Physicists have discovered a new form of stable atomic matter where a super-intense laser field yanks an atom's electron back and forth so fast that the electron doesn't fly out of the atom as it normally would.
Extensive supercomputer simulations demonstrating this result appear in today's issue of Science.
"This is a reversal of a very long trend that began with Einstein in 1905," says Joseph Eberly, professor of physics and optics at the University of Rochester. "A dramatic shift in viewpoint is required to explain the new physics of atoms in very strong laser fields."
Einstein won the Nobel Prize for his explanation of the photoelectric effect, which describes how atoms emit electrons when the atoms are bombarded by particles of light known as photons. Since then physicists have assumed that the more light they pump into an atom, the more readily electrons shoot out of it. But now Eberly and Kenneth Kulander of Lawrence Livermore National Laboratory have found that this is not so. Under super- intense laser light, the process reverses: The atom stabilizes and electrons become less likely to leave the atom.
"You could say that our work was prompted by the development of high-power, short-pulse lasers over the last decade, allowing us to look at questions no one has thought to ask before," says Kulander.
The extra energy the photons bring to the atom dramatically distorts the electron's "orbit" into a straight line as the electron is yanked back and forth by the laser's oscillating electric field, like a ping pong ball ricocheting back and forth between two paddles. In Eberly's and Kulander's studies, the laser forces the electron to travel 100 times further from the atom than it normally would.
The new orbit cuts down the electron's chances of shooting away from the atom. "As it races from one end of the atom to the other, the electron is rarely near the nucleus, which is the only place an electron can collect the momentum necessary to escape the atom," says Kulander.
In fact, says Eberly, the orbit of the electron is no longer defined by its attraction to the nucleus. "The electron has been freed from the nucleus, but now it's bound by the electric force from the laser light wave."
Kulander says electrons leave an atom most easily when the strength of the laser equals the force of an atom's nucleus, a laser intensity around 1016 watts per square centimeter. At higher intensities, the process reverses. His and Eberly's simulations centered around laser intensities 10 to 100 times stronger.
Such lasers bombard each atom with a stream of up to a billion photons in just one picosecond, or one-trillionth of a second. Today's most powerful lasers operate at this level, and many scientists are looking for laboratory evidence of the phenomenon.
"The photons are coming so thick and so fast that the atom must consider them as a group, as a wave," says Eberly. "The atom doesn't have time to recover from one photon before the next one hits it -- the photons are actually overlapping."
Scientists are already finding applications for closely related processes. These include above-threshold ionization and high harmonic generation, where scientists have found ways to make atoms produce high-energy electrons and photons. These could provide a source of short-wavelength light like X-ray lasers for use in a variety of applications, including advanced lithography and the study of biological materials, Kulander says.
Atomic stabilization by super-intense lasers was first suggested by Marvin Mittleman of City College of New York and Mihai Gavrila of FOM Institute in Amsterdam. Eberly and Kulander say that while the idea was greeted with skepticism just a few years ago, more physicists are accepting the concept as evidence mounts.
This work was sponsored by the U.S. Department of Energy.
tr