Polarization-tuned Dynamic Color Filters Incorporating a Dielectric-loaded Aluminum Nanowire Array

Resonant-mode engineering for additive reflective structural colors with high brightness and high color purity

We present quad-layered reflective structural color filters generating vivid additive primary colors by controlling a mode number in a Fabry-Perot (FP) cavity and an anti-reflective (AR) coating layer, thus accomplishing high spectral contrast which is highly demanded in creating sharp colors. The reflection brightness of fabricated structural color filters is over 78% and a color gamut is comparable to the standard color gamut (sRGB). Higher-order resonant modes are exploited yielding a narrow passband with strong suppression of the reflection at shorter and longer wavelength ranges for a green color, while red and blue colors are produced by employing fundamental resonant modes. Besides, the structural color filters maintain both high brightness and high color purity at oblique incidence angles up to 40° due to a small angle of refraction by a cavity medium with high refractive index. Moreover, a large-scale fabrication is enabled owing to the simplicity of a device structure, where thin film deposition is used. The scheme presented in this work may open the door to a number of applications, such as reflective displays, imaging devices, colored photovoltaics, and decorations.

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.

Sodium-Based Concave Metasurfaces for High Performing Plasmonic Optical Filters by Templated Spin-on-Sodiophobic-Glass

Optical filters have aroused tremendous excitement in advanced photonic instruments and modern digital displays due to their flexible capability of spectrum manipulation. Plasmonic metasurfaces of narrow bandwidth, high spectral contrast, and robust structure tolerance are highly desired for optical filtration (especially in the visible regime) but rather challenging as large spectral broadening from intrinsic ohmic loss and design/fabrication deviations. Here the high-performing sodium-based metasurfaces are demonstrated for optical filtration across 450 to 750 nm by unique structure design of spatially decoupled concave surfaces and precise fabrication through templated solidification of liquid metals. Thanks to the distinct suppression of metallic loss as well as fabrication tolerance of interfacial structures, the as-prepared concave metasurfaces enable a minimum linewidth of ≈15 nm, a maximal optical contrast of ≈93%, and a high measure-to-design spectral match ratio ≈1500. These results have for the first time pushed the operation wavelengths of sodium-based plasmonic devices from infrared to visible which in turn demonstrates the capability of filling the blank of commercial dielectric optical filters thus far.

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.

Manipulation of resonance orders and absorbing materials for structural colors in transmission with improved color purity

We present an improved color purity of additive transmissive structural color filters by controlling a resonance order and by inserting a highly absorbing material. The proposed structure consists of a single metal sandwiched by two transparent dielectric media serving as a cavity to minimize the ohmic loss in the metal mirrors, which is distinctly different from a conventional Fabry-Perot (FP) cavity that is in general designed to have two metal mirrors. Low reflections at an air-dielectric interface cause a quality-factor of a resonance to be reduced, causing a degraded color purity, which can be improved by employing a 1st order resonance that exhibits a narrower bandwidth than a fundamental FP resonant mode (0th order). For a red color with the improved purity, introducing an ultrathin absorbing layer in the middle of a top cavity enables the 1st resonance to be trivially influenced while selectively suppressing a 2nd order resonance appearing at the shorter wavelength region. Moreover, angle-insensitive performances up to 60° are attained by utilizing a cavity material with high index of refraction. Besides, the fabrication of the structural coloring devices involves a few deposition steps, thus rendering the approach suitable for applications over the large area. The described concept could be applied to diverse applications, such as colored solar panels, sensors, imaging devices, and decorations.

Optical Transmission Plasmonic Color Filter with Wider Color Gamut Based on X-Shaped Nanostructure

Extraordinary Optical Transmission Plasmonic Color Filters (EOT-PCFs) with nanostructures have the advantages of consistent color, small size, and excellent color reproduction, making them a suitable replacement for colorant-based filters. Currently, the color gamut created by plasmonic filters is limited to the standard red, green, blue (sRGB) color space, which limits their use in the future. To address this limitation, we propose a surface plasmon resonance (SPR) color filter scheme, which may provide a RGB-wide color gamut while exceeding the sRGB color space. On the surface of the aluminum film, a unique nanopattern structure is etched. The nanohole functions as a coupled grating that matches photon momentum to plasma when exposed to natural light. Metals and surfaces create surface plasmon resonances as light passes through the metal film. The plasmon resonance wavelength can be modified by modifying the structural parameters of the nanopattern to obtain varied transmission spectra. The International Commission on Illumination (CIE 1931) chromaticity diagram can convert the transmission spectrum into color coordinates and convert the spectrum into various colors. The color range and saturation can outperform existing color filters.

