Lasetron to produce zeptosecond flashes

PHILIP BALL

The latest super-powered lasers could help make a lasetron © Lawrence Livermore National Laboratory

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 s).

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.

Flash glance

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 pulses², and in November a group of scientists in Austria, Germany and Canada made bursts of soft X-rays about 650 attoseconds long³.

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 (10^-12 s).

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.

Emission control

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, laser-like radiation.

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.

1. Kaplan, A. E. & Shkolnikov, P. L. Lasetron: a proposed source of powerful nuclear-time-scale electromagnetic bursts. Physical Review Letters, 88, 074801 (2002).
2. Paul, P. M. et al. Observation of a train of attosecond pulses from high harmonic generation. Science, 292, 1689 (2001).
3. Hentschel, M et al. Attosecond metrology. Nature, 414, 509 - 513 (2001).

© Nature News Service / Macmillan Magazines Ltd 2002