Using high-energy laser pulses,
physicists have recently broken time down
to attoseconds—fractions of a second so small
the digits on a clock would seem
to go on forever. We're used to seeing
Olympic skiers win events by
hundredths of a second. A skier who
won by a single attosecond would
be ahead by less than the width of an atom—
less even than a proton. Insignificant as they sound,
such time frames are opening new windows
onto chemical reactions and
other impossibly speedy events.
Time just got shorter. Granted, it was pretty short already: Four years ago, physicists managed to create a pulse of laser light lasting only five femtoseconds, or five quadrillionths (5 x 10-15) of a second. In everyday photography, a camera flashbulb can "stop time" at about 1/1,000 of a second—fast enough to freeze the swing of a baseball batter, if not a speeding fastball. Likewise, the femtosecond "flashbulb" enabled scientists to observe phenomena never before seen in freeze-frame: vibrating molecules, the binding of atoms during chemical reactions, and other ultrasmall, ultrafleeting events.
But ultrafast is not good enough. All kinds of important things can happen between one quadrillionth of a second and the next, and if your flashbulb is too slow, you'll miss out. So scientists have been pressing on, punching the clock, hurrying to create even tinier windows of time through which to study the physical world. Recently, an international team of physicists finally succeeded in breaking the so-called femtosecond barrier. With a complex, high-energy laser, they generated a pulse of light little more than half a femtosecond long—650 attoseconds, to be precise. The attosecond (10-18 second) has long existed as a theoretical entity, but this is the first time anyone has actually seen it. It's a newfound slice of time—a tiny one but with gargantuan potential.
"This is the real timescale of matter," says Paul Corkum, a physicist with the Steacie Institute for Molecular Sciences in Ottawa and one of the principal investigators in the study. "We're gaining the ability to look at the microworld of atoms and molecules on its own terms."
Although the fact is rarely appreciated, humans function in—and rely upon—several different timescales simultaneously. The average human heart beats once per second. Lightning strikes in a hundredth of a second. A home computer executes a single software instruction in nanoseconds, or billionths of a second. Circuits have switching times in picoseconds, or trillionths of a second. The shorter time gets, the harder it is to keep up with.
The invention of the laser in the 1960s offered a boost to scientists struggling to keep pace. The most common type of laser works by exciting the atoms of a noble gas like neon. (Other lasers work with solids, such as rubies, or even with organic dyes.) As the atoms "relax" and their electrons fall back into place, the gas glows at a characteristic wavelength of light—visible, microwave, red, or blue; it all depends on the atom involved. A laser forces the light waves to travel in unison and focuses the glow into an intense beam of light.
Creating a laser pulse is trickier. Physicists first use tiny mirrors to make a light beam run back and forth across itself inside a laser. Where the waves of light interfere—where their peaks and valleys coincide—spots of darkness and spots of light result. Tiny, ultrafast shutters can then be used to eliminate all but a single wavelength. Voilà, a pulse of light.
By the late 1980s, the laser pulse had reached a record brevity of six femtoseconds. (For a rough sense of scale, one femtosecond is to 90 seconds what 90 seconds is to the age of the universe.) No longer were researchers relegated to watching before-and-after pictures of chemical reactions; now they could watch slow-motion movies of the intermediate states. In the years since, a new science, femtochemistry, has come to focus on the mechanics of photosynthesis and other molecular reactions. In 1999 Ahmed Zewail, at Caltech, won the Nobel Prize in chemistry for a series of elegant experiments that revealed how chemical bonds break and re-form over a timescale of 100 to 200 femtoseconds.
The femtosecond pulse isn't just a camera shutter or a flashbulb; it has evolved into a powerful tool. It is superb for drilling tiny holes: Its energy is deposited so quickly, there's no time for the surrounding material to heat up, so there's less mess and inefficiency. Also, femtosecond pulses are only about a thousandth of a millimeter long. (In contrast, a pulse of light one second long would stretch from Earth to the moon.) Think of them as tiny bombs. They can be focused to strike just below the surface of a transparent material without actually piercing it. Femtosecond pulses are being used to etch optical waveguides inside panes of glass—a development that could revolutionize data storage and telecommunications. Femtosecond researchers are developing a new method of laser eye surgery that operates directly on the cornea without damaging the tissue above it.
"It's a way of putting your hand inside biological materials, and doing so with very little energy," Corkum says.
In short, the femtosecond is great for handling whole atoms and molecules. But for the physicist interested in electrons, which are far smaller, lighter, and faster than the atomic nuclei they swarm around, that timescale is just too slow. "We're interested in taking this a step further," says Ferenc Krausz, a principal investigator of the study and a physicist at the Photonics Institute at the Vienna University of Technology.
