Electrons Take New Shape Inside Unconventional Metal


Two Light-Trapping Techniques Combine for the Best of Both Worlds

Of all the moonbeam-holding chip technologies out there, two stand the tallest: the evocatively named whispering gallery mode microrings, which are easy to manufacture and can trap light of many colors very efficiently, and photonic crystals, which are much trickier to make and inject light into but are unrivaled in their ability to confine light of a particular color into a tiny space—resulting in a very large intensity of light for each confined photon.

Recently, a team of researchers at JQI struck upon a clever way to combine whispering gallery modes and photonic crystals in one easily manufacturable device. This hybrid device, which they call a microgear photonic crystal ring, can trap many colors of light while also capturing particular colors in tightly confined, high-intensity bundles. This unique combination of features opens a route to new applications, as well as exciting possibilities for manipulating light in novel ways for basic research.

“There are potential applications, like single photon sources and quantum gates,” says Adjunct ProfessorScanning electron microscope image of a novel photonic microring with micron-scale gears patterned inside a larger circle. (Credit: Kartik Srinivasan/JQI)Scanning electron microscope image of a novel photonic microring with micron-scale gears patterned inside a larger circle. (Credit: Kartik Srinivasan/JQI) Kartik Srinivasan, who is also a fellow of the National Institute of Standards and Technology (NIST). “But a part of it is also fun electromagnetism and fun optical phenomena in these devices.”

The team introduced their device in a paper published in the journal Nature Photonics in 2021, and they showed off more of what it can do in a paper published recently in the journal Physical Review Letters.  

Whispering gallery mode (WGM) microrings are named after the gallery inside St. Paul’s Cathedral, a masterpiece of Baroque architecture that towers over London. Whispers in the Cathedral can be heard anywhere within the gallery because the sound gets trapped by the round walls and reflected back inside. Similarly, optical WGMs trap light in a ring, typically about a tenth of a millimeter in diameter, made of silica or another material that is transparent to optical light. Light of the right color travels round and round the ring many thousands of times before leaking out, producing a high light intensity in a small volume. Building a WGM microring that traps the desired color with minimal loss, as well as getting the light into the ring, is relatively straightforward for a wide range of colors.

Photonic crystals can confine light to much smaller volumes—sometimes less than one wavelength across. They achieve this with a carefully crafted periodic structure made up of a grid of holes or posts in a chip. The regular grid reflects light of a very specific color, and a small, intentionally introduced imperfection in the grid—called a defect—accumulates the light within the surrounding reflecting grid, trapping it in a tiny space. Photonic crystals are unrivaled in comparison to WGMs in terms of the light intensity they can create per photon, but they require very detailed electromagnetic design and precise manufacturing to implement in practice. Moreover, photonic crystals that can trap multiple colors have been challenging to realize.

The new hybrid ring is easy to manufacture and guide light into like WGMs, but it also provides extra localization for particular colors, like photonic crystals. The design of this hybrid is surprisingly simple. The researchers created a regular microring out of silicon nitride, a hollow circle much like the gallery in St. Paul’s Cathedral. To add a photonic crystal element, they cut notches into the inside wall of their ring, making it resemble a gear. It turned out that adding the gear notches inside the ring didn’t reduce the number of times the light would go around before leaking out—the ring trapped light just as well as before. Moreover, to add a defect, the researchers simply modified the size of a few of the notches.  Finally, the microgears confine just a few colors of light into tight bundles, while allowing other colors to circle around the microring freely.

“People have been saying for a long time that microrings and photonic crystals have complementary strengths, and so it would be great to put them together to get the best of both worlds,” Srinivasan says. “But in general, when people put them together this didn’t happen – sometimes you could even get the worst of both worlds. The notion that you can stick a photonic crystal into a microring with this kind of strength and modulation, while retaining a high quality factor (low loss), has actually been rather surprising for a lot of people, myself included.”

In their combined design, Srinivasan’s team showed that they could confine the light into a space more than ten times smaller than previous WGMs, enabling a higher optical intensity than in conventional WGMs. And they preserved some of best qualities of the WGMs, including a high quality factor (the light going around the ring several thousand times before leaking out) and the ease of getting light into and out of the ring. Perhaps most importantly, the design and manufacture of these hybrid devices remains straightforward for different colors of light and other parameters.

