New Laser Experiment Spins Light Like a Merry-go-round

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Category: Research News

Published: Wednesday, February 28 2024 05:33

In day-to-day life, light seems intangible. We walk through it and create and extinguish it with the flip of a switch. But, like matter, light actually carries a little punch—it has momentum. Light constantly nudges things and can even be used to push spacecraft. Light can also spin objects if it carries orbital angular momentum (OAM)—the property associated with a rotating object’s tendency to keep spinning.

Scientists have known that light can have OAM since the early 90s, and they’ve discovered that the OAM of light is associated with swirls or vortices in the light’s phase—the position of the peaks or troughs of the electromagnetic waves that make up the light. Initially, research on OAM focused on vortices that exist in the cross section of a light beam—the phase turning like the propeller of a plane flying along the light’s path. But in recent years, physicists at UMD, led by UMD Physics Professor Howard Milchberg, have discovered that light can carry its OAM in a vortex turned to the side—the phase spins like a wheel on a car, rolling along with the light. The researchers called these light structures spatio-temporal optical vortices (STOVs) and described the momentum they carry as transverse OAM.

“Before our experiments, it wasn’t appreciated that particles of light—photons—could have sideways-pointing OAM,” Milchberg says. “Colleagues initially thought it was weird or wrong. Now research on STOVs is rapidly growing worldwide, with possible applications in areas such as optical communications, nonlinear optics, and exotic forms of microscopy.”

In an article published on Feb. 28, 2024, in the journal Physical Review X, the team describes a novel technique they used to change the transverse OAM of a light pulse as it travels. Their method requires some laboratory tools, like specialized lasers, but in many ways, it resembles spinning a playground merry-go-round or twisting a wrench.Similarities exist between spinning everyday items, like a playground merry-go-round, and spinning vortices of light. Image credit: Martin Vorel

“Because STOVs are a new field, our main goal is gaining a fundamental understanding of how they work. And one of the best ways to do that is to mess with them,” says Scott Hancock, a UMD physics postdoctoral researcher and first author of the paper. “Basically, what are the physics rules for changing the transverse OAM of a light pulse?”

In previous work, Milchberg, Hancock and colleagues described how they created and observed pulses of light that carry transverse OAM, and in a paper published in Physical Review Letters in 2021, they presented a theory that describes how to calculate this OAM and provides a roadmap for changing a STOV’s transverse OAM.

The consequences described in the team’s theory aren’t so different from the physics at play when kids are on a playground. When you spin a merry-go-round you change the angular momentum by pushing it, and the effectiveness of a push depends on where you apply the force—you get nothing from pushing inwards on the axle and the greatest change from pushing sideways on the outer edge. The mass of the merry-go-round and everything on it also impact the angular momentum. For instance, kids jumping off a moving merry-go-round carry away some of the angular momentum, making the merry-go-round easier to stop.

The team’s theory of the transverse OAM of light looks very similar to the physics governing the spin of a merry-go-round. However, their merry-go-round is a disk made of light energy laid out in one dimension of space and another of time instead of two spatial dimensions, and its axis is moving at the speed of light. Their theory predicts that pushing on different parts of a merry-go-round light pulse can change its transverse OAM by different amounts and that if a bit of light is scattered off a speck of dust and leaves the pulse then the pulse loses some transverse OAM with it.

The team focused on testing what happened when they gave the transverse OAM vortices a shove. But changing the transverse OAM of a light pulse isn’t as easy as giving a merry-go-round a solid push; there isn’t any matter to grab onto and apply a force. To change the transverse OAM of a light pulse, you need to flick its phase.

As light journeys through space, its phase naturally shifts, and how fast the phase changes depends on the index of refraction of the material that the light travels through. So Milchberg and the team predicted that if they could create a rapid change in the refractive index at selected locations in the pulse as it flew by, it would flick that portion of the pulse. However, if the entire pulse passes through the area with a new index of refraction, they predicted that there would be no change in OAM—like having someone on the opposite side of a merry-go-round trying to slow it down while you are trying to speed it up.

To test their theory, the team needed to develop the ability to flick a small section of a pulse moving at the speed of light. Luckily, Milchberg’s lab already had invented the appropriate tools. In multiple previous experiments, the group has manipulated light by using lasers for the rapid generation of plasmas—a phase of matter in which electrons have been torn free from their atoms. The process is useful because the plasma brings with it a new index of refraction.

