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, e.g., 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-Pérot 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.

Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, we explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics. We report a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half pitch using two-photon polymerization lithography (TPL). Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promises a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices.

The rapid development of additive manufacturing has fueled a revolution in various research fields and industrial applications. Among the myriad of advanced three-dimensional printing techniques, two-photon polymerization lithography (TPL) uniquely offers a significant electron microscope (SEM) images of SMP structures before and after programming and after recovery, respectively. Reproduced under terms of the CC-BY license. [114] Copyright 2021, The Authors, published by Nature Publishing Group. b) Stiff SMPs for high-resolution reversible nanophotonics. I-II) The deformed and recovered nanopillars under optical microscope and SEM, respectively. Reproduced with permission. [115] Copyright 2022, American Chemical Society. c) Vapor-responsive photonic arrays. I-IV) Optical image of a grid array in air, in water vapors ~ 20 s, saturation with water vapors ~ 88s, and after the gas flow stopped, respectively.Reproduced under terms of the CC-BY license. [116] Copyright 2021, The Authors, published by Royal Society of Chemistry. d) SEM image of a 40-layer face-cantered-cubic photonic structure printed by SU-8. Reproduced with permission. [117] Copyright 2004, Nature Publishing Group. e) SEM image of helices with an axial pitch of 800 nm and a radius of 800 nm fabricated by the two-step absorption photoresist. Reproduced with permission. [118]

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 lowrefractive-index nanopillar to demonstrate grayscale inversion and color desaturation and steganography at the level of single nanopillars.

Color changes can be achieved by straining photonic crystals or gratings embedded in stretchable materials. However, the multiple repeat units and the need for a volumetric assembly of nanostructures limit the density of information content. Inspired by surface reliefs on oracle bones and music records as a means of information archival, here, surface‐relief elastomers are endowed with multiple sets of information that are accessible by mechanical straining along in‐plane axes. Distinct from Bragg diffraction effects from periodic structures, trenches that generate color due to variations in trench depth, enabling individual trench segments to support a single color, are reported. Using 3D printed cuboids, trenches of varying geometric parameters are replicated in elastomers. These parameters determine the initial color (or lack thereof), the response to capillary forces, and the appearance when strained along or across the trenches. Strain induces modulation in trench depth or the opening and closure of a trench, resulting in surface reliefs with up to six distinct states, and an initially featureless surface that reveals two distinct images when stretched along different axes. The highly reversible structural colors are promising in optical data archival, anti‐counterfeiting, and strain‐sensing applications.