figureDonna Strickland at the 2018 Nobel Prize Award Ceremony in Stockholm, Sweden. [Alexander Mahmoud / © Nobel Media AB 2018]

figureDonna Strickland in the lab at the University of Rochester. [University of Rochester]

You may have heard of Donna Strickland. Last year, the ultrafast-laser legend joined a very small, exclusive club. In December, on the anniversary of Arthur Nobel’s death, she became a Nobel laureate.

Her role in the groundbreaking discovery of chirped pulse amplification (CPA), a technique that generates ultrashort, high-intensity laser pulses that can be used in applications ranging from eye surgery to micromachining to cutting-edge particle physics, has rocketed the 2013 OSA President to fame. And while any person earning the world’s most prestigious science award warrants public notice, Strickland’s win has attracted particular excitement and press attention owing to one conspicuous fact: she is only the third woman to win the physics prize in its 118-year history.

Yet another compelling detail of her story has received less attention—not only did she help invent a technique now being used to amplify laser pulses up to the petawatt level, she did it before she had even earned her Ph.D. This detail elevates Strickland to an even smaller club of Nobel laureates recognized for student achievements.

It’s not unheard of in the community, but it does afford Strickland a unique perspective. The University of Waterloo, Canada, physics professor has spent much of the time since last fall reliving her own education, and she’s arrived at some interesting insights on science and scholarship while strolling down memory lane.

Hooked on lasers

The story of how Strickland ended up at the University of Rochester, N.Y., USA, where she was fated to meet her mentor and co-prize winner Gérard Mourou, begins with an unexpected statement: “I was unbelievably shy.” If you’ve met Strickland, this confession may be a little hard to swallow. She laughs often and skips effortlessly from heavy explanations of laser science to light-hearted quips.

Not only did she help invent a technique now being used to amplify laser pulses up to the petawatt level, she did it before she had even earned her Ph.D.

Nonetheless, Strickland says that she was this close to being a math major at the University of Waterloo—a comfortable choice, where her sister and best friend attended—before she pivoted and instead pursued a B.Eng. in engineering physics at McMaster University. Strickland credits this decision to her desire to branch out and meet new people, but her choice to focus on lasers was all “gut instinct.” After all, she asks, “they’re so fun, who wouldn’t want to study lasers?”

So where does one go for graduate education once they’ve become hooked on lasers? Strickland asked this exact question of some McMaster grad students and was promptly directed to Rochester’s Institute of Optics. She met Mourou in her very first week there, when she toured the laser lab. The strong preparation she had received in the undergraduate Canadian engineering curriculum allowed Strickland to breeze through her first few months of classes at Rochester. She decided to put her free time to good use, and began working 10 hours a week with Mourou in his lab later that Fall.

Strickland’s very first project in Mourou’s lab brought another fateful meeting. “I had to learn to machine,” she says, “so the very first thing I did that first week, I met this group of three electrical-engineering students, and they needed a box to be built over their laser heads.” One of those electrical-engineering students turned out to be Doug Dykaar, her future husband.

figureDonna Strickland (far right) poses with her family on vacation in 1985, the same year CPA was invented. [Courtesy of Donna Strickland]

From HHG to CPA

Strickland’s road to CPA actually began with another physics problem—high-harmonic generation (HHG), a nonlinear-optics process in which a target, after multi­photon absorption by a high-intensity laser pulse, re-emits light at a high harmonic of the pulse’s optical frequency. In the early 1980s, HHG was just a few years old, and the method was opening up new research avenues in nonlinear optics and proving to be a key source of ultrashort coherent light. In particular, OSA Honorary Member Stephen E. Harris had proposed a method for achieving the difficult feat of HHG above the second and third harmonics—and, at Mourou’s suggestion, Strickland decided to tackle ninth-order harmonic generation, a then-unexplored frontier, as her thesis project.

But she kept hitting a roadblock: how to get the intensity needed from the 30-millijoule, 30-picosecond laser she had in the lab. The answer, she knew, was compressing the pulse, to pack more energy into a shorter time. But when she tried to compress and amplify the pulses, another nonlinear-optics effect, whole-beam self-focusing—which leads to imperfect pulse compression as well as damage to the amplifier—frustrated her goal.

By 1984, that problem had set the (lab) table for CPA. “In my recollection,” Strickland says, “people were doing pulse compression of Nd:YAG, and it was already showing that if you wanted to do really good compression, you had to do stretching first.” So, Mourou and Strickland took that background, and used it as the foundation for a new way to amplify laser pulses.

In their scheme, a 150-ps pulse from a mode-locked Nd:YAG laser would first pass through a length of optical fiber. The fiber acted as a dispersive element, stretching, or “chirping,” the pulse in time. The researchers then used a Nd:Glass regenerative amplifier to beef up the stretched pulse’s energy. Finally, a double-grating setup re-compressed the pulse to a 1.5-ps width. The result was a short pulse that packed in substantially greater energy—and, thus, gigawatts of power—without burning out the laser amplifier. “That,” Strickland says, “was the original CPA.”

Simple process, staggering implications

Perhaps because she’s had to explain it so often, Strickland insists that, at its core, the pioneering technique is simple. “I think that’s partly the beauty of CPA,” Strickland reflects, “It really is simplistic in its whole, in what it needed to do.” But in this case, simple didn’t mean limited. According to Strickland, she and Mourou were aware from the beginning of the invention’s potential. “At no time did we think we’re stopping at a gigawatt,” Strickland says, “This wasn’t just to do something; this was to really change the field of high-intensity laser physics.”

