Lasetron to produce
world's shortest bursts of light could make nuclear
|The latest super-powered
lasers could help make a lasetron|
|© Lawrence Livermore
Meet the Lasetron. It's a tool physicists in the
United States have dreamt up to chop light into smaller
pieces than ever before. If it works, pulses of light
lasting just a few zeptoseconds could soon be looking
inside atomic nuclei. A zeptosecond is
one-billion-trillionth of a second (10-21
The lasetron could also generate a magnetic pulse
many times stronger than any other available on Earth -
about ten billion times stronger than the geomagnetic
field. This could be used to study exotic astrophysical
environments like those on neutron stars.
Lasetrons don't exist yet, but in theory they would
create fleeting flashes by focusing high-power lasers
onto tiny particles or very thin wires. So say Alexander
Kaplan of Johns Hopkins University in Baltimore,
Maryland, and Peter Shkolnikov of the State University
of New York at Stony Brook1.
Other researchers have recently trimmed pulses of
electromagnetic radiation to a few hundred attoseconds
or billion-billionths of a second (10-18 s).
Last June a European team made a train of 250-attosecond
and in November a group of scientists in Austria,
Germany and Canada made bursts of soft X-rays about 650
Flashes this short are in demand for probing events
in the atomic world. The making and breaking of chemical
bonds, or the oscillation of an atom at the end of a
spring-like bond, typically take place on timescales of
femtoseconds (10-15 s) to picoseconds
Femtosecond laser pulses are now routine. They are
used in a kind of ultrafast flash photography that
catches atomic motions. If such pulses can be made
bright enough, scientists hope to watch entire molecules
changing shape during a chemical reaction.
Attosecond time resolution makes it possible to study
the movement of electrons. The electrons that bind atoms
together move even faster than the atoms themselves,
adjusting almost instantly to atomic motions.
But events briefer still occur within the nuclei at
the heart of atoms. Here the distances are even shorter,
and things happen in a zeptosecond or less. Following
processes such as neutrons and protons coming together
to form a nucleus, or coming apart during nuclear
fission, needs zeptosecond flashes to illuminate a
sequence of freeze-frames.
Kaplan and Shkolnikov hope to watch these processes
by exploiting the way that subatomic particles such as
electrons emit radiation when accelerated into a
circular path with a speed close to that of light. This
emission is called synchrotron radiation.
If the light of extremely powerful (petawatt or
1015 W) lasers is circularly polarized,
Kaplan and Shkolnikov suggest that it should stimulate
electrons to move in a tight helix and emit radiation in
zeptosecond bursts. The researchers show that many
electrons can be stimulated in this way at the same
time, creating bright zeptosecond pulses of coherent,
In a lasetron, the stimulating laser light would fall
on a thin wire and the ultrashort pulses would be
emitted sideways - in both of the directions at right
angles both to the incoming laser and the wire.