Augmented reality (AR) glasses, lenses thinner than a human hair, and holograms floating above your fingertips may sound like technologies from science fiction. At the heart of these emerging technologies, however, lies a nanoscale optical device known as a "metasurface." Made of nanostructures smaller than the wavelength of light, metasurfaces can precisely control the direction, color, and other properties of light. Despite their remarkable capabilities, their complex structures have long posed a major challenge to researchers.
A research team led by Professor Junsuk Rho and Dr. Seokwoo Kim of POSTECH has discovered hidden repeating patterns within seemingly disordered nanostructures, opening a new route toward faster and more accurate analysis and design of metasurfaces. The study was published in the international journal Nature Communications.
To achieve a desired optical function, metasurfaces are designed by gradually varying the size, shape, or orientation of individual nanostructures. As these structures become increasingly different from one another, however, the periodicity of the overall surface is lost, making it difficult to calculate how the device interacts with light. If a structure is periodic, researchers can analyze only a small portion of it and predict the behavior of the entire device. In contrast, an aperiodic structure may require an enormous number of individual nanostructures to be calculated separately.
A metasurface measuring several centimeters across can contain more than one billion nanostructures, making rigorous computer simulations extremely time-consuming. Conventional approaches have therefore often relied either on simplified assumptions or on computationally demanding large-scale simulations.
The research team focused on the idea that even a structure that appears highly complex may contain an underlying repeating order. They found a clue in the Moiré pattern1, a large-scale interference pattern that appears when two regular patterns with slightly different alignments or spacings are overlaid. A familiar example can be seen when two layers of mosquito netting or thin curtains are placed on top of each other.
The researchers interpreted the basic lattice of a metasurface and the additional spatial variation introduced to redirect light as two overlapping patterns. They then mathematically demonstrated that when the orientation and spacing of these patterns satisfy specific conditions, a large-scale repeating pattern emerges even within a structure that initially appears aperiodic.
In simple terms, the team identified a "small repeating unit" that can represent an enormous array of seemingly different nanostructures. As a result, it is no longer necessary to calculate the entire large-area metasurface directly. Even for a device containing more than one billion nanostructures, its overall properties can be analyzed by calculating only a small unit consisting of several tens of nanostructures.
Based on this framework, the research team fabricated actual metasurface devices and confirmed that light was controlled in the predicted directions. The method also enabled the researchers to clearly distinguish light traveling in the intended direction from unwanted diffraction and scattering channels, allowing metasurface performance to be analyzed rapidly and accurately.
Because the underlying principle is based on wave physics, the framework is not limited to light and can potentially be extended to systems involving sound, electrons, and mechanical vibrations. The approach is therefore expected to provide a powerful foundation for faster and more precise development of next-generation optical devices, including AR and VR components and high-performance metalenses.
Professor Junsuk Rho said, "By uncovering hidden order within structures that appear disordered, this framework provides a powerful tool for accelerating the development of next-generation optical devices."
This work was supported by the Ministry of Science and ICT of Korea and the National Research Foundation of Korea through research programs including the Mid-Career Researcher Program and the Sejong Science Fellowship.
