Researchers at the Massachusetts Institute of Technology (MIT) in the US, in collaboration with colleagues from the University of Central Florida and universities in China and France, have developed a new method for using a glass-based composite to fabricate flexible, stretchable photonic devises that could promise applications such as skin-mounted optical biomedical sensors or flexible electronics.
“The key idea is to configure the device geometry such that the structure can become highly flexible and stretchable — even if the constituent materials are stiff in their bulk state — without disrupting the optical functions,” explains Juejun Hu, the Merton C. Flemings Associate Professor of Materials Science and Engineering at MIT. Namely, the team has created Euler-spiral waveguide structures (which Hu says are in a way similar to helical spring coils) to enable better stretchability while minimizing optical loss.
Overcoming challenges with previous flexible photonics
Scientists around the world have been working on various ways to design and fabricate flexible photonic technologies. Yet, the path to successful, commercially viable products has been difficult. “Similar designs have previously been used in stretchable electronics,” says Hu. “But our work manages to overcome new challenges such as mitigating optical losses resulting from the bent structures.”
Moreover, the professor notes that they have also developed a method to directly deposit and fabricate low-loss dielectric photonic structures on elastomers, which he says had been a standing challenge due to the large thermal expansion of elastomers and, hence, excessive thermal stress.
Ruggedness with chalcogenide glass
Most current photonic devices are fabricated from rigid materials on rigid substrates. But the expert believes those are inherently unsuitable for applications that require soft, skin-like materials. Most soft materials, on the other hand, have a low refractive index, which, according to information from MIT, leads to a poor ability to confine a light beam. So, instead of using a soft, stretchable material, Hu and his colleagues tried an entirely novel method and instead formed a stiff material — in this case a thin layer of a glass called chalcogenide — into a spring-like coil. The architecture of this glass coil allows it to stretch and bend freely while maintaining its desirable optical properties. “We created a composite via configurational, geometric design, which exhibits remarkably enhanced mechanical ruggedness,” Hu reports. The result is a flexible and very transparent material with a high refractive index. MIT says tests have shown that such spring-like configurations on polymer substrate can undergo thousands of stretching cycles without losing their optical performance.
Asked what is truly new about his team’s approach, Hu answered: “The optical design for stretchability and low loss, the monolithic integration capability on elastomers and the resulting first single-mode stretchable photonics.”
Enabling new applications
The research could enable new kinds of optical technologies that had not been feasible before. Hu names examples: “Stretchable and transparent optical sensors that can be conformally integrated on or wrapped around curved surfaces (e.g., lens, cockpit windows, domes, human skins), for example, for strain/stress sensing, data transmission or monitoring of biophysical signals.”
Regarding the next steps for Hu and his team, he says, “We are now working on fully integrated flexible and conformal sensors for human skin imaging and interrogation.”
The work is reported in two related papers: “Monolithically integrated stretchable photonics,” which, according to MIT, is slated for publication soon in Light: Science and Applications, and “Chalcogenide glass-on-graphene photonics,” already published in Nature Photonics.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today