Power/Performance Bits: April 20

Multiplexing twisted light; flexible body heat harvesting; carbon quantum dots.


Multiplexing twisted light
Researchers from University of California San Diego and University of California Berkeley found a way to multiplex light by using discrete twisting laser beams from antennas made up of concentric rings.

“It’s the first time that lasers producing twisted light have been directly multiplexed,” said Boubacar Kanté, an Associate Professor at UC Berkeley’s Department of Electrical Engineering and Computer Sciences. “We’ve been experiencing an explosion of data in our world, and the communication channels we have now will soon be insufficient for what we need. The technology we are reporting overcomes current data capacity limits through a characteristic of light called the orbital angular momentum. It is a game-changer with applications in biological imaging, quantum cryptography, high-capacity communications and sensors.”

Orbital angular momentum, or OAM, is a property of light that could offer exponentially greater capacity for data transmission. It can be compared to the vortex of a tornado, the researchers said.

“The vortex in light, with its infinite degrees of freedom, can, in principle, support an unbounded quantity of data,” said Kanté. “The challenge has been finding a way to reliably produce the infinite number of OAM beams. No one has ever produced OAM beams of such high charges in such a compact device before.”

The team used topological antennas, so that the essential properties are retained even when the device is twisted or bent.

To make the topological antenna, the researchers used electron-beam lithography to etch a grid pattern onto indium gallium arsenide phosphide and then bonded the structure onto a surface made of yttrium iron garnet. The researchers designed the grid to form quantum wells in a pattern of three concentric circles, with the largest about 50 microns in diameter, to trap photons. The design created conditions to support a phenomenon known as the photonic quantum Hall effect, which describes the movement of photons when a magnetic field is applied, forcing light to travel in only one direction in the rings.

“People thought the quantum Hall effect with a magnetic field could be used in electronics but not in optics because of the weak magnetism of existing materials at optical frequencies,” said Kanté. “We are the first to show that the quantum Hall effect does work for light.”

By applying a magnetic field perpendicular to their two-dimensional microstructure, the researchers successfully generated three OAM laser beams traveling in circular orbits above the surface. The study further showed that the laser beams had quantum numbers as large as 276, referring to the number of times light twists around its axis in one wavelength.

The researchers were able to generate three OAM laser beams traveling in circular orbits above the surface by applying a magnetic field perpendicular to their two-dimensional microstructure.

“In our study, we demonstrated this capability at telecommunication wavelengths, but in principle, it can be adapted to other frequency bands,” said Kanté. “Even though we created three lasers, multiplying the data rate by three, there is no limit to the possible number of beams and data capacity.”

Next, the team will try to make quantum Hall rings that use electricity as power sources.

Flexible body heat harvesting
Engineers at North Carolina State University are working to improve self-powered wearable devices. Their flexible thermoelectric generator (TEG) is designed to be worn on the wrist, harvesting heat energy from the human body to power sensors that monitor health information such as heart rate.

While flexible TEGs are more comfortable and provide better skin contact than their rigid counterparts, they have so far lagged in performance and efficiency.

The team first designed a flexible TEG in 2017, using the same semiconductor elements used in rigid devices and connecting them electrically in series using liquid-metal interconnects made of EGaIn, a non-toxic alloy of gallium and indium. EGaIn provided both metal-like electrical conductivity and stretchability. The entire device was embedded in a stretchable silicone elastomer.

In 2020, the researchers managed to improve the thermal engineering while increasing the density of the semiconductor elements responsible for converting heat into electricity. One of the improvements was a high thermal conductivity silicone elastomer that encapsulated the EGaIn interconnects.

Most recently, they added aerogel flakes to the silicone elastomer to reduce the elastomer’s thermal conductivity. Experimental results showed that this reduced the heat leakage through the elastomer by half.

NC State’s flexible heat harvesting device shows better efficiency at retaining heat to power the device. (Credit: Mehmet Ozturk / NC State)

“The addition of aerogel stops the heat from leaking between the device’s thermoelectric ‘legs,'” said Mehmet Ozturk, an NC State professor of electrical and computer engineering. “The higher the heat leakage, the lower the temperature that develops across the device, which translates to lower output power.”

The team believes the approach provides a low-cost opportunity to existing rigid thermoelectric module manufacturers to enter the flexible thermoelectric market.

“The flexible device reported in this paper is performing an order of magnitude better than the device we reported in 2017 and continues to approach the performance of rigid devices,” added Ozturk. The researchers plan to continue work on improving the device’s efficiency.

Carbon quantum dots
Researchers from the University of Illinois Urbana-Champaign and the University of Maryland Baltimore County are looking into ways to make carbon-based quantum dots commercially viable.

Light-emitting quantum dots are useful for electronics and medical applications but are typically comprised of toxic or expensive metals. Nontoxic carbon-based dots are easy to produce, but they emit less light.

The team decided to investigate the quality of individual carbon-based dots. “Coming into this study, we did not know if all carbon dots are only mediocre emitters or if some were perfect and others were bad,” said Martin Gruebele, a chemistry professor at Illinois. “We knew that if we could show that there are good ones and bad ones, maybe we could eventually find a way to pick the perfect ones out of the mix.”

However, observing quantum dots is difficult: they are less than 10nm in diameter and, when excited, decide whether to fluoresce in a matter of in picoseconds. “Using our previously developed single-molecule absorption scanning tunneling microscope, we could only image excited states with no time resolution,” Gruebele said. “In this study, however, we can now record quantum dots while in their excited state by combining true nanometer space resolution with femtosecond time resolution.”

When the carbon quantum dots are excited, they either emit light or expel the energy as heat before lighting up.

“We found that in bulk populations, approximately 20% of a given population of carbon nanodots are perfect emitters, while about 80% have a very short light emission state before expelling heat,” Gruebele said. “Being able to see that there are different populations tells us that it may be possible to purify carbon dot populations by selecting only the perfect light emitters.”

The researchers said the imaging technology also could allow them to investigate why some dots never light up and potentially improve the synthesis process to only produce perfect light-emitting carbon dots.

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