A special device material with versatile activity for 6G mobile communications using the 'terahertz (THz) bandwidth' has been developed, and is expected to be utilized in future terahertz and infrared hybrid communications.

The Nano Optics Laboratory of the Department of Physics at UNIST (President Yong-Hoon Lee) announced on the 6th that it has developed a ‘new metasurface’ that can control electrical conductivity while maintaining transparency in a wide range of terahertz (THz) by patterning vanadium dioxide (VO₂) into a slit array.

Metasurfaces are two-dimensional materials engineered to have properties that do not exist in nature, and the metasurface developed this time has properties suitable for use as a component in 6G communications.

Vanadium dioxide is a material known for its state changing depending on temperature near room temperature (67℃). Because it can be a metal that conducts electricity well or an insulator that does not conduct electricity, it is expected to be good for use as an active metasurface element.

For this reason, many research groups around the world are working hard to develop patterning technology to create patterns on vanadium dioxide and use them as devices.

On the other hand, existing vanadium dioxide thin film patterning methods had limitations as they relied on 'reactive ion etching (RIE)', which can cause damage, or 'drop casting', which has poor reproducibility.

In this study, we devised a method to utilize the photolithography process, which creates patterns on materials using light in semiconductor manufacturing.

This method involves creating a vanadium metal pattern using photolithography, then heating it and growing vanadium dioxide through thermal oxidation, which reacts with oxygen in the air. This method enables large-area vanadium dioxide patterning free from etching damage.

The research team created a vanadium oxide metasurface with a slit array structure with a period smaller than the wavelength of terahertz waves, and implemented a terahertz wave region transparent electrode whose electrical properties can be controlled. When measuring the optical and electrical properties at temperatures ranging from room temperature to 100℃, the conductivity of the vanadium dioxide portion changed by several thousand times.

On the other hand, the broadband transmittance of terahertz waves through the metasurface remained consistently high and transparent even when a phase transition occurred.

“When vanadium dioxide is in an insulating state, it has a refractive index similar to that of a sapphire substrate, resulting in high transmittance,” said Professor Deok-Hyung Lee of the Department of Physics at UNIST, the lead researcher. “When this material changes to a metal state, the light collected at the slit compensates for the reduced transmittance in vanadium dioxide, maintaining high transmittance.”

The research team also confirmed that near-infrared (NIR) light penetrating the metasurface can be selectively modulated based on its state. This demonstrates its potential for multispectral applications.

“The multifunctional terahertz wave tunable transparent electrode developed in this study can be utilized in multispectral applications such as terahertz wave and near-infrared hybrid communications,” said Yang Hyo-sim, a doctoral student at Seoul National University and the first author of the study. “The etching-free patterning process will be useful for mass production of vanadium dioxide-based metasurfaces.”

This study, supported by the National Research Foundation of Korea, was published online on August 7 in Laser & Photonics Reviews, a world-renowned academic journal in the field of optics.