Colorimetric histology using plasmonically active microscope slides

The human eye can distinguish as many as 10,000 different colours but is far less sensitive to variations in intensity¹, meaning that colour is highly desirable when interpreting images. However, most biological samples are essentially transparent, and nearly invisible when viewed using a standard optical microscope². It is therefore highly desirable to be able to produce coloured images without needing to add any stains or dyes, which can alter the sample properties. Here we demonstrate that colorimetric histology images can be generated using full-sized plasmonically active microscope slides. These slides translate subtle changes in the dielectric constant into striking colour contrast when samples are placed upon them. We demonstrate the biomedical potential of this technique, which we term histoplasmonics, by distinguishing neoplastic cells from normal breast epithelium during the earliest stages of tumorigenesis in the mouse MMTV-PyMT mammary tumour model. We then apply this method to human diagnostic tissue and validate its utility in distinguishing normal epithelium, usual ductal hyperplasia, and early-stage breast cancer (ductal carcinoma in situ). The colorimetric output of the image pixels is compared to conventional histopathology. The results we report here support the hypothesis that histoplasmonics can be used as a novel alternative or adjunct to general staining. The widespread availability of this technique and its incorporation into standard laboratory workflows may prove transformative for applications extending well beyond tissue diagnostics. This work also highlights opportunities for improvements to digital pathology that have yet to be explored.

Direct fabrication and characterization of gold nanohole arrays

We demonstrate a one-step fabrication method to realize desired gold (Au) nanoholes arrays by using a one-photon absorption based direct laser writing technique. Thanks to the optically induced thermal effect of Au material at 532 nm excitation wavelength, the local temperature at the laser focus area can reach as high as 600°C, which induces an evaporation of the Au thin film resulting in a metallic nanohole. By controlling the laser spot movement and exposure time, different two-dimensional Au nanoholes structures with periodicity as small as 500 nm have been demonstrated. This allows obtaining plasmonic nanostructures in a single step without needing the preparation of polymeric template and lift-off process. By this direct fabrication technique, the nanoholes do not have circular shape as the laser focusing spot, due to the non-uniform heat transfer in a no-perfect flat Au film. However, the FDTD simulation results and the experimental measurement of the transmission spectra show that the properties of fabricated plasmonic nanoholes arrays are very close to those of ideal plasmonic nanostructures. Actually, the plasmonic resonance depends strongly on the periodicity of the metallic structures while the heterogeneous form of the holes simply enlarges the resonant peak. Furthermore, it is theoretically demonstrated that the non-perfect circular shape of the Au hole allows amplifying the electromagnetic field of the resonant peak by several times as compared to the case of perfect circular shape. This could be an advantage for application of this fabricated structure in laser and nonlinear optics domains.

Artificial Structural Colors and Applications

Structural colors are colors generated by the interaction between incident light and nanostructures. Structural colors have been studied for decades due to their promising advantages of long-term stability and environmentally friendly properties compared with conventional pigments and dyes. Previous studies have demonstrated many artificial structural colors inspired by naturally generated colors from plants and animals. Moreover, many strategies consisting of different principles have been reported to achieve dynamically tunable structural colors. Furthermore, the artificial structural colors can have multiple functions besides decoration, such as absorbing solar energy, anti-counterfeiting, and information encryption. In the present work, we reviewed the typical artificial structural colors generated by multilayer films, photonic crystals, and metasurfaces according to the type of structures, and discussed the approaches to achieve dynamically tunable structural colors.

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.

