Enter the attosecond. Theorists long suspected that a femtosecond-size pulse of visible light is in fact composed of—and can be subdivided into—several attosecond-length light pulses, much as a musical note contains many harmonic tones. The problem is measuring them. Electromagnetic harmonics are very weak, with wavelengths in the ultraviolet and X-ray range—too short to be detected.
With a modified interferometer—a special light filter for lasers—Krausz and his coworkers set off to hunt for attoseconds. They fired ultrabrief (seven-femtosecond) laser bursts of red light into a stream of neon atoms, thereby stripping the electrons free of their neon nuclei. The electrons were then carried along by the laser pulse and almost instantly smashed back into the neon nuclei. The effect was a bit like that of shooting a bell. The collision, as predicted, produced high-frequency harmonic tones of X rays and extreme ultraviolet rays. The physicists then filtered the harmonic light, allowing only select bursts of X rays—including one only 650 attoseconds long—to pass.
No sooner had the physicists caught an attosecond pulse than they demonstrated its usefulness. They aimed an attosecond pulse and a longer pulse of red light into a gas of krypton atoms. The attosecond pulse excited the krypton atoms, kicking electrons free; then the red-light pulse hit the electrons and took a reading of their energy. By adjusting the time delay between the two pulses, the scientists gained a very precise measurement—within a matter of attoseconds—of how long it takes the electron to decay. Never before had electron dynamics been studied on so short a timescale.
The experiment set the physics world buzzing. "Attoseconds will give us a new way to think about electrons," says Louis Dimauro, a physicist at Brookhaven National Laboratory. "They become a new probe of matter that will then be applied across the sciences. The age of attophysics has begun."
Eventually, physicists hope to do more than just watch electrons gain and lose energy. "We can think of using attosecond pulses not only to trace these processes but also to control the relaxation of an atom following its excitation," Krausz says. "It's very exciting." For example, by controlling the means by which atoms, at attosecond timescales, release X rays, one might build an efficient X-ray laser, long a dream among physicists. The semiconductor industry, which has a thirst for speeding up computer chips, transistors, and other electronic devices, likewise wouldn't mind getting a taste of some attoseconds. "We know that every other advance that's led to shorter pulses has led to major advances," Corkum says. "This is the next step."
Of course, one day, perhaps not so very far in the future, even the speedy attosecond will fail to satisfy. Electrons will look downright poky. "As you go into smaller structures of matter, inside the atomic nucleus, processes become even faster," Krausz says. "In nuclear physics, the natural timescale is several orders of magnitude faster—in the realm of zeptoseconds," or sextillionths of a second.
In the meantime, physicists will have to manage with the little free time they've gained already. One can imagine them getting carried away: filling up their hard drives with electron home videos, clogging the airwaves with attosecond flicks that seem to yawn for seconds—for eternity, basically. Corkum assures that won't happen: "In practice, we're only looking at a reasonable period of time." In small time, he says, just like in the big time, viewer boredom still sets the limits. "My brother-in-law recently sent some movies of their baby," Corkum says. "It was fun at first, but after 15 minutes—wow, that's a lot of time."
How far is a second?
Time has been chopped so fine by modern physicists that its subdivisions are getting harder and harder to grasp. One way to keep them in perspective is to imagine a road trip from Los Angeles to New York City—2,787 miles—that takes only one nanosecond to complete. In one picosecond, the car would make one thousandth of the trip—about 2.8 miles, or all the way to East Los Angeles. In one femtosecond, it would have gone a millionth of the way, or less than two car lengths. An attosecond—the shortest time interval now measurable—would account for only a billionth of the trip: about a fifth of an inch.
Were the car to keep on going for the relative eternity of a full second, it could make half a billion trips to New York and back. At that pace, it would be traveling at 16 million times the speed of light. Of course, the driver could just as well choose to ignore New York entirely and head into orbit. Were he to spend the entire second leisurely circling Earth, he would make it around the planet about 112 million times. Were he to circle the solar system instead, he'd manage about 120 tours of the sun on Pluto's orbital path.
Sky photograph by Bill Frymire/Masterfile.
The Vienna University of Technology's attophysics page has a number of links to relevant papers: info.tuwien.ac.at/photonik.
A rather technical description of attosecond pulse generation, from the Max Planck Institute for Quantum Optics: www.mpq.mpg.de/lpg/research/attoseconds/attosecond.html.
An accessible explanation of Ahmed Zewail's work in femtochemistry can be found at www.nobel.se/chemistry/laureates/1999/press.html.
Read more about Ahmed Zewail's work with femtosecond pulses in "Welcome to Femtoland," by Gary Taubes, Discover, February 1994, page 82.
A little rusty on metric prefixes and conversions? Get some remedial help at www.metric.fsworld.co.uk/siprefix.htm.