Light science - Berkeley Engineering (2024)

Boubacar Kanté takes on tough challenges to drive innovation in optoelectronics

The ability to commandeer light is both an ancient dream of humanity and a science and engineering challenge that has never been more relevant.

Harnessing light to transfer more information at higher speed has become increasingly critical as data centers reach the limits of copper-wire connections. To solve the perils of climate change, researchers are looking to lasers that can deliver an enormous shot of power, triggering fusion reactions for clean energy. Lasers are also needed in manufacturing, to see processes inside the human body, for wrangling quantum phenomena for a variety of practical applications — and much more.

Many potential applications of lasers have remained locked or unwieldy for the past 60 years due to fundamental limitations of the technology. But now, that has begun to change, thanks to researchers like Boubacar Kanté, the Chenming Hu Professor of Electrical Engineering and Computer Sciences.

Tackling tough challenges comes second nature to Kanté. Among his many achievements from the seemingly impossible category, he is perhaps best known for work developing a new type of semiconductor laser — the Berkeley Surface Emitting Laser (BerkSEL).

When first announced in 2022 in a landmark paper, BerkSEL technology resolved a decades-long challenge in wave-physics: The ability to emit a single mode of light while scaling up in size and power. The new type of laser means that increases in size do not have to result in a loss of coherence, which is light of a single wavelength being beamed out in one direction.

Coherence is what enables lasers to be more powerful and to cover longer distances, making them suitable for a range of applications. Researchers typically use external mechanisms, such as a waveguide, to amplify laser beams and circumvent loss of coherence.

But using another medium to amplify laser light takes up a lot of space, Kanté says. By eliminating the need for external help, the BerkSEL can be smaller, thus increasing the efficiency of computer chips and other components in a range of consumer and industrial applications that rely on lasers.

“Researchers have been trying since the 1960s to build a single-mode laser that can be scaled up in size and power,” says Kanté. “Now, we have met the challenge, demonstrating both of these qualities in a laser. This will likely stand as one of the most important papers published by my group.”

Rushin Contractor (Ph.D.’23 EECS), one of Kanté’s former graduate researchers who now works with him on commercial development of the BerkSEL, recalls the early days when they first began research on the new laser.

Kanté “assigned me to look at some possible ideas — what were some developments in physics discovered five or 10 years ago but never really applied for anything? Then I found that maybe there’s a certain combination of electromagnetic fields that can give rise to these [BerkSEL-like] effects,” Contractor says. “But I was also asking the question, what if we make this into a realistic device? Will it work?”

It did. And now, experts say the BerkSEL will lead to more powerful and efficient lasers for industrial materials processing; communication networks; military applications; small, unmanned spacecraft propulsion; and semiconductor lasers for carbon-free fusion energy. Moreover, the new strategy may also provide insights for longstanding problems in basic physics research.

“You have to choose well”

The BerkSEL and Kanté’s other research successes are part of his personal journey, one that spans cultures and continents. He says he has been interested in science since his childhood in Gabon, where his family moved shortly after his birth in Mali. His father, a high school science teacher, provided early inspiration; Kanté’s young life was steeped in learning and the values of discipline and hard work.

Following undergraduate studies in science in France, Kanté says, he began to zero-in on the problems he found interesting and wanted to tackle.

“At first you’re interested in everything,” he says of the challenges. “But then you realize you can’t do them all. You have to choose well in order to contribute to science and engineering.”

For Kanté, those contributions began in earnest with a 2009 paper reporting a nonmagnetic metamaterial cloak at microwave frequencies. He co-authored the paper as part of his doctoral studies at the University of Paris Saclay.

Cloaking, or the ability to render objects invisible, is an ancient human dream, Kanté explains. The rise of metamaterials — engineered materials with unique electromagnetic properties offering extreme control over optical fields — has awakened new efforts to create invisibility cloaks that would have numerous applications, especially related to national security.

Kanté and his coauthors created an invisibility cloak in free space that had the biggest concealed region reported at the time. They did so with split ring resonators, devices embedded in a silicone matrix operating at microwave frequencies and based on electric rather than magnetic response. The nonmagnetic approach was key, allowing for the scaling of the cloak from microwave to optical regions of the spectrum.

More than a mathematical curiosity

Following his doctoral studies, Kanté’s first faculty position took him to UC San Diego, where his research interests evolved from the microwave and radar parts of the spectrum to optics. He would go on to demonstrate the world’s first topological laser based on the quantum Hall effect for light, and his interest in wave physics — in particular, a concept in quantum mechanics called bound states in the continuum (BICs) — began a series of pathbreaking discoveries in light science.

For many years, Kanté explains, BICs were regarded as little more than a mathematical curiosity. In time, however, researchers came to understand BICs as a wave phenomenon that could exist outside the purely theoretical realm of quantum mechanics. These bound states were shown to occur in many different fields of wave physics, including acoustics, microwaves and nanophotonics.

What attracted the attention of researchers like Kanté is that BIC waves remain perfectly confined, or bound, in open systems. They will not escape like other waves in an open system. Being able to confine laser light in this way would be highly advantageous. The ability to elicit lasing action from a BIC, however, remained elusive.