Dielectric Resonance-Based Optical Metasurfaces: From Fundamentals to Applications

Optical metasurface as a booming research field has put forward profound progress in optics and photonics. Compared with metallic-based components, which suffer from significant thermal loss and low efficiency, high-index all-dielectric nanostructures can readily combine electric and magnetic Mie resonances together, leading to efficient manipulation of optical properties such as amplitude, phase, polarization, chirality, and anisotropy. These advances have enabled tremendous developments in practical photonic devices that can confine and guide light at the nanoscale. Here we review the recent development of local electromagnetic resonances such as Mie-type scattering, bound states in the continuum, Fano resonances, and anapole resonances in dielectric metasurfaces and summarize the fundamental principles of dielectric resonances. We discuss the recent research frontiers in dielectric metasurfaces including wavefront-shaping, metalenses, multifunctional and computational approaches. We review the strategies and methods to realize the dynamic tuning of dielectric metasurfaces. Finally, we conclude with an outlook on the challenges and prospects of dielectric metasurfaces.

Permalloy nanowires/graphene oxide composite with enhanced conductive properties

Carbon–metal-based composites arise as advanced materials in the frontiers with nanotechnology, since the properties inherent to each component are multiplexed into a new material with potential applications. In this work, a novel composite consisting of randomly oriented permalloy nanowires (Py NWs) intercalated among the sheets of multi-layered graphene oxide (GO) was performed. Py NWs were synthesized by electrodeposition inside mesoporous alumina templates, while GO sheets were separated by means of sonication. Sequential deposition steps of Py NWs and GO flakes allowed to reach a reproducible and stable graphene oxide-based magnetic assembly. Microscopic and spectroscopic results indicate that Py NWs are anchored on the surface as well as around the edges of the multi-layered GO, promoted by the presence of chemical groups, while magnetic characterization affords additional support to our hypothesis regarding the parallel orientation of the Py NWs with respect to the GO film, and also hints the parallel stacking of GO sheets with respect to the substrate. The most striking result remains on the electrochemical performance achieved by the composite that evidences an enhanced conductive behaviour compared to a standard electrode. Such effect provides an approach to the development of permalloy nanowires/graphene oxide-based electrodes as attractive candidates for molecular sensing devices.

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.

Plasmonic color filter based on a hetero-metal-insulator-metal grating

Plasmonic color filters are expected to be candidates for application to complementary metal-oxide-semiconductor (CMOS) image sensor arrays with reduced pixel size, owing to the subwavelength mode volume of plasmons. Designs of metallic gratings based on the guided-mode resonance effect suffer from the sideband transmission issue due to high-order diffraction. Here, we propose a plasmonic color filter structure based on a hetero-metal-insulator-metal grating. The guided mode, in resonance with the second-order diffraction, is highly attenuated by the forbidden band, such that the sideband transmission can be suppressed. As calculated by using the transfer matrix method and the finite-difference time-domain method, the Al-ZnO-Ag waveguide-based structure presents a color filter characteristic with the peak transmittance greater than 70% and the peak wavelength tunable in the visible light band. It may find application in displays, image sensors, and biomedical imaging technologies.

Linewidth study of pixelated aluminum nanowire gratings on polarization performance

Nowadays, nanowire gratings are widely used in various applications such as imaging sensors and high-resolution microscopes. Structure parameters are the main factors that affect the optical performance of the gratings. This work aims to present the influence of the linewidth of pixelated aluminum nanowire gratings with a fixed period on the transmittance and extinction ratio in the visible region. By controlling the exposure doses of electron beam lithography (EBL), different linewidths of pixelated aluminum nanowire gratings with a period of 170 nm were fabricated. The significant effects of linewidth difference on the polarization performance were verified by the simulations of finite-difference time-domain (FDTD) software. The simulations were divided into two parts: the discussion of the pure aluminum without considering oxidation and the discussion of the surface aluminum being oxidized into the aluminum oxide. An optical system was built to evaluate the performance of the fabricated structures. The results show that the trends of the measurement results are consistent with that of simulation. This work will give a guide to the fabrication and evaluation of the nanowire gratings.

