Nearly 50-meter Laser Experiment Sets Record in Campus Hallway

It's not at every university that laser pulses powerful enough to burn paper and skin are sent blazing down a hallway. But that’s what happened in UMD’s Energy Research Facility, an unremarkable looking building on the northeast corner of campus. If you visit the utilitarian white and gray hall now, it seems like any other university hall—as long as you don’t peek behind a cork board and spot the metal plate covering a hole in the wall.A laser is sent down a UMD hallway in an experiment to corral light as it makes a 45-meters-long journey.A laser is sent down a UMD hallway in an experiment to corral light as it makes a 45-meters-long journey.

But for a handful of nights in 2021, UMD Physics Professor Howard Milchberg and his colleagues transformed the hallway into a laboratory: The shiny surfaces of the doors and a water fountain were covered to avoid potentially blinding reflections; connecting hallways were blocked off with signs, caution tape and special laser-absorbing black curtains; and scientific equipment and cables inhabited normally open walking space.

As members of the team went about their work, a snapping sound warned of the dangerously powerful path the laser blazed down the hall. Sometimes the beam’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tang. Each night, a researcher sat alone at a computer in the adjacent lab with a walkie-talkie and performed requested adjustments to the laser.

Their efforts were to temporarily transfigure thin air into a fiber optic cable—or, more specifically, an air waveguide—that would guide light for tens of meters. Like one of the fiber optic internet cables that provide efficient highways for streams of optical data, an air waveguide prescribes a path for light. These air waveguides have many potential applications related to collecting or transmitting light, such as detecting light emitted by atmospheric pollution, long-range laser communication or even laser weaponry. With an air waveguide, there is no need to unspool solid cable and be concerned with the constraints of gravity; instead, the cable rapidly forms unsupported in the air. In a paper accepted for publication in the journal Physical Review XPhysical Review X the team described how they set a record by guiding light in 45-meter-long air waveguides and explained the physics behind their method.

The researchers conducted their record-setting atmospheric alchemy at night to avoid inconveniencing (or zapping) colleagues or unsuspecting students during the workday. They had to get their safety procedures approved before they could repurpose the hallway.

“It was a really unique experience,” says Andrew Goffin, a UMD electrical and computer engineering graduate student who worked on the project and is a lead author on the resulting journal article. “There's a lot of work that goes into shooting lasers outside the lab that you don't have to deal with when you're in the lab—like putting up curtains for eye safety. It was definitely tiring.”

 Left to right Eric Rosenthal, a physicist at the U.S. Naval Research Laboratory; Anthony Valenzuela, a physicist at the U.S. Army Research Lab; and Goffin align optics at a porthole in the wall in order to send the laser beam from the lab down the hallway. The white dotted lines show the approximate beam path before and after the optics redirected it. Left to right Eric Rosenthal, a physicist at the U.S. Naval Research Laboratory; Anthony Valenzuela, a physicist at the U.S. Army Research Lab; and Goffin align optics at a porthole in the wall in order to send the laser beam from the lab down the hallway. The white dotted lines show the approximate beam path before and after the optics redirected it. All the work was to see to what lengths they could push the technique. Previously Milchberg’s lab demonstrated that a similar method worked for distances of less than a meter. But the researchers hit a roadblock in extending their experiments to tens of meters: Their lab is too small and moving the laser is impractical. Thus, a hole in the wall and a hallway becoming lab space.

“There were major challenges: the huge scale-up to 50 meters forced us to reconsider the fundamental physics of air waveguide generation, plus wanting to send a high-power laser down a 50-meter-long public hallway naturally triggers major safety issues,” Milchberg says. “Fortunately, we got excellent cooperation from both the physics and from the Maryland environmental safety office!”

Without fiber optic cables or waveguides, a light beam—whether from a laser or a flashlight—will continuously expand as it travels. If allowed to spread unchecked, a beam’s intensity can drop to un-useful levels. Whether you are trying to recreate a science fiction laser blaster or to detect pollutant levels in the atmosphere by pumping them full of energy with a laser and capturing the released light, it pays to ensure efficient, concentrated delivery of the light.

Milchberg’s potential solution to this challenge of keeping light confined is additional light—in the form of ultra-short laser pulses. This project built on previous work from 2014 in which his lab demonstrated that they could use such laser pulses to sculpt waveguides in the air.

The short pulse technique utilizes the ability of a laser to provide such a high intensity along a path, called a filament, that it creates a plasma—a phase of matter where electrons have been torn free from their atoms. This energetic path heats the air, so it expands and leaves a path of low-density air in the laser’s wake. This process resembles a tiny version of lighting and thunder where the lightning bolt’s energy turns the air into a plasma that explosively expands the air, creating the thunderclap; the popping sounds the researchers heard along the beam path were the tiny cousins of thunder.

But these low-density filament paths on their own weren’t what the team needed to guide a laser. The researchers wanted a high-density core (the same as internet fiber optic cables). So, they created an arrangement of multiple low-density tunnels that naturally diffuse and merge into a moat surrounding a denser core of unperturbed air.

The 2014 experiments used a set arrangement of just four laser filaments, but the new experiment took advantage of a novel laser setup that automatically scales up the number of filaments depending on the laser energy; the filaments naturally distribute themselves around a ring.

The researchers showed that the technique could extend the length of the air waveguide, increasing the power they could deliver to a target at the end of the hallway. At the conclusion of the laser’s journey, the waveguide had kept about 20% of the light that otherwise would have been lost from their target area. The distance was about 60 times farther than their record from previous experiments. The team’s calculations suggest that they are not yet near the theoretical limit of the technique, and they say that much higher guiding efficiencies should be easily achievable with the method in the future.