In the new experiment, the team used a laser to make narrow columns of plasma, which they called transient wires, that are small enough and flash into existence quickly enough to target specific regions of the pulse mid-flight. The index of refraction of a transient wire plays the role of a child pushing the merry-go-round.

The researchers generated the transient wire and meticulously aligned all their beams so that the wire precisely intercepted the desired section of the OAM-carrying pulse. After part of the pulse passed through the wire and received a flick, the pulse reached a special optical pulse analyzer the team invented. As predicted, when the researchers analyzed the collected data, they found that the refractive index flick changed the pulse’s transverse OAM.

They then made slight adjustments in the orientation and timing of the transient wire to target different parts of the light pulse. The team performed multiple measurements with the transient wire crossing through the top and bottom of two types of pulses: STOVs that already carried transverse OAM and a second type called a Gaussian pulse without any OAM at all. For the two cases, corresponding to pushing an already spinning or a stationary merry-go-round, they found that the biggest push was achieved by applying the transient wire flick near the top and bottom edges of the light pulse. For each position, they also adjusted the timing of the transient wire laser on various runs so that different amounts of the pulse traveled through the plasma and the vortex received a different amount of kick.Researchers who previously generated vortices of light that they describe as “edge-first flying donuts” have now performed experiments where they disturb the path of the vortices mid-flight to study changes to their momentum. Image credit: Intense Laser-Matter Interactions Lab, UMD

The team also showed that, like a merry-go-round, pushing with the spin adds OAM and pushing against it removes OAM. Since opposite edges of the optical merry-go-round are traveling in opposite directions, the plasma wire could fulfill both roles by changing its position even though it always pushed in the same direction. The group says the calculations they performed using their theory are in excellent agreement with the results from their experiment.

“It turns out that ultrafast plasma provides a precision test of our transverse OAM theory,” says Milchberg. “It registers a measurable perturbation to the pulse, but not so strong a perturbation that the pulse is completely messed up.”

The team plans to continue exploring the physics associated with transverse OAM. The techniques they have developed could provide new insights into how OAM changes over time during the interaction of an intense laser beam with matter (which is where Milchberg’s lab first discovered transverse OAM). The group plans to investigate applications of transverse OAM, such as encoding information into the swirling pulses of light. Their results from this experiment demonstrate that the naturally occurring fluctuations in the index of refraction of air are too slow to change a pulse’s transverse OAM and distort any information it is carrying.

“It's at an early stage in this research,” Hancock says. “It's hard to say where it will go. But it appears to have a lot of promise for basic physics and applications. Calling it exciting is an understatement.”

Story by Bailey Bedford

In addition to Milchberg, and Hancock, graduate student Andrew Goffin and UMD physics postdoctoral associate Sina Zahedpour were co-authors.

The Many Wonders of Uranium Ditelluride

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Category: Research News

Published: Monday, February 26 2024 09:24

In the menagerie of exotic materials, superconductors boast their own vibrant ecosystem.

All superconductors allow electricity to flow without any resistance. It’s their hallmark feature. But in many cases, that’s where the similarities end.

Some superconductors, like aluminum, are conventional—run-of-the-mill, bread-and-butter materials that are well understood and hold no surprises. Others are deemed unconventional: They are not yet fully understood, but that seem to follow a known pattern. But one material—uranium ditelluride (UTe2)—defies classification, continuously baffling scientists with a plethora of unexpected behaviors. 

“At first, we thought this was going to be another interesting superconductor like some other uranium compounds that have been studied in the past,” says Johnpierre Paglione, a professor of physics at the University of Maryland (UMD) and the director of the Quantum Materials Center (QMC). “But at this point, it's gone beyond that. And it's become a much richer example of how crazy a superconductor can get.”

Most superconductors start doing their resistance-less thing when they get super cold. But temperature is only one of the knobs available to researchers studying a material in the lab. Some materials slip into superconductivity when you dial in other aspects of their environment, like the pressure they’re subjected to or the strength of a magnetic field they’re bathed in. UTe2 isn’t fussy about these properties, and it happily hosts superconductivity in all kinds of different situations. And as researchers continue studying the material, they are finding more questions than answers. 

