Manipulating the magnetic response to light in natural materials

When light moves from one material into another, it bends — like how a partially submerged object appears distorted under water when viewed from above. What if, instead of bending, a material could change the light so much that the material was no longer visible at all?

In a study published in Physical Review A, University of Wisconsin­–Madison researchers have shown for the first time that a similar response can be obtained and manipulated in naturally-occurring materials. The findings have implications from the development of “perfect” lenses for improved microscopy to Harry Potter-esque invisibility cloaks.

Visible light is made of both magnetic and electric fields, and the refractive index of a material — how much it bends the light — is determined by how the material interacts with those two fields. Nearly all materials we encounter in everyday life, though, interact entirely with light’s electric field.

profile photo of Zach Buckholtz
Zach Buckholtz

Researchers have spent the past two decades developing artificial materials that more strongly interact with light’s magnetic field by manipulating the refractive index. With a strong enough response, the material could eventually have a negative refractive index, leading to unique optical properties. However, the response in synthetic materials is limited by the size of their repeating units. A naturally-occurring crystal that has much smaller unit cells is likely a better choice.

“Part of producing a negative refractive index is that the material needs to have a strong response to both electric and magnetic fields, so the big challenge is getting that magnetic response in natural materials,” explains Zach Buckholtz, a graduate student in UW–Madison physics professor Deniz Yavuz’s group and lead author of the study. “A few years ago, we published a paper showing that the crystal we’re working with has a magnetic response, and in this study, we were able to manipulate the response.”

The natural material Buckholtz is working with is a silicon-based crystal, which in general is optically ordinary, except that it has been “doped” with the rare earth metal Europium. Rare earth metals are unique in that they contain an abundance of electrons in the atoms’ outer energy shells. Those electrons can then work together to create a bigger magnetic response, but only if they are all in tune with each other.

“If you have some magnetic response and a much larger electric response to light, you can connect those two responses,” Buckholtz says. “To get to a negative refractive index from there, you have to set up coherences between the energy levels, meaning you have to make sure all those energy levels are oscillating together.”

the apparatus hosting the rare earth metal doped crystal glows orange, and the background of the photo has a green fluorescence hue from the input laser
The experimental setup in the Yavuz lab. The orange glow is from the fluorescing crystal, with the green laser providing the green hue outside the chamber.

To show they can manipulate the magnetic response, Buckholtz and Yavuz did two things. First, because the crystal is a mix of ions with slightly different electron responses, they needed to set up their experimental system to select for one class of ion. This uniformity allows for a cleaner interpretation of the results.

“We send a laser into the crystal, and then measure how much of the light is transmitted. But because the crystal isn’t perfect, instead of seeing a narrow peak for the transitions, you’ll see a really broad transition,” Buckholtz explains. “So, we do this procedure known as spectral hole burning to clear out the ions we don’t want and then we’ll be left with just one transition, which is necessary to move on to experiments that involve coherence.”

Next, they wanted to show if they could increase the magnetic response. To do so, they needed to take those selected ions, put them in coherence, and then measure the response compared to ions not in coherence. In these experiments, they shined one (a probe beam) or two (probe and coupling beam) wavelengths of laser at the ions. Both lasers excite electrons in the ions to a higher-energy state, and the scientists can again measure how might light is transmitted through the crystal as a readout of the electron transitions.

“With just the probe beam, we see just the normal transition, and that’s what we did in our previous study. But with the coupling beam added in, it connects and adds another transition state in there,” Buckholtz says. “If those states are in coherence, they cancel each other out, and we see that effect as a peak in transmitted light, which means the index of refraction is going toward zero.”

Buckholtz notes that the magnetic response they see is not yet large enough to produce the materials with interesting new optical properties they are hoping for. Still, he says, this work provides a path forward to continue manipulations to improve the response, such as investigating different rare earth metals.