But then, in 2017, Kanté led a team that reported room temperature lasing action from an optically pumped BIC.

The BIC laser Kanté developed is made of a thin semiconductor membrane consisting of indium, gallium, arsenic and phosphorus. The membrane is structured as an array of nano-sized cylinders suspended in air. The cylinders are interconnected by a network of supporting bridges, which provide mechanical stability to the device. By powering the membrane with a high frequency laser beam, Kanté and coworkers induced the BIC system to emit its own lower-frequency laser beam at telecommunication frequency.

The technology, he says, could revolutionize the development of surface lasers, making them more compact and energy-efficient for applications in communications, computing, sensing and more. The new BIC lasers could also be developed as high-power lasers for industrial and defense applications.

A wide range of interests

Since Kanté’s arrival at Berkeley in 2019, his research interests have included areas of wave-matter interaction, from microwave to optical wavelengths, and related fields such as antennas, nanophotonics, novel materials and quantum optics.

Continuing his interest in topological lasers, Kanté’s group, in 2021, demonstrated the emission of discrete twisting laser beams from antennas made up of concentric rings, roughly equal to the diameter of a human hair, and placed on silicon chips.

The work is based on the orbital angular momentum (OAM) of light, a property that has attracted the interest of researchers because it offers exponentially greater capacity for data transmission. One way to think about OAM, Kanté says, is to compare it to the vortex of a tornado.

A larger quantum number means light can expand its ‘vocabulary’

By applying a magnetic field perpendicular to the ring microstructure, Kanté’s team generated three OAM laser beams traveling in circular orbits above the surface of a chip. The laser beams had as many as 276 twists of light — referred to as their quantum number — in one wavelength, around an axis.

A larger quantum number means light can expand its “vocabulary,” Kante says. His group has demonstrated the capability at telecommunication wavelengths, but it could also be adapted to other frequency bands. And though his team created just three lasers, multiplying the data-rate capacity by three, he says there is no limit to the possible number of beams and data capacity.

Kanté’s work in topological lasers is just one example of his wide range of scientific interests. Students from a variety of backgrounds in science have become aware of his research group, and it is a draw for those looking for the cutting edge.

“He’s definitely a very hands-on group leader,” says electrical engineering and computer sciences doctoral student Emma Scott Martin, who works on designing photonic crystal lasers. “He’s always [wanting to know] what everyone is up to. Especially as a new student who doesn’t have as much experience, that’s definitely a positive thing.”

Advancing quantum optics

Indeed, when he presents his work at scientific meetings, or just in conversation, Kanté’s passion for work in the laboratory is obvious. At one recent meeting, he gave a whirlwind presentation of his group’s work in semiconductor lasers, including his breakthroughs in confining light in nanocavities. Just last year, his group demonstrated the first on-demand quantum light source using silicon — the material upon which millions of tiny electronic devices are manufactured each day, and the most “scalable” optoelectronic material known. Among the exciting potential applications is a source of photons for a quantum internet.

A quantum internet at scale, Kanté explains, would require not only a bright and efficient quantum light source but also photons that can propagate in existing optical fibers without being absorbed. No light source available today can meet that high bar.

The on-demand silicon quantum light source developed by the Kanté Lab is the first experimental work demonstrating integration of a single silicon atomic emissive center, known as the G center, directly in a silicon nanophotonic cavity. For the first time, his team embedded an atomic defect in silicon the size of an atom (1 angstrom) in a silicon photonic cavity (1 micron) with the size of less than one-tenth of a human hair.

The cavity forces the atom to be brighter, and it emits photons at a faster rate, Kanté says. “Those are necessary ingredients for scalable quantum light sources for the future [quantum] internet.”

Manufacturing of the single-photon emitters involves a controlled fabrication sequence, starting with a commercial-grade silicon wafer that is carbon implanted. The implantation is followed by lithography, etching and thermal annealing — all standard processes available in today’s semiconductor foundries. Kanté says his team has overcome some of the key challenges, but improvements are needed, and many questions are yet to be answered.

A 2023 study on this work was led, in part, by Walid Redjem, who joined Kanté’s group in 2020 as a postdoctoral researcher after completing his Ph.D. in quantum optics. At first, Redjem wondered how he would fit in with the rest of the lab, as Kanté’s research up to that point aligned more with classical optics. But he says Kanté’s solution was simple: Redjem was to start a new area (of quantum optics) in his group.

“He is always trying to explore, always ready to try new things,” Redjem, now at State University of New York at Albany, says of Kanté. “He doesn’t want to sit in one place.

“It wasn’t easy every day,” Redjem continues. “But in the end, I have learned a lot [from Kanté] about how to think about science, how to work with the best people, always trying to be impactful, trying to push the limits.”

Kanté says, “I tell people the problems I want to solve, and while I may not have the entire solution, I can tell you why I think the problems can be solved.” He says at Berkeley he has found the support and the students with capabilities “for all of this to come together for discovery.”