“This one material seems to do 100 different things,” says Nicholas Butch, who is a physicist at the National Institute of Standards and Technology (NIST) and a member of QMC. “Somebody asked me after one of my talks ‘What right does one material have to do all these things?’ and I said ‘Right?’”

Butch and Paglione, together with colleagues at UMD, NIST, QMC and elsewhere, have been at the forefront of exploring the many wonders of UTe2. Postdocs Shang Ran and Corey Frank, working at both NIST and QMC have spearheaded many of the efforts, from discovering superconductivity in the material to testing samples at National High Magnetic Field Laboratory facilities around the country and experimenting with different preparation techniques. And the buzz around UTe2 is catching on: QMC has been sharing the samples they synthesize with researchers at other universities, including the University of Illinois at Urbana-Champaign and Cornell University, and further study by these groups resulted in the discovery of yet more unexpected behaviors. Uranium ditelluride (UTe2)

A serendipitous discovery

Back in 2018, UMD and NIST postdoc Shang Ran was trying to synthesize U7Te12—a mixture of uranium and tellurium that’s predicted to have intriguing magnetic properties. Instead, Ran kept accidentally making UTe2. He found some literature from the 1960s suggesting UTe2 might have some interesting magnetic properties as well, and after consulting Butch, the two decided to cool it down anyway to see what would happen. Ran stuck the sample into a special helium-powered refrigerator. To his surprise, superconducting currents started to flow.

“We accidentally synthesized this uranium ditelluride, and it turned out it’s a superconductor. So, miracle!” Ran says. “That certainly brought excitement to the community and to our research.”

Ran became captivated with UTe2, and the team went on to poke and prod at it to try to understand its superconducting properties. To start, they set out to explore one of the key behaviors for any superconductor—its response to a magnetic field.  

In superconductors, electrons floating around in the material couple up, forming what’s known as Cooper pairs. These pairs act in concert with each other, and with the other pairs around them, allowing the electrons to flow without resistance. However, a strong magnetic field can break up the pairs, destroying the superconducting magic. One of the main signatures of a superconductor is how much magnetic juice it can withstand, and Ran and his collaborators set out to find this landmark for uranium ditelluride. 

To their surprise, uranium ditelluride remained a superconductor as they turned the field all the way up to the maximum power they has access to in the lab—20 tesla. That’s the combined magnetic strength of about two thousand fridge magnets, or ten times the magnetic field in an MRI machine. “I was shocked when [graduate student Chris Eckgerg] showed me the data,” says Ran. “I asked him ‘Did you measure correctly?’ We measured again and it was all correct. So, we realized, okay, there's some very strange thing going on.”

It wasn’t until they brought the material to the National High Magnetic Field Laboratory in Tallahassee, Florida that they finally found a magnet strong enough to tear apart UTe2’s Cooper pairs: It took an astounding 35 tesla to break the bond. For comparison, the first superconductor ever discovered—mercury—loses its superconductivity at a mere 0.1 tesla. This tipped off Ran, Butch, and the others that UTe2 was no conventional superconductor. They guessed that the electrons inside UTe2 form Cooper pairs in an unusually resilient way.

A special kind of dance

The electrons in a superconductor are kind of like a group of couples on a dance floor. In conventional superconductors, the electron pairs dance together in a straight line, a simple dance where partners mirror each other known as spin-singlet pairing. This synchronized movement allows them to glide effortlessly across the dance floor without any hindrance. However, in some unconventional superconductors, the electron pairs dance in swirly circles, spinning around each other as they glide across the dance floor. This unique dance style, known as spin-triplet pairing, gives them a different kind of coordination.

One consequence of this swirly dance pattern is that breaking up the partners with a magnetic field is much harder, which would explain the high magnetic field UTe2 could withstand. To check if that was going on inside UTe2, the QMC team collaborated with the group of Yuji Furukawa at Iowa State University. The Iowa team used their best techniques for distinguishing between the electron dance patterns, nuclear magnetic resonance spectroscopy. These studies confirmed Ran’s suspicions that UTe2 is a rare spin-triplet superconductor. 

Fewer than a dozen materials are suspected of spin-triplet pairing, and the other candidates are difficult to study—they are either hard to synthesize reliably or they only become superconducting under intense pressures or extremely low temperatures. Uranium ditelluride appears to be the most user-friendly spin-triplet superconductor to date, presenting a rare opportunity for researchers. 

