Microscopic springs transform fragile silicon into a flexible terahertz metamaterial

Researchers at Osaka Research Institute of Industrial Science and Technology, in collaboration with universities in Osaka and Tohoku, have created a flexible silicon material capable of adjusting its response to terahertz waves through mechanical manipulation. By introducing microscopic spiral springs into the silicon, the scientists made the fragile material adaptable to bending and stretching.

Terahertz waves, which occupy a space between microwaves and light in the frequency spectrum, have potential applications in material analysis and future wireless communication networks. While silicon can be used to create low-loss optical components in the terahertz range, those components are rigid and can be easily damaged by physical impact.

Dr. Hidemasa Yamane, a senior researcher at ORIST, explained that the team's breakthrough was to incorporate flexibility directly into the silicon structure itself. The microscopic springs enable the material to bend and stretch while retaining its optical properties for terahertz waves. The researchers used microelectromechanical systems (MEMS) fabrication techniques to etch spiral-shaped gaps into a 200-µm-thick silicon wafer, forming spring arms approximately 10 µm wide.

This self-supporting structure does not require additional backing materials like polymers or metals and its effective refractive index can be altered by deformation. When the structure is pulled, the gaps between the silicon arms expand, reducing the effective refractive index and enabling the phase delay of terahertz waves to be altered by changing the extent of stretching.

Mechanical deformation also allows for tuning the birefringence of the material, which refers to the difference in effective refractive index for different polarizations. This discovery opens up new possibilities for terahertz silicon photonics, potentially leading to the development of tunable terahertz integrated circuits using MEMS technology.