Researchers at the Joint Quantum Institute (JQI) have created the first silicon chip that can reliably constrain light to its four corners. The effect, which arises from interfering optical pathways, isn't altered by small defects during fabrication and could eventually enable the creation of robust sources of quantum light.

That robustness is due to topological physics, which describes the properties of materials that are insensitive to small changes in geometry. The cornering of light, which was reported June 17 in Nature Photonics, is a realization of a new topological effect, first predicted in 2017.

A new, grooved silicon chip keeps light in the corners using the physics of quadrupoles and topology. (Credit: E. Edwards/JQI)

In particular, the new work is a demonstration of quadrupole topological physics. A quadrupole is an arrangement of four poles—sinks and sources of force fields such as electrical charges or the poles of a magnet. You can visualize an electric quadrupole by imagining charges on each corner of a square that alternate positive-negative-positive-negative as you go along the perimeter.

The fact that the cornering arises from quadrupole physics instead of the physics of dipoles—that is, arrangements of just two poles—means it a higher-order topological effect.

Although the cornering effect has been observed in acoustic and microwave systems before, the new work is the first time it’s been observed in an optical system, says Associate Professor and JQI Fellow Mohammad Hafezi, the paper’s senior author. "We have been developing integrated silicon photonic systems to realize ideas derived from topology in a physical system," Hafezi says. "The fact that we use components compatible with current technology means that, if these systems are robust, they could possibly be translated into immediate applications."

In the new work, laser light is injected into a grid of resonators—grooved loops in the silicon that confine the light to rings. By placing the resonators at carefully measured distances, it's possible to adjust the interaction between neighboring resonators and alter the path that light takes through the grid.

The cumulative effect is that the light in the middle of the chip interferes with itself, causing most of the light injected into the chip to spend its time at the four corners.

Light doesn’t have an electric charge, but the presence or absence of light in a given resonator provides a kind of polar behavior. In this way, the pattern of resonators on the chip corresponds to a collection of interacting quadrupoles—precisely the conditions required by the first prediction of higher-order topological states of matter.

To test their fabricated pattern, Hafezi and his colleagues injected light into each corner of the chip and then captured an image of the chip with a microscope. In the collected light, they saw four bright peaks, one at each corner of the chip.

To show that the cornered light was trapped by topology, and not merely a result of where they injected the lasers, they tested a chip with the bottom two rows of resonators shifted. This changed their interactions with the resonators above, and, at least theoretically, changed where the bright spots should appear. They again injected the light at the corners, and this time—just as theory predicted—the lower two bright spots showed up above the rows of shifted resonators and not at the physical corners.

Despite the protection from small changes in resonator placement offered by topology, a second, more destructive fabrication defect remains in these chips. Since each resonator isn't exactly the same, the four points of light at the corners all shine with slightly different frequencies. This means that, for the moment, the chip may be no better than a single resonator if used as a source of photons—the quantum particles of light that many hope to harness as carriers of quantum information in future devices and networks.

"If you have many sources that are forced by topology to spit out identical photons, then you could interfere them, and that would be a game-changer," says Sunil Mittal, the lead author of the paper and a postdoctoral researcher at JQI. "I hope this work actually excites theorists to think about maybe looking for models that are insensitive to this lingering disorder in resonator frequencies."

Story by Chris Cesare

Hafezi and Mittal also have affiliations in the Department of Electrical and Computer Engineering, as well as the Institue for Research in Electronics and Applied Physics. Hafezi is also an associate professor in the Department of Physics.

A portion of the Apollo 15 lunar laser ranging retroreflector array, as placed on the Moon and photographed by D. Scott. Credits: NASA/D. Scott

In an Eos article titled, "Seeing the Light," UMD Physics Research Scientist and Professor Emeritus Doug Currie describes the current retroreflectors installed on the Moon and his proposal to send a new module as part of an upcoming lunar mission. The article details how retroreflectors installed on the Moon during the Apollo 11, 14 and 15 missions are still used today, but that improvements are needed.

 Photo Credit: CTAO/M-A. Besel/IAC (G.P. Diaz)/ESOIn an interview with SciTech Europa, Fermi Project Scientist at the Astroparticle Physics Laboratory, Astrophysics Science Division of NASA’s Goddard Space Flight Center and University of Maryland Adjunct Associate Professor Julie McEnery answers questions about the Cherenkov Telescope Array (CTA) and its impact on astrophysics. She also explores the integration of Fermi Gamma ray Space Telescope and CTA. 

Gaurang Yodh, a University of Maryland physics professor from 1961-88, died on June 3 at the age of 90. Yodh earned his Ph.D. at the University of Chicago in 1955, working with Herb Anderson and Enrico Fermi. After appointments at Stanford, the Tata Institute and the Carnegie Institute, he joined the UMD physics faculty in 1961. In his long career researching particle physics and cosmic rays, his contributions included developing improved radiation detectors for particle detection and developing ground-based water Cherenkov gamma ray telescopes to study gamma rays and search for sources of cosmic rays.

Yodh was a Fellow of the American Physical Society, the American Association for the Advancement of Science and the UK Institute of Physics.

He was an extraordinarily accomplished sitar player, and while in College Park offered a course in Indian classical music performance that helped launch the UMD ethnomusicology program. 

While a professor at the University of California, Irvine, Yodh established the Yodh Prize for outstanding achievement in cosmic rays and astroparticle physics. Jordan Goodman, who earned his doctorate under Yodh in 1978, received this award in 2017.

Mohammad Hafezi has been named a finalist for the 2019 Blavatnik National Awards for Young Scientists.

He is one of 31 researchers competing for three Blavatnik National Laureate Awards in the categories of Physical Sciences and Engineering, Chemistry and Life Sciences, and is one of 10 finalists in Physical Sciences and Engineering. Each of the three National Laureates will win $250,000—the world’s largest unrestricted prize for early-career scientists. The awards are sponsored by the Blavatnik Family Foundation and the New York Academy of Sciences.

"Starting during his time as a postdoc in the Joint Quantum Institute, Hafezi has established himself as a world leader in marrying topology, many body physics and photonics," said Steve Rolston, chair of the Department of Physics. "With appointments in physics and engineering,  he is helping to catalyze UMD's efforts to transition quantum physics to quantum technology."

Now in its 13th year, the Blavatnik National Awards for Young Scientists recognize the past accomplishments and the future promise of the most talented faculty-rank scientists and engineers aged 42 years and younger at America’s top academic and research institutions. This year, the Blavatnik National Awards received an unprecedented 343 nominations from 169 academic and research centers across 44 states—a record in all three categories. The three 2019 National Laureates will be announced June 26.

Inspired by the concept of topology in mathematics and its prevalence in electronic quantum materials, Hafezi’s innovative work has addressed a critical problem of inevitable nanofabrication defects. These imperfections have plagued the reliability and performance of optical devices in nanophotonics and quantum optics for years. Hafezi has shown that like electrons, photons under a given set of conditions can also be made insensitive to both the shape and defects in an optical device. This discovery has garnered immense interest in the optics community and spurred a new field of topological photonics. Hafezi is an associate professor with affiliations in the Department of Electrical and Computer Engineering, Department of Physics, Joint Quantum Institute, and Institute for Research in Electronics and Applied Physics.