“This is the only triplet superconductor I know that can be studied by so many different probes,” says Vidya Madhavan, a condensed matter physicist and professor at the University of Illinois at Urbana-Champaign (UIUC) who is a longtime collaborator of the QMC team. 

In addition to satisfying a physicist’s basic curiosity, spin-triplet superconductors might be useful as platforms for quantum computing. Spin-triplet pairing is a necessary ingredient for a yet rarer property that hasn’t been confirmed in any superconductor to date—a non-trivial topology. If spin-triplet pairing imbues electron couples with killer dance moves, a non-trivial topology warps the whole dance floor with curves and twists, radically changing the dance patterns of all the couples en masse. 

In the months following the discovery of UTe2’s swirly dance patterns, some evidence suggested that UTe2 might not only be a spin-triplet superconductor but also possess that topological special sauce. The evidence is not yet conclusive, but researchers are hard at work trying to sort this out, as well as understand more about what makes UTe2 tick. And their sleuthing keeps turning up more surprises. 

Superconductivity raised from the dead (and the never-born)

Ran and his labmates were wondering why 35 tesla seemed to be the magic number that broke superconductivity in UTe2. In search of clues, they went back to the National High Magnetic Field Lab.  They kept turning up the magnetic field even higher, looking at how the non-superconducting chunk responded. They also tilted the sample, putting the magnetic field off-kilter from UTe2’s natural crystal structure. 

Shockingly, as they kept rotating the sample, superconductivity reappeared at a field of 40 tesla. This was strange. Turning the field up really high killed the superconductivity, but if you kept going it came back to life. This phenomenon was termed Lazarus superconductivity after the biblical figure raised from the dead. Lazarus superconductivity is extremely rare, though not entirely unprecedented. It’s cropped up in a handful of materials before, and scientists think they have plausible mechanisms for explaining the effect. But none of those mechanisms seemed applicable to UTe2. 

In 2020, Ran joined the physics department at Washington University in St Louis, passing the torch of Butch’s QMC lab to a new postdoctoral researcher, Corey Frank. Frank had just completed her PhD in solid-state chemistry—the perfect background for mastering different ways of concocting the UTe2 crystal. She played with the initial concentrations of the starting materials as well as precise techniques and temperatures of preparation. Among other things, Frank developed a protocol for making UTe2 samples that are just shy of superconducting by making them intentionally just a bit dirty, peppering the crystals with purposefully introduced defects. These defects gum up the pathways by which electrons pair up and find their dance partners, preventing the development of superconductivity. “You can learn a lot about a phase by studying what kills it,” Frank says. 

Frank and her colleagues made a purposefully dirty sample and took it with them on another trip to the National High Magnetic Field Laboratory, this time in Los Alamos, New Mexico. They stuck the sample into the huge magnets and cranked up the field. Once the field was high enough and the sample had the right orientation, the resistance through the material dropped to zero—superconductivity was revived. 

“I was so excited,” Frank recalls. “You're not allowed to jump when you're on the platform of a high-field magnet, but I had to get down from the magnet so I could jump. It was amazing.”

This was completely unprecedented. In all the previous Lazarus superconducting materials, the mechanism behind the rebirth was presumed to involve recreating the conditions at a low magnetic field. Here, recreating conditions at a low magnetic field would not result in superconductivity because the samples had intentional defects, and yet there it was—superconductivity raised not from the dead, but from the never-born, a high-field superconducting phase all its own. The team reported this phenomenon last year in a preprint.

“We know how high field superconductivity works, the rules that govern that, and this one breaks those rules,” Frank says. “So the fact that we have this much more robust high-field phase is wild. I cannot overemphasize how unexpected it is.” 

The authors have some ideas of what could be causing this behavior, and they say further experiments are needed to figure out if those ideas are correct. For now, the experiments are on hold as they require even stronger magnetic fields than the National High Magnetic Field Laboratory currently offers. In the meantime, the QMC team is still studying how this superconductivity dies, comparing their revived samples to others in search of a pattern. 

Making waves 

Over many years Ran, Frank, and other members of the Butch lab have mastered the dual feats of growing pure uranium ditelluride crystals and studying their overall behavior—superconductivity, response to magnetic fields, and more. But they lacked the tools and expertise to zoom in on the microscopic, atom-by-atom behavior of UTe2. So they’ve enlisted the help of Vidya Madhavan’s team at UIUC.