“We have a magnetic response, we can set up coherence, and we can manipulate the response,” Buckholtz says. “Now, we want to increase the scale of the response to with a goal of eventually making the refractive index below zero.”

The Milky Way’s satellites help reveal link between dark matter halos and galaxy formation

This story was adapted from one originally published by Nathan Collins at SLAC. Read the original version here.

Just as the sun has planets and the planets have moons, our galaxy has satellite galaxies, and some of those might have smaller satellite galaxies of their own. To wit, the Large Magellanic Cloud (LMC), a relatively large satellite galaxy visible from the southern hemisphere, is thought to have brought at least six of its own satellite galaxies with it when it first approached the Milky Way, based on recent measurements from the European Space Agency’s Gaia mission.

Astrophysicists believe that dark matter is responsible for much of that structure, and now researchers with the Dark Energy Survey — including University of Wisconsin­–Madison assistant professor of physics Keith Bechtol and his research group — have drawn on observations of faint galaxies around the Milky Way to place tighter constraints on the connection between the size and structure of galaxies and the dark matter halos that surround them. At the same time, they have found more evidence for the existence of LMC satellite galaxies and made a new prediction: If the scientists’ models are correct, the Milky Way should have an additional 150 or more very faint satellite galaxies awaiting discovery by next-generation projects such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time.

profile photo of keith bechtol
Prof. Keith Bechtol

Two new studies, forthcoming in the Astrophysical Journal and available as preprints (pre-print 1; pre-print 2), are part of a larger effort to understand how dark matter works on scales smaller than our galaxy.

“The ultra-faint galaxies that orbit the Milky Way are small clouds of dark matter with just enough stars to see that they exist. They are nearly invisible, but if spotted, they make excellent natural laboratories to study dark matter,” Bechtol says. “We hope to learn what dark matter is made of, how it was produced in the early Universe, and what relationship it has to the known particle species.”

Shining galaxies’ light on dark matter

Astronomers have long known the Milky Way has satellite galaxies, including the Large Magellanic Cloud, which can be seen by the naked eye from the southern hemisphere, but the number was thought to be around just a dozen or so until around the year 2000. Since then, the number of observed satellite galaxies has risen dramatically. Thanks to the Sloan Digital Sky Survey and more recent discoveries by projects including the Dark Energy Survey (DES), the number of known satellite galaxies has climbed to about 60.

Such discoveries are always exciting, but what’s perhaps most exciting is what the data could tell us about the cosmos. “For the first time, we can look for these satellite galaxies across about three-quarters of the sky, and that’s really important to several different ways of learning about dark matter and galaxy formation,” said Risa Wechsler, director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). Last year, for example, the DES team used data on satellite galaxies in conjunction with computer simulations to place much tighter limits on dark matter’s interactions with ordinary matter.

Now, the team is using data from a comprehensive search over most of the sky to ask different questions, including how much dark matter it takes to form a galaxy, how many satellite galaxies we should expect to find around the Milky Way and whether galaxies can bring their own satellites into orbit around our own – a key prediction of the most popular model of dark matter.

Hints of galactic hierarchy

The answer to that last question appears to be a resounding “yes.”

The possibility of detecting a hierarchy of satellite galaxies first arose some years back when DES detected more satellite galaxies in the vicinity of the Large Magellanic Cloud than they would have expected if those satellites were randomly distributed throughout the sky. More data was needed to conclusively attribute this excess to galaxies that arrived at the Milky Way with the Large Magellanic Cloud.

map of observational areas from the DES and Pan-STARRS survey shows which regions of the sky were covered in this analysis. Areas darkened out were not covered, white areas were where data was collected and from where it was analyzed
The two studies probed, analyzed and characterized data from years of observation in two sky surveys.

In the first published study, the DES group combined observations from DES with those from the Pan-STARRS survey, together covering 75% of the sky, to test this hypothesis. The DES data represents nearly 40,000 images from a 500-million-pixel camera collected over three years from a telescope in Chile.

