image: Figure 1 Working principle and experimental characterization of time-programmable coloration. Under broadband white light illumination, the designed 3D metastructures precisely modulate the optical field to generate predefined colors and patterns in the far field. The enlarged inset illustrates the modulation mechanism of the structural colors produced by the 3D metastructures, where nenv1, nenv2 and nenv3 denote the environmental refractive indices corresponding to different device working environments. Morphological characterization of the chameleon-inspired patterned 3D metastructures is shown with scanning electron microscopy (SEM) images of the complete pattern and a magnified local region (bottom left). Time-programmable continuous color tuning achieved under varying environmental refractive indices is illustrated on the right, where T1–T7 represent the corresponding programmable time points. The bottom insets depict representative R/G/B output states under three distinct refractive index conditions.
Credit: Zhi-Yong Hu et al.
With the development of the information era, encryption technologies have become a critical foundation for high-security applications such as military communications, medical privacy, and commercial secrets. Although traditional electronic encryption is mature, its reliance on mathematical algorithms makes it vulnerable to quantum-computing attacks, and loose key-management mechanisms can introduce additional weaknesses. Optical encryption, grounded in physical principles, can reduce the risk of quantum decryption at the source and enables high-dimensional encoding by jointly exploiting multiple degrees of freedom such as wavelength, polarization, and phase, thereby enhancing security. Micro- and nanostructures, capable of precisely manipulating optical fields without dyes and offering high stability and environmental friendliness, are regarded as ideal carriers for optical encryption and also exhibit potential for multi-physical-field responsiveness. However, most existing dynamic optical encryption strategies rely on single-parameter-triggered binary color switching, which limits information capacity and is difficult to defend against brute-force analyses based on multiple parameters. Moreover, if the carrier is not destroyed after information retrieval, secondary leakage risks remain. Current destruction approaches often depend on chemical reactions or external energy input, suffer from substrate and environmental constraints, and may raise concerns such as contamination and toxicity. Therefore, developing new encryption technologies that integrate continuous color-gamut control, multidimensional dynamic encoding, and self-destruction capability is a key challenge for overcoming the binary-state bottleneck and building highly secure information carriers.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Din Ping Tsai from Department of Electrical Engineering and State Key Laboratory of Optical Quantum Materials, City University of Hong Kong, Hong Kong 999077, China, and co-workers have proposed a metastructure-based structural-color control strategy enabled by precise programming of geometric parameters. By accurately designing the key geometric parameters of meta-atoms, the structural color can be predictably tuned across the visible spectrum, achieving wide-gamut structural-color output. Experimental results demonstrate that, within the same material and fabrication framework, high-resolution monochromatic and multicolor device printing can be realized, showing good color consistency and reproducibility and providing a solid basis for subsequent information encoding.
Building on this, the research team developed a deep-learning-based recognition scheme for structural-color anti-counterfeiting labels. A convolutional neural network is employed to automatically extract the color-distribution and spatial-texture features of structural-color patterns, enabling rapid discrimination among different label categories. Compared with conventional approaches that rely on manually set thresholds or single features, this scheme exhibits stronger generalization under complex imaging conditions. The results show that high recognition accuracy can still be maintained under disturbances such as background variations, defocus, rotation, and local contamination, thereby improving the practicality and reliability of structural-color labels in real-world anti-counterfeiting applications.
Moreover, by tuning the refractive index of the surrounding liquid, the metastructure’s spectral response can be continuously modulated, allowing the structural color to evolve over time following a prescribed trajectory and enabling time-programmable dynamic coloration. Furthermore, capillary forces generated during liquid evaporation are exploited to induce irreversible collapse of the nanostructures, permanently altering the carrier after display or readout and thereby achieving irreversible information erasure. This approach realizes both dynamic modulation and erasure without additional energy input or complex chemical reactions, reducing the risk of residual information leakage after readout.
Based on the above capabilities of continuous color-gamut control, time-programmable modulation, and self-destruction, we develop a multilayer information-encryption and stepwise decoding strategy. Different time windows correspond to different decoding outcomes, enabling a single carrier to output multiple messages sequentially, thereby substantially expanding the key space and information capacity and enhancing resistance to brute-force cracking and multi-parameter analytical attacks. Meanwhile, the self-destructive erasure mechanism renders the carrier irreversibly invalid after readout, endowing the encryption system with a physical burn-after-reading security property. This proof of concept offers a new design paradigm for high-capacity, high-security, and erasable optical encryption and anti-counterfeiting devices.
Article Title
Time-programmable coloration via 3D metastructures for optical encryption