In her lab, Madhavan has a scanning tunneling microscope (STM). An STM works by bringing a bit of metal tapered down to a tiny, fine point extremely close to the surface of a sample—so close that electrons from the sample can hop over to the conducting tip, or vice versa. By measuring how many electrons make the jump, scientists can learn a lot about the microscopic structure of a material, including where the electrons are on the surface of the sample.

The Butch group sent Madhavan a sample, and Anuva Aishwarya, a graduate student at UIUC who led the study, placed a sample of UTe2 into the scanning tunneling microscope. The team cooled the material just shy of its superconducting temperature, and they stumbled upon another surprise: The electrons didn’t follow the ups and downs of UTe2’s crystal structure. Instead, they clustered together and then apart, forming waves of charge frozen into the surface with a pattern all their own.

These kinds of charge density waves are uncommon but not unprecedented. However, the measurements performed by Ran, Frank or others at QMC didn’t show any indication that a charge density wave might be found in UTe2. To Madhavan and her team, this came out of nowhere.

To try to understand what they were seeing, Aishwarya and her lab mates probed the behavior of these waves in different temperatures and magnetic fields. They found that, in a magnetic field, the charge density wave seemed intimately related to superconductivity itself. As they turned up the field, the charge density wave broke down at precisely the same field strength as superconductivity. This tipped off Madhavan and her collaborators at UIUC that maybe this wave had some relationship to the superconductivity in uranium ditelluride.

If you want to pick out individual electrons, a regular STM is great. But if you want to peer inside the dance patterns of electron couples in a superconductor, you need an STM armed with a special kind of tip—one that is itself a superconductor. The team of Seamus Davis at Cornell University had just such a superconducting tip. They became intrigued by Madhavan’s results and got in on the action. They obtained another sample from the QMC team and stuck it in their specialized STM. They found that the electron pairs behaved similarly to the lone electrons. Here, too, the pairs clustered together and apart, forming a so-called pair density wave with the same beat as the charge density wave observed by Madhavan. This is the first time such a pair density wave has been found in a spin-triplet superconductor.

As with many aspects of UTe2, the origins of the charge and pair density waves remain far from clear. But, Ran comments, these waves are a fairly common feature in unconventional spin-singlet superconductors. This may provide clues for how all these different strange superconductors are connected. “We eventually need to understand unconventional superconductivity overall,” Ran says. “And having this common theme I think is very important for theorists.”

While theorists are hard at work trying to crack the puzzle of unconventional superconductivity, Ran, Butch, and other researchers are continuing to explore all that UTe2 has in store. “It's really rich. It’s a great place to explore,” says Butch. “This one material underscores how little we know about spin triplet physics. It’s as if we are writing textbooks about it right now. So that's actually very exciting.”

Story by Dina Genkina

Aaron Sternbach Combines Light and Matter to Push Experimental Boundaries

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Category: Department News

Published: Monday, February 26 2024 09:24

Aaron Sternbach, a new assistant professor in the Department of Physics at the University of Maryland, is an expert in combining light and material properties to produce unique results. His experiments have allowed him to spy on elusive quantum interactions that play out on extremely small and fast scales.

“I study quantum materials with light,” Sternbach said. “When I encounter something I can’t see with light because of common ‘limits,’ I study the physics behind these limits and try to push them further. That is a really fun part of the job. In some cases, that approach can lead to new opportunities.” 

Since he was a kid, Sternbach enjoyed math and physics. But even as he enrolled to study physics as an undergraduate at Boston University, he wasn’t certain that he wanted to pursue a career in physics. During his freshman and sophomore years, he worked in an astrophysics lab where he spent most of his time helping design measurement devices. But after almost two years of work, his efforts hadn’t produced a physical device. The project wasn’t progressing at a pace he found satisfying, so he considered switching majors to study medicine in his junior year. Aaron Sternbach

That changed after he spent his summer working in physicist Richard Averitt’s lab. He assisted with an experiment that used light to manipulate the electrical properties of a quantum material. In the project, light drove the material from being a poorly conducting insulator to an electrical conductor. Achieving the transition required focusing the light into a smaller spot than is possible using lenses or other common techniques. Instead, it required taking advantage of the material’s structure. 