The raw DES data was run through a series of data compressions, including a final step led by Bechtol’s group, to identify and catalog individual stars, including their color, which infers temperature, and how far away they are.

“We throw the star catalog into our search algorithms, which are responsible for identifying small groups of stars that are clustered in space and have similar colors and brightness. There’s a particular distribution for what we expect the stars to look like in ultrafaint galaxies,” says UW-Madison physics graduate student Mitch McNanna. “Even then we’re not 100 percent sure that we’ve found a real galaxy, so we also collect spectroscopic observations to measure the doppler motion of the stars. Hopefully we see the group of stars is moving in a way that’s different from the rest of the stars in the Milky Way halo.”

The team, including Alex Drlica-Wagner at Fermilab, produced a model of which satellite galaxies are most likely to be seen by current surveys, given where they are in the sky as well as their brightness, size and distance.

In the second study, led by others in the DES team including Ethan Nadler at Stanford University and collaborators, the team took the findings of the latest satellite census and analyzed computer simulations of millions of possible universes. Those simulations model the formation of dark matter structure that permeates the Milky Way, including details such as smaller dark matter clumps within the Milky Way that are expected to host satellite galaxies. To connect dark matter to galaxy formation, the researchers used a flexible model that allows them to account for uncertainties in the current understanding of galaxy formation, including the relationship between galaxies’ brightness and the mass of dark matter clumps within which they form.

Those components in hand, the team ran their model with a wide range of parameters and searched for simulations in which LMC-like objects fell into the gravitational pull of a Milky Way-like galaxy. By comparing those cases with galactic observations, they could infer a range of astrophysical parameters, including how many satellite galaxies should have tagged along with the LMC. The results were consistent with Gaia observations: Six satellite galaxies should currently be detected in the vicinity of the LMC, moving with roughly the right velocities and in roughly the same places as astronomers had previously observed. The simulations also suggested that the LMC first approached the Milky Way about 2.2 billion years ago, consistent with high-precision measurements of the motion of the LMC from the Hubble Space Telescope.

Galaxies yet unseen

In addition to the LMC findings, the team also put limits on the connection between dark matter halos and galaxy structure. For example, in simulations that most closely matched the history of the Milky Way and the LMC, the smallest galaxies astronomers could currently observe should have stars with a combined mass of around a hundred suns, and about a million times as much dark matter. According to an extrapolation of the model, the faintest galaxies that could ever be observed could form in halos up to a hundred times less massive than that.

And there could be more discoveries to come: If the simulations are correct, there are around 150 more satellite galaxies – more than double the number already discovered – hovering around the Milky Way. The discovery of those galaxies would help confirm the researchers’ model of the links between dark matter and galaxy formation, and likely place tighter constraints on the nature of dark matter itself.

Shared experiences: Conference for women in physics brings UW undergrads together

group photo in front of an astronomy and galaxy-themed backdrop

UW–Madison sophomore Haley Stueber did not always know she wanted to study physics, but she had an inkling.

“I started taking astronomy and physics classes in high school, and what really got me into physics was the astronomy aspect,” Stueber says. “I was always of the mindset that I wanted to keep learning, and I felt like the realm where I could do that was space, because the universe is so big.”

Like most women interested in pursuing a physics major, Stueber noticed something when she started college.

a small lecture hall with only women undergraduate students
Unlike most of their physics lectures, the undergrads who attend CUWiP sessions are surrounded by only women and gender minorities.

“All of my physics courses are predominantly male,” she says. “It was intimidating at first. I’ve definitely gotten more used to it, but it still just kinda sucks looking around the room and being like, ‘Alright, there’s one woman over there, one in that corner, and me.’” She notes that the male majority in her classes has not been largely problematic, but it would be nice to have more of a female presence.

Stueber’s experiences are similar to those of many women physics and physical science majors. According to Joelle Corrigan, a physics graduate student and president of GMaWiP, a UW–Madison organization for Gender Minorities and Women in Physics, only around 20 percent of undergraduate physics majors are women.

