFC nanocomposite thin films to a color filter, heat mirror, semi-transparent electrode, and electromagnetic shield. In addition, the fabrication of a large-area film (400 × 700 mm²) showed that the approach applies highly to industries.

Full Color and Grayscale Painting with 3D Printed Low-Index Nanopillars

Sculpting nanostructures into different geometries in either one or two dimensions produces a wide range of colorful elements in microscopic prints. However, achieving different shades of gray and control of color saturation remain challenging. Here, we report a complete approach to color and grayscale generation based on the tuning of a single nanostructure geometry. Through two-photon polymerization lithography, we systematically investigated color generation from the basic single nanopillar geometry in low-refractive-index (n < 1.6) material. Grayscale and full color palettes were achieved that allow decomposition onto hue, saturation, and brightness values. This approach enabled the "painting" of arbitrary colorful and grayscale images by mapping desired prints to precisely controllable parameters during 3D printing. We further extend our understanding of the scattering properties of the low-refractive-index nanopillar to demonstrate grayscale inversion and color desaturation and steganography at the level of single nanopillars.

Narrow-band, low-sideband plasmonic filter of asymmetric bi-layer metallic nanoslit arrays

We propose a narrow-band plasmonic filter with low sidebands in the VIS-NIR regime, consisting of two closely spaced, optically thin layers of asymmetric metallic nanoslit arrays that have equal periods but different slit widths. Based on numerical simulations, we clarify that the filtering characteristics in the transmission spectrum is mainly due to intercoupled local plasmon resonance (LPR) modes in the top- and bottom-layer nanoslits and in-plane waveguiding surface plasmon resonance (SPR) modes bound to the top and bottom metal structure layers respectively. The intercoupled LPR modes boost the transmission in a way that the adjoining nanoslits in the top and bottom metal layers act as optical antennas efficiently receiving and emitting light via intermediate plasmon modes, while the in-plane SPR modes at neighboring wavelengths suppress the transmission, so as to shape the passband peak. It's important that asymmetry of the nanoslits helps to improve quality factor of the intercoupled LPR mode and thus to reduce the passband width. Also, asymmetry of the bi-layer metallic nanoslit arrays helps to suppress the sidebands that are relevant to the higher-order in-plane SPR modes at shorter wavelengths. In the spectrum at longer wavelengths, non-resonant transmission of light is suppressed by increased total thickness of the structured metal layers. Furthermore, a two-dimensional version of the filter structure is presented, demonstrating similar filtering characteristics that can be optimally used for arbitrarily polarized or unpolarized light.

Bright Field Structural Colors in Silicon-on-Insulator Nanostructures

Structural coloration with artificially nanostructured materials is emerging as a prospective alternative to traditional pigments for the high resolution, sustainable recycling, and long-time durability. However, achieving bright field structural colors with dielectric nanostructures remains a great challenge due to the weak scattering in an asymmetric environment. Here, we demonstrate all-dielectric bright field structural colors with diffraction-limited resolution on the silicon-on-insulator platform. The backscattering is strongly enhanced from the constructive interference between Mie resonances of individual Si antennas and Fabry-Perot resonances supported by the SiO₂ layer. The fabricated colors with varying hues and saturations show strong insensitivity with respect to the interparticle spacing and, remarkably, the viewing angle under resonant conditions. Compared with creating a quasi-homogeneous environment, our strategy is solid and complementary metal-oxide semiconductor integrable, paving the way for practical applications of structural colors in nanoscale color printing, microdisplays, and imaging.

