Plasma Guides Maintain Laser Focus

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Category: Research News

Published: Friday, August 14 2020 08:17

In science fiction, firing powerful lasers looks easy—the Death Star can just send destructive power hurtling through space as a tight beam. But in reality, once a powerful laser has been fired, care must be taken to ensure it doesn’t get spread too thin.

If you’ve ever pointed a flashlight at a wall, then you’ve observed a more mundane example of this diffusion of light. The farther you are from the wall, the more the beam spreads, resulting in a larger and dimmer spot of light. Lasers generally expand much more slowly than a flashlight, but the effect is important when the laser travels a long way or must maintain a high intensity.

Whether your goal is blowing up a planet to achieve galactic domination or, more realistically, accelerating electrons to incredible speeds for physics research, you’ll want as tight and powerful a beam as possible to maximize the intensity. For terrestrial experiments, researchers can use devices called waveguides, like the optical fibers that might be carrying internet throughout your neighborhood, to transport a laser while keeping it contained to a narrow beam. The distinct core and outer shell—or cladding—of a waveguide keep the laser from spreading out. But if the laser pulse is too intense you run into a problem—it will destroy an optical fiber in a thousandth of a nanosecond.

Lasers are used to create an indestructible optical fiber out of plasma that helps researchers confine a separate laser pulse as it travels through the plasma. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)

Researchers at the University of Maryland, led by UMD Physics Professor Howard Milchberg, have developed an improved technique to make waveguides that can withstand the power of intense lasers. In a paper published on August 14, 2020 in Physical Review Letters, they demonstrated how powerful pulses can be transmitted along a waveguide that is created by firing weaker laser pulses into a cloud of hydrogen. They predict that the technique, developed with support from the US Department of Energy High Energy Physics program and the National Science Foundation, will be a powerful tool in high-energy particle acceleration experiments.

“A plasma waveguide can be a powerful tool for a variety of fields,” says Bo Miao, a co-author of the paper and UMD physics postdoctoral associate. “I’m excited that the experiment finally worked out after two years of hard work of alternating delight and frustration.”

Their technique relies on building a waveguide from a plasma—a gas where the electrons have been torn from the nuclei of the atoms.

“A plasma waveguide has all the structure of an optical fiber, the classic core, the classic cladding,” says Milchberg. “Although in this case, it's indestructible. The hydrogen plasma forming the waveguide is already ripped up into its protons and electrons, so there's not much more violence you can do to it.”

In the early 1990s, Milchberg and colleagues developed a related technique to use lasers to create plasma waveguides for other, more intense, lasers. In this earlier technique a laser beam is sent into a gas; as it travels, it rips electrons from their atoms along the beam, creating a plasma tunnel that is warmer than the surrounding gas. Due to its heat, the plasma expands, forming a low-density plasma core surrounded by a high-density wall formed by the shockwave from the plasma’s rush outward.

This structure is precisely what is needed for a waveguide, but the method has a pitfall—researchers can’t craft the core and wall independently. To get the wall to have the necessary thickness and density of electrical charges to function as a waveguide, required the core to be kept too dense for particle acceleration applications.

In the new paper, the team demonstrates an improved method that lets them craft the wall and core independently. Their insight was to use two specialized laser beams—called Bessel beams—to craft the waveguide. The first laser is a simple Bessel beam that forms the low-density core while causing less heating than the previous method.

Caption: On the left is a cross section of the intensity of the Bessel beam responsible for creating the low-density plasma core. On the right is a cross section of the intensity of the Bessel beam that creates the high-density plasma wall. The left image is 50 micrometers across and the right image is 100 micrometers across. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)But the second laser beam is more exotic. It is a hollow tube of light that allows them to build the wall of the waveguide by creating additional plasma from the gas surrounding the plasma core. Since the second laser pulse can match the shape of the high-density wall, they can tailor it without impacting the conditions of the core.

