A team of researchers has introduced a new multidimensional sampling theory aimed at addressing the drawbacks of flat optics. This study not only highlights the limitations of traditional sampling theories when designing metasurfaces but also introduces a groundbreaking anti-aliasing technique that greatly improves optical performance.
Led by Professor Junsuk Rho at POSTECH, along with M.S./Ph.D. students Seokwoo Kim, Joohoon Kim, Kyungtae Kim, and Minsu Jeong from the Department of Mechanical Engineering, the research team has crafted this innovative multidimensional sampling theory to tackle issues associated with flat optics. Their findings, which provide insights into the limitations of conventional sampling theories in metasurface design and offer a novel anti-aliasing solution that significantly boosts optical performance, were published in Nature Communications.
Flat optics is an advanced technology that manipulates light at a nanoscale by engineering ultra-thin surfaces with tiny structures. Unlike traditional optical systems that depend on bulky lenses and mirrors, flat optics allows for the creation of ultra-compact and high-performance optical devices. This advancement is particularly important for the miniaturization of smartphone cameras (helping to reduce the “camera bump”) and for innovations in augmented reality (AR) and virtual reality (VR) technologies.
Among the most promising applications of flat optics are metasurfaces, which utilize millions of nanostructures to carefully sample and control how light’s phase is distributed. In this context, sampling refers to the conversion of continuous optical signals into discrete data points—similar to how the human brain processes what we see by quickly capturing multiple images each second to create the illusion of motion. However, traditional sampling methods face significant challenges. Low sampling rates can result in aliasing artifacts, which distort images and reduce optical efficiency. A classic example is the wagon-wheel effect, where a spinning wheel in a video seems to rotate backward or stay still due to a low frame rate. This aliasing phenomenon presents a critical challenge in metasurface design, impairing optical efficiency and accuracy.
For many years, researchers have leaned on the Nyquist sampling theorem to anticipate and alleviate aliasing. However, the POSTECH team found that while Nyquist’s theorem serves well in digital signal processing, it doesn’t fully address the optical complexities encountered in metasurfaces. Although Nyquist theory successfully establishes frequency limits for digital applications, it doesn’t accurately predict or prevent optical distortion in metasurfaces, which must take into account both the intricate nanostructures and the wave properties of light.
To resolve this challenge, the team introduced a novel multidimensional sampling theory that takes into consideration both the two-dimensional lattice structure of metasurfaces and the wave characteristics of light. Their research, for the first time, demonstrated that the geometric relationship between a metasurface’s nanostructured lattice and its spectral properties is vital for its optical performance. By manipulating the lattice rotation and incorporating diffraction elements, the team devised an anti-aliasing method that minimizes noise and improves light manipulation. This approach enabled them to significantly reduce optical noise across a wide spectrum—from visible to ultraviolet light—and to showcase high-numerical-aperture (NA) metalenses and wide-angle meta-holograms operating in the ultraviolet range. This work not only reshapes the theoretical basis for optical metasurfaces but also alleviates fabrication challenges, paving the way for more practical high-resolution ultraviolet metasurfaces and those with high numerical aperture.
Professor Junsuk Rho noted the importance of their findings: “This work opens up new avenues for next-generation flat optical devices, such as high-NA metalenses and wide-angle meta-holograms. Our sampling theory is highly adaptable, covering wavelengths from microwaves to extreme ultraviolet. Creating optics for short-wavelength ultraviolet light demands extremely precise fabrication, making this research area quite challenging. However, our discoveries significantly reduce these fabrication requirements, thereby unlocking fresh opportunities in ultraviolet metasurfaces.”
This research received funding from POSCO, Samsung Electronics, the Ministry of Science and ICT, and the National Research Foundation of Korea.
