Laser-generated "lenses" could enable recording of molecular dynamics

JUNE 30, 2009--A research team at the University of Nebraska-Lincoln has figured out a possible way to observe and record the behavior of matter at the molecular level. Working with Nobel laureate Ahmed Zewail (chemistry, 1999) of the California Institute of Technology (Pasadena, CA), they developed mathematical models to show that laser beams create ultra-high-speed "temporal lenses" that would be capable of making "movies" of molecular processes for biomedical imaging and other applications.

The finding, published by the Proceedings of the National Academy of Sciences, describes "lenses" made not of glass but of laser beams, which would keep pulses of electrons from dispersing and instead focus them on a target. The timescales required are measured in femtoseconds (quadrillionths of a second) and attoseconds (quintillionths of a second).

Herman Batelaan, associate professor of physics at UNL, said the process is analogous to filming the flight of a thrown baseball. First, the camera lens must be focused properly or the ball will appear blurred. Second, the camera has to have a fast shutter speed or the seams of the ball will appear streaked.

But the analogy stops there. And the timescales involved are "daunting," Batelaan said. "A crisp image of the seams of a thrown baseball can be made with a strobe pulse of about one 10 millionth of a second. Taking a crisp image of an atom in a molecule is much more demanding. Pulses that are a billion times shorter than that are needed. Anything produced to date is 50 times slower than that and making movies of most molecules has stayed out of reach.

"The new idea is that a temporal lens exists and obeys the same laws as a spatial lens. That's what we showed in this paper. Nobody had ever used a temporal lens to get a higher resolution."

The physicists modeled two types of lenses. One was a temporal "thin" lens created using one laser beam that could compress electron pulses to less than 10 femtoseconds. The second was a "thick" lens created using two counterpropagating laser beams that showed the potential of compressing electron pulses to reach focuses of attosecond duration.

"The thick lens will give the best value, but it's much more complex, because the attosecond regime is three orders of magnitude smaller than the femtosecond regime," said Shawn Hilbert, the paper's lead author and a May Ph.D. graduate under Batelaan.

"It's great that we were able to take a very simple idea--at least in physics--and produce a paper like this," Hilbert said. "The lensmakers equation is something you learn in intro to optics, so pretty much any undergrad in physics learns this stuff. But now we've applied it to time, which no one has ever thought of in this way before. You could explain this to intro undergraduate students and they would get the idea. It's very simple, but it's also very powerful."

In addition to Hilbert (who will begin a tenure-track faculty position at Texas Lutheran University in the fall) Batelaan and Zewail, the other co-authors of the paper are Cornelis "Kees" Uiterwaal, associate professor of physics at UNL, and Brett Barwick, a postdoctoral scholar with Zewail at Cal Tech. Barwick earned his Ph.D. at UNL, studying quantum optics with Batelaan from 2002 to 2007, before going on to Zewail's lab, and provided the critical bridge to bring the collaboration about.

"He (Zewail) called us and said, 'Hey, do want to work with us?'" Batelaan said. "He knew that with Brett, we had developed techniques that allowed us to explore this kind of research. He asked if we could go further with this application because the Cal Tech people want to use it to make movies of molecular processes.
"We've been looking through cloudy lenses in a pair of glasses. Now, we've figured out that there is such a thing as clear glasses. I think it's really cool."

For more information see the paper, Temporal lenses for attosecond and femtosecond electron pulses, at the Proceedings of the National Academy of Sciences site.

Posted by Barbara G. Goode, barbarag@pennwell.com, for BioOptics World.

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