A material’s response to light is dictated by its internal structure, and the project was looking at just one of the countless possible materials that scientists can find in nature or deliberately engineer. Sternbach became hooked on physics when he started to explore simulations of materials as part of the project, and he never looked back.

"I found it fascinating that engineering light could totally change the properties of a quantum material," Sternbach said. "I started playing with all sorts of simulations to try to understand how far this approach could go."

He started to wish he could watch the transitions between insulator and conductor in experiments as they played out in real space and real time. In 2013, that desire led him to graduate school at UC San Diego, where he joined the lab of Dimitri Basov. Basov had recently been investigating new techniques that used material properties to focus light into unusually small spots. His early results showed that the approach could be useful for observing and learning about quantum materials. In the middle of his graduate studies, Sternbach moved to Columbia University when Basov relocated his lab there.

Working with Basov, Sternbach helped develop a new observation technique that can observe very quick changes while also getting around a rule in physics called the diffraction limit. The diffraction limit is the inevitable result of the way that waves, including light, spread—diffract—when they pass the edge of an object and then keep spreading as they travel. For devices that use lenses and apertures to manipulate light, the diffraction limit imposes strict constraints both on the smallest spot a beam of light can be focused into and on the smallest features that the device can be used to clearly distinguish. However, by using the structure of a material to continually influence light, researchers can circumvent the diffraction limit and build new tools for manipulating and observing the microscopic world. 

The observation technique that Sternbach helped develop simultaneously uses the material’s structure to herd light along paths that beat the diffraction limit and uses very short flashes of light to accurately capture quickly unfolding events as they play out over time. To get clear snapshots of rapidly changing experiments, the team used extremely short flashes of light, providing a clear view of brief periods instead of capturing a blurry image like an overexposed photograph. 

“Learning to interact with data and gaining an intuition for what you're seeing is like learning a new language,” Sternbach said. “You learn fantastic approaches to see parts of the world that are way beyond a native human scale.”During positive refraction (green) the path of incoming light (blue) will bend, but it will never cross the dotted line perpendicular to the interface of the two materials. In rarer circumstances, called negative refraction (red), the light sharply turns and continues on the same side of the dotted line. (Credit: Bailey Bedford, UMD)

As part of his graduate work, Sternbach got to apply the technique he’d developed to observe materials transforming in real space and in real time after light was used to initiate a change.

After completing his degree, he continued to work with Basov as a postdoctoral researcher. In a new project, they incorporated an additional way that light and matter can interact. They studied polaritons—particle-like combinations of light and matter with characteristics of both. Since the matter portion of a polariton contributes mass, polaritons behave more like matter than normal light: They can carry significantly more momentum than light and can be more tightly confined into a beam than freely propagating light. 

Sternbach and his colleagues wanted to observe a particular type of polariton, called a hyperbolic polariton, that travels through the bulk of a material along a specific type of constrained path. In an article published in the journal Science in 2021, the team shared how they created polaritons by hitting a layered material with a pulse of light and then used their new technique to observe polaritons and follow their journey through the material. Their measurements revealed details about quantum states that were crucial to the polaritons’ existence and that only existed in the material for trillionths of a second. 

Following that experiment, Sternbach and his colleagues studied hyperbolic polaritons that moved between two different adjacent materials. They investigated two naturally occurring materials that were known to produce polaritons and revealed that a polariton’s path would bend in an unusual way as it passed across the interface between the two materials. 

Normally when light travels between two materials, such as water and air, its path bends slightly based on the fact that it travels at different speeds in each material. This bending of light—called refraction—is why a straight straw placed in a glass of water looks like it bends at the interface. 

In an article published in the journal Science in 2023, Sternbach and his colleagues showed that when they properly oriented the two materials, the polaritons at the interface didn’t refract normally. 

Most materials produce positive refraction, where light is deflected a bit but is limited in how far it can swerve to either side. Positive refraction is similar to a simple dive into a swimming pool: The diver’s direction will change some when they move into the water, but as they continue down, they also keep moving forward. 

Sternbach and his colleagues observed their polaritons bending in a more drastic way, called negative refraction. During negative refraction, a beam almost does a U-turn. While it continues down into the new material, it also travels backwards, like a diver who instantly makes a sharp turn as they hit the water so that they end up under the diving board instead of in front of it. 