“In our work it’s basically the purest, simplest photonic crystal,” says Xiyuan Lu, an assistant research scientist at NIST and JQI and an author on both publications.  “Which is why you don't need to carry out any simulation. You can know [how to design properties] intuitively.”

After adding the microgear notches to the device last year, the team went on to extend its capabilities and detailed the performance in their more recent work. They put multiple defects into the notch pattern, with each defect created by making a few of the gear teeth shorter than the surrounding ones. Each defect confines light to a small fraction of the circumference of the microring, much like in a photonic crystal. They were able to put up to four defects into the same microring, confining light in four places and building up high intensities in a tightly confined space.

They found another unique feature of this microgear approach. The microgear can control different colors of light in different ways at the same time. Certain colors will get trapped in the defects and confined to a volume much smaller than the ring itself. At the same time, other colors can circulate freely around the microring, unconfined by the defects but still influenced by the gear structure, giving researchers extra control over the light beam.

In a normal WGM, the electromagnetic field that makes up a beam or a pulse of light wraps around the microring, forming a standing wave. If you were to ride along this wave, it would take you up and down along the edge of the ring, going through a number of peaks and troughs before dropping you back where you started. Although the number of peaks and troughs can be predicted, where exactly in the ring they will line up is completely random.

“If everything is symmetric, light can stand anywhere it likes,” says Lu. “But now we can control it.”

By placing the microgears and defects, the researchers can control exactly where in the microring the peaks and troughs of the free-floating color will end up. And they can even wrap it around in unintuitive ways, creating something akin to a Möbius strip out of light—a circular structure you’d have to traverse twice in order to end up where you started.

In addition to fun with electromagnetism, these microgears open up possible applications in several realms, including non-linear optics, where light interacts with the matter it travels through to produce new colors and directions.

“In photonic crystals, you can kind of engineer one mode pretty well,” Srinivasan says. “But it’s difficult to engineer multiple modes simultaneously. With this device, we can envision mixing between different colors of light that we can really engineer the modes of while having these additional resources of strong confinement and high intensity.”

Another promising application is in the realm of cavity quantum electrodynamics: the fundamental study of the interactions between atoms and light. The approach is to trap single atoms or quantum dots near a localized, intense beam of light and study their behavior. This also allows for the control of quantum matter with light.

“We have a platform now where it’s straightforward for us to have multiple sites within one of these resonators that can host single quantum emitters,” Srinivasan says.

These potential applications have not been demonstrated yet, but the researchers are confident that this new tool will find many uses. Among its strongest advantages is how easy it is to design, fabricate and work with.

“In our case, the platform seems to be quite forgiving,” Lu says. “If you do anything new, chances are it can work well.”

Original story by Dina Genkina: https://jqi.umd.edu/news/two-light-trapping-techniques-combine-best-both-worlds

In addition to Lu and Srinivasan, authors on the papers included Mingkang Wang, a postdoctoral associate at NIST; Feng Zhou, a research associate at NIST; Andrew McClung, a former postdoctoral researcher at the University of Massachusetts Amherst now at Raytheon; Marcelo Davanco, a research scientist at NIST; and Vladimir Aksyuk, the project leader in the Photonics and Optomechanics Group at NIST.

Molecular Tug-of-war Gives Cells Their Shape

In a new study, University of Maryland researchers have demystified the process by which cells receive their shape—and it all starts with a protein called actin.

Actin is a key component of the cytoskeleton that provides structure to cells, much like how our skeletons support our bodies. However, unlike our skeleton, the actin cytoskeleton is a highly malleable structure that can rapidly assemble and disassemble in response to biochemical and biophysical cues. 

It is well known that actin can form both 3D spherical shell-like structures that protect cells from external pressure and 2D rings that modify intracellular functions. But whenever researchers tried to recreate these structures outside the cell, they almost always ended up with clusters of actin. No one knew why—until now.

The researchers used computer simulations to show that actin and its partner protein, myosin, engage in a tug-of-war, with myosin trying to trap actin in local clusters and actin attempting to flee. If actin wins, actin filaments escape myosin’s pulling force and spontaneously form rings and spherical shells. If myosin wins, the actin network collapses and forms dense clusters. 