“At no time did we think we’re stopping at a gigawatt. This wasn’t just to do something; this was to really change the field of high-intensity laser physics.” —Donna Strickland

Strickland also says the research process leading to CPA “ran more smoothly than most” and was “fairly straightforward” once the project took off. And, remarkably, the initial invention of CPA by Strickland and Mourou—which turned out to be the seminal technique enabling ultrafast lasers—was done without grant money; virtually everything for the project was donated. “I think we had to buy the gratings,” Strickland recalls, “and that was it.”

Still, even though the research never went completely off the rails, a few wrenches were thrown into the works. One celebrated example involved the fiber used as the dispersive element in the setup.

Mourou—who, Strickland says, had a gift for getting others to provide material for projects—contacted Corning and persuaded the firm to part with a 2.5-km spool of single-mode, 9-micron-core specialty fiber for the experiment. “I spent a day of my life unspooling and re-spooling the fiber” onto a new spool in a film can, Strickland says—and she found that when she attempted to put light through the fiber, “nothing came out the other end.” Further investigation showed that the fiber had broken cleanly around midway through the expanse.

The result: The experiment had to make do with a 1.4-km length of fiber—which turned out to be just fine for stretching the pulse. “You might think, I was just a great grad student, I sat down and calculated how long it should be, but no,” she admits. “I broke the fiber, but luckily that didn’t matter in the end.”

Getting to the “big thing”

Strickland vividly remembers the night (or, more accurately, wee hours of the morning) that the team finally got CPA to work. The researchers used a low-tech “sewing machine auto correlator” to see what was happening in real time, and they used a streak camera to measure the system’s pulse duration. “I still remember it was a Thursday night,” says Strickland, “I know that because we celebrated Friday!” Even in the moment, Strickland felt the gravity of the event. “You do know it’s going to be big,” she says. “Not big like it’s turned out to be big. But we knew, as a group, that this was the big thing.”

With that big thing in the bag, the race to publish the results was on—as Strickland and Mourou knew that, in 1985, they weren’t the only ones working on this problem. They submitted the paper in July to Optics Communications, a journal known for its relatively quick turnaround. A few months later, they interviewed a postdoc who asked them to explain the key figure in their paper. “He had a copy of our paper that we didn’t even know was out,” Strickland laughs, “that’s how prepared he was.”

But when she saw the figure in the paper, her humor quickly turned to panic. The published figure was not from their paper—indeed, it “was not even from optics at all!” The paper was hastily republished that December, through the sheer willpower of Mourou. An erratum was not going to cut it. “If you had seen a cartoon of Gérard” at that time, says Strickland, “he would have steam coming out of his ears.”

figureDonna Strickland at CLEO 2019. [OSA]

The “perennial grad student”

Strickland graduated from Rochester a few short years after CPA’s (re)publication. With both CPA and her Ph.D. under her belt, the future Nobel laureate launched into a 35-year-and-counting career in ultrafast-laser science, starting with a postdoctoral research fellowship in the lab of Paul Corkum at the National Research Council Canada. After the short-pulse Ti:sapphire laser came out in 1990, CPA systems were suddenly attainable for smaller labs; the door was opened for table-top terawatt lasers. “At that point, it started to take off” Strickland recalls, and CPA has been part of her work ever since.

She’s kept herself busy in the decades between her doctoral research and her 2018 Nobel Prize; Strickland currently runs the ultrafast-laser group at Waterloo. Perhaps that’s why it feels so strange for her, as a scientist with a long, accomplished career, to be thrust into the spotlight for research that took place so long ago. “I have become the perennial grad student now,” quips Strickland, “even though I’m 60 this year. I’m in everyone’s mind as the grad student.”

Surreal as it may be for the mother of three—one son, one daughter, and CPA, she jokes—to be an honorary student at 60, Strickland has found that the position makes her more approachable to current graduate students. She’s gained a cult status in their ranks. “After Gérard and I will give a talk,” she says, “it’s all the grad students who want to talk to me.”

Parting advice

As a professor and mentor many times over, Strickland has fostered enough careers to qualify her to dispense advice. Here is what she has to say:

First, don’t ask her how to win a Nobel Prize. “Really,” Strickland says, “I’ve had some questions that are like that, though!”

Second, she is adamant that “if you can imagine yourself doing anything else, go do it.” When she asked herself this question as a student, she realized she couldn’t see herself doing anything other than being a “laser jock,” so she stuck to her guns. She knows it’s not easy, especially in graduate school; she even recalls a couple times that she considered quitting her Ph.D. “But I think if it’s what you really want to do, you’ll face the hurdles.”

Third, Strickland also encourages students not to judge their work by socially contrived metrics of success. “People do find this inspiring,” she says, but “I would hope that people aren’t thinking, well, what do I do to get a Nobel Prize? Because that’s a very artificial thing to try to aim for.” What’s more, Strickland has noticed a worrisome mounting pressure on researchers for citations. “It’s funny,” she says, reflecting on her 1985 paper, “we knew it was going to be an important paper, but I don’t think we were thinking thousands of citations. We just thought it would change the high-intensity laser field.”

So there you have it. Take Strickland’s advice and don’t aim for the Nobel. Just work hard—and revolutionize your field.

Molly Moser is OPN’s associate editor.