In an effort to support and retain women in physics, the American Physical Society hosts the Conference for Undergraduate Women in Physics (CUWiP). This year, 10 UW–Madison undergraduates, along with Corrigan and physics grad student Abigail Shearrow, attended the Midwest regional CUWiP, held January 17-19 and hosted by the University of Minnesota–Twin Cities.

“It is a very rare, empowering experience to be in a packed auditorium filled with women and gender minorities all excited about physics,” Corrigan says of the annual conference, which she first attended as an undergraduate. “They have many talks from amazing female scientists, sharing their work and providing role models to many students who may not have seen successful females in that role previously.”

Stueber concurs with Corrigan that it was helpful to see and meet women in the research labs they toured or heard speak during the conference — every presenter was a women scientist. Katy Jurgella, a junior astrophysics and geology major who is only now taking her first physical science course with a female professor this spring, agrees.

“Every presenter who talked about their research also gave an overview of their life story. If you see just their research, you’re like, ‘Oh, wow, this woman has a PhD in astrophysics, I’ll never get there,’” Jurgella says. “But then they mention they were born on a farm and I was like, ‘I was born on a farm, too!’ It was inspiring to me.”

women from many Midwest schools stand behind their schools' banners on a stair case where the conference was held
Students from all over the Midwest attended the regional CUWiP, hosted by the University of Minnesota–Twin Cities.

While one focus of the conference was on research, an equal emphasis was given to professional development, including topics that often strongly apply to women.

Junior AMEP major Gabby Every says, “I went to breakout sessions this year on imposter syndrome, negotiation techniques for women specifically, well-being, work-life balance, and one on grad school. It was a catch-all of issues faced by women specifically.”

Anna Gerosolina, a junior astrophysics and chemistry major who currently has no plans to attend graduate school, says the professional development sessions were very helpful because they did not solely focus on women in academia issues.

“There was one talk about being a woman in the workforce in general, and how you need to be a little more aggressive. But it’s a hard balance because a lot of times we come across as bossy even though it comes across as great when guys are aggressive,” Gerosolina says. “That really stuck with me. It was basically, just stop apologizing for existing. And I didn’t even realize how much I did that.”

UW undergrad women at a table during the conference dinner
UW women physical sciences undergraduates enjoy a conference meal together.

The Midwest regional CUWiP was held January 17-19, just before the spring semester began. The students who attended have already noticed a difference in how they approach their courses, professors, and classmates.

“Even two weeks into the semester, I’ve noticed I’m better at asking questions in class because I’m less afraid,” says Jurgella. “At the conference, they stressed, ‘Don’t be embarrassed if you don’t know something, because no one knows anything!’ It’s helped me remain humble, but I’m also less embarrassed now to ask about something I don’t know.”

All the women spoke of the support they now have from their fellow attendees, such as studying together, working together on projects, and just sharing experiences as women in the physical sciences.

“The conference is a great environment because sometimes I forget how reserved I can be in a room full of dominant male voices,” says Every. “Once you’re surrounded by all these women who feel the same way and have gone through similar things, you come out of your shell and talk about things that really matter to you.”

Adds Gerosolina, “These are women I can study with and not be mansplained about how to do basic physics. We even have a Snapchat group chat now!”

Baha Balantekin elected APS Speaker of the Council

Profile photo of Baha BalantekinBaha Balantekin, the Eugene P. Wigner Professor of Physics at the University of Wisconsin–Madison, has been elected Speaker of the Council for the American Physics Society (APS). He will serve as Speaker-elect in 2020, Speaker in 2021, and Speaker Emeritus in 2022.

The APS Speaker of the Council presides over the Council, a body of elected leadership within the professional society. The Speaker also serves on the APS Board of Directors as well as presiding over the Council’s Steering Committee.

