单 界 面 多 重 反 射 干 涉 结 构 色 原 理 与 应 用(特 邀)

Significance Structural colors, originating from the physical interaction between light and micro/nanostructures, have emerged as a pivotal and rapidly advancing field in photonics and materials science. Unlike traditional pigment-based colors that rely on molecular absorption and are prone to fading and environmental toxicity, structural colors offer remarkable advantages including high saturation, environmental friendliness, and superior stability. Among the diverse physical mechanisms for generating structural colors-such as photonic crystals, waveguide modes, and various resonances-the phenomenon of single-interface multibounce reflection interference has recently gained prominence as a uniquely simple and versatile approach. Its core principle leverages multiple reflections and interference of light within a single microscale concave interface to generate vivid structural colors. This mechanism fundamentally breaks away from the reliance on precise periodic nanostructures, offering instead a pathway to vibrant colors through geometrically tunable, single-element micro-optics. The significance of this technology lies in its exceptional flexibility: it is compatible with a wide range of materials-from polymers, liquid crystals, and hydrogels to soft matter systems-and can be realized using cost-effective, scalable fabrication techniques. These attributes make it a powerful platform for a new generation of applications in smart displays, advanced anti-counterfeiting, high-sensitivity optical sensors, and adaptive photonic devices, positioning it at the forefront of intelligent material design. The optical foundation of this structural color mechanism is elegantly rooted in the accumulation of optical path differences (OPDs) as light undergoes multiple reflections along curved interfaces, leading to constructive and destructive interference for specific wavelengths. Two primary optical configurations have been systematically elucidated and experimentally validated. The first is based on total internal reflection (TIR), where light traveling from a higher-refractive-index medium to a lowerindex one undergoes complete reflection at a concave interface when the incident angle exceeds the critical angle; the multiple reflections along different paths within the interface create distinct OPDs and resulting interference colors. The second, more recently explored configuration utilizes metallized curved surfaces, where a reflective metal coating applied to a concave microstructure enables efficient multibounce reflection without the stringent angle and refractive-index constraints of TIR. This approach not only enhances reflectivity and optical efficiency but also allows for greater design freedom, including the use of lower-refractive-index materials to optimize color saturation by fine-tuning the OPD. Progress A key driver for the advancement of this field is its exceptional compatibility with a diverse portfolio of micro-fabrication strategies, broadly categorized into bottom-up and top-down approaches. Bottom-up methods excel in forming functional microstructures from liquid or colloidal precursors. Microfluidics enables the generation of highly uniform droplets and complex emulsion templates from materials like liquid crystals and hydrogels, serving as dynamic, color-tunable micro-reactors. Inkjet printing offers a digital, maskless route for depositing transparent polymer inks to form micro-domes with precisely controlled curvature, enabling full-color printing and stimuli-responsive color changes. Self-assembly of colloidal particles provides a low-cost route for creating large-area periodic structures that support multibounce interference, though with inherent limitations in structural complexity. Conversely, top-down and replication methods provide superior control over microstructure geometry and integration. Grayscale lithography stands out for its ability to fabricate high-precision, continuously curved 3D microstructures by spatially modulating the exposure dose in photoresist, which is ideal for crafting customized optical interfaces. Subtractive manufacturing, including various etching techniques, allows for the precise removal of material to create well-defined micro-concave arrays with pixel-level color control. Template replication techniques efficiently mass-produce functional structures by molding and transferring from a master template, proving highly effective for polymers and hydrogels. Furthermore, thermal reflow post-processing transforms lithographically patterned micro-posts into smooth, spherical-cap micro-domes, significantly improving optical quality and enabling dynamic color response through mechanical deformation. The compelling optical properties and fabrication versatility of these structural colors have unlocked a spectrum of innovative applications across multiple domains. In the realm of display and visualization, they enable static high-resolution color images and, more importantly, dynamic displays that respond to external stimuli such as mechanical stress, temperature, or humidity, paving the way for low-power, “paper-like” display technologies and intelligent signage. For information encryption and anti-counterfeiting, the technology offers unparalleled security features. Stimuliresponsive systems-activated by specific solvents, magnetic fields, humidity, or polarized light-enable information to be hidden and revealed on demand, creating dynamic, high-capacity encryption platforms. The inherent angle-dependency of the colors and the ability to create hidden images that only appear under specific lighting conditions further elevate the barrier against counterfeiting, making them ideal for protecting high-value goods and secure documents. In optical sensing, these structural colors function as intuitive, colorimetric sensors. Changes in the local environment-such as mechanical strain, variations in humidity, or the presence of chemical vapors-induce morphological or refractive index changes in responsive materials like hydrogels or elastomers, which in turn shift the interference condition and produce a visible color change. This provides a direct visual readout for real-time monitoring in applications ranging from flexible electronic skins to environmental sensing. Conclusions and Prospects In conclusion, single-interface multibounce reflection interference structural colors represent a mature yet dynamically evolving frontier in photonic materials. While significant progress has been made in understanding its principles and developing fabrication methodologies, several challenges remain to be addressed to fully realize its potential. These include the simultaneous optimization of color saturation, brightness, and gamut; the development of efficient inverse design capabilities, potentially powered by machine learning, to bridge the gap between desired optical response and optimal structure; and the enhancement of mechanical durability and environmental stability for long-term practical use. Future advancements are likely to be driven by the convergence of multiple physical mechanisms-such as hybridizing interference with plasmonic or Mie resonant effects-to create richer color palettes and novel functionalities. The deep integration of intelligent computational design with automated digital manufacturing will accelerate the development of application-specific devices. Concurrently, the exploration of novel dynamic and responsive material systems will unlock new possibilities in adaptive camouflage, wearable health monitors, and bio-integrated optical devices. Through continued interdisciplinary collaboration across optics, materials science, and engineering, this vibrant technology is poised to make the critical transition from laboratory demonstration to widespread industrial application, fundamentally shaping the future of intelligent photonics and sustainable optical materials.