While structural colors are ubiquitous in nature, saturated reds are mysteriously absent. This long-standing problem of achieving Schrödinger’s red demands sharp transitions from “stopband” to a high-reflectance “passband” with total suppression of higher-order resonances at blue/green wavelengths. Current approaches based on nanoantennas are insufficient to satisfy all conditions simultaneously. Here, we designed Si nanoantennas to support two partially overlapping quasi–bound-states-in-the-continuum modes with a gradient descent algorithm to achieve sharp spectral edges at red wavelengths. Meanwhile, high-order modes at blue/green wavelengths are suppressed via engineering the substrate-induced diffraction channels and the absorption of amorphous Si. This design produces possibly the most saturated and brightest reds with ~80% reflectance, exceeding the red vertex in sRGB and even the cadmium red pigment. Its nature of being sensitive to polarization and illumination angle could be potentially used for information encryption, and this proposed paradigm could be generalized to other Schrödinger’s color pixels.

Dielectric optical nanoantennas are promising as fundamental building blocks in next generation color displays, metasurface holograms, and wavefront shaping optical devices. Due to the high refractive index of the nanoantenna material, they support geometry-dependent Mie resonances in the visible spectrum. Although phase change materials, such as the germanium-antimony-tellurium alloys, and post-transition metal oxides, such as ITO, have been used to tune antennas in the near-infrared spectrum, reversibly tuning the response of dielectric antennas in the visible spectrum remains challenging. In this paper, we designed and experimentally demonstrated dielectric nanodisc arrays exhibiting reversible tunability of Mie resonances in the visible spectrum. We achieved tunability by exploiting phase transitions in Sb2S3 nanodiscs. Mie resonances within the nanodisc give rise to structural colors in the reflection mode. Crystallization and laser-induced amorphization of these Sb2S3 resonators allow the colors to be switched back and forth. These tunable Sb2S3 nanoantenna arrays could enable the next generation of high-resolution color displays, holographic displays, and miniature LiDAR systems.

Dielectric optical nanoantennas play an important role in color displays, metasurface holograms, and wavefront shaping applications. They usually exploit Mie resonances as supported on nanostructures with high refractive index, such as Si and TiO2. However, these resonances normally cannot be tuned. Although phase change materials, such as the germanium-antimony-tellurium alloys and post transition metal oxides, such as ITO, have been used to tune optical antennas in the near infrared spectrum, tunable dielectric antennae in the visible spectrum remain to be demonstrated. In this paper, we designed and experimentally demonstrated tunable dielectric nanoantenna arrays with Mie resonances in the visible spectrum, exploiting phase transitions in wide-bandgap Sb2S3 nano-resonators. In the amorphous state, Mie resonances in these Sb2S3 nanostructures give rise to a strong structural color in reflection mode. Thermal annealing induced crystallization and laser induced amorphization of the Sb2S3 resonators allow the color to be tuned reversibly. We believe these tunable Sb2S3 nanoantennae arrays will enable a wide variety of tunable nanophotonic applications, such as high-resolution color displays, holographic displays, and miniature LiDAR systems.

Tunable Mie Resonances in the Visible Spectrum Li Lu, Zhaogang Dong, Febiana Tijiptoharsono, Ray Jia Hong Ng, Hongtao Wang, Soroosh Daqiqeh Rezaei, Yunzheng Wang, Hai Sheng Leong, Joel K. W. Yang* and Robert E. Simpson Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-03 Innovis, 138634 Singapore Institute of High Performance Computing, A*STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, #16-16 Connexis, 138632 Singapore

Zhaogang Dong*, Lei Jin, Soroosh Daqiqeh Rezaei, Hao Wang, Yang Chen, Febiana Tjiptoharsono, Jinfa Ho, Sergey Gorelik, Ray Jia Hong Ng, Qifeng Ruan, Cheng-Wei Qiu, and Joel K. W. Yang* Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-03 Innovis, 138634 Singapore Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583, Singapore College of Electronic and Information Engineering, Hangzhou Dianzi University, Hangzhou, China, 310018 Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore Singapore Institute of Food and Biotechnology Innovation, A*STAR (Agency for Science, Technology and Research), 31 Biopolis Way, #01-02 Nanos, Singapore 138669

Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, for example, plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry-Perot interference, and Rayleigh-Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved. We discuss mechanisms to tune structural colors and review recent advances in dynamic structural colors. In the end, we propose the concept of a universal pixel that could be crucial in realizing next-generation displays based on nanophotonic structural colors.

