Dynamic structural color based on tunable optical resonance plays a key role in applications including encryption visualization, camouflage and colorimetric sensing. However, the current design requires either complex growth processes of the high-quality tunable materials or complicated circuit designs. This work makes a humidity-swelling hydrogel layer for metal–insulator–metal (MIM) structure in the dynamic multi-color display. Here, polyvinyl alcohol (PVA) hydrogel structure is patterned through grayscale e-beam lithography and the controlled PVA thickness leads the programmable reflective resonance covering the entire visible range. By varying the ambient humidity between 9.8 and 90.1% RH, the reflective resonance of the structure is tailored across a wavelength range over 100 nm. Our materials platform of humidity-sensitive hydrogel resist presents a novel approach of the stepwise and reversible optical tunability for photonic devices.

Although various artificial dyes and pigments have been invented, certain application fields need structural colors because they can last for centuries even under harsh conditions. Here, we report that the antireflective Ag brochosomes (soccer-ball-like microscale granules covered by nanobowls) become colorful when the nanobowls on the Ag brochosomes are filled by polystyrene (PS) nanospheres. The color originates from the enhanced electromagnetic resonances of the PS nanospheres by the surrounding metallic nanobowls, suggested by both the experimental and the simulation results. The color is determined by the size of the PS nanospheres. We can tailor the color simply by reducing the diameter of the PS nanospheres via the plasma etching treatment. The color intensity of the Ag brochosomes filled with PS nanospheres shows weak dependence on the observing angles, benefiting from their spherical shape. Plasma etching treatment of the Ag brochosomes filled with PS nanospheres through different masks can design various structural color patterns. The simple fabrication process and the easy processability make the Ag brochosomes filled with PS nanospheres have promising applications in the structural color fields.

Control over the directionality of thermal emission plays a fundamental role in efficient heat transport. Although nanophotonic technologies have demonstrated the capability for angular‐selective thermal emission, achieving asymmetric directional thermal emission in reciprocal systems with energy directed to a single output angle remains challenging due to symmetric band dispersion. In this work, we present a general strategy for achieving asymmetric directional thermal emission in reciprocal systems. With periodic perturbation and broken mirror symmetry, metasurfaces behave as resonant metagratings whose resonances can be diffracted to symmetric output angles with distinct efficiency, allowing for high emissivity towards a single direction. The asymmetric directional thermal emitter is experimentally demonstrated at mid‐infrared wavelengths with high emissivity (ε = 0.61) at the observation angle of +30°, and low emissivity (ε<0.3) at other angles. This work highlights the potential for manipulating the directionality of thermal emission, which holds promise for developing ultrathin customized thermal sources and impacts on various thermal‐engineering applications.This article is protected by copyright. All rights reserved

Long‐wave infrared (LWIR) of 8–14 μm is an imperative atmospheric transmission window for applications such as infrared detectors, radiative cooling, and infrared stealth. Attempts to manipulate thermal emission in the LWIR band consist of either directional emission but narrowband or LWIR emission but non‐directional. Directional thermal emission covering the entire LWIR band has remained elusive, so far. Here, a whole LWIR directional thermal emitter is introduced consisting of top epsilon‐near‐zero (ENZ) films (SiO2/SiO/Al2O3), a dielectric gap (Ge), and bottom ENZ films (TiO2/Ta2O5) on a metal. The emitter exhibits high emissivity (>0.9) in p‐polarization at specific directions (72°–82°) covering the entire 8–14 μm atmospheric window. Additionally, infrared encryption information on the emitter can only be observed in p‐polarization in the oblique direction. This approach introduces a new route toward simultaneous control of bandwidth and directionality of thermal emission and has inclusive applications such as infrared camouflage and energy management.

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