“Basically, the version of the technique that was used up until our paper is very constrained in the size of the guide, the length and other parameters,” says Linus Feder, a co-author of the paper and a UMD physics graduate student. “This new technique is highly adaptable and tunable. It just does away with a lot of the restrictions on the types of laser beams you can guide.”

The researchers demonstrated that the improvement allowed them to guide a laser for 30 centimeters in a tight beam—about 50% farther than previous experiments that used wider, 20-centimeter plasma waveguides created with a different technique.

Milchberg says their waveguide is like a long hypodermic needle and that the older method was more like a drinking straw. With the smaller guide, the laser’s energy is packed into a much smaller area, resulting in a much higher intensity.

“The only reason we were limited to 30 centimeters was lab geometry and not having enough laser energy,” says Milchberg. “But with more laser energy, there's no obstacle to us doing this for a couple of meters.

The new method may increase the practicality of using plasma waveguiding of intense laser pulses to accelerate charged particles for high energy physics experiments. The group is planning experiments to confirm their predictions of how the process will work with more powerful lasers.

The paper was selected as an editors’ suggestion and was highlighted in Physical Review Focus.

Story by Bailey Bedford: This email address is being protected from spambots. You need JavaScript enabled to view it.

In addition to Milchberg, Feder, and Miao, graduate students Andrew Goffin and Jaron Shrock were co-authors.
This research was supported by US Department of Energy (DESC0015516) and the National Science Foundation (PHY1619582).

RESEARCH CONTACTS
Bo Miao: This email address is being protected from spambots. You need JavaScript enabled to view it.
Linus Feder:This email address is being protected from spambots. You need JavaScript enabled to view it.

Ott Elected Foreign Member of the Academia Europaea

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Category: Department News

Published: Friday, August 07 2020 01:53

Professor Ed Ott has been elected a foreign member of the Academia Europaea for his outstanding achievements and international scholarship as a researcher. He is globally known for his pioneering contributions in nonlinear dynamics and chaos theory.

Ott is a University of Maryland Distinguished University Professor and holder of the Yuen Sang and Yu Yuen Kit So Endowed Professorship in nonlinear dynamics. He received the 2014 Julius Edgar Lilienfeld Prize of the American Physical Society, and in 2017, the Lewis Fry Richardson Medal of the European Geosciences Union. Also in 2017, he was selected for the Jürgen Moser Lecture, sponsored by the Society for Industrial and Applied Mathematics (SIAM).  

Ott is a Fellow of the Society for Industrial and Applied Mathematics, the American Physical Society and the Institute of Electrical and Electronics Engineers. He received his B.S. in Electrical Engineering at The Cooper Union and his M.S. and Ph.D. in Electrophysics from Polytechnic Institute, then enjoyed a postdoctoral fellowship at the Department of Applied Mathematics and Theoretical Physics of Cambridge University. Returning stateside, he joined the Electrical Engineering faculty at Cornell. He left Ithaca in 1979 to join the Department of Physics and Department of Electrical Engineering on this campus. He is a member of the Institute for Research in Electronics and Applied Physics (IREAP), and has held appointments at the Naval Research Lab and what is now the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara.

The Academia Europaea fosters excellence in scholarship in the humanities, law, social and hard sciences.  

Lila Snow, 1927 - 2020

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Category: Department News

Published: Friday, August 07 2020 01:33

Lila Snow, a noted local artist and television host, died on July 13. She was the wife of George Snow, a UMD professor of high energy physics from 1958 to 1992.

Lila, a native New Yorker, earned a degree in chemistry from Brooklyn College and studied art at the Corcoran School, American University and the University of Maryland.

In 1972, Lila and George co-taught the first Women’s Studies course on this campus. After her husband’s death in 2000, Lila established the George A. Snow Memorial Award, to acknowledge the paucity of women in the field of physics and encourage greater participation. The award has highlighted exceptional efforts within the department, including outreach, mentoring and innovation.

In 2016, Lila donated two original artworks, Particle Picture and Scienza, for display near the high energy group’s offices in the Physical Sciences Complex.