Periodic planar Fabry-Perot nanocavities with tunable interference colors based on filling density effects

Structural colors of high performance and economically feasible fabrication are desired in various applications. Herein, we demonstrate that reflective full-color filters based on the interference effect can be realized in periodic Fabry-Perot (F-P) nanocavity arrays of the same thickness. Enabled by simply adjusting the nanocavity size and array period, the resonant wavelengths can be successively tuned in the whole visible light range, which is mainly attributed to the varied effective refractive index introduced by the different filling density of the F-P nanocavity. Compared to the plasmonic colors utilizing the similar nanostructures, the proposed interference colors offer unique advantages of higher color contrast, wider gamut, and lower fabrication requirements. Besides, these color filters do not involve modulating the vertical dimensions of the F-P nanocavities, which is conducive to the monolithic integration of multicolor optical cavities and their large-area applications in consumable products combined with replica patterning techniques, such as nanoimprinting and soft lithography.

Young's double-slit-like diffraction stemming from scattering of long-ranged surface plasmon polaritons along distinct ±Z-faces' metallic layers in indium-tin-oxide coated Fe doped LiNbO3

Young's double-slit-like diffraction was seen on a viewing screen placed perpendicularly to a sharply cut edge of a Z-cut iron doped LiNbO₃ (LN) slab coated with indium-tin-oxide (ITO) films. The high contrast fringes observed confirm two sets of visible long-ranged surface plasmon polaritons propagating along the two ITO-LN interfaces distinctly over 5 mm path length with well-kept coherency, apart from metal uses. The indices of refraction measured with polarimetry from the ±Z-faces and changing transmission spectra obtained are consistent with the physical picture, along with dynamics of the very first reflection from the -Z-face under varying polarization angles between the two incident laser beams onto the slab.

Plasmonic modes in cylindrical nanoparticles and dimers

We present analytical expressions for the resonance frequencies of the plasmonic modes hosted in a cylindrical nanoparticle within the quasi-static approximation. Our theoretical model gives us access to both the longitudinally and transversally polarized dipolar modes for a metallic cylinder with an arbitrary aspect ratio, which allows us to capture the physics of both plasmonic nanodisks and nanowires. We also calculate quantum mechanical corrections to these resonance frequencies due to the spill-out effect, which is of relevance for cylinders with nanometric dimensions. We go on to consider the coupling of localized surface plasmons in a dimer of cylindrical nanoparticles, which leads to collective plasmonic excitations. We extend our theoretical formalism to construct an analytical model of the dimer, describing the evolution with the inter-nanoparticle separation of the resultant bright and dark collective modes. We comment on the renormalization of the coupled mode frequencies due to the spill-out effect, and discuss some methods of experimental detection.

Spectral Modulation through the Hybridization of Mie-Scatterers and Quasi-Guided Mode Resonances: Realizing Full and Gradients of Structural Color

Metasurfaces made up of subwavelength arrays of Mie scatterers can be engineered to control the optical properties of incident light. The hybridization of the fundamental Mie resonances with lattice resonances greatly enhances the scattering cross-section of individual Mie scatterers. Through careful design of the locations of these hybridized modes using two differently engineered hydrogenated amorphous silicon nanorods, we numerically calculate and experimentally fabricate two examples of full color printing; one with spectral colors comparable to the Adobe RGB gamut, and another with gradients of color. We identify and characterize the mechanisms behind each and provide a framework that can be used to design any all-dielectric metasurfaces of subwavelength Mie scatterers for spectral modulation.

Demonstration of > 2π reflection phase range in optical metasurfaces based on detuned gap-surface plasmon resonators

Plasmonic metasurfaces, representing arrays of gap-surface plasmon (GSP) resonators and consisting of arrays of metal nanobricks atop thin dielectric layers supported by thick metal films, constitute an important subclass of optical metasurfaces operating in reflection and enabling the realization of numerous, diverse and multiple, functionalities. The available phase variation range is however limited to being \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<\! 2\pi$$\end{document}<2π, a circumstance that complicates the metasurface design for functionalities requiring slowly varying phases over the whole range of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2\pi$$\end{document}2π, e.g., in holographic applications. The available phase range also determines the wavelength bandwidth of metasurfaces operating with linearly polarized fields due to the propagation (size-dependent) nature of the reflection phase. We suggest an approach to extend the phase range and bandwidth limitations in the GSP-based metasurfaces by incorporating a pair of detuned GSP resonators into a metasurface elementary unit cell. With detailed simulations related to those for conventional single-resonator metasurfaces and proof-of-concept experiments, we demonstrate that the detuned-resonator GSP metasurfaces designed for beam steering at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${900}\,\,\hbox {nm}$$\end{document}900nm wavelength exhibit the extended reflection phase and operation bandwidth. We believe that the considered detuned-resonator GSP metasurfaces can advantageously be exploited in applications requiring the design of arbitrary phase gradients and/or broadband operation with linearly polarized fields.

