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Photos by Evan Krape, courtesy of Tingyi Gu | Illustrations by Joy Smoker

UD Engineering’s Tingyi Gu and Fellow Researchers Build Cutting-Edge Computing Devices

On the third floor of Du Pont Hall at the University of Delaware, electrical engineers analyze delicate centimeter-sized computer chips on a large optical table surrounded by oscilloscopes, lenses and lasers. These researchers are busy collecting data on the ability of these chips to convert light waves into electrical signals, with the goal of figuring out how to make the next batch of chips they make even faster, more energy efficient, or with increased computing capabilities.

It’s here in the lab Tingyi Guassistant professor at Department of Electrical and Computer Engineering, where researchers are pushing the boundaries of the field of integrated photonic devices. Using a high-risk, high-reward research strategy, Gu’s research group has made progress in developing new chip designs and applying unique materials for a wide range of optical communication applications, detection and computing.

Light control

Photonic devices are those that can create, control, or sense light, and photonic integrated circuits are capable of using light for even more complex functions such as data analysis. Gu, who began working in this field as a graduate student, focuses on improving photonic integrated circuits through basic research, with an emphasis on developing new chip designs and investigation of how materials from other applications could be incorporated into photonic devices.

“For me and my students, we are less likely to read an article and change something to show a slightly better benefit. Instead, we’re trying to find something that can be more revolutionary by trying to fundamentally change the way we do things,” Gu said of his group’s research strategy. “It’s a higher risk approach, but it’s more fun to explore that rather than trying to repeat what others have done or make incremental progress.”

Two examples of how Gu’s research strategy has led to advances in the field of photonics can be found in two of his group’s early 2022 papers, one published in Nature Communication and the other in Advanced materials.

Scaling

In 2019, Gu and graduate student Zi Wang developed a on chip Transformer optics design principle for robust wavefront control on an integrated photonics platform, which can be used for complex processes related to other fields such as quantum optics.

Now the last of the bunch Nature communication document demonstrates how advanced computing capabilities can be integrated directly onto these photonic chips. “In 2019, our device had very simple components, like the Fourier transform. Now, with nearly a thousand pre-programmed elements, the integrated metasystem can handle uncertainties in spectral domains, which is a milestone of integrated photonic processors compared to its electronic counterpart,” Gu said.

Wang, who is now a postdoctoral fellow at National Institute of Standards and Technology (NIST), said that scaling up their original design, which would also make it compatible with manufacturing processes, was the hardest part of this recent article. “The structure was designed with a gradient backpropagation method, which cost a lot of time and computational resources in our original design,” Wang said. “But I found that our structure has a particular symmetry, and by using symmetry in mathematical calculation, the calculation became much easier.”

Using this idea, researchers found they could use the diffraction of light to perform complex calculations and data analysis. “And because each of the programmable components is much smaller than conventional chips, you can pack a lot more of them into the same chip area,” Gu said.

Gu added that this paper is an example of how new design approaches can help researchers use existing manufacturing methods to create chips that are more powerful than current cutting-edge technologies. “There is much greater potential for integrated photonic circuits, not just in the same way they have been used and studied for decades, and even current circuits now have limits that we can break,” said she declared.

Create new (optical) memories

A second article, published in Advanced materials, shows how Gu’s lab is taking inspiration from materials in other applications to assess whether they could be used for photonic memory, which relies on light instead of magnetism to store information.

Known as optical memristive devices, these rewritable memory storage platforms have the potential to reduce overall power consumption, but currently rely on a slow process related to material phase changes, or state physics of the material (the most common being solid, liquid and gas). Phase shift is how optical devices store memory, but here the phase shift process requires a transition between an amorphous phase (which doesn’t have much structure, like a pile of grains of sand) and a crystalline phase ( which is very structured, like a close-up on a snowflake).

Creating a compact yet controllable phase shifter for photonic integrated circuits has remained a challenge because currently available materials for optical devices only change phase very slowly and at extremely high temperatures.

In this paper, the group investigated indium selenide (In2Se3), a material commonly used in electronic devices but which has not been widely incorporated into optical applications, to see if it could create optical memory by passing from one crystalline phase to another instead of from one crystalline phase to another. amorphous phases.

In this study, lead author Tiantian Li, a former UD postdoc who is now an associate professor at Xi’an University of Posts and Telecommunications, found for the first time that the phase transition mechanism of indium selenide was different from that originally theorized based on simulated results. The researchers then used this theoretical knowledge to phase-shift different crystal states, creating optical memories using short nanosecond light pulses.

“Optical phase change materials have attracted a lot of interest due to the promising application in optical computing,” Li said of the impacts of this work. “The high power consumption of the phase-change material influences the computational speed of the neural network, and our material promises to break this bottleneck.”

Beyond their applications for the field of photonics, these two articles also show the importance of creativity and unique sources of inspiration in this field. “We try to leverage other resources and combine knowledge from different fields,” Gu said. “In the Nature Communication paper, we were inspired by people doing image classifications for machine learning, and we brought that to our integrated photonics platform, and for the Advanced materials paper, we were inspired by chemists who study phase transition mechanisms.

The future of photonics

Current members of the Gu lab are busy continuing progress on these and other projects, all focused on improving the current state of the art in photonic devices.

This innovative work includes three key but challenging phases: simulation, where different chip designs are evaluated using computer software; manufacturing, where the actual chips are manufactured to UD nanofabrication facility; and testing, where they take the chips back to the lab to see how well they perform compared to what was predicted by the simulation.

For Yahui Xiao, a doctoral student working on photonic crystals, doing this type of research, which requires knowledge ranging from fundamental physics to fabrication, has provided her with significant graduate school experience, especially since she plans to looking forward to a career in this type of “hybrid” research in optical and nanophotonic engineering.

“I would like to get a glimpse of current technology by understanding the underlying physics,” she said. “Here at UD, we have the nanofabrication facility, and those fabrication skills are the ones that we can use when we go into the industry, because we can do the whole fabrication process.”

Doctoral student Dun Mao, who is working on the indium selenide project, says that although research in this field can be difficult, it is encouraging to be able to make breakthroughs and obtain good results. “The most exciting part is when we observe an interesting phenomenon from our experiments that can make a device faster or more efficient,” he said.

Gu added that while there are many unanswered research questions in the field of photonics that their group could answer, the work in his lab is still driven by the interests and passions of his students. “We try to take higher-risk approaches in the lab, and sometimes it’s good, sometimes it’s not as good, but I think students learn a lot,” Gu said.

Both Mao and Xiao said that Gu’s support had been instrumental in their success in graduate school so far, and Xiao added that having Gu as a mentor in a typically male-dominated field had also been an inspiration to her. “Dr. Gu is a very good example for me – I can learn a lot from her, she is really successful in this field, and with good projects, sponsors and relationships. Overall, she helped me really encouraged.

The complete list of co-authors on the Nature Communication the article features Zi Wang, Lorry Chang, Feifan Wang, Tiantian Li (now an associate professor at Xi’an University of Posts and Telecommunications), and Tingyi Gu.

The complete list of co-authors on the Advanced materials the article includes Chris J. Benmore and Ganesh Sivaraman of Argonne National Laboratory and Tiantian Li of UD (now an associate professor at Xi’an University of Posts and Telecommunications), Yong Wang, Wei Li, Dun Mao , Igor Evangelista, Huadan Xing, Qiu Li, Feifan Wang, Anderson Janotti, Stephanie Law and Tingyi Gu.

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