A Finer Cut of Light

JR Minkel

Focus

A Finer Cut of Light

February 8, 2002• Phys. Rev. Focus 9, 8

The world’s most powerful lasers may be able to produce light pulses 100,000 times shorter than today’s shortest.

Figure caption — **Big Beam Machine.** Extremely powerful lasers may be capable of producing light pulses measured in zeptoseconds ( $10^{- 21}$ s) –short enough to watch events in the atomic nucleus. The Rutherford Appleton Laboratory’s Vulcan laser facility in the UK is being upgraded and could test the new scheme (computer rendering shows new target area).

If you’ve ever seen dripping water droplets caught in a strobe light, you know roughly how physicists take snapshots of the fastest processes at work in an atom. A short enough pulse of light can probe the motions of molecules or even the electrons around single atoms. But no one has figured out how to take such “movies” of the interactions responsible for fission, fusion, and other nuclear events. Now, in the 18 February print issue of PRL, researchers propose a way of breaking the nuclear barrier with pulses so short they would be measured in zeptoseconds $(10^{- 21}$ s). The method involves blasting a small wire with light from one of the high power laser facilities now under construction.

The shortest laser pulses are now in the femtosecond $(10^{- 15}$ s) range, which is fast enough to watch vibrations of molecules. In the early 1990s, however, theorists reasoned that linearly polarized laser bursts focused onto an inert gas could chop electromagnetic waves into even finer pieces. The electrons of atoms caught in the laser light would oscillate, emitting 100-attosecond $(10^{- 16}$ s) or shorter bursts in the process. Researchers performed the first experiments based on these pulses just last year.

Now Alexander Kaplan of Johns Hopkins University and Peter Shkolnikov of the State University of New York at Stony Brook have proposed a way to go one better. Their calculations show that circularly polarized light from petawatt $(10^{15}$ watt) lasers–which are currently under construction around the world–could induce incredibly short blasts of synchrotron-like radiation. Accordingly, they call their proposal the “lasetron.”

Electrons caught in such a beam would rotate rapidly along with the spinning electric field of the laser light, which would cause them to pour out a tight cone of radiation. Viewed edge on, they would appear to flash for just a zeptosecond every time the cone of light came around again, as if from a miniature lighthouse. If the electrons were in a thin wire, the setup would act as an antenna, spitting out zeptosecond bursts twice every laser cycle at right angles to the incoming beam and wire. According to the uncertainty principle, these ultrashort pulses would have a very wide spectrum of energies–with some photons in the gamma radiation range, having more than 1 MeV of energy.

“This work is very far behind the horizon–but who knows,” Kaplan says. “The only thing I can do as a theorist is to wave a red flag or scream into [an experimenter’s] face.” One problem, he notes, is that detecting the ultraquick flashes all by themselves will be difficult. Fortunately, the moving electrons should act as a current and produce a massive magnetic $f i e l d - - 10^{6}$ Tesla, comparable to the field around a white dwarf star–for a few femtoseconds at a time. This field would scatter a beam of neutrons, which may give a much simpler, indirect test for the light pulses, as well as provide an interesting test bed for magnetic studies, Kaplan adds.

The lasetron idea does indeed pose tough challenges to experimentalists, says Ferenc Krausz of the Vienna University of Technology. The two big ones will be generating a single pulse and measuring its duration. But the challenge is exciting, Krausz adds, as “Kaplan and Shkolnikov may have opened up a fascinating new direction of research in ultrafast science.”

–JR Minkel

JR Minkel is a freelance science writer in New York City.