Research Bits: Jan. 3

Printing electronics on curved surfaces

Researchers from North Carolina State University have demonstrated a new technique for directly printing electronic circuits onto curved and corrugated surfaces. They have used the technique to create prototype “smart” contact lenses, pressure-sensitive latex gloves, and transparent electrodes.

“There are many existing techniques for creating printed electronics using various materials, but limitations exist,” said Yong Zhu, professor of Mechanical and Aerospace Engineering at NC State. “One challenge is that existing techniques require the use of polymer binding agents in the ‘ink’ you use to print the circuits. This impairs the circuit’s conductivity, so you have to incorporate an additional step to remove those binding agents after printing. A second challenge is that these printing techniques typically require you to print on flat surfaces, but many applications require surfaces that aren’t flat.”

“We’ve developed a technique that doesn’t require binding agents and that allows us to print on a variety of curvilinear surfaces,” said Yuxuan Liu, a Ph.D. student at NC State. “It also allows us to print the circuits as grid structures with uniform thickness.”

The first step in the new technique is to create a template for the relevant application that incorporates a specific pattern of microscale grooves. The template is then used to replicate that pattern in a thin elastic polymer film. Researchers then attach the thin polymer film to the relevant substrate, which can be flat or curved. At this point, the tiny grooves in the polymer are filled with a liquid solution containing silver nanowires. The solution is allowed to dry at room temperature, leaving behind silver nanowires in a soft material with the desired shape and circuit pattern.

To demonstrate the technique, the researchers created three proof-of-concept prototypes. One was a “smart” contact lens with built-in circuits, which could be used to measure the fluid pressure of the eye, which is relevant for some biomedical applications. One was a flexible, transparent electrode with circuits printed in a grid pattern, which could be used in solar cells or on touch panels. The third is a latex glove that has circuits printed on it that serve as pressure sensors, which has applications in robotics and human-machine interface applications.

“We think this could be scaled up pretty easily, in terms of manufacturing,” Zhu said. “We’re open to talking with industries who are interested in exploring this technique’s potential.”

High-efficiency optical comb

Researchers from Harvard University developed an electro-optic frequency comb that is 100-times more efficient and has more than twice the bandwidth of previous state-of-the-art versions.

“Our device paves the way for practical optical frequency comb generators and opens the door for new applications, said Marko Lončar, professor of electrical engineering at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). “It also provides a platform to investigate new areas of optical physics.”

In 2019, Lončar and his lab demonstrated a stable, on-chip frequency comb that could be controlled with microwaves. This electro-optical frequency comb, built on a lithium niobate platform, spanned the entire telecommunications bandwidth but was limited in its efficiency. In 2021, the team developed a coupled resonators device to control the flow of light and used it to demonstrate on-chip frequency shifters — a device that can change the color of light with nearly 100% efficiency. The latest research applies the two concepts to address the challenge in resonator based electro-optic frequency combs — efficiency-bandwidth tradeoff.

“We demonstrated that by combining these two approaches — the coupled resonator with the electro-optical frequency comb — we could improve the efficiency a lot without sacrificing bandwidth. In fact, we actually improved bandwidth,” said Yaowen Hu, a research assistant at SEAS.

“We found that when you improve the performance of the comb source to this level, the device starts operating in an entirely new regime that combines the process of electro-optic frequency comb generation with the more traditional approach of a Kerr frequency comb,” said Mengjie Yu, a former postdoctoral fellow at SEAS and now an assistant professor at the University of Southern California.

This new comb can generate ultrafast femtosecond pulses at high power. Together with the high-efficiency and broadband, this device can be useful for applications in astronomy, optical computing, ranging, and optical metrology.

Photosynthesis photodetector

Researchers from the University of Michigan developed a new type of high-efficiency photodetector inspired by the photosynthetic complexes plants use to turn sunlight into energy.

“Our devices combine long-range transport of optical energy with long-range conversion to electrical current,” said Stephen Forrest from the University of Michigan. “This arrangement, analogous to what is seen in plants, has the potential to greatly enhance the power generation efficiency of solar cells, which use devices similar to photodetectors to convert sunlight into energy.”

The team was able to generate polaritons in an organic thin film. “A polariton combines a molecular excited state with a photon, giving it both light-like and matter-like properties that allow long-range energy transport and conversion,” said Forrest. “This photodetector is one of the first demonstrations of a practical optoelectronic device based on polaritons.”

To create a photodetector based on polaritons, the researchers had to design structures that allow polariton propagation over long distances in an organic semiconductor thin film. They also had to figure out how to integrate a simple organic detector into the propagation region in a way that would produce efficient polariton-to-charge conversion.

Due to the unusual structure of the detector they had to develop a way to accurately quantify the results and put them in the context of conventional detectors.

The results showed that the new photodetector is more efficient at converting light to electrical current than a comparable silicon photodiode. It can also gather light from areas about 0.01 mm² and achieve conversion of light to electrical current over distances of 0.1 nm. This distance is three orders larger than the energy transfer distance of photosynthetic complexes.

The researchers found insights into how polaritons propagate in open structures with a single mirror. The new device also allowed the first measurements of how efficiently incident photons can be converted to polaritons.

“Our work shows that polaritons, in addition to being interesting science, are also a goldmine of applications yet to be discovered,” said Forrest. “Devices such as ours provide an unusual, and possibly unique, method to understand the fundamental properties of polaritons and to enable yet to be imagined ways to manipulate light and charge.”