“If we had a longer hallway, our results show that we could have adjusted the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is an author on the paper. “But we got our guide right for the hallway we have.”Distributions of the laser light collected after the hallway journey without a waveguide (left) and with a waveguide (right). Distributions of the laser light collected after the hallway journey without a waveguide (left) and with a waveguide (right).

The researchers also did shorter eight-meter tests in the lab where they investigated the physics playing out in the process in more detail. For the shorter test they managed to deliver about 60% of the potentially lost light to their target.

The popping sound of the plasma formation was put to practical use in their tests. Besides being an indication of where the beam was, it also provided the researchers with data. They used a line of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy going into making the waveguide translates to a louder pop).

The team found that the waveguide lasted for just hundredths of a second before dissipating back into thin air. But that’s eons for the laser bursts the researchers were sending through it: Light can traverse more than 3,000 km in that time.

Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their air waveguides. They also plan to guide different colors of light and to investigate if a faster filament pulse repetition rate can produce a waveguide to channel a continuous high-power beam.

“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications”, Milchberg says. “Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”

Story by Bailey Bedford. Images by Intense Laser-Matter Interactions Lab, UMD.

In addition to Milchberg, Goffin and Tartaro, Aaron Schweinsburg and Anthony Valenzuela from the DEVCOM Army Research Lab, and Eric Rosenthal from the Naval Research Lab are also authors and Ilia Larkin, a former UMD graduate student and current systems engineer at KLA, is a co-lead author.

Publication information: https://journals.aps.org/prx/accepted/8707dK4dIb91a60bb6df4e56bdc44a53b2267be80

PI affiliations: Howard Milchberg is jointly appointed to the departments of Physics and Electrical and Computer Engineering and is affiliated with the Institute for Research in Electronics and Applied Physics.

This work is supported by the Office of Naval Research (N00014-17-1-2705 and N00014-20-1-2233), the Air Force Office of Scientific Research and the JTO (FA9550-16-1-0121, FA9550-16-1-0284, and FA9550-21-1-0405), the  Army Research Lab (W911NF1620233) and the Army Research Office (W911NF-14-1-0372).

Electrons Take New Shape Inside Unconventional Metal

 

Faculty, Staff, Student and Alumni Awards & Notes

We proudly recognize members of our community who recently garnered major honors, began new positions and more.

Faculty and Staff 
  • Alessandra Buonanno recevied the 2022 Tomalla Prize for her research in gravitational wave physics.
  • Josiland Chambers received the department's Staff Excellence Award.
  • The American Physical Society (APS) has honored Professor Emeritus Alex Dragt with the 2023 Robert R. Wilson Prize for Achievement in the Physics of Particle Accelerators
  • Clay Daetwyler received the Lorraine DeSalvo Chair's Award for Outstanding Service.
  • Sankar Das Sarma has again been included on Clarivate Analytics list of Highly Cited Researchers, a compilation of influential names in science.
  • Manuel Franco Sevilla received the CMNS Board of Visitors’ Junior Faculty Award.
  • Jim Gates was quoted in a The New York Times story about the Webb telescope and again about Ukraininan scientists.
  • Alexey Gorshkov was elected a Fellow of Optica. He also spoke with the Caltech Heritage Project about quantum science advances.
  • Xiangdong Ji was quoted in Physics World.
  • Müge Karagöz was elected Vice President of Chesapeake Section of the American Association of Physics Teachers (CSAAPT) for a 1-year term. 
  • Dan Lathrop was quoted in Physics Today.
  • Wolfgang Losert was elected a Fellow of the American Association for the Advancement of Science
  • Rob McIntire received the department's Sibylle Sampson Award.
  • Ed Ott retired on December 31
  • Lester Putnam joined the department as a machinist.
  • Bruce Rowley from the machine shop.
  • Naomi Russo received the department's Staff Excellence Award.
  • Roald Sagdeev gave the John S. Toll Lecture.
  • Bonnie Seal-Filiatreau received the department's Staff Excellence Award.
  • Stephanie Williams joined the department as an academic advisor.
  • Victor Yakovenko and the LecDemo operation were credited by Sam Teitelbaum (BS., '10) for inspiring a love of physics.
  •  At a recent luncheon, Physics staff members were recognized for their years of service to the state of Maryland
    • Allen Monroe, 40 years
    • Doug Bensen, 35 years
    • Sally Megonigal, 35 years
    • Jack Touart, 30 years
    • Bonnie Seal-Filiatreau, 25 years
    • Cheryl Ekstrom, 20 years
    • Melissa Britton, 15 years
    • Mark Conners, 15 years
    • LaVita Williams, 15 years
    • Roy Arunkumar, 10 years
    • Omar Torres, 5 years
 Students
 Alumni
  • Siegfried Bleher (Ph.D., '89) accepted a position at Middlebury College.
  • Marten L. denBoer (Ph.D., '79) was named Radford University’s interim provost for academic year 2022-23.
  • Emily Edwards (Ph.D, '09) was elected a Fellow of the American Physical Society
  • Alexei Fedotov (Ph.D. ’97) was elected a Fellow of the American Physical Society.
  • Alan Henry (B.S., '02) discussed his book "Seen, Heard and Paid" with Ebony magazine.  
  • Tian Li (Ph.D., '17), of the University of Tennessee at Chattanooga, is developing a new microscopy technique.  
  • Slava Merkin (Ph.D. ’04) of the APL Center for Geospace Storms is the PI of a new NASA $15M, five-year research project understand and predict space weather.  
  • Ana Maria Rey (Ph.D. ’04) was featured by Optica in The (Atomic) Clockmaker. 
Department News
In Memoriam

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.