Power/Performance Bits: June 16

One-directional optical
Researchers from University of Pennsylvania, Peking University and Massachusetts Institute of Technology developed a design for optical devices that radiate light in only one direction, which could reduce energy consumption in optical fiber networks and data centers.

Light tends to flow in a single direction optical fibers, but while most of the light passing through on-chip couplers continues on to the fiber, some travels back in the opposite direction. According to the researchers, a large portion of energy consumption in data traffic is due to this radiation loss. Previous studies reported a minimum loss of 25% at each interface between optical fibers and chips.

“Sometimes you may need to pass five interfaces, and loss cascades to 80% if you use existing devices. In fact, extra energy and elements are needed to amplify and relay the signal again and again, which introduces noise, lowers signal-to-noise ratio, and, ultimately, reduces communication bandwidth,” said Jicheng Jin, a PhD student at Penn.

The researchers discovered that breaking left-right symmetry in their device reduced this loss to zero. “The conclusion was a little unexpected. Since we want the coupler to radiate towards the top rather than the bottom, it seems natural to break up-down symmetry,” said Jin. “However, it turns out we must also make it left-right asymmetric to completely suppress the bottom radiation.”

For a coupler with both up-down and left-right symmetries, there is one charge on each side forbidding the radiation in the vertical direction.  “Imagine it as two-part glue. By breaking the left-right symmetry, the topological charge is split into two half charges, and the two-part glue is separated so each part can flow. By breaking the up-down symmetry, each part flows differently on the top and the bottom, so the two-part glue combines only on the bottom, eliminating radiation in that direction. It’s like a leaky pipe has been fixed with a topological two-part glue,” said Bo Zhen, assistant professor at Penn.

The team eventually settled on a design with a series of slanted bars, which break left-right and up-down symmetries at the same time. To fabricate these structures, the researchers developed a novel etching method, with silicon chips placed on a wedge-like substrate. This allows etching to occur at a slanted angle, whereas standard etchers can only create vertical side walls. After fabricating the device, the researchers were able to confirm the low energy loss that was predicted by their theory.

“Experimentally, we found that radiation towards the bottom is 500 times weaker than radiation towards the top in our device, minimizing energy loss,” says Jin. “In addition, our design is very robust to fabrication errors. One limiting factor in our current design is the bandwidth. Right now, we can cover about 26nm, which is decent but not optimal, and our next step is to further improve this to 200nm.”

The team hopes to further develop this etching technique to be compatible with existing industrial processes, and the researchers expect applications that can both help light travel more efficiently at short distances, such as between an optical fiber cable and a chip in a server, and over longer distances, such as long-range Lidar systems. “This method is relatively clear and straightforward, so I think everyone can easily get this reliable structure,” said Jin.

Solar windows
Researchers from the ARC Centre of Excellence in Exciton Science at Monash University and CSIRO built a semi-transparent perovskite solar cell that can be incorporated into window glass to provide some of a building’s power.

The team used an organic semiconductor that can be made into a polymer and used it to replace a commonly used solar cell component, Spiro-OMeTAD, which develops a watery coating resulting in low stability.

“Rooftop solar has a conversion efficiency of between 15 and 20%,” said Professor Jacek Jasieniak from Exciton Science. “The semi-transparent cells have a conversion efficiency of 17%, while still transmitting more than 10% of the incoming light, so they are right in the zone. It’s long been a dream to have windows that generate electricity, and now that looks possible.”

There are trade-offs, of course. “The solar cells can be made more, or less, transparent. The more transparent they are, the less electricity they generate, so that becomes something for architects to consider,” said Jasieniak. He added that solar windows tinted to the same degree as current glazed commercial windows would generate about 140 watts of electricity per square meter.

The team is now working on scaling up the manufacturing process, noted Anthony Chesman, research scientist at CSIRO. “We’ll be looking to develop a large-scale glass manufacturing process that can be easily transferred to industry so manufacturers can readily uptake the technology.”

The researchers say that the additional expense of adding the semi-transparent solar cells would be marginal for large windows in high-rise buildings, where windows are already pricey.

“These solar cells mean a big change to the way we think about buildings and the way they function. Up until now every building has been designed on the assumption that windows are fundamentally passive. Now they will actively produce electricity,” said Jasieniak. “Planners and designers might have to even reconsider how they position buildings on sites, to optimize how the walls catch the sun.”

Next, the researchers plan to build a tandem device that uses both perovskite and organic solar cells. As far as commercialization, Jasieniak said “that will depend on how successful scaling of the technology will be, but we are aiming to get there within 10 years.”