Plasmonic colours are structural colours that emerge from resonant interactions between light and metallic nanostructures. The engineering of plasmonic colours is a promising, rapidly emerging research field that could have a large technological impact. We highlight basic properties of plasmonic colours and recent nanofabrication developments, comparing technology-performance indicators for traditional and nanophotonic colour technologies. The structures of interest include diffraction gratings, nanoaperture arrays, thin films, and multilayers and structures that support Mie resonances and whispering-gallery modes. We discuss plasmonic colour nanotechnology based on localized surface plasmon resonances, such as gap plasmons and hybridized disk–hole plasmons, which allow for colour printing with sub-diffraction resolution. We also address a range of fabrication approaches that enable large-area printing and nanoscale lithography compatible with complementary metal-oxide semiconductor technologies, including nanoimprint lithography and self-assembly. Finally, we review recent developments in dynamically reconfigurable plasmonic colours and in the laser-induced post-processing of plasmonic colour surfaces. Plasmonic colours can be used to colour large surfaces, can be mass-produced and dynamically reconfigured, and can provide sub-diffraction resolution. In this Review, basic properties of plasmonic colours, different platforms supporting them and recent developments in the field are discussed.

Plasmonic colour generation

Metasurfaces enable manipulation of light propagation at an unprecedented level, benefitting from a number of merits unavailable to conventional optical elements, such as ultracompactness, precise phase and polarization control at deep subwavelength scale, and multifunctionalities. Recent progress in this field has witnessed a plethora of functional metasurfaces, ranging from lenses and vortex beam generation to holography. However, research endeavors have been mainly devoted to static devices, exploiting only a glimpse of opportunities that metasurfaces can offer. We demonstrate a dynamic metasurface platform, which allows independent manipulation of addressable subwavelength pixels at visible frequencies through controlled chemical reactions. In particular, we create dynamic metasurface holograms for advanced optical information processing and encryption. Plasmonic nanorods tailored to exhibit hierarchical reaction kinetics upon hydrogenation/dehydrogenation constitute addressable pixels in multiplexed metasurfaces. The helicity of light, hydrogen, oxygen, and reaction duration serve as multiple keys to encrypt the metasurfaces. One single metasurface can be deciphered into manifold messages with customized keys, featuring a compact data storage scheme as well as a high level of information security. Our work suggests a novel route to protect and transmit classified data, where highly restricted access of information is imposed.

https://advances.sciencemag.org/content/advances/4/6/eaar6768.full.pdf

Addressable metasurfaces for dynamic holography and optical information encryption

The achievement of structural color has shown advantages in large-gamut, high-saturation, high-brightness, and high-resolution. While a large number of plasmonic/dielectric nanostructures have been developed for structural color, the previous approaches fail to match all the above criterion simultaneously. Herein we utilize the Si metasurface to demonstrate an all-in-one solution for structural color. Due to the intrinsic material loss, the conventional Si metasurfaces only have a broadband reflection and a small gamut of 78% of sRGB. Once they are combined with a refractive index matching layer, the reflection bandwidth and the background reflection are both reduced, improving the brightness and the color purity significantly. Consequently, the experimentally demonstrated gamut has been increased to around 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020. Meanwhile, high refractive index of silicon preserves the distinct color in a pixel with 2 × 2 array of nanodisks, giving a diffraction-limit resolution. Here, the authors fabricate a metasurface with high brightness and large gamut structured colors by combining a silicon metasurface with a refractive index matching layer. The experimentally demonstrated gamut is 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020.

/pdf/all-dielectric-metasurface-for-high-performance-structural-nst8v5pwnp.pdf

All-dielectric metasurface for high-performance structural color.

Localized optical resonances in metallic nanostructures have been increasingly used in color printing, demonstrating unprecedented resolution but limited in color gamut. Here, we introduce a new nanostructure design, which broadens the gamut while retaining print resolution. Instead of metals, silicon nanostructures that exhibit localized magnetic and electric dipole resonances were fabricated on a silicon substrate coated with a Si3N4 index matching layer. Index matching allows a suppression of substrate effects, thus enabling Kerker’s conditions to be met, that is, sharpened transitions in the reflectance spectra leading to saturated colors. This nanostructure design achieves a color gamut superior to sRGB, and is compatible with CMOS processes. The presented design could enable compact high-resolution color displays and filters, and the use of a Si3N4 antireflection coating can be readily extended to designs with nanostructures fabricated using other high-index materials.

Printing Beyond sRGB Color Gamut by Mimicking Silicon Nanostructures in Free-Space.