Charge accumulation resulting in metallization of II-VI semiconductor (ZnX X = O, S, Se) films neighboring polar liquid crystal molecules and their surface plasmonic response in the visible region

The surfaces of some IIB-VI semiconductors (ZnX, X = O, S, Se) are metallized by neighboring highly polar and atomically vertically aligned (VA) liquid crystal (LC) molecules. Owing to polar catastrophe, the charge carriers swarm in an extremely thin layer and the density can achieve 4.86 × 1028 m-3 close to the LC layer, which can be regarded as a 2-dimensional electron gas (2DEG). Using density functional theory (DFT), it was found that the dielectric functions of the modified layer become negative in the visible region. This indicates the semiconductor/LC platform is an ideal active plasmonic candidate, apart from the lossy metal constituents. Experimentally, after mediation with phase gratings written in the LC system, surface plasmon polaritons (SPPs) can be excited at the semiconductor surface and localized charges are gathered in an adjacent LC layer. With the help of the enhanced static electric field from the metallic surface, significantly more 2D diffraction orders in many rows and columns and a huge energy transfer between the laser beams and SPPs was observed, which is consistent with the metallization results and the bidirectional coupling between the SPPs and incident lights. The generalization of the II-VI semiconductors means the system has great promise for use in practical applications owing to the ultra-low loss. The novel insights regarding this combination with liquid crystals will be beneficial for real-time holographic displays and the study of tunable epsilon near zero points.

Asymmetrical 1st reflection trend owing to metallization difference at ± Z-faces in indium tin oxide coated Fe-doped lithium niobate

The charges accumulated at the interface of indium tin oxide (ITO) and iron doped lithium niobate (LN) in an extremely thin layer because of semiconductor band alignment were analyzed with ab initio theory. The formation of 2D electron gas makes the interface metallic and the excitation of surface plasmon polaritons (SPPs) possible. In experiments, diametrically opposite trends of the very first reflection (VFR) on the ± Z-faces of ITO coated Fe-LN slabs were observed and associated with the differences in metallization and the photovoltaic charge accumulation there. Microscopically, the electric environments of the two ITO/LN interfaces differ greatly owing to spontaneous polarization and photovoltaic fields, which alter the band structures and band alignment, resulting in phase gratings with a π-shift difference recorded at the two interfaces. This affects the opposite energy coupling between the SPPs and laser beams and results in the dramatically opposite trends of VFR.

Nanostructured Color Filters: A Review of Recent Developments

Color plays an important role in human life: without it life would be dull and monochromatic. Printing color with distinct characteristics, like hue, brightness and saturation, and high resolution, are the main characteristic of image sensing devices. A flexible design of color filter is also desired for angle insensitivity and independence of direction of polarization of incident light. Furthermore, it is important that the designed filter be compatible with the image sensing devices in terms of technology and size. Therefore, color filter requires special care in its design, operation and integration. In this paper, we present a comprehensive review of nanostructured color filter designs described to date and evaluate them in terms of their performance.

