A research team at Pohang University of Science and Technology (POSTECH) has developed a stretchable photonic device capable of controlling light wavelengths in all directions. According to the researchers, the work establishes a foundational technology for advanced photonic devices with potential in various industrial applications. Conventional displays and image sensors produce colors by blending red, green, and blue whereas structural color technology produces colors through the interaction of light with microscopic nanostructures, resulting in more vivid and diverse displays of color. Traditional color mixing techniques, which use dyes or luminescent materials, are limited to passive and fixed color representation. In contrast, tunable color technology dynamically controls nanostructures corresponding to specific light wavelengths, allowing for the free adjustment of pure colors. Schematic representation of a flexible, stretchable photonic device capable of structural color tuning in both long and short wavelength directions are shown. Courtesy of POSTECH. Previous research has primarily been limited to unidirectional color tuning, typically shifting colors from red to blue. Reversing this shift from blue to red, which has a longer wavelength, has been a significant challenge. Current technology only allows adjustments towards shorter wavelengths, making it difficult to achieve diverse color representation in the ideal free wavelength direction. Therefore, there is need for a device capable of bidirectional and omnidirectional wavelength adjustment to maximize the utilization of wavelength control technology. The team led by professor Su Seok Choi and PhD candidate Seungmin Nam addressed these challenges by integrating chiral liquid crystal elastomers (CLCEs) with dielectric elastomer actuators (DEAs). CLCEs are flexible materials capable of structural color changes, while DEAs induce flexible deformation of dielectrics in response to electrical stimuli. The team optimized the actuator structure to allow both expansion and contraction, combining it with CLCEs, and developed a highly adaptable stretchable device. This device can freely adjust the wavelength position across the visible spectrum, from shorter to longer wavelengths and vice versa. In their experiments, the researchers demonstrated that their CLCE-based photonic device could control structural colors over a broad range of visible wavelengths (from blue at 450 nm to red at 650 nm) by using electrical stimuli. This represents a significant advancement over previous technologies, which were limited to unidirectional wavelength tuning. “This technology can be applied in displays, optical sensors, optical camouflage, direct optical analogue encryption, biomimetic sensors, and smart wearable devices, among many other applications involving light, color, and further broadband electromagnetic waves beyond visible band,” Choi said. Choi and his team plan to expand the application scope of the technology through continued research. The work was published in Light: Science & Applications (www.doi.org/10.1038/s41377-024-01470-w).
