[POSTECH research team develops a thin metasurface color filter that displays vivid images.]

[The filter provides 120–170 times higher resolution (60,000-86,000 PPI) than a smartphone display (500 PPI).]

A research team at POSTECH (Pohang University of Science and Technology, President: Moo Hwan Kim) unveiled a new technology that can display clear color images on a filter that is as thin as three strands of hair. The team has developed a color filter, which provides 120–170 times higher resolution than high-end smartphone screens. The filter is made using metasurfaces, a type of engineered flat material with properties that are not found in nature.

The research team was led by Professor Junsuk Rho of the Department of Mechanical Engineering and the Department of Chemical Engineering; Ph.D. candidates Trevon Badloe and Joohoon Kim, and Dr. Inki Kim (currently a professor at the Department of Biophysics at Sungkyunkwan University) in the Department of Mechanical Engineering; Professor Young-Ki Kim and Ph.D. candidate Won-Sik Kim in the Department of Chemical Engineering, and Professor Wooksung Kim of the Department of Electrical Engineering. Their work was recently published in the international journal Light: Science & Applications.

The metasurface consists of a periodic array of nanostructures and is receiving attention because it produces vivid images with bright structural colors.^*1 Previous studies have produced vivid color images with structural colors using dielectric materials or structural colors of plasmonic materials with a metal structure. However, commercialization of these technologies requires a flexible printing method that can dynamically tune colors based on a specific stimulation.

The research team has developed a metasurface color filter that can freely turn the three primary colors on and off with high chroma and brightness. The team paid attention to the fact that controlling the three primary colors—red, green and blue—at a pixel level enables the flexible expression of colors.

One of the noticeable features of this color filter is the ability of each pixel to create a wide variety of colors. This is realized by tuning the brightness of the three primary colors independently and blending them together. Therefore, the brightness of various colors, including black and white, can be varied continuously. This differentiates it from other color filters, which cannot vary the brightness or express multiple colors in each pixel.

This color filter has pixels that are as small as 420 nm (nanometer, 1nm=10-9m) for red, 360 nm for green and 300 nm for blue. Its resolution is as high as 60,000 PPI (pixel per inch, the number of pixels in an inch) for red, 70,000 PPI for green and 85,000 PPI for blue. The displays used for the latest smartphones have about 500 PPI. Compared to this, the resolution of the filter is 120–170 times higher. The research team has succeeded in expressing vivid images on a 200–300-micrometer (㎛)-thin filter, which is similar to the thickness of two or three strands of hair.

Academia is also paying great attention to this research outcome, as it allows vivid images to be expressed on a light and thin display. Moreover, it is expected that the filter can be used for cryptographic applications and for deflective displays because the color of individual pixels can be controlled.

This research was supported by the Samsung Research Funding & Incubation Center for Future Technology, the POSCO-POSTECH-RIST Convergence Research Center, and the National Research Foundation of Korea.

DOI: https://doi.org/10.1038/s41377-022-00806-8

1. Structural color
A color generated through reflection and interference by a periodic array of structure in transparent materials. The color observed depends on the light from the outside and the viewing angle.