Structural color printing with a dielectric layer coated on a nanotextured metal substrate: simulation and experiment

The printing of plasmonic structural colors relies on noble metal nanostructures fabricated on Si, glass, or plastic substrates. This paper presents a simple surface structure for producing vivid structural colors directly from common metal substrates. The structure is formed by texturing the surface of stainless steel (STS) via imprinting and coating it with a dielectric layer. Diverse colors are generated simply by varying the thickness of the dielectric layer. The colors arise from surface plasmon resonance and guided-mode resonance of the incident light, which are excited on the textured STS surface and inside the dielectric layer, respectively. A finite-difference time-domain simulation shows that 500 nm is the optimum texture periodicity with regard to the tunability and vividness of the colors. This is experimentally verified by printing many differently colored images on the surface of STS substrates with a texture period of 500 nm. The proposed structure/method does not require a nanofabrication technique such as electron-beam lithography or focused ion beam etching. The results of the study provide a facile route for producing vivid structural colors on metals, which may find various applications, including surface decoration, product identification, anti-counterfeiting, and perfect absorbers.

Transmitted plasmonic colors with different overlays utilizing the Fano-resonance

This study develops a large-area pixelated filter that can achieve colors covering the entire visible range with a fixed period under normal incidence. Vivid colors as blue, green, and yellow (peak efficiency of ~60%) are experimentally achieved based on a Fano-resonance by altering the overlay's refractive index, which is highly sensitive to the surrounding material. Furthermore, the feasibility of using this device in large-area color printing and index sensors is discussed in detail, wherein a large-area (3 cm × 3 cm) logo and a figure of merit of 254 are achieved. Therefore, this developed structure can be regarded as an alternative to traditional periodic-dependent structure colors, which can also be performed as index sensors with high sensitivity.

Bright and vivid plasmonic color filters having dual resonance modes with proper orthogonality

The mode orthogonality fundamentally influences the scattering spectra of multi-resonance systems, such as plasmonic color filters. We show that planar arrays of silver nanostructures with dual localized surface plasmon resonances and the right mode orthogonality can function as transmissive RGB color filters with peak transmittances higher than 70%, and color gamut areas larger than 90% of the sRGB space. These are the brightest and most saturated of all designs proposed thus far. We present the Pareto frontier from designs with more than 80% peak transmittance, to designs that achieve a color gamut larger than 120% of the sRGB space.

Resonant laser printing of bi-material metasurfaces: from plasmonic to photonic optical response

Metasurfaces are nanostructured surfaces with engineered optical properties - currently impacting many branches of optics, from miniaturization of optical components to realizing high-resolution structural colors. The optical properties of metasurfaces can be traced to the individual meta-atoms, which set the nature of the optical response, e.g., plasmonic for metallic meta-atoms or photonic for dielectric meta-atoms. Combining multiple types of responses opens up new horizons in design of optical materials, but has so far been avoided due to the fabrication difficulties associated with constructing a metasurface composed of several meta-atom materials. Here, we present a multi-material design approach by optically post-processing a metasurface constructed from self-assembled polystyrene spheres coated with silver. Using our concept of resonant laser printing, we locally alter the initial plasmonic response of the meta-atoms to a pure photonic response. Our work constitutes a conceptually different way of designing metasurfaces and can pave the way for realizing multi-material metasurfaces on large areas while being cost effective.

Design and Simulation of Active Frequency-selective Metasurface for Full-colour Plasmonic Display

In this paper, we report a full-colour plasmonic pixel by incorporating a low-index buffer layer and an EO material layer with a gap surface plasmon-based metasuface. The reflection spectra can be modulated by an external voltage bias with a reflectivity higher than 60% when filtering red, green and blue primary light. Vivid colour can be generated by mixing the three primaries in time sequence. Brightness can be tuned by the duty cycle of bright and dark state. Theoretical calculations demonstrate that the switchable pixels we designed can achieve a gamut overlapping 80% area of NTSC colour space and a contrast ratio of 10.63, 26.11 and 2.97 for red, green and blue when using a white quatom-dot-enhancement-film backlit.