Spectrally sharp metasurfaces for wide-angle high extinction of green lasers

In optical nanostructures used as artificial resonance-based color filters, there is unfortunate universal trade-off between spectral sharpness and angular tolerance as well as maximum extinction. We rigorously derive the maximum performance bounds of wavelength-rejection filters realized by single-layer plasmonic metasurfaces with a dominant resonance and weak near-field coupling, and propose a multi-layer approach to overcome these single-layer limits and trade-offs. We also present a realistic example that has a narrow full-width-at-half-maximum bandwidth of 24 nm with 10 dB extinction at 532 nm with good angular tolerance up to 60°. The performance of the proposed metasurface is close to the general theoretical bound.

Reflection and transmission two-way structural colors

Management of reflection and transmission two-way structural colors is significant in color displays, projections, and anticounterfeiting. Here, inspired by the Lycurgus Cup, we fabricated photonic crystals with opal and inverse opal structures with controlled thickness, which show reflection and transmission two-way structural colors. In order to balance the reflection and transmission intensities, we first studied the effect of the order layer thickness on the reflection and transmission spectra and found that a thickness of about 5 μm can help the structural colors achieve high saturation in both directions. The photonic crystal film built with 295 nm SiO2 spheres shows bright red and green structural colors in the reflection and transmission directions, respectively. These two-way colors can be projected onto substrates, similar to a transflective color filter. The color displays can be tuned by adjusting the angle between the incident light and the sample. Furthermore, we also patterned the photonic crystal film with two-way structural colors, which shows clear patterns and rich colors in both directions. The photonic crystals assembled on a small wine glass display two-way structural colors similar to those of the Lycurgus Cup. More importantly, a flexible inverse opal photonic crystal film with two-way structural colors was also fabricated, which can be applied in multimode anticounterfeiting. This work will greatly expand the application field of photonic crystals in double sided displays, transflective color filters and anticounterfeiting.

Stretchable Biaxial and Shear Strain Sensors Using Diffractive Structural Colors

Structural colors that can be changed dynamically, using either plasmonic nanostructures or photonic crystals, are rapidly emerging research areas for stretchable sensors. Despite the wide applications of various techniques to achieve strain-responsive structural colors, important factors in the feasibility of strain sensors-such as their sensing mechanism, stability, and reproducibility-have not yet been explored. Here, we introduce a stretchable, diffractive, color-based wireless strain sensor that can measure strain using the entire visible spectrum, based on an array of cone-shaped nanostructures on the surface of an elastomeric substrate. By stretching or compressing the substrate, the diffractive color can be tuned according to the changing grating pitch. Using the proposed method, we designed three types of strain-sensing modes: large-deformation (maximum 100%) tensile strain, biaxial 2D strain, and shear strain (maximum 78%). The strain sensors were fabricated, and applicability to strain-sensing was evaluated.

Printing a Wide Gamut of Saturated Structural Colors Using Binary Mixtures, With Applications in Anticounterfeiting

Use of colloidal suspensions to generate structural colors has the potential to reduce the use of toxic metals or organic pigments in inkjet printing, coatings, cosmetics, and other applications, and is a promising avenue to create large-scale nanostructures that produce long-lasting colors. However, expanded use of structural colors requires a reduction in coffee-ring effects during printing, which currently requires intricately patterned substrates or high particle concentrations, and diversification of colors to compete with conventional printing inks. Here, we treat substrate surfaces with cold plasma to facilitate spontaneous assembly of particles into colloidal nanostructures, reducing the need for highly concentrated particle suspensions. Moreover, by employing binary mixtures, we can tune the lightness of the hue produced or change the hue itself, allowing us to cover wider regions of color space. We demonstrate the use of this cold-plasma approach on a variety of substrates, favoring substrate diversity on which printing can be performed. This methodology enables creation of high-resolution, complex designs and opens a path for extending the limits of anticounterfeiting applications by using binary mixtures.

