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
(1015 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 field--106 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
