A research team led by Professor Junsuk Rho at POSTECH (Pohang University of Science and Technology) has developed a secure hologram platform that operates solely based on the wavelength of light and the spacing between metasurface layers. The technology makes hacking and counterfeiting virtually impossible, and is expected to be widely adopted for security cards, anti-counterfeiting, and military communications. With a growing number of hacking incidents and data breaches, the limitations of digital security are becoming increasingly evident. No matter how sophisticated an encryption scheme is, as long as it exists as code, it is difficult to completely eliminate the risk of intrusion. Motivated by this challenge, the team proposed a new approach that uses the physical conditions of light itself as a security key.
At the core of this innovation is the “metasurface,” an ultrathin optical device that arranges microscopic structures to control light. By illuminating a metasurface, a holographic image can be reconstructed in free space. However, conventional holograms have typically been limited in that a single device could store only one piece of information. To overcome this limitation, the researchers designed a “modular diffractive deep neural network,” applying concepts from artificial neural networks to optical structures. In this architecture, light propagation and interference autonomously perform computations, enabling information processing using light alone—without electrical power or electronic chips. Each metasurface functions as a layer of the neural network, and the team trained the system such that entirely different outputs emerge when layers are used individually versus when they are combined. For example, illuminating the metasurface with a specific wavelength reconstructs an ID hologram, while a different wavelength produces a completely different image. Another layer might reconstruct QR-code information. In other words, each layer independently stores distinct information.
The technology’s true potential appears when two or more metasurfaces are combined. When two layers are positioned at a precise separation and illuminated with a specific wavelength, an encrypted hologram—corresponding to a password—appears. If the wavelength or the interlayer spacing deviates even slightly, the information remains hidden. In this way, the color of light and the distance between layers function as a “physical password.” Notably, in theory, as the number of wavelengths (m) and the number of metasurface layers (N) increase, the number of information channels grows exponentially as m(2ⁿ−1). This suggests that the security levels and combinations of information achievable within a single device can be expanded virtually without limit.
The team expects the technology to be applicable to anti-counterfeiting labels for IDs and passports, secure military and diplomatic documents, and next-generation optical communications. “By using the physical properties of light itself as a security key, this study could fundamentally reshape the paradigm of conventional digital security,” said Professor Junsuk Rho. “As digital technologies become more advanced, our results highlight that physical security can ultimately provide the strongest solution.”
This research was carried out by Professor Junsuk Rho’s team at POSTECH (Department of Mechanical Engineering; Department of Chemical Engineering; Department of Electrical Engineering; and the Graduate School of Convergence Science and Technology). The work was published in Advanced Functional Materials , an international journal in materials science and nanotechnology, and was supported by the POSCO-POSTECH-RIST Convergence Research Institute and the National Research Foundation of Korea.
Advanced Functional Materials
Recomposable Layered Metasurfaces for Wavelength-Multiplexed Optical Encryption via Modular Diffractive Deep Neural Networks
29-Nov-2025