Research Bits: July 26

Photonic computing with polarization

Researchers at the University of Oxford and University of Exeter developed a method that uses the polarization of light to maximize information storage density and computing performance using nanowires.

The researchers note that different polarizations of light do not interact with each other, allowing each to be used as an independent information channel. “We all know that the advantage of photonics over electronics is that light is faster and more functional over large bandwidths. So, our aim was to fully harness such advantages of photonics combining with tunable material to realize faster and denser information processing,” said June Sang Lee, DPhil student in the Department of Materials, University of Oxford.

They developed a hybridized-active-dielectric (HAD) nanowire using a hybrid glassy material which shows switchable material properties upon the illumination of optical pulses. Each nanowire shows selective responses to a specific polarization direction, so information can be simultaneously processed using multiple polarizations in different directions. They said that the new photonic design could be more than 300 times faster and denser than current electronic chips.

“This is just the beginning of what we would like to see in future, which is the exploitation of all degrees of freedoms that light offers, including polarization to dramatically parallelize information processing. Definitely early-stage work — our speed estimates still need research to verify them experimentally — but super exciting ideas that combine electronics, non-linear materials and computing,” said Harish Bhaskaran, professor of applied nanomaterials at the University of Oxford.

Harvesting energy from multiple sources

Scientists at Nanyang Technological University Singapore (NTU Singapore) developed a stretchable and waterproof fabric that turns energy generated from body movements into electrical energy.

While many energy-harvesting devices capture either piezoelectric (when a material is pressed or squashed) or triboelectric (when a material encounters friction with another material) energy, the new fabric can make use of both.

A key component of the fabric is a polymer that, when pressed or squeezed, converts mechanical stress into electrical energy. It is also made with stretchable spandex as a base layer and integrated with a rubber-like material to keep it strong, flexible, and waterproof.

A proof-of-concept experiment showed that a 3cm by 4cm piece of the fabric generated enough electrical energy to light up 100 LEDs, or 2.34 watts per square meter of electricity. Washing, folding, and crumpling the fabric did not cause any performance degradation, and it could maintain stable electrical output for up to five months.

Lee Pooi See, materials scientist and NTU Associate Provost Professor, said, “There have been many attempts to develop fabric or garments that can harvest energy from movement, but a big challenge has been to develop something that does not degrade in function after being washed, and at the same time retains excellent electrical output. In our study, we demonstrated that our prototype continues to function well after washing and crumpling. We think it could be woven into t-shirts or integrated into soles of shoes to collect energy from the body’s smallest movements, piping electricity to mobile devices.”

Lee continued, “Despite improved battery capacity and reduced power demand, power sources for wearable devices still require frequent battery replacements. Our results show that our energy harvesting prototype fabric can harness vibration energy from a human to potentially extend the lifetime of a battery or even to build self-powered systems. To our knowledge, this is the first hybrid perovskite-based energy device that is stable, stretchable, breathable, waterproof, and at the same time capable of delivering outstanding electrical output performance.”

Stretchy displays

Researchers from Stanford University, Lawrence Berkeley National Laboratory, and the University of Southern Mississippi propose a way to make stretchable color displays.

Zhitao Zhang, a postdoctoral scholar at Stanford, found a yellow-colored light-emitting polymer called SuperYellow that became soft and pliable and also emitted brighter light when mixed with a type of the stretchy plastic polyurethane. “If we add polyurethane, we see SuperYellow form nanostructures. These nanostructures are really important. They make the brittle polymer stretchable, and they make the polymer emit brighter light because the nanostructures are connected like a fishnet.”

The group then created elastic red, green, and blue light-emitting polymers.

Combining the different materials was a challenge. Zhenan Bao, chemical engineer and professor in the School of Engineering at Stanford, said of combining the materials, “Electronically, they have to match each other to give us high brightness. But then, they also need to have similarly good mechanical properties to allow the display to be stretchable. And finally, for the fabrication, Zhitao had to figure out a way to stack the layers together so that the process will not degrade the brightness.”

The final display contains seven layers. Two outer layers are substrates that encapsulate the device. Moving inward are two electrode layers, each followed by charge transporting layers. Finally, the light-emitting layer sits sandwiched in the center.

When electricity runs through the display, one electrode injects positive charges into the light-emitting layer while the other injects negatively charged electrons into it. When the two types of charges meet, they bond and go into an energetically excited state. Almost immediately after, the state returns to normal by producing a photon.

The team said that the resulting all-polymer film can be adhered to an arm or finger and doesn’t rip during bending or flexing. One application would be wearable trackers that have their display directly attached to the skin.