Efficient generation of anti-Stokes emission within nanometric volumes enables the design of ultracompact, miniaturized photonic devices for a host of applications. Many subwavelength crystals, such as metal nanoparticles and two-dimensional layered semiconductors, have been coupled with plasmonic nanostructures for augmented anti-Stokes luminescence through multiple-harmonic generation. However, their upconversion process remains inefficient due to their intrinsic low absorption coefficients. Here, we demonstrate on-chip, site-specific integration of lanthanide-activated nanocrystals within gold nanotrenches of sub-25 nm gaps via bottom-up self-assembly. Coupling of upconversion nanoparticles to subwavelength gap-plasmon modes boosts 3.7-fold spontaneous emission rates and enhances upconversion by a factor of 100 000. Numerical investigations reveal that the gap-mode nanocavity confines incident excitation radiation into nanometric photonic hotspots with extremely high field intensity, accelerating multiphoton upconversion processes. The ability to design lateral gap-plasmon modes for enhanced frequency conversion may hold the potential to develop on-chip, background-free molecular sensors and low-threshold upconversion lasers.

Unlike dye-based colorants, for which dilution results in a decrease in color saturation, a reduction of nanostructure density in plasmonic prints could increase color saturation instead. This interesting observation can be explained by the absorption cross-section of the nanostructure being larger than its physical cross-section. In this paper, we demonstrate the correlation between absorption cross-section and nanostructure density and use it to realize saturated colors by fabricating metal-insulator-metal aluminum nanostructures that support gap-surface plasmons (GSPs). We obtained structures with absorption cross-sections that exceed 10 times their physical cross-sections. The large absorption cross-sections of the GSP structures herald a color-mixing scheme where nanostructures of different hues are combined within subpixels at a constant pitch. The pitch is chosen such that the total absorption cross-section of individual constituents of the cell occupies the unit size area. Using a constant pitch of 320 nm, hence preserving the print resolution, our structures exhibit 45% coverage of the sRGB color space. By employing absorption cross-sections of the nanostructures, we produced black and saturated green pixels, which have been challenging to achieve in plasmonic color printing. The effects of square and hexagonal arrangements on color saturation are investigated, and point mixing effects are observed between individual nanostructures.

We present results on colors of metal disk arrays viewed under a darkfield microscope and show that the darkfield colors can be manipulated independently of the brightfield colors. We investigate the appearance of colors as disks are clustered to form a new array with double the pitch and a basis of four disks. These structures of aluminum disks on aluminum oxide on aluminum have resonances in the visible spectrum, so by arranging them in small tight clusters, a coupled plasmon resonant mode is produced at shorter wavelengths. This feature causes a reflectance minimum and leads to an increase in the gamut of darkfield colors produced. These colors are tuned by changing the size of the disk and the inter-disk gap within the clusters. Interestingly, the intensities of the reflectance peaks also demonstrate good agreement with the Fourier series coefficients for square waves. Polarization-tunable colors are also demonstrated by designing rectangular arrays that have dissimilar periods along the two orthogonal axes of the array, and a four-level security tag is fabricated that encodes images for viewing under brightfield, darkfield (both x and y polarization), and infrared illumination.

