"The More You Bend, the Stronger the Light"...Ultrathin Optical Device with Triple Signal When Flexed Developed [Reading Science]

UNIST and Ajou University Joint Research Team Develops Flexible Optical Transducer Device

Performance Maintained After 190 Bending Cycles... Potential for Wearable Optical Sensor Applications

A Korean research team has developed an ultrathin optical transducer device that generates stronger light signals the more it is bent. This research overturns the existing limitations of flexible electronic and optical devices, which typically experience diminished performance when bent. The findings are expected to be applied to the development of wearable optical sensors and next-generation flexible optical devices in the future.

On May 17, Ulsan National Institute of Science and Technology (UNIST) announced that the research team led by Professors Hyungryul Park and Seon Namgoong from the Department of Physics collaborated with Professor Younghwan Ahn’s research team from the Department of Physics at Ajou University to develop a flexible optical transducer device capable of controlling the intensity of optical signals through bending.

Flexible optical converter that amplifies light signals by bending. Provided by the research team.

The research results were published online in the international journal Science Advances on May 8 (local time).

The device developed this time has an "optical conversion" function, which emits light at half the wavelength of the incident light. For example, it converts 800-nanometer (nm) infrared light into a 400 nm second harmonic generation (SHG) signal.

While optical conversion technology is widely used in lasers and precision optical equipment, previous devices required thick optical media, which imposed limits on miniaturization and flexibility.

"The More You Bend, the Narrower the Nanogap, the Greater the Concentration of Light"

The research team solved this problem by combining a monolayer of molybdenum disulfide (MoS₂), an ultrathin two-dimensional semiconductor material, with a metallic nanogap structure.

The developed device has a structure where a metallic thin film and molybdenum disulfide are sequentially stacked on a flexible substrate. Notably, an ultrafine gap (nanogap) about 20 nm wide is formed between the metallic films, serving to strongly concentrate light.

When the device is bent inward, the distance between the nanogaps narrows, forming a strong plasmonic near-field, which in turn increases the optical signal intensity. Conversely, when bent outward, the gap widens and the signal weakens.

Structure and measurement results of a flexible optical transducer device where the spacing of metal nanogaps changes according to bending deformation, thereby adjusting the intensity of the optical signal. When bent inward, the nanogap narrows and the signal strengthens; when bent outward, the gap widens and the signal weakens. The actual changes in nanogap width were also confirmed through microscopic and electron microscopic analyses. Provided by the research team

Experimental results showed that when about 1.2% compressive deformation was applied, the SHG signal increased approximately threefold compared to before deformation. The research team explained that, based on the nanogap area where the light is actually concentrated, the device demonstrated up to about 8,400 times the local enhancement effect compared to molybdenum disulfide on a flat gold thin film.

First author Sharma Sobhagyam commented, "Whereas deformation was previously considered a factor that weakens optical signals, this study shows that deformation itself can be used as a means of signal control."

Performance Maintained at 95% Even After 190 Bending Cycles

The research team also confirmed high durability in repeated bending experiments. Even after more than 190 bending cycles, the optical signal maintained over 95% of its initial value.

Furthermore, Raman spectroscopy analysis revealed that the nanogap structure partially disperses the force transferred to the molybdenum disulfide, thereby reducing material damage.

Photo of the research team. (From left) Hyungryul Park, Professor at UNIST; Younghwan Ahn, Professor at Ajou University; and Sharma Sobhagyam, Researcher at UNIST. Provided by UNIST

Professor Hyungryul Park stated, "This device can be applied not only to flexible optical devices but also to the development of strain sensors that read signals varying according to bending state. It can also serve as a platform for researching deformation-based property changes in ultrathin materials."

The research team expects that this technology will be applied in the future development of wearable optical sensors, tunable optical modulators, and ultracompact frequency conversion devices, as well as next-generation flexible optoelectronic and integrated optical systems.

This content was produced with the assistance of AI translation services.