Polarization-dependent wide-angle color filter incorporating meta-dielectric nanostructures

A metadielectric nanostructure with narrow cavities is proposed, behaving as a reflective color filter for TM-polarized light while as a broadband reflector for TE-polarized light. By varying the cavity depth or changing the incident light polarization, reflective colors of the proposed structure cover the entire visible spectrum conveniently. In particular, the reflections of this proposed structure show good angular tolerance up to 50° for both polarizations. Furthermore, it can display colors even with two grating slits, which shows high printing resolution up to 70555 dpi, having great potential for applications of a large area color imaging and anticounterfeiting devices.

Eight Inch Wafer-Scale Flexible Polarization-Dependent Color Filters with Ag-TiO2 Composite Nanowires

In this study, 8 in. wafer-scale flexible polarization-dependent color filters with Ag-TiO₂ composite nanowires have been fabricated using nanoimprint and E-beam evaporation. The filters change their color via a simple rotation of the polarizer. In addition, the color of the filter can be controlled by altering the thickness of the Ag and TiO₂ nanowires deposited on the polymer patterns. Polarization-dependent color filters were realized by selective inhibition of transmission using the plasmonic resonance at the insulator/metal/insulator nanostructure interface, which occurs at particular wavelengths for the transverse magnetic polarizations. Special colors, including purple, blue, green, yellow, and pink, could be obtained with high transmission beyond 65% by varying the thickness of the deposited Ag and TiO₂ nanowires on the periodic polymer pattern under transverse magnetic polarization. In addition, a continuous color change was achieved by varying the polarization angle. Last, numerical simulations were implemented in comparison with the experimental results, and the mechanism was explained. We believe that this simple and cost-effective method can be applied to processes such as anticounterfeiting and holographic imaging as well as to color displays.

Single-Step Laser Plasmonic Coloration of Metal Films

Utilization of structural colors produced by nanosized optical antennas is expected to revolutionize the current display technologies based on an inkjet or a pigmentation-based color printing. Meanwhile, the versatile color-mapping strategy combining the fast single-step single-substrate fabrication cycle with low-cost scalable operation is still missing. We propose lithography-free pure optical approach based on a direct local ablative reshaping of the gold film with nanojoule (nJ)-energy femtosecond laser pulses. Plasmon-color printing at a resolution up to 2.5 × 10⁴ dots per inch satisfying the current visualization demands and data storage capacity is achieved. By controlling only the applied pulse energy, wide gamut of colors in scattering regime was reproduced via tuning the size of the printed nanovoids, which have a polarization- and shape-dependent localized plasmon-mediated scattering. Additionally, brightness of a single pixel was gradually adjusted via varying of the spacing between the printed nanovoids. The presented experimental demonstration opens a new direction toward plasmon-color printing for various applications where durability is required: low-cost cryptography, security tagging, and ultracompact optical data storage.

Emerging optical properties from the combination of simple optical effects

Structural color arises from the patterning of geometric features or refractive indices of the constituent materials on the length-scale of visible light. Many different organisms have developed structurally colored materials as a means of creating multifunctional structures or displaying colors for which pigments are unavailable. By studying such organisms, scientists have developed artificial structurally colored materials that take advantage of the hierarchical geometries, frequently employed for structural coloration in nature. These geometries can be combined with absorbers-a strategy also found in many natural organisms-to reduce the effects of fabrication imperfections. Furthermore, artificial structures can incorporate materials that are not available to nature-in the form of plasmonic nanoparticles or metal layers-leading to a host of novel color effects. Here, we explore recent research involving the combination of different geometries and materials to enhance the structural color effect or to create entirely new effects, which cannot be observed otherwise.

Plasmonic- and dielectric-based structural coloring: from fundamentals to practical applications

Structural coloring is production of color by surfaces that have microstructure fine enough to interfere with visible light; this phenomenon provides a novel paradigm for color printing. Plasmonic color is an emergent property of the interaction between light and metallic surfaces. This phenomenon can surpass the diffraction limit and achieve near unlimited lifetime. We categorize plasmonic color filters according to their designs (hole, rod, metal-insulator-metal, grating), and also describe structures supported by Mie resonance. We discuss the principles, and the merits and demerits of each color filter. We also discuss a new concept of color filters with tunability and reconfigurability, which enable printing of structural color to yield dynamic coloring at will. Approaches for dynamic coloring are classified as liquid crystal, chemical transition and mechanical deformation. At the end of review, we highlight a scale-up of fabrication methods, including nanoimprinting, self-assembly and laser-induced process that may enable real-world application of structural coloring.