Actin (shown in magenta and in box “a”) and myosin (shown in green and in box “b”) are depicted in the actin rings of live T cells (box “c”). Box “d” provides a snapshot of MEDYAN simulations, which resemble the actin ring found in T cells. Credit: Haoran Ni.  Actin (shown in magenta and in box “a”) and myosin (shown in green and in box “b”) are depicted in the actin rings of live T cells (box “c”). Box “d” provides a snapshot of MEDYAN simulations, which resemble the actin ring found in T cells. Credit: Haoran Ni.  

“Actin rings and spherical shells are ubiquitous in almost all cell types across species. We think that understanding the mechanism behind the formation of these structures unlocks the door to how cells sense and respond to their environment,” said Garegin Papoian, a co-author of the study and a UMD Monroe Martin Professor in the Department of Chemistry and Biochemistry and the Institute for Physical Science and Technology (IPST).

Their findings, published Oct. 21, 2022 in the journal eLife, could have important implications for human health. Because actin rings are central to our bodies’ ability to fight off foreign cells—with defects potentially resulting in impaired immunity or autoimmune disorders—the findings of this study could aid the development of future drugs.

Actin monomers can be thought of as railroad cars, which link up to form a train-like actin filament. These actin trains move through the cell because of a process called treadmilling. Also at play are the myosin motors, which pull oppositely oriented trains toward each other. Papoian, Qin Ni (Ph.D. ’21, chemical engineering) and biophysics Ph.D. student Haoran Ni believed that a competition between myosin’s pulling force and the rate of treadmilling was responsible for the formation of actin rings.

Fine-tuning these parameters in living cells is not possible, so the researchers turned to a simulation software called MEDYAN, developed by the Papoian Lab. MEDYAN uses physics and chemistry rules to simulate the dynamics of cytoskeletal proteins. They simulated an actin and myosin network (collectively referred to as actomyosin) in a thin disc and spherical shell.

They found that if the actin trains move slowly, the myosin pulling force causes traffic jams, which are the actomyosin clusters that have been observed in networks reconstituted outside cells. On the other hand, if the actin trains move fast, they can escape myosin’s pull. Once they reach the boundary of the disc, myosin’s pulling force makes the actin trains turn, preventing a head-on collision with the disc edge. Repeated occurrence of these events results in all the trains moving in a circle along the perimeter of the disc, which forms the actin ring.

Further analysis offers a thermodynamic theory to explain why cells form rings and shells. According to the laws of physics, systems favor the lowest energy configuration. Myosin proteins generate a lot of mechanical energy by bending actin filaments, which can only be released if actin can run away and relax. In living cells, actin’s ability to move fast enough to escape myosin and run to the edge allows for this built-up energy to be released, allowing for the formation of rings or shells, which, thermodynamically speaking, is the lowest energy configuration.

“The reason rings were not previously seen outside the cell is because actin just wasn’t moving fast enough,” Papoian said. “Myosin was winning 10 times out of 10.”

Together with Professor Arpita Upadhyaya and physics graduate student Kaustubh Wagh, biological sciences graduate student Aashli Pathni and biophysics graduate student Vishavdeep Vashisht, the team set out to test this model in living cells by turning their attention to T cells, where rings naturally form.

T cells are the cells in our body that hunt down foreign cells. When they recognize a cell as foreign and become activated, the T cell cytoskeleton rapidly reorganizes itself to form an actin ring at the cell-cell interface. Starting with cells that had formed rings, the researchers investigated the effect of perturbing actin and myosin using high-resolution live-cell imaging.

Reducing the actin train speed resulted in dissolution of the ring into small clusters, while increasing myosin’s pulling force led to rapid contraction of the ring, in remarkable agreement with associated simulations.

As a follow-up to this study, the team plans to add more complexity to the model and include other cytoskeletal components and organelles.

“We have been able to capture one fundamental aspect of cytoskeletal organization,” Papoian said. “Piece by piece, we plan to build a computational model of a complete cell using fundamental principles from physics and chemistry.”