Multi-frequency surface plasmons supported with a nanoscale non-uniform 2D electron gas formed due to a polar catastrophe at the oxide interface, dispersions, diffractions, and beyond

Recently, 2D electron gases (2DEGs) formed at oxide interfaces are drawing increasing attention as they cause a myriad of intriguing phenomena. As ideal platforms in supporting surface plasmon polaritons (SPPs) without metallic constituents, such 2DEGs are favorable in non-linear plasmonics for ultra-low total Joule dissipation. Convincingly, an increase in the interfacial electron density (IIED) formed at the interface of indium-tin-oxide and LiNbO3 composite slab is responsible for a number of interesting phenomena, which are hardly explained with the conventional photorefractive theoretical framework but can be satisfactorily elucidated via SPP excitation and resultant colossal non-linear effects. Since the polar-catastrophe-led IIED is universal to all combinations of highly polar ferroelectric oxides (FOs) and less polar transparent conducting oxides (TCOs), a systematic theoretical treatment of an FO/TCO system is pivotal to a variety of promising applications. In this study, the nanometer scale 2DEG at the FO/TCO interface is illustrated theoretically with the Thomas-Fermi screening picture, by taking into account the spontaneous polarization, along with related boundary conditions. The local plasma frequency of 2DEGs can be increased up to the UV regime for the composite slabs discussed, which are suitable for highly desirable visible applications. The SPP dispersion relationship was given for the 2DEG layer sandwiched between the FO slab and the unmodified TCO layer. To further take the non-uniform nature of IIED into account, dramatic dispersions of dielectric permittivity and index of refraction were simulated with a very broad range, hinting at different ways for meeting phase matching conditions and slowing the light for non-linear plasmonic applications, which are confirmed experimentally.

Nanosphere-Aggregation-Induced Reflection and Its Application in Large-Area and High-Precision Panchromatic Inkjet Printing

Artificial structural colors have attracted more and more attention due to their high photostability, low toxicity, and brilliant colors. Inkjet printing of photonic crystals or amorphous photonic structures can realize large-scale structural color patterns, while plasma printing of metals can achieve high-precision color images. However, still no method is available to fabricate structural color patterns on both a large scale and with high precision. Here, nanosphere-aggregation-induced reflection (NAIR) is first theoretically and experimentally demonstrated and vivid full-spectrum structural color can be generated based on NAIR. Dramatically different from photonic crystals, the accumulation of only a few monodisperse dielectric spheres with an appropriate refractive index and diameter can produce bright structural colors, which makes high resolution possible. By introducing commercial inkjet printers, this aggregate structure can be constructed at high speed in a large scale. Importantly, the color mixing is easily performed by simultaneously applying spheres with different sizes, which allow us to sophisticatedly control the generated color. The demonstrated NAIR printing paves the way toward a full-spectrum, large-scale, and high-precision structural color, offering great potential for daily commercial utilization.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO₂ metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

A Plasmonic Painter's Method of Color Mixing for a Continuous Red-Green-Blue Palette

The ability of mixing colors with remarkable results had long been exclusive to the talents of master painters. By finely combining colors in different amounts on the palette, intuitively, they obtain smooth gradients with any given color. Creating such smooth color variations through scattering by the structural patterning of a surface, as opposed to color pigments, has long remained a challenge. Here, we borrow from the painter's approach and demonstrate color mixing generated by an optical metasurface. We propose a single-layer plasmonic color pixel and a method for nanophotonic structural color mixing based on the additive red-green-blue (RGB) color model. The color pixels consist of plasmonic nanorod arrays that generate vivid primary colors and enable independent control of color brightness without affecting chromaticity by simply varying geometric in-plane parameters. By interleaving different nanorod arrays, we combine up to three primary colors on a single pixel. Based on this, two- and three-color mixing is demonstrated, enabling the continuous coverage of a plasmonic RGB color gamut and yielding a palette with a virtually unlimited number of colors. With this multiresonant color pixel, we show the photorealistic printing of color and monochrome images at the nanoscale, with ultrasmooth transitions in color and brightness. Our color-mixing approach can be applied to a broad range of scatterer designs and materials and has the potential to be used for multiwavelength color filters and dynamic photorealistic displays.