Liquid-crystal tunable color filters based on aluminum metasurfaces

Designing color pixels using plasmonic nanostructures and metasurfaces has become a luring area of research in recent years. Here, we experimentally demonstrated the voltage tunability of a dynamic plasmonic color filter by using an aluminum grating integrated with the nematic liquid crystal (LC). Along with a typical substrate coated with rubbed polyimide film, the aluminum grating itself serves as a molecular alignment layer to form a twisted LC cell. This hybrid structure allows electrically controlled transmission color by applying the voltage. A significant spectral tunability of such a device has been demonstrated by applying the small voltage from 0 to 4 V_rms.

Polarization tunable all-dielectric color filters based on cross-shaped Si nanoantennas

Polarization sensitive and insensitive color filters have important applications in the area of nano-spectroscopy and CCD imaging applications. Metallic nanostructures provide an efficient way to design and engineer ultrathin color filters. These nanostructures have capability to split the white light into fundamental colors and enable color filters with ultrahigh resolution but their efficiency can be restricted due to high losses in metals especially at the visible wavelengths. In this work, we demonstrate all-dielectric color filters based on Si nanoantennas, which are sensitive to incident-wave polarization and, thus, tunable with the aid of polarization angle variation. Two different information can be encoded in two different polarization states in one nanostructure. The nanoantenna based pixels are highly efficient and can provide high quality of colors, in particular, due to low losses in Si at optical frequencies. We experimentally demonstrate that a variety of colors can be achieved by changing the physical size of the nonsymmetric cross-shaped nanoantennas. The proposed devices allow to cover an extended gamut of colors on CIE-1931 chromaticity diagram owing to the existence of high-quality resonances in Si nanoantennas. Significant tunability of the suggested color filters can be achieved by varying polarization angle in both transmission and reflection mode. Additional tunability can be obtained by switching between transmission and reflection modes.

Plasmonic nanohole electrodes for active color tunable liquid crystal transmissive pixels

Plasmonic pixels have been shown to offer numerous advantages over pigment-based color filters used in modern commercial liquid crystal (LC) displays. However, wideband dynamic tunability across the visible spectrum remains challenging. We experimentally demonstrate transmissive electrically tunable LC-nanohole pixels operating across the visible spectrum with unpolarized input light. An ultrathin Al nanohole electrode is designed to exhibit a polarized spectral response based on surface plasmon resonances. An output analyzer in combination with a nematic LC layer enables pixel color to be electronically controlled through an applied voltage across the device, where LC reorientation leads to tunable mixing of the relative contributions from the plasmonic color input. The nanostructured Al layer, acting as a combined electrode, polarizer, and functional color filter, is highly promising for electro-optic display applications.

Free-standing plasmonic metal-dielectric-metal bandpass filter with high transmission efficiency

Plasmonic spectrum filtering devices based on metallic nanostructures have attracted wide attention due to their good reliability, ease of fabrication, and wideband tunability. However, the presence of thick substrate significantly limits the structure's longitudinal size for further optoelectronic integration and reduces the devices' performance. Here we propose and demonstrate an ultra-thin plasmonic bandpass filter based on free-standing periodic metal-dielectric-metal stack geometry working in the near-infrared wavelength range. The coupling between free-space electromagnetic waves and spatially confined plasmonic modes in the designed structure is systematically investigated. As demonstrated in the calculation and experiment, the free-standing plasmonic filters have more than 90% transmission efficiency and superior angular tolerance. The experimental results are in good agreement with the theoretical calculations. These artificial nanostructured filtering devices may find potential applications in the extremely compact device architectures.