Artwork often needs to be verified for authenticity during transactions. Electronic tags and labels are easily spotted and hence counterfeited. A covert and effective tag needs to be: (1) microscopic, (2) camouflaged into the surroundings, and (3) contain multiple sets of information. Here, we developed aluminum (Al) nanostructures that resonate across the ultraviolet (UV) to infrared (IR) spectra for use in micro-tags with varying colors of similar brightness and containing two sets of information in the visible and IR. Native Al2O3 on Al films was measured to be ~4–7 nm thick, enabling resonances to be supported by Al disks with diameters merely ~1/6th of the wavelength at the fundamental mode. Through accurate modeling of the nanostructures and high-resolution electron-beam lithography, we designed and fabricated a printed micro-tag on silicon. This micro-tag requires image processing to extract a quick response (QR) code in the visible, and 1.2 μm IR illumination (or visible light darkfield imaging) to extract a covert barcode. The compact and multi-spectral encoding of prints demonstrated here is particularly suited for discreet tagging of art and high-value merchandise.

When a microlens array is aligned and overlaid on an array of patterns with similar periodicity, a highly magnified image of the patterns is observed. This effect, known as moiré magnification, is used to reveal micropatterns that are unresolvable by the naked eye. These patterns are typically limited by print resolution to single color patterns. Here, we demonstrate the potential to selectively reveal more than one set of color patterns. By rotating a microlens array relative to a print containing three overlapping arrays of structural color patterns in 10° steps, each pattern array can be distinctly revealed with minimal crosstalk. This rotation-selective effect of moiré magnification is not seen in conventional microscopy. An advantage is that the moiré images are observable by the naked eye under incoherent illumination. We leverage nanoscale three-dimensional printing by using the two-photon lithography process to produce structural color pattern arrays in a single lithographic step with precisely aligned color pixels. We believe that this work can have applications in precise rotational-alignment tools, covert security documents, and information multiplexing devices.

Through numerical simulations, we investigate the correlation between the absorption cross-section and the color saturation of plasmonic nanostructures of varying density. Understanding this correlation, enables the prediction of an optimal nanostructure separation, or combinations of different nanostructure sizes for plasmonic color printing applications. Here, we use metal-insulator-metal (MIM) aluminum nanostructures that support gap-plasmons. Large absorption cross-sections were observed that exceed twelve times the physical cross-section of the nanostructure disks. We derive a set of equations to determine the optimal separation for a periodic array using the absorption cross-section of an individual structure to realize saturated colors. Using the optimum pitch and enabled by the large absorption cross-sections of our structures, we employ color mixing strategies to realize a wider color gamut. The simulated color gamut exceeds the sRGB gamut for some colors, and includes dark tones. Color mixing using structures with large absorption cross-sections is a practical approach to generate a broad range of colors, in comparison to fabricating structures with continuously varying sizes.

Thin-film and isolated metal nanostructures with localized plasmon resonances in the visible spectrum are promising for commercial adoption of ultrahigh resolution color printing. As metallization processes used in printing industries tend to produce thick and continuous metal coatings, challenges remain in designing structures with similar local resonances to produce colors. Here, we demonstrate color elements through surface textures on an encapsulated silver surface thus bringing plasmonic colors a step closer to commercial printing processes. We probe the chromatic range of protrusions and indentations, two possible architectures that could form on an all-metal surface, and further investigate the modulation of color when the protrusions evolve into the reversed structures of indentations. Notably, the indentations generate pitch-dependent colors; albeit protrusions generate size-dependent colors that are superior. Building upon this blueprint for all metal plasmonic color printing, we highlight the spectral tunability of these complementary geometries through an oxidation robust color microprint. We envision that these structures will enable economically feasible ultrahigh resolution color creation with metallic finishes on plastic films.

Optical resonances in metallic nanostructures are promising in enabling high-resolution plasmonic color prints, color filters, and in rendering colors for plastic consumer products. However, nanostructure patterning approaches have relied on charged-particle beam lithography, with limited throughput. For the purpose of visually evaluating colors spanning a large parameter space, it is important to develop a rapid and cost-effective approach to patterning large areas. The speed at which the parameter space is explored experimentally needs to be comparable to the time it takes to run full electromagnetic simulations. Here, we used a bottom up approach to cost-effectively create periodic nanostructures on centimeter-scale samples. Upon further processing, this approach produced more complex geometries, such as rings and domes, compared to the standard structures consisting of metal disks. By adjusting various geometric parameters, vivid colors with an expanded gamut were obtained.