Other creations grace the permanent collections of Radford University, the American Association for the Advancement of Science, the American University Museum and the Philadelphia Museum of Judaica, as well as locations in Argentina, Italy and Japan.

She hosted The Art Scene on Montgomery Municipal Cable Television for two decades.

The sculpture Bradford near the UMD chemistry building, created by Lila Katzen, was donated to the UMD campus by George and Lila Snow.

In addition to art, Lila was interested in languages, and studied in Geneva, Paris, Rome, Bologna and Sendai. As a child, she was a double dutch jump rope champion; as an adult, a standup comedian at varied venues including physics conferences, nightclubs and the National Museum of Women in the Arts.  She penned a memoir, With A Name Like Tuchmacher..., and in 2003 received a Lifetime Achievement Award from Brooklyn College.

Lila is survived by Zachary Snow, of Rhinebeck, N.Y.; Andrew Snow of Chevy Chase, and Sara Snow of Vancouver, British Columbia; eight grandchildren; and three great-grandchildren.

Gifts in support of Washington artists can be made in her memory to the American University Museum. Please note “In memory of Lila Snow” in the correspondence.

On the third floor of the PSC hang two donated works by Lila Snow. The larger, Particle Painting (oil on canvas) was created in 2015. The smaller, Scienza, commemorates Maryland’s contribution to the  collaborative efforts at CERN. It is a collage of street posters from Bologna added to an Iarocci tube, which is a component of many sorts of detectors.

Lila Snow and Mildred Goldberger in 1975 at The Princeton Inn, where Lila was a comedian.

Chih-Li Tu, C.N. Yang, Lila and George Snow in 1997.

Scientists See Train of Photons in a New Light

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Category: Research News

Published: Tuesday, August 04 2020 05:33

Flashlight beams don’t clash together like lightsabers because individual units of light—photons—generally don’t interact with each other. Two beams don’t even flicker when they cross paths.

But by using matter as an intermediary, scientists have unlocked a rich world of photon interactions. In these early days of exploring the resulting possibilities, researchers are tackling topics like producing indistinguishable single photons and investigating how even just three photons form into basic molecules of light. The ability to harness these exotic behaviors of light is expected to lead to advances in areas such as quantum computing and precision measurement.

In a paper recently published in Physical Review Research, Adjunct Associate Professor Alexey Gorshkov, Joint Quantum Institute (JQI) postdoctoral researcher Przemyslaw Bienias, and their colleagues describe an experiment that investigates how to extract a train of single photons from a laser packed with many photons.

In the experiment, the researchers examined how photons in a laser beam can interact through atomic intermediaries so that most photons are dissipated—scattered out of the beam—and only a single photon is transmitted at a time. They also developed an improved model that makes better predictions for more intense levels of light than previous research focused on (greater intensity is expected to be required for practical applications). The new results reveal details about the work to be done to conquer the complexities of interacting photons.A cloud of Rydberg atoms can scatter most light to whittle a laser down to train of individual photons. But photons can get re-absorbed within the larger control beam making things more complicated. (Credit: Przemyslaw Bienias, University of Maryland)

“Until recently, it was basically too difficult to study anything other than a few of these interacting photons because even when we have two or three things get extremely complicated,” says Gorshkov, whi is also a physicist at the National Institute of Standards and Technology and Fellow of the Joint Center for Quantum Information and Computer Science. “The hope with this experiment was that dissipation would somehow simplify the problem, and it sort of did.”

Trains, Blockades and Water Slides

To create the interactions, the researchers needed atoms that are sensitive to the electromagnetic influence of individual photons. Counterintuitively, the right tool for the job is a cloud of electrically neutral atoms. But not just any neutral atoms; these specific atoms—known as Rydberg atoms—have an electron with so much energy that it stays far from the center of the atom.

