Laser-Focused: How Minjung Son’s Research Brings Together Photonics and Chemistry for a Brighter Future

by Jack Osmond, Photos by Chris McIntosh

Assistant Professor of Chemistry Minjung Son loves shooting lasers at things for a living––just check her Twitter bio for proof. She stumbled upon her passion for lasers, or ultrafast spectroscopy, as an undergraduate. After taking classes in physical chemistry and quantum mechanics, she joined a research group where she got the chance to work with laser spectroscopy on small molecules, like porphyrins, for the first time. 

“I learned how to get my hands dirty. I just loved it. All these different colors in a dark lab, and we can actually measure some super fast processes that our eyes can’t capture. I just got fascinated by that aspect.” From that point on, she knew she wanted to pursue her researching with a graduate degree.

While doing her PhD at MIT, Son fell in love in Boston and decided she wanted to continue doing research in the city. That led her to the BU Photonics center, where she’s continuing her work with spectroscopy and physical chemistry. While she’ll remain as a permanent faculty member at the center, her lab space is temporary – she is currently waiting for her permanent lab space to be finished in the Life Science and Engineering Building. Despite the temporary arrangement, Son jokes that she doesn’t want to leave: “I really like being here, and I kind of want to stay here forever if I can.”

“This is such a nice place where I can interface with a lot of engineers and scientists who are all working on optics and photonics,” Son continues. “Being here and interacting with other faculty and students who are similarly minded and excited about optics, spectroscopy, and photonics is really great.”

Where Son differs from other faculty at the Photonics Center, however, is in her focus on chemistry. Yet Son’s main focus on physical chemistry lies at the intersection of the Photonics Center’s disciplines. “It’s really interdisciplinary by definition […] We can talk to physicists like we’re physicists, but we can talk to chemists making molecules.” Son describes how her team uses “physical tools from physics and optics, but we actually apply those tools to study interesting molecular systems, which is more like a chemist.”

Much of Son’s research focuses on understanding light-harvesting molecular systems. Such systems can be both natural, like a plant photosynthesizing sunlight, or artificial, like a solar panel. These light-harvesting molecular reactions happen on the femtosecond timescale––that’s one quadrillionth of a second. These light-harvesting systems absorb energy from the sun and put it towards functional work, making these reactions essential to understanding how humans can better capture and use solar energy. The only way to capture these reactions is to use ultrafast spectroscopy, which Son describes as, “using lasers or super short laser pulses to take snapshots of the pathways of those processes, energy transfer and electron transfer, so we can actually time-resolve the mechanism and the time scales of those processes.”

Despite the small scale of these reactions, Professor Son’s research understanding how they work makes a big impact on society. For example, the engineers and scientists in charge of making artificial light-harvesting systems (like solar cells) need to understand these molecular systems to enhance the utility of said systems. “They’re not going to know [the systems] without the photophysical parameters that we measure.” 

“We’re providing the industry people, or the material scientists, with the design principles of how to optimize those device structures,” Son explains, “so that [the molecular structures] can be building blocks for real world applications and devices for things like LEDs.”

Professor Son’s research doesn’t just influence industry, however. Her research is also critical for optimizing natural photosynthetic systems, most commonly found in plants. According to Son, these “are actually the most efficient light-harvesters on this planet,” and hold potential to combat worldwide food and energy crises.

“If you think about the ever growing demand for energy and food globally, photosynthesis has been proposed as a way to maybe address the problem, or the demand, because if you can figure out how to optimize the molecular-level photosynthetic mechanism, then people who are doing field studies have found that we can increase crop yields by 30% […] so for that we have to delve into the molecular level stuff, which only we can do.”

With all the talk of harvesting light, Professor Son is thrilled to be based in the Photonics Center, as she describes her current research as “really, really photonics-y.” Presently, her team is working on polaritons: pairs of reflective gold or silver mirrors that trap light inside of them. When Son and her team put natural photosynthetic proteins inside of these mirror pairs, they interact with the intense light bouncing back and forth between the mirrors. Son and her team then use spectroscopy to gain further insight into how the interaction alters the photophysical pathways of the light-harvesting molecular system. This knowledge is essential in both optimizing natural systems and creating artificial ones.

“This is the prime example of how photonics can be interfaced with traditional physical chemistry research,” Son says, “I’m super excited to see what actually comes out of this research.”

Son’s two undergraduate research assistants, Gabe Russo and Christopher Kretschman, are essential to her work. Russo, a chemistry major, works on actually building these polaritons. “He’s doing a lot of the material science stuff,” Son explains, such as making the mirrors and gluing them together. Part of this work includes finding the optimal distance between the two mirrors to create the strongest interactions.

Chris Kretschman, a physics and chemistry major, “is naturally really interested in working with lasers,” much like Son. Kretschman spends a lot of time in the laser lab, helping Professor Son align the optics and measuring the pulse diagnostics, putting together a white-light generation setup. Compared to Russo, Kretschman’s work relies more heavily on optics and photonics related research. If these two roles seem different, it only aligns with Son’s philosophy as a mentor. 

“I just want to be a mentor who lets people do what they’re most excited about,” she says. “I typically ask them and they pick two or three things, then I give each person an individual project that isn’t the same as everyone else. But they’re all broadly related under the same umbrella project, so they can always talk to each other.”

In addition to her “really terrific” undergraduate students, Son is also recruiting graduate students. To Son, the most important aspect she looks for in research assistants is passion. 

“You’ll find something you love about research because our work is so interdisciplinary,” she says. Including chemistry, physics, biophysics, programming, and coding. “If you have passion, and if you look at my website and that kind of stuff is what you’re excited about, you can talk to me and we can go from there.”

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