POSTECH Develops Technology Beyond RGB Colour Limits
Researchers in South Korea have developed a colour technology that can directly generate and control colour and the wavelength of light, moving beyond the conventional red, green and blue (RGB) method used in televisions and smartphones.
The technology uses tunable nanostructures rather than mixing three fixed primary colours. It is being developed as a foundational technology for next-generation optoelectronics, including displays, augmented reality, holograms, optical semiconductors and image sensors.
Choi Soo-seok, a professor of electrical engineering at POSTECH, led the work. Choi was selected as the September winner of the Korea Science and Technology Award, hosted by the Ministry of Science and ICT and co-organised by the National Research Foundation of Korea and Seoul Economic Daily. He discussed the technology on 2 September.
For decades, display technologies have used a principle in which three fixed RGB primary colours are combined to produce different colours. The method became an industry standard because combinations of the three primary colours can stably reproduce countless colours.
The approach also has a fundamental limitation. Because colours are created by combining multiple light sources, reproducing the high-purity colours found in nature becomes more difficult as materials, pixel design, and manufacturing processes grow more complex.
Choi and his research team looked to butterflies and chameleons for a different approach. These animals produce vivid structural colours using regular microstructures measured in nanometres that selectively reflect light at specific wavelengths, rather than relying on pigment mixing.
When the spacing of these structures changes, the wavelength of reflected light also changes. The change in wavelength produces a corresponding change in the colour visible. Reproducing the complex nanostructures found in nature artificially, however, can require complex nanofabrication processes.
The POSTECH team addressed this problem using chiral nanostructures. In these structures, molecules spontaneously arrange themselves into a helical pattern. The team used this molecular structure to reproduce the principle through which structural colours are produced in nature.
The periodicity of the molecular structure can be adjusted to create and alter the desired colour and wavelength directly. This makes it possible to control colour and wavelength without the complex nanofabrication needed to reproduce intricate natural structures.
One of the technology’s applications is photonic e-skin, a material designed to mimic the colour-changing behaviour of chameleon skin. When the material is stretched, the spacing of its nanostructure changes. This alters the wavelength of reflected light and, consequently, the colour produced by the material.
The researchers also established a relationship between the degree of stretching and the wavelength of the colour. This allows the extent of an object’s deformation to be read and analysed directly through colour, rather than the material simply changing colour when it is stretched.
The team has also developed a tunable colour filter that can select and display a desired colour with a voltage of about 2 volts. Conventional displays require a separate filter for each colour, including red, green and blue. The new filter uses a single chiral nanostructure to produce multiple colours, with the colour changing as the voltage is adjusted.
The researchers later applied the same principle to lasers. The approach allows the colour of light emitted from a single laser to be freely varied across the visible spectrum, from red to violet.
The technology can reproduce higher colour purity than typical organic light-emitting diode or quantum dot technologies. This makes it suitable for next-generation optical technologies that require sharp and precise light, including augmented reality and holograms.
Choi’s research grew from questions and experience accumulated during his career in industry. He spent 19 years at LG Display, where he encountered the process complexity, optical loss and colour-purity limits associated with RGB colour mixing.
After moving to POSTECH in 2019, Choi began full-scale research into next-generation displays based on chiral nanostructures. The research has also included work with LG Display on stretchable displays. The team has worked with Samsung Electronics for several years on research aimed at extending optical technologies into semiconductors and image sensors.
Choi is now also pursuing research into optical encryption, which moves beyond reproducing colour to embedding information in light itself. The approach assigns different colour, wavelength and polarisation information to each pixel, while electrical stimuli or material deformation can serve as conditions for decryption.
The research team can layer multiple levels of information onto a single image and read them only under specific conditions. The approach could be applied to anti-counterfeiting and authentication, high-density optical data storage and optical information processing.
The work is also being considered in the context of the growing importance of artificial intelligence. Existing encryption technologies are largely electrical and digital, while the research explores physical information channels that differ from those methods.
Light can carry colour, intensity, wavelength, polarisation, and spatial patterns simultaneously. These characteristics provide multiple forms of information that can be used in optical encryption, extending the research beyond direct colour control to the use of light itself as an information medium.