“It is an honor to be elected, for me and for the UW,” Balantekin says. “Speaker of the Council is another public face of APS besides the Presidential line.”

APS is the professional society of not only physicists in the United States, but also has a worldwide membership. According to the mission statement, APS exists to advance and diffuse the knowledge of physics for the benefit of humanity, to promote physics, and to serve the broader physics community. APS relies on volunteers to serve in leadership positions, such as Speaker of the Council, to advance its mission.

“Having Prof. Balantekin in the leadership role in the American Physics Society is a matter of pride for our department, and we are happy to share his leadership skills with the wider physics community,” says Sridhara Dasu, chair of the UW–Madison department of physics.

Balantekin was elected to the role at the annual election meeting of the APS Board and Council, held in early November 2019. He has been a Fellow of APS since 1994. He is currently completing his second year on the Council of Representatives and his first year on the Board of Directors, to which he was elected last year.

Wesley Smith honored for achievements in particle physics

The American Physical Society (APS) announced today, Oct 22, that Wesley Smith, a University of Wisconsin–Madison Professor Emeritus of Physics and former Bjorn Wiik Professor of Physics, has earned the 2020 W.K.H. Panofsky Prize in Experimental Particle Physics.

The Panofsky Prize recognizes “outstanding achievements in experimental particle physics,” and is the top APS award in that field.

Smith developed systems that enabled the discovery of the Higgs boson, a previously elusive particle believed to give mass to all matter. Smith led a team of over 100 scientists on the CMS experiment trigger system that captured the data for the Higgs’ discovery at the Large Hadron Collider (LHC) in Geneva.

“In the experiment at the LHC, proton beams collide 40 million times per second, and each time the beams cross, detectors record a snapshot. That’s over a megabyte of data, 40 million times per second. You can’t store it all,” Smith explains. “The problem is, one in 10 trillion of those collisions actually has a Higgs in it, and you don’t want to lose any of them. So how do you do this?”

To sift through the enormous amount of data, Smith and his team developed a triggering system. The two-step mechanism quickly filters through the first set of data, using high speed electronics to take one billion collisions per second and identify interesting patterns in up to 100,000 of them. Then, the second step uses thousands of computing nodes to filter the data down to 1,000 collisions per second — a reasonable amount of data that can be stored.

Wesley Smith holds a large electronics board full of circuits and wires
Professor Wesley Smith shows the electronics of the trigger system which led to the discovery of the Higgs Boson. Smith led the team that designed and developed the trigger system.

“We designed a large amount of electronics that runs incredibly fast, and it had to be programmable and flexible because we didn’t know what we were going to find,” Smith says. “We’re basically throwing out 99.9999% of the data, and keeping a tiny fraction where all the physics has to come out.”

Smith, who had previously led a team charged with a similar task for the Zeus experiment in Hamburg, Germany, was asked to lead the CMS triggering team in 1993. The Higgs was discovered in June 2012.

Even though a major goal of the CMS experiment was realized, physicists have much left to learn about the Higgs, which means studying more Higgs events. Continuing plans for the experiment, set to go online in the mid-2020s, involve increasing the amount of proton collisions by a factor of 10, resulting in 10 times more data per second. As Smith was finishing his career, he worked on the initial prototyping for an even more advanced triggering system to filter through larger data sets.

“This award means a great deal to me because it’s the recognition of my colleagues, of the team of people who contributed, and because it recognizes this particularly challenging area of detector development and particle physics experimentation which had to be solved in order to do physics at the LHC,” Smith says.

UW–Madison physics department chairperson Sridhara Dasu, who trained with Smith before beginning his faculty position, nominated Smith for the award.

“Professor Smith is recognized as the world-leading expert in the design, construction and operations of the trigger electronics system for hadron colliders,” Dasu says. “The trigger system is at the very heart of particle physics experiments, requiring the very best talent. Professor Smith is the leader in training those best experimenters.”