No Limits for Light or Science

Seeking the shade of the palm, date, and tamarisk trees at the Furnace Creek Golf Course, it’s impossible to miss the Sierra Nevada Mountains towering over the nation’s lowest, hottest, and driest golf course. On the other hand, it’s easy to miss the 5,740 solar panels tucked among the greenways, although a few stray shots have hit the panels. Gathering up the intense sunlight of Death Valley, the solar farm provides a third of the electricity used at the adjacent resort. The solar panels tilt to follow the sun through the day, making them more efficient. But even with numerous sunny days, materials limit solar panel efficiency. Today’s panels convert just about 30 percent of the light they gather into electricity.     

Re-imagining materials for solar panels and so much more demands curious people who care about big problems. That’s the team at the Photonics at the Thermodynamic Limits (PTL) Center, an Energy Frontier Research Center (EFRC) funded by the Department of Energy’s Office of Science.

“Curiosity-driven science is key,” said Jennifer Dionne, PTL Director and assistant professor at Stanford University, about 250 miles west of Death Valley.

The PTL team’s curiosity is paying off as they learn how materials can work with light more efficiently. That efficiency—to the very limits of what materials can handle—could result in light working harder in re-imagined solar panels and better in fiber optic cables. It could also mean new, unusual jobs for light, such as replacing water or caustic coolants in refrigerators, enabling a different type of energy storage, or optical propulsion. Dionne believes her center will make major scientific breakthroughs about light and its behavior in the next five years.

Curiouser and curiouser.

If scientists want to get more work out of light, they need to control—very precisely—how it behaves. To uncover that behavior, they must follow tiny bits of light as they zip through, circle around, and collide with atomic-scale structures. That means using cutting-edge tools at places like the SLAC National Accelerator Laboratory.

If the tools don’t exist, the PTL team builds them. For example, PTL member and doctoral student Fariah Hayee and her colleagues modified an electron microscope to pick out each atom and see how it emits light in a particular structure.

“It’s like looking through the hole in Alice in Wonderland,” said Hayee. 

Having looked at the data, PTL scientists must make sense of what they see. That can mean trying to look at the data from different perspectives. For example, Jenny Coulter, a graduate student with PTL, walked into Dionne’s office one day and suggested studying light as if it flowed like water, sliding around obstacles and flooding too-small channels. Dionne agreed. That need to know more led to more conversations, and now, PTL scientists are studying the hydrodynamics of light, in addition to the classical and quantum mechanics of light.

Asking questions is vital, as is a playful spirit. “In some ways, my children have reinforced what it means to be a scientist,” said Dionne of her two children. “Toddlers are incredibly creative and curious. That curiosity leads to creative play and enables them to learn many new insights about the world. I think many of the greatest scientific breakthroughs came from scientists experimenting with a creative, curious, and almost playful spirit in the lab.”

Collaboration bolsters curiosity at PTL. “It’s not an individual adventure; it is anything but,” said Prineha Narang, an assistant professor at Harvard University and a lead PTL theorist. “An EFRC without synergy wouldn’t go very far.”

Some of that collaboration has occurred on running trails. “When we had our EFRC kickoff [meeting], we had all these outdoor activities,” said Narang. “We bond over running and biking together.”

That bonding also happens during regular online seminars attended by the whole team. “Because there is a lot of different expertise [at these gatherings], you can try a really primitive idea and work with others to fill it out,” said Hayee. “That’s critical when you’re working with such complex materials and at the intersection of light, heat, and electrons.”

Because PTL’s team includes graduate students and principal investigators at five universities in three different time zones, smaller teams at each university get together. Coffee and bagels are often involved.

Pick a color.

At Coupa Cafe on Stanford’s campus, PTL students and faculty from the university gather every week to talk over breakfast. One week, Hayee started chatting with Leo Yu, also a student at Stanford, about a popular material called hexagonal boron nitride, also known as white graphene. This material is similar to graphene in that it is super thin. Unlike graphene, it also emits blue, green, and red light. Nobody knew why the color varied.

Curious.

Yu went into the lab and made ultra-thin wafers of white graphene. Hayee characterized the wafer’s defects, one at a time, under an electron microscope outfitted with light detection capabilities. They analyzed the data and brought in other folks as well, including Narang who modeled the results. Their curiosity led them to discover that fork-like defects and the motion of electrons across the surface determines the light’s color. Such work offers insights into materials that could, one day, serve as the backbone of ultra-secure communication systems.

That’s the great puzzle.

Learning how light and materials interact—or should interact—drives the PTL team as does the potential reward. “When you do basic science for DOE, you get to go big,” said Narang. “You get to try to change the world.”

PTL — which began in 2018 and is located/based at Stanford University — brings together scientists from California Institute of Technology, Harvard University, University of California at Berkeley, and University of Illinois Urbana-Champaign.

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