Original story:https://cmns.umd.edu/news-events/features/5000

This article is adapted from text provided by Qin Ni and Kaustubh Wagh.

The research paper, “A tug of war between filament treadmilling and myosin induced contractility generates actin rings,” was published in eLife on Oct. 21, 2022.  

This work was supported by the National Science Foundation (Award Nos. CHE-1800418, PHY-1806903 and PHY-1607645) and the National Institutes of Health (Award No. R01 GM131054). This story does not necessarily reflect the views of these organizations.

Media Relations Contact: Emily C. Nunez, 301-405-9463, This email address is being protected from spambots. You need JavaScript enabled to view it.

UMD Team Leads a New Test of Universality of Leptons at the LHCb Experiment

The LHCb collaboration has presented a new test of the universality of the electroweak properties of leptons.

Nearly seven years of analysis of LHCb data by University of Maryland physicists Phoebe Hamilton and Hassan Jawahery and CERN collaborator Greg Ciezarek led to the results unveiled October 18, 2022 at a seminar at CERN and presented the next day at a flavor physics workshop at CERN.The results reflect analysis of seven years of data by Phoebe Hamilton and Hassan Jawahery and their CERN collaborator, Greg Ciezarek.The results reflect analysis of seven years of data by Phoebe Hamilton and Hassan Jawahery and their CERN collaborator, Greg Ciezarek.

In 2015, the LHCb team reported on a test of lepton universality with the measurement of a key observable. But the new results represent the first simultaneous measurements of two correlated observables at the LHC collider, significantly improving the sensitivity to new physics effects. This is particularly important for the extensions of the Standard Model that contain additional Higgs bosons. The results are consistent with previous measurements, which hinted at deviation from lepton universality.  The combined values are at 3.2 standard deviation from the Standard Model. 

These studies are a major theme of the physics program of the Maryland group in LHCb with the current and the future data with the upgraded detector. Over the past decade they have carried out a broad program of studies of lepton flavor universality in decays of particles containing the b quark, which have been published in PRL and highlighted in the CERN Courier and Symmetry magazine.  Professor Manuel Franco Sevilla, whose PhD thesis work at BaBar provided the first hint of deviation of these observables from universality, has recently co-authored a comprehensive review of the past studies and the prospects for future measurements in Review of Modern Physics


Quantum Gases Keep Their Cool, Prompting New Mysteries

Quantum physics is a notorious rule-breaker. For example, it makes the classical laws of thermodynamics, which describe how heat and energy move around, look more like guidelines than ironclad natural laws.

In some experiments, a quantum object can keep its cool despite sitting next to something hot that is steadily releasing energy. It’s similar to reaching into the oven for a hot pan without a mitt and having your hand remain comfortably cool.

For an isolated quantum object, like a single atom, physicists have a good idea why this behavior sometimes happens. But many researchers suspected that any time several quantum objects got together and started bumping into each other the resulting gang of quantum particles would be too disorganized to pull off this particular violation of the laws of thermodynamics.

A new experiment led by David Weld, an associate professor of physics at the University of California, Santa Barbara (UCSB), in collaboration with Professor Victor Galitski of the Joint Quantum Institute, shows that several interacting quantum particles can also keep their cool—at least for a time. In a paper(link is external) published Sept. 26, 2022 in the journal Nature Physics, Galitski, who is also a Chesapeake Chair Professor of Theoretical Physics in the Department of Physics at UMD, and the researchers at UCSB describe the experiment, which is the first to explore this behavior, called dynamical localization, with interactions included.

The experiment builds on theoretical predictions made by Galitski and his colleagues, and the results reveal mysteries for the researchers to pursue concerning what the particles are doing in the experiment. Uncovering exactly how the particles can break a revered law of thermodynamics might provide significant insight into how quantum effects and interactions combine—and those insights might find uses in the designs of quantum computers, which will necessarily contain many interacting particles.Equipment at the University of California, Santa Barbara used to create clouds of Lithium atoms. It was used to study how atoms absorb energy when they have various levels of interaction with each other. (Credit: Tony Mastres, UCSB)Equipment at the University of California, Santa Barbara used to create clouds of Lithium atoms. It was used to study how atoms absorb energy when they have various levels of interaction with each other. (Credit: Tony Mastres, UCSB)

“The big question is whether this phenomenon can survive in systems which are actually of interest,” Galitski says. “This is the first exploration of the fate of this very interesting phenomenon of non-heating as a function of interactions.”