The team’s experiment revealed that producing negative refraction in the experiment depended on getting the top layer turned at just the right orientation to the bottom layer. The team went on to use negative refraction to create a tiny container for trapping light. They demonstrated that when the polaritons were reflected at the exposed surfaces of each material, the negative index of refraction allowed the polaritons to become stuck in a loop that is much smaller than the wavelength of the light outside the materials. 

Now that Sternbach has joined UMD, he plans to continue this line of research in his own lab, where he hopes to create a supportive environment for students. He is currently looking for new students to join him in exploring quantum materials and the complex interactions that can be engineered between light and matter. 

“I always felt that exploring curiosities and doing things that I really enjoyed doing was enough,” Sternbach said. “And I think that was a good rule of thumb. It's allowed me to explore this direction, which is basic research, freely and grow in whatever direction nature allows. I'm very excited to see where this goes in the future at Maryland.”

Story by Bailey Bedford

The Sternbach group is always looking for exceptional graduate and undergraduate students as well as postdoctoral researchers who wish to join the team. Those interested may reach out to him by email at This email address is being protected from spambots. You need JavaScript enabled to view it..

Solving the Mystery of the Stinky Vapor Plumes on Campus

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Category: Department News

Published: Monday, February 26 2024 09:24

Billowing white columns of vapor rise silently from sidewalks and manholes around the University of Maryland campus when it gets chilly. These mysterious plumes are sometimes accompanied by a strange odor or a lingering warmth. Like ghosts, the hazy wisps seem to come and go without much explanation. But what’s the cause of this foggy, strange smelling campuswide phenomenon and where is it coming from? 

“It’s definitely not intentional or desirable,” said UMD Physics Professor Daniel Lathrop. “You’re not supposed to see or smell it at all. In fact, it’s just one component of a much bigger problem. This visible vapor is a sign that our underground utilities infrastructure is aging and inadvertently helping to cause water and heat waste across our campus.”Dan Lathrop. Credit: Georgia Jiang

Like a leaky pipe, this leaking vapor can cause serious problems including infrastructure damage, energy waste and pollution to the natural areas surrounding UMD. 

The university’s infrastructure—some of which dates back over 150 years—naturally deteriorated over time. That includes the decades-old tri-generation energy system that the majority of the campus runs on today.

“Our current energy generation system was installed in 1999. It heats, cools and powers over 250 campus buildings,” explained Lathrop, who also holds joint appointments in the Departments of Geology and Physics, the Institute for Physical Science and Technology, and the Institute for Research in Electronics and Applied Physics. “A natural gas-fired turbine generates most of the electricity we use and the heat that this turbine produces is recaptured to produce steam, which produces additional electric power. The steam is then also used to heat and cool our buildings.” 

Gregg Garbesi, assistant director of utilities and energy management at UMD’s Facilities Management, says the visible vapor comes when liquid (like groundwater, city water leaks, stormwater leaks, etc.) physically contacts the underground steam line. He compares the campus’ visible vapor to steam caused by pouring cold water into a hot pan on a stove. He explains that the odd smells accompanying the vapor occur when organic materials come into contact with the leaked vapor, which is generated underground or in a steam manhole. 

According to Garbesi, seeing these vapor plumes signals energy loss. 

“For every pound of vapor generated, a pound of steam is condensed in the steam pipes,” he said. “All the energy that went into making that pound of steam—which could have been used to power campus—is lost, and if 100% of the condensate doesn’t return to the energy plant, that water is also lost.”

“We’re wasting about a million gallons of water a day now,” Lathrop added. “The loss in 2019 alone is estimated to be 700 million gallons, costing us an estimated $4 to $6 million a year. And that’s just the financial price tag that we know about.”

That’s why Lathrop is leading a multidepartment, cross-disciplinary Grand Challenges team that aims to map and pinpoint locations where steam is being lost and get a better look at challenges with energy consumption, water quality, methane emissions and air quality on UMD’s campus. With colleagues and students from the Departments of Geology, Atmospheric and Oceanic Science, Geographical Sciences, and Environmental Science and Technology, Lathrop hopes to tackle these interrelated issues by pinpointing the biggest problems and providing concrete solutions.

