Etched Microscopic Springs Make Brittle Silicon Stretch and Tune Terahertz Waves
Osaka researchers carved spiral springs into a silicon wafer, producing a self-supporting material whose response to terahertz light changes as it is pulled.
Silicon is hard and brittle, and a team in Japan has found a way to make it bend and stretch by carving it into microscopic springs.
Researchers at the Osaka Research Institute of Industrial Science and Technology, working with the University of Osaka and Tohoku University, built a flexible, stretchable silicon material whose response to terahertz waves can be tuned mechanically. The work is published in the journal Optica.
Terahertz waves have frequencies between those of microwaves and visible light. They hold potential for inspecting materials and for future wireless communications. Silicon is widely used to make low-loss optical components in the terahertz range, but those parts are rigid and can easily break under mechanical impact. That limits where they can be used.
"The key was to build flexibility into the silicon structure itself," said Dr. Hidemasa Yamane, a senior research scientist at ORIST. "The tiny springs allow the structure to bend and stretch, while the same silicon network also provides optical functionality for terahertz waves."
To make the material, the team used microelectromechanical systems fabrication techniques. They etched spiral-shaped gaps all the way through a silicon wafer 200 micrometers thick, leaving spring arms about 10 micrometers wide. The resulting structure holds itself up and needs no polymer or metal backing. Its effective refractive index, which depends on the relative proportions and arrangement of silicon and air, changes when the structure is deformed.
That is what makes it tunable. "When we pull on the structure, the gaps between the silicon arms broaden," Yamane explained. "Increasing the proportion of air reduces the effective refractive index. We can therefore change the phase delay of terahertz waves by changing how much the structure is stretched."
The team measured the effect with terahertz time-domain spectroscopy. Stretching the pattern by 12.6% reduced the effective refractive index by about 6% for light polarized perpendicular to the stretching direction. For polarization parallel to the stretch, the reduction was 8%. Because the two polarizations respond differently, deformation also tunes the material's birefringence, which is the difference in effective refractive index between the two polarizations.
The design works because the silicon-to-air ratio and the structure's directional properties both shift as the spirals open up. In micrographs of the sample, the dark through-etched gaps visibly widen as the material is stretched along one direction.
"Mechanical deformation opens a new avenue for terahertz silicon photonics," said Professor Masayuki Fujita of Tohoku University and the University of Osaka. "Further study will lead to the development of tunable terahertz integrated circuits using MEMS technology."
The result is a laboratory demonstration of one material, not a finished device. But it points to terahertz optics that can be reshaped on demand and that survive bending, which rigid silicon parts cannot do.