The atoms become photon intermediaries when these electrons are pushed to their extreme, remaining just barely tethered to the atom. With the lone, negatively charged electron so far out, the central electrons and protons are left contributing a counterbalancing positive charge. And when stretched out, these opposite charges make the atom sensitive to the influence of passing photons and other atoms. In the experiment, the interactions between these sensitive atoms and photons is tailored to turn a laser beam that is packed with photons into a well-spaced train.

The cloud of Rydberg atoms is kind of like a lifeguard at a water park. Instead of children rushing down a slide dangerously close together, only one is allowed to pass at a time. The lifeguard ensures the kids go down the slide as a steady, evenly spaced train and not in a crowded rush.

Unlike a lifeguard, the Rydberg atoms can’t keep the photons waiting in line. Instead they let one through and turn away the rest for a while. The interactions in the cloud of atoms form a blockade around each transmitted photon that scatters other photons aside, ensuring its solitary journey.

To achieve the effect, the researchers used Rydberg atoms and a pair of lasers to orchestrate a quantum mechanical balancing act. They selected the frequency of the first laser so that its photons would be absorbed by the atoms and scattered in a new direction. But this is the laser that is whittled down into the photon train, and they needed a way to let individual photons through.

That’s were the second laser comes in. It creates another possible photon absorption that quantum mechanically interferes with the first and allows a single photon to pass unabsorbed. When that single photon gets through, it disturbs the state of the nearby atoms, upsetting the delicate balance achieved with the two lasers and blocking the passage of any photons crowding too closely behind.

Ideally, if this process is efficient and the stream of photons is steady enough, it should produce a stream of individual photons each following just behind the blockade of the previous. But if the laser is not intense enough, it is like a slow day at the waterpark, when there is not always a kid eagerly awaiting their turn. In the new experiment, the researchers focused on what happens when they crowed many photons into the beam.

Model (Photon) Trains

Gorshkov and Bienias’s colleagues performed the experiment, and the team compared their results to two previous models of the blockade effect. Their measurements of the transmitted light matched the models when the number of photons was low, but as the researchers pushed the intensity to higher levels, the results and the models’ predictions started looking very different. It looked like something was building up over time and interfering with the predicted, desired formation of well-defined photon trains.

The team determined that the models failed to account for an important detail: the knock-on effects of the scattered photons. Just because those photons weren’t transmitted, doesn’t mean they could be ignored. The team suspected the models were being thrown off by some of the scattered light interacting with Rydberg atoms outside of the laser beam. These additional interactions would put the atoms into new states, which the scientists call pollutants, that would interfere with the efficient creation of a single photon train.

The researchers modified one of their models to capture the important effects of the pollutants without keeping track of every interaction in the larger cloud of atoms. While this simplified model is called a “toy model,” it is really a practical tool that will help researchers push the technique to greater heights in their larger effort to understand photon interactions. The model helped the researchers explain the behavior of the transmitted light that the older models failed to capture. It also provides a useful way to think about the physics that is preventing an ideal single photon train and might be useful in judging how effectively future experiments prevent the undesirable affects—perhaps by using cloud of atoms with different shapes.

“We are quite optimistic when it comes to removing the pollutants or trying to create less of them,” says Bienias. “It will be more experimentally challenging, but we believe it is possible.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/scientists-see-train-photons-new-light

In addition to Bienias and Gorshkov, James Douglas, a Co-founder at MEETOPTICS; Asaf Paris-Mandoki, a physics researcher at Instituto de Física, Universidad Nacional Autónoma de México; JQI postdoctoral researcher Paraj Titum; Ivan Mirgorodskiy; Christoph Tresp, a research and development employee at TOPTICA Photonics; Emil Zeuthen, a physics professor at the Niels Bohr Institute; Michael J. Gullans, a former JQI postdoctoral researcher and current associate scholar at Princeton University; Marco Manzoni, a data scientist at Big Blue Analytics; Sebastian Hofferberth, a professor of physics at the University of Southern Denmark; and Darrick Chang, a professor at the Institut de Ciencies Fotoniques, were also co-authors of the paper.