“Methane emissions, carbon dioxide emissions and pipeline water loss from our campus play a big role in stream water and air contamination nearby,” he said. “It’s our goal to measure these impacts and figure out how we can remediate these challenges to reduce our campus’ climate footprint and improve the environment here.”

Diagnosing and treating an aging energy system

When Lathrop began this project in spring 2023, his first step was to work with Facilities Management to learn more about the campus’ utilities-related challenges, particularly the steam issues caused by underground pipelines. He soon realized that his ongoing U.S. National Science Foundation-funded research—to develop sensors capable of detecting geophysical anomalies—could play a key part in this new project. 

“My lab was originally developing geophysical sensors to look for landmines and unexploded ordnance, but we recognized that this same system could be used to find buried utilities,” Lathrop explained, referring to a project that was named a UMD Invention of the Year in May 2022. 

Licensed drone pilot and physics senior Meyer Taffel is test-flying a drone over campus to magnetically map UMD's underground utilities. Credit: Dan LathropUsing these sensors and historic infrastructure blueprints provided by Facilities Management, Lathrop and his team sketched out an extensive map of steam-emitting sources on campus. They concluded that UMD’s energy system had at least 55 active steam vents. 

A group of undergraduate physics majors taking Lathrop’s PHYS 499X class added to those findings when they hand-built a calorimeter (an instrument that measures heat from chemical and physical reactions) in fall 2023 as a course project. 

The students used the calorimeter to identify dozens of campus hot spots and learned that some of these accidental vents lost more than 60 kilowatts of heat energy via escaped condensation. For reference, a standard incandescent light bulb uses about 60 watts of electricity on average—meaning that the steam and heat leaking from some of these vents could power approximately 1,000 of those light bulbs. 

Lathrop estimates that this loss alone could equate to approximately $50,000 per year but he believes that identifying and analyzing the factors that led to the hot spots was invaluable.

“This is a team effort and having everyone on the same page can really make a difference when it comes to fixing things on campus and allocating resources appropriately. Our work has already started to help the university identify and patch sites on campus with water leaks,” he said. “We were able to prevent local building damage and protect students, staff and faculty from potential safety hazards.”

Putting together pieces of a bigger puzzle

Other members of the Grand Challenges project team are also on the ground looking for ways to locate and address the campus’ interconnected water and air quality problems.

Geology graduate student Julia Famiglietti and Geology Chair and Professor James Farquhar tracked gas leaks on campus through the sewer system to create a methane inventory (list of methane sources and sinks). They identified a contamination source underground that they suspect is related to the aging steam system.

“We think that the contamination has something to do with steam additives accidentally coming into contact with and reacting to methane,” explained Famiglietti, who sampled the air in campus sewers at least once a month to determine the composition of gases found in emissions. “We’re focusing on understanding the chemistry behind this contamination signature and directly discussing with the steam plant management personnel how to address the problem.” 

Geology Associate Professor Karen Prestegaard and Professor Sujay Kaushal found similar results while working with student groups to monitor the water quality of streams and storm drains. They discovered that water discharging from steam vents into storm drains had noticeably higher pH values and higher salt contamination than natural rainwater—potentially encrusting pipes with buildup and impacting wildlife in the Paint Branch waterways. 

Other efforts to gather and analyze the researchers’ environmental data include Project Greenhouse, in which First-Year Innovation & Research Experience (FIRE) undergraduates are drafting a sustainable methane budget for UMD.

Lathrop hopes that this multiyear campuswide team effort will help UMD reduce water and steam waste (and the associated environmental and fiscal costs) by 30% by the end of the project in June 2026. He believes this project can serve as a model for other research efforts to analyze similar urban environmental impacts for cities or large organizations like universities. 

“Our campus is a microcosm of urban and suburban environmental impacts, so evaluating UMD’s impact on the environment leads to a better understanding of how humans impact climate on a local, national and global level,” Lathrop said. “We’re all doing our part to protect our campus and the people living and working here.” 

Written by Georgia Jiang

 Other UMD faculty members involved in the Remediation of Methane, Water, and Heat Waste Grand Challenges project include Atmospheric and Oceanic Science Professor Russell Dickerson, Environmental Science and Technology Associate Professor Stephanie Yarwood, FIRE Assistant Clinical Professor Danielle Niu, Geographical Sciences Assistant Professor Yiqun Xie, and Geology Professors Michael Evans and Vedran Lekic.