Full Color Generation Using Silver Tandem Nanodisks

Plasmonic effects associated with metallic nanostructures have been widely studied for color generation. It became apparent that highly saturated and bright colors are hard to obtain, and very small nanostructures need to be fabricated. To address this issue, in this study, we employ metal-insulator-metal sandwich nanodisks that support enhanced in-phase electric dipole modes, which are blue-shifted with respect to a single metal disk. The blue shift enables the generation of short wavelength colors with larger nanostructures. The radiation modes hybridize with the Wood's anomaly in periodic structures, creating narrow and high-resonance peaks in the reflection and deep valleys in the transmission spectra, thus producing vivid complementary colors in both cases. Full colors can be achieved by tuning the radius of the nanodisks and the periodicity of the arrays. Good agreement between simulations and experiments is demonstrated and analyzed in CIE1931, sRGB, and HSV color spaces. The presented method has potential for applications in imaging, data storage, ultrafine displays, and plasmon-based biosensors.

Actively addressed single pixel full-colour plasmonic display

Dynamic, colour-changing surfaces have many applications including displays, wearables and active camouflage. Plasmonic nanostructures can fill this role by having the advantages of ultra-small pixels, high reflectivity and post-fabrication tuning through control of the surrounding media. However, previous reports of post-fabrication tuning have yet to cover a full red-green-blue (RGB) colour basis set with a single nanostructure of singular dimensions. Here, we report a method which greatly advances this tuning and demonstrates a liquid crystal-plasmonic system that covers the full RGB colour basis set, only as a function of voltage. This is accomplished through a surface morphology-induced, polarization-dependent plasmonic resonance and a combination of bulk and surface liquid crystal effects that manifest at different voltages. We further demonstrate the system's compatibility with existing LCD technology by integrating it with a commercially available thin-film-transistor array. The imprinted surface interfaces readily with computers to display images as well as video.

Energy transfer and depolarization in the photoluminescence of a plasmonic molecule

We report a comprehensive experimental study of the polarization dependence between excitation and photoluminescence (PL) emission from single dolmen-like metallic nanostructures that exhibit both Fano-like and Lorentz-like plasmon resonances. Though the PL spectra of this plasmonic "molecule" also exhibit the Fano and Lorentz signature, the emitted photons do not maintain the same polarization as the excitation. Surprisingly, the degree of depolarization correlates closely to the resonant excitation of the constituent atoms (single nanorod). More specifically, the excitation of a transverse plasmon mode results in a depolarized emission through the longitudinal plasmon modes of the constituent nanorods. In view of the recent evidence of on-resonant plasmon induced excitations in generating hot electrons, our results suggest that depolarized PL emissions could be enhanced through hot-electron decay. Both macroscopic and microscopic mechanisms are proposed to well-understand the excitation wavelength dependent depolarized photoluminescence behaviors in the plasmonic molecule. Our results lay a foundation for applying the depolarized photoluminescence of complex plasmonic nanostructures in polarization engineering.

Polarization-independent split bull's eye antennas for infrared nano-photodetectors

Split bull’s eye (SBE) antennas exhibit much larger extraordinary optical transmission and strong polarization dependence rather than bull’s eye (BE) antennas in the infrared range due to the introduced sub-wavelength slit. Here, we demonstrate a dual-split bull’s eye (DSBE) antenna, which consists of two sub-wavelength slits crossing through the center of the BE antenna with an intersection angle θ. The polarization dependence in transmission can be flexibly tailored by adjusting the intersection angle, following a cos² (Φ + θ/2) angular dependence on polarization angle Φ. When θ = 90°, the DSBE antenna yields high and polarization-independent transmission enhancement over the entire infrared spectrum. It presents highly promising applications for polarization-insensitive photodetectors and other optoelectronic devices.

Single-step fabrication of thin-film linear variable bandpass filters based on metal-insulator-metal geometry

A single-step fabrication method is presented for ultra-thin, linearly variable optical bandpass filters (LVBFs) based on a metal-insulator-metal arrangement using modified evaporation deposition techniques. This alternate process methodology offers reduced complexity and cost in comparison to conventional techniques for fabricating LVBFs. We are able to achieve linear variation of insulator thickness across a sample, by adjusting the geometrical parameters of a typical physical vapor deposition process. We demonstrate LVBFs with spectral selectivity from 400 to 850 nm based on Ag (25 nm) and MgF₂ (75-250 nm). Maximum spectral transmittance is measured at ∼70% with a Q-factor of ∼20.