For a single particle, physicists have the math to explain how quantum mechanical waves of probability sway and crash together in just the right way that crests and troughs meet and cancel out any possibility of the particle absorbing energy. Galitski and his colleagues decided to tackle the more complicated case of investigating if the same behavior can occurs when multiple particles interact. They predicted that in the right circumstances repeated kicks of energy would warm up the collection of particles but that at a certain point the temperature would plateau and refuse to go up anymore.

The next natural step was to confirm that this behavior can happen in a lab and that their math wasn’t missing some crucial detail of reality. Fortunately, the idea intrigued Weld, who had the right experimental equipment for testing the theory—almost. His lab can set up quantum particles with the needed interactions and supply of energy to attempt to defy thermodynamics; they used lasers to trap a quantum gas of lithium atoms and then steadily pumped energy at the atoms with laser pulses.

But there was a catch: To keep the math manageable, Galitski’s theory was calculated for particles confined to live on a one-dimensional line, and it’s not easy for Weld and his team to keep the cloud of atoms that tightly constrained. Atoms in a gas naturally explore and interact in three dimensions even when confined in a slender trap. The team made their cloud of atoms long and narrow, but the extra wiggle room tends to significantly impact the quantum world of atoms.

“With just a few discussions, the basic picture of what we wanted to do was clear quite quickly,” Weld says. “Though the experiment turned out to be quite challenging and it took a lot of effort in the lab to make it all work!"

While Weld’s lab couldn’t do the experiment in one dimension, they could easily control how strong the interactions were between atoms. So, the team started with the well-understood case where particles weren’t interacting and then observed how things changed as they increased the interaction strength.

“So, they didn't actually do exactly what we wanted them to do, in a one-dimensional system, because they just don't have one-dimensional systems,” Galitski says, “but they did what they could do. There is this kind of a tension, which is common, that what’s easy theoretically is usually difficult experimentally, and vice versa.”

When the particles weren’t interacting, the researchers saw the expected result: the particles heating up a little before reaching a constant temperature. Then, when they adjusted the experiment so that the atoms could interact a little, they still saw the temperature plateau at the same level. But unlike in the one dimensional theory, the atoms eventually started heating up again—although not as quickly as predicted by normal thermodynamics. When they increased the level of interactions, the temperature plateaued for a shorter time.

While Galitski’s one-dimensional theory doesn’t describe the exact experiment performed, another theory seems to have some luck explaining the sluggish heating that follows the plateaus. That theory applies to very cold groups of particles that have formed a Bose Einstein condensate, a phase of matter where all the particles share the same quantum state. The equations that describe Bose-Einstein condensates can predict the rate of the slow heating—despite that very heating meaning that the atoms shouldn’t be describable as a Bose Einstein condensate.

“So, in some sense, it's a double mystery,” Galitski says. “We actually don't know why it goes this way, but there is a theory which is not supposed to work but kind of works.”

The observed plateaus prove that interactions don’t always force particles to bow to the decrees of thermodynamics. Efforts to push experiments to test the predictions for particles constrained to one dimension and to push the theory to explain the three-dimensional experiments might not only reveal new quantum physics but could also lead to the development of new research tools. If the physics behind these experiments can be untangled, perhaps the plateaus will one day be extended and can be used to design new and better quantum technologies.

“Mysteries are always good because they lead oftentimes to new discoveries,” Galitski says. “What would be nice is to see whether you can stabilize the dynamic localization—this plateau—under some protocols and conditions. That's what they're working on. And it's important because it would preserve quantum information.”


In addition to Galitski and Weld, former UCSB physics student Alec Cao; UCSB graduate students Roshan Sajjad, Ethan Q. Simmons, Jeremy L. Tanlimco and Eber Nolasco-Martinez; former UCSB postdoctoral researcher Hector Mas; and UCSB postdoctoral researchers Toshihiko Shimasaki and H. Esat Kondakci were also co-authors of the paper.