A clearer future: POSTECH research team reduce light noise to push the boundaries of flat optics

A research team at POSTECH, led by Professor Junsuk Rho (Departments of Mechanical Engineering, Chemical Engineering, Electrical Engineering, and the Graduate School of Convergence Science and Technology), along with M.S./Ph.D. students Seokwoo Kim, Joohoon Kim, Kyungtae Kim, and Minsu Jeong (Department of Mechanical Engineering), has developed a novel multidimensional sampling theory to overcome the limitations of flat optics. Their study not only identifies the constraints of conventional sampling theories in metasurface design but also presents an innovative anti-aliasing strategy that significantly enhances optical performance. Their findings were published in Nature Communications.

Flat optics is a cutting-edge technology that manipulates light at the nanoscale by patterning ultra-thin surfaces with nanostructures. Unlike traditional optical systems that rely on bulky lenses and mirrors, flat optics enables ultra-compact, high-performance optical devices. This innovation is particularly crucial in miniaturizing smartphone cameras (reducing the “camera bump”) and advancing AR/VR technologies.

Metasurfaces, one of the most promising applications of flat optics, rely on hundreds of millions of nanostructures to precisely sample and control the phase distribution of light. Sampling, in this context, refers to the process of converting analog optical signals into discrete data points—similar to how the human brain processes visual information by rapidly capturing multiple images per second to create continuous motion perception. However, traditional sampling methods come with challenges. When the sampling rate is too low, aliasing artifacts occur, leading to distorted images and optical inefficiencies. A well-known example is the wagon-wheel effect, where a spinning wheel in a video appears to move backward or freeze due to insufficient frame rates. This aliasing issue is a major limitation in metasurface design, significantly reducing optical efficiency and precision.

For decades, researchers have relied on the Nyquist sampling theorem to predict and mitigate aliasing. However, the POSTECH team discovered that Nyquist’s theorem, while useful for digital signal processing, does not fully account for the optical complexities of metasurfaces. While Nyquist theory effectively defines frequency limits for digital signal processing, it fails to accurately predict or prevent optical distortion in metasurfaces, which must account for both the complex nanostructure of metasurfaces and the wave nature of light.

To address this limitation, the team developed a new multidimensional sampling theory that incorporates both the two-dimensional lattice structure of metasurfaces and the wave properties of light. Their research, for the first time, revealed that the geometric relationship between a metasurface’s nanostructured lattice and its spectral profile plays a crucial role in determining optical performance. By adjusting the lattice rotation and integrating diffraction elements, the team introduced an anti-aliasing strategy that minimizes noise and enhances light control. Using this approach, they successfully reduced optical noise across a broad spectrum—from visible light to ultraviolet wavelengths—and demonstrated high-numerical-aperture (NA) metalenses and wide-angle meta-holograms functioning in the ultraviolet regime. This study not only redefines the theoretical framework for optical metasurfaces but also relaxes fabrication constraints, making high-resolution ultraviolet and high-numerical-aperture metasurfaces more feasible.

Professor Junsuk Rho emphasized the significance of their discovery: “This research opens new possibilities for next-generation flat optical devices, including high-NA metalenses and wide-angle meta-holograms. Our newly developed sampling theory is highly versatile, spanning wavelengths from microwaves to extreme ultraviolet. Short-wavelength ultraviolet optics require extremely precise fabrication, making research in this area highly challenging. However, our findings significantly ease these fabrication demands, unlocking new opportunities in ultraviolet metasurfaces.”

This research was supported by POSCO, Samsung Electronics, the Ministry of Science and ICT, and the National Research Foundation of Korea.