Angle-tolerant polarization-tuned color filter exploiting a nanostructured cavity

A polarization-mediated color filter featuring a high angular tolerance is proposed incorporating a metal-dielectric-metal etalon based on a nanostructured cavity, where a one-dimensional subwavelength grating of a high refractive index is embedded in a base layer of a low refractive index. The aim of the nanostructured cavity is mimicking of the equivalent birefringent medium whereby different effective refractive indices are exhibited depending on the incident polarization. As the transmission peak of the etalon is effectively tuned through the tailoring of the refractive index of the cavity, the proposed filter is capable of providing a continuum of vivid output colors through a dynamic control of the polarization. The effective medium theory is chiefly applied for an investigation of the birefringent characteristics of the nanostructured cavity. A dielectric overlay that acts as an anti-reflection coating is specifically adopted for the etalon to enhance the transmission efficiency. The proposed polarization-tuned filter evidently provides a high transmission of ~71% and a high angular tolerance of ~35° in conjunction with a wide polarization-mediated color tuning.

Continuously Tunable, Polarization Controlled, Colour Palette Produced from Nanoscale Plasmonic Pixels

Colour filters based on nano-apertures in thin metallic films have been widely studied due to their extraordinary optical transmission and small size. These properties make them prime candidates for use in high-resolution colour displays and high accuracy bio-sensors. The inclusion of polarization sensitive plasmonic features in such devices allow additional control over the electromagnetic field distribution, critical for investigations of polarization induced phenomena. Here we demonstrate that cross-shaped nano-apertures can be used for polarization controlled color tuning in the visible range and apply fundamental theoretical models to interpret key features of the transmitted spectrum. Full color transmission was achieved by fine-tuning the periodicity of the apertures, whilst keeping the geometry of individual apertures constant. We demonstrate this effect for both transverse electric and magnetic fields. Furthermore we have been able to demonstrate the same polarization sensitivity even for nano-size, sub-wavelength sets of arrays, which is paramount for ultra-high resolution compact colour displays.

Trans-Reflective Color Filters Based on a Phase Compensated Etalon Enabling Adjustable Color Saturation

Trans-reflective color filters, which take advantage of a phase compensated etalon (silver-titania-silver-titania) based nano-resonator, have been demonstrated to feature a variable spectral bandwidth at a constant resonant wavelength. Such adjustment of the bandwidth is presumed to translate into flexible control of the color saturation for the transmissive and reflective output colors produced by the filters. The thickness of the metallic mirror is primarily altered to tailor the bandwidth, which however entails a phase shift associated with the etalon. As a result, the resonant wavelength is inevitably displaced. In order to mitigate this issue, we attempted to compensate for the induced phase shift by introducing a dielectric functional layer on top of the etalon. The phase compensation mediated by the functional layer was meticulously investigated in terms of the thickness of the metallic mirror, from the perspective of the resonance condition. The proposed color filters were capable of providing additive colors of blue, green, and red for the transmission mode while exhibiting subtractive colors of yellow, magenta, and cyan for the reflection mode. The corresponding color saturation was estimated to be efficiently adjusted both in transmission and reflection.

Static and Dynamic Magnetization of Gradient FeNi Alloy Nanowire

FeNi binary nanowires with gradient composition are fabricated by the electrodeposition method. The energy dispersive spec-trometer line-sweep results show that the composition changes gradually along the wire axis. The gradient FeNi nanowires exhibit polycrystalline and crystal twinning at different areas along the nanowire axis, with a textured face-centered cubic structure. The static and dynamic magnetization properties are characterized by a hysteresis loop and ferromagnetic reso-nance with pumping frequencies from 12– 40 GHz. The linear dispersion of the pumping frequency vs: the resonance field has been observed with the applied bias field higher than the saturation field, corresponding to the hysteresis loop. The field-sweep linewidths decrease with increasing pumping frequency, and the frequency-sweep linewidths stay nearly constant at the unsaturated region. The linewidth is a Gilbert type at the saturated state, with damping of 0.035 ± 0.003. Compared with the damping of the homogeneous composition FeNi nanowire (a = 0.044 ± 0.005), the gradient FeNi nanowire may have less eddy current damping, which could make it an alternative candidate for spintronics and microstrip antennas.