Laser physicist’s tenacity led to key technical advances
Tenio Popmintchev, 39
University of Colorado Boulder
Experimental physics is not for the fainthearted. One tiny error — or a concatenation of many — can keep a complicated experiment from working smoothly. Fortunately, Tenio Popmintchev has the tenacity for it.
Popmintchev, a laser physicist at the JILA institute at the University of Colorado Boulder, thinks nothing of running an experiment for 72 hours straight, or spending years tinkering with a finicky set of high-powered lasers, or shipping the entire setup to Vienna to re-create the experiments with collaborators there. A commitment to detail drives Popmintchev’s rising career, says his adviser and mentor Henry Kapteyn. “Tenio is not intimidated by what might go wrong in an experiment, and is very good at identifying and investigating the unknowns that might be holding an experiment back,” Kapteyn says.
At age 39, Popmintchev has already played a key role in inventing the first tabletop X-ray laser, which uses short pulses of light to illuminate the nature of matter. Its bright beams promise to probe everything from the movements of electrons and atoms within DNA to the folding of proteins in extraordinary detail. It would be relatively cheap and multipurpose, a Swiss army knife made of light that many researchers could use. “The same kind of revolution that happened with lasers in the 1960s is happening now in X-ray science,” Popmintchev says.
He began his drive toward physics early, while growing up in the town of Kazanlak in central Bulgaria. He was planning to study mathematics until a high school teacher cannily told him that physicists were the best mathematicians of all. And with that, the teacher had a fresh recruit for the national physics Olympiad team.
The same kind of revolution that happened with lasers in the 1960s is happening now in X-ray science.
— Tenio Popmintchev
Popmintchev went on to take honorable mention in an International Physics Olympiad in ninth grade and a bronze medal in 11th grade. He still speaks about that teacher fondly. “We used to solve problems the whole day long,” he says. “It was a lot of fun.”
In college, he started to explore the world of lasers with a physicist who had trained under the same teacher. They worked on infrared lasers for cosmetic surgery, and Popmintchev found his niche in coaxing the best out of experimental equipment. For his Ph.D. work, Popmintchev moved to one of the world’s top labs for studying ultrafast lasers, established in Boulder by Kapteyn and his collaborator and wife, Margaret Murnane.
Like a strobe light revealing the motion of dancers under a disco ball, ultrafast lasers can “freeze” atoms and molecules by illuminating them with every flash. Kapteyn and Murnane’s group uses lasers that pulse on the order of attoseconds, or billionths of a billionth of a second. “One attosecond is to a second as a second is to the age of the universe,” Popmintchev says. That superfast stop-motion means that scientists can glimpse atoms and molecules interacting with one another.
Popmintchev’s research aimed to push these frontiers past the usual wavelengths and into the higher energies of X-rays. Unlike infrared or ultraviolet light, X-rays can penetrate objects to reveal internal structure, like dental X-rays highlighting cavities. But making enough X-rays, with enough power, can require enormous, billion-dollar machines that accelerate electrons to high speeds.
Popmintchev wanted to find a way to make X-ray lasers accessible to more scientists. He turned to a method called high-harmonic generation, which was discovered in 1987 when researchers noticed that under certain conditions their lasers efficiently generated shorter wavelengths of light. The technique had been used mainly with ultraviolet lasers, but Popmintchev and colleagues realized that infrared lasers could be coaxed to produce X-ray pulses if they were sent beaming through pressurized gas.
So he built a cylinder that could fit in the palm of a hand and contains helium gas at 50 times atmospheric pressure. When laser light hits the high-pressure gas, it strips electrons off the helium. Each electron accelerates away from and then back toward its charged helium atom. When the electron crashes back into the helium, it releases extra energy gained from this acceleration as higher-energy X-rays. By tweaking the pressure of the gas and the intensity of the laser, Popmintchev could get the X-ray emissions to move in phase with one another, producing a coherent beam that his team could control with exquisite precision.
In this setup, a long-wavelength laser is sent through a waveguide of compressed helium gas to produce a bright coherent X-ray beam. The grating demonstrates that the beam includes energies across the X-ray spectrum (far right) and so has many applications.
Researchers had not thought they could do such a thing with high-harmonic generation. But Popmintchev persisted, tinkering with the lasers until they did what he wanted. Trillions of atoms shone together in X-ray harmony. He got not only his Ph.D. but also a paper in Science in 2012.
Today his team includes another Popmintchev — his younger brother Dimitar. Both of them went through the same physics schooling in Kazanlak, then at the university in Sofia. Tenio is the older brother who made his mark first, known for his frequent collaborations with others. Dimitar is quieter and prefers to work on his own. Dimitar and Tenio work and live together, around the clock. “He is a great scientist and a great brother,” Dimitar says.
For now, the Popmintchev brothers are working on ways to beef up their lasers’ performance and power. In Science in December, they described a way to tweak high-harmonic generation to produce X-rays even more efficiently. Perfectionists do not rest on their laurels.
Popmintchev is now leaving Boulder to start up laboratories at the University of California, San Diego and in Vienna. Eventually he hopes to make the tabletop X-ray laser a possibility for any lab — an accessible, affordable X-ray microscope for nanotechnology, biology and other applications. “This will be one of the ultimate light tools,” he says. “This is like your personal X-ray laser that you can modify the way you want.”
T. Popmintchev et al. Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers. Science. Vol. 336, June 8, 2012. doi: 10.1126/science.1218497.
D. Popmintchev et al. Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiple ionized plasmas. Science. Vol. 350, Dec. 4, 2015. doi: 10.1126/science.aac9755.