High-resolution multicolor printing based on pixelated optical nanostructures is of great importance for promoting advances in color display science. So far, most of the work in this field has been focused on achieving static colors, limiting many potential applications. This inevitably calls for the development of dynamic color displays with advanced and innovative functionalities. In this Letter, we demonstrate a novel dynamic color printing scheme using magnesium-based pixelated Fabry-Perot cavities by gray scale nanolithography. With controlled hydrogenation and dehydrogenation, magnesium undergoes unique metal and dielectric transitions, enabling distinct blank and color states from the pixelated Fabry-Perot resonators. Following such a scheme, we first demonstrate dynamic Ishihara plates, in which the encrypted images can only be read out using hydrogen as information decoding key. We also demonstrate a new type of dynamic color generation, which enables fascinating transformations between black/white printing and color printing with fine tonal tuning. Our work will find wide-ranging applications in full-color printing and displays, colorimetric sensing, information encryption and anticounterfeiting.

Dynamic Color Displays Using Stepwise Cavity Resonators.

Reflective Color Filters and Monolithic Color Printing Based on Asymmetric Fabry–Perot Cavities Using Nickel as a Broadband Absorber

This study demonstrates a full-color printing concept based on the interference effect in pixelized metal–dielectric–metal Fabry–Perot (FP) resonance cavities. The pixel color for printing is determined by the thickness of the dielectric layer in each microscale FP cavity. Abundant colors with controllable brightness and saturation are achieved by varying both the thickness and the filling density of the FP cavities using grayscale lithography. Enabled by the wide color gamut, a vivid full-color image can be reproduced at the microscopic scale with high resolution by correlating the colors with the dimensional parameters of the FP cavities through a layout-generation algorithm. The colorization strategy based on interference effects provides a new opportunity to use artificial structures for color printing and also has the potential to be scaled up for large-volume application in consumable products using replica patterning techniques such as grayscale photolithography, nanoimprinting, and soft lithography.

Microscopic Interference Full-Color Printing Using Grayscale-Patterned Fabry–Perot Resonance Cavities

Visible-light color filters using patterned nanostructures have attracted much interest due to their various advantages such as compactness, enhanced stability, and environmental friendliness compared with traditional pigment or dye-based optical filters. While most existing studies are based on planar nanostructures with lateral variation in size, shape, and arrangement, the vertical dimension of structures is a long-ignored degree of freedom for the structural colors. Herein, we demonstrate a synthetic platform for transmissive color filter array by coordinated manipulations between height-varying nanocavities and their lateral filling fractions. The thickness variation of those nanocavities has been fully deployed as an alternative degree of freedom, yielding vivid colors with wide gamut and excellent saturation. Experimental results show that the color-rendering capability of the pixelated nanocavities can be still retained as pixels are miniaturized to 500 nm. Crosstalk between closely spaced pixels of a Bayer color filter arrangement was calculated, showing minimal crosstalk for 1 µm2 square subpixels. Our work provides an approach to designing and fabricating ultracompact color filter arrays for various potential applications including stained-glass microprints, microspectrometers, and high-resolution image sensing systems.

/pdf/stepwise-nanocavity-assisted-transmissive-color-filter-array-kztqjzhc0u.pdf

Stepwise-Nanocavity-Assisted Transmissive Color Filter Array Microprints

We demonstrated a tunable structural color filter based on an asymmetric Fabry-Perot cavity employing germanium antimony tellurium alloy Ge2Sb2Te5 (GST) as a switchable ultrathin lossy layer. The color tunability and switch mechanism of our designed structure were investigated by both simulation and analytical approaches. Both numerical simulations and analytical results show that the tunable reflective colors can be generated through the reversible phase transition of GST from amorphous to crystalline. Additionally, the generated colors possess high brightness, high saturation, and a wide gamut. Our designed structure will inspire phase-transition-based systems' potential applications in colorimetric sensing, smart windows, full-color printing and displays, anti-counterfeiting, and data encryption.

Yanming Zhou

Papers

Dynamic Color Displays Using Stepwise Cavity Resonators.

Reflective Color Filters and Monolithic Color Printing Based on Asymmetric Fabry–Perot Cavities Using Nickel as a Broadband Absorber

Microscopic Interference Full-Color Printing Using Grayscale-Patterned Fabry–Perot Resonance Cavities

Stepwise-Nanocavity-Assisted Transmissive Color Filter Array Microprints

Tunable reflective color filters based on asymmetric Fabry-Perot cavities employing ultrathin Ge2Sb2Te5 as a broadband absorber.