Photonic Devices Go Flexible

A new material produced by MIT researchers can be repeatedly stretched without losing its optical properties. (Source: MIT)

Researchers at MIT and several other insti­tutions have developed a method for making photonic devices that can bend and stretch without damage. The devices could find uses in cables to connect computing devices, or in diag­nostic and moni­toring systems that could be attached to the skin or implanted in the body, flexing easily with the natural tissue. The findings, which involve the use of a specia­lized kind of glass – chalco­genide –, are developed by Juejun Hu and more than a dozen others at MIT, the Uni­versity of Central Florida, and univer­sities in China and France.

Hu says that many people are interested in the possi­bility of optical techno­logies that can stretch and bend, especially for applications such as skin-mounted moni­toring devices that could directly sense optical signals. Such devices might, for example, simul­taneously detect heart rate, blood oxygen levels, and even blood pressure. Photonics devices process light beams directly, using systems of LEDs, lenses, and mirrors fabri­cated with the same kinds of processes used to manu­facture electronic micro­chips. Using light beams rather than a flow of electrons can have advan­tages for many appli­cations; if the original data is light-based, for example, optical processing avoids the need for a conver­sion process.

But most current photonics devices are fabri­cated from rigid materials on rigid substrates, Hu says, and thus have an “inherent mismatch” for appli­cations that “should be soft like human skin.” But most soft materials, including most polymers, have a low refrac­tive index, which leads to a poor ability to confine a light beam. Instead of using such flexible materials, Hu and his team took a novel approach: They formed the stiff material – in this case a thin layer of chalco­genide – into a spring-like coil. Just as steel can be made to stretch and bend when formed into a spring, the archi­tecture of this glass coil allows it to stretch and bend freely while main­taining its desirable optical proper­ties.

“You end up with something as flexible as rubber, that can bend and stretch, and still has a high refractive index and is very transparent,” Hu says. Tests have shown that such spring-like confi­gurations, made directly on a polymer substrate, can undergo thousands of stretching cycles with no detec­table degra­dation in their optical performance. The team produced a variety of photonic components, inter­connected by the flexible, spring-like wave­guides, all in an epoxy resin matrix, which was made stiffer near the optical components and more flexible around the wave­guides.

Other kinds of stretchable pho­tonics have been made by embedding nanorods of a stiffer material in a polymer base, but those require extra manu­facturing steps and are not compa­tible with existing photonic systems, Hu says. Such flexible, stretchable photonic circuits could also be useful for appli­cations where the devices need to conform to the uneven surfaces of some other material, such as in strain gauges. Optics tech­nology is very sensitive to strain, according to Hu, and could detect defor­mations of less than one-hundredth of 1 percent.

This research is still in early stages; Hu’s team has demon­strated only single devices at a time thus far. “For it to be useful, we have to demon­strate all the components inte­grated on a single device,” he says. Work is ongoing to develop the tech­nology to that point so that it could be commer­cially applied, which Hu says could take another two to three years. (Source: MIT)

Reference: L. Li et al.: Monolithically Integrated Stretchable Photonics, Light: Sci. & App. 7, e17138 (2018); DOI: 10.1038/lsa.2017.138

Link: Photonics Materials Group, Massachusetts Institute of Technology MIT, Cambridge, USA

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