快猫短视频

FERENC KRAUSZ鈥橲 picture looks just the way you might draw a light wave 鈥� a brief wiggle on a dark background, with an ethereal, transient quality to it. But it becomes all the more impressive when you realise it isn鈥檛 a drawing of a light wave. It is the first ever photograph of one.

The photo was published last August to surprisingly little fanfare, considering that Hungarian-born Krausz is already a scientific celebrity for work on ultra-fast lasers. Yet he says this picture probably represents the most profound scientific achievement of his life, for one simple reason: 鈥淭his is the most direct experimental evidence for light being an electromagnetic wave,鈥� he says.

Until now evidence for the wave nature of light has been circumstantial. In 1801, Thomas Young carried out his famous double-slit experiment in an attempt to find out if light was made of waves or particles. He saw that passing a beam of sunlight through two parallel narrow slits casts a pattern of light and dark stripes. His findings were taken as evidence that light is made of waves 鈥� the stripes are caused by light waves spreading out from each slit and interfering with each other, in the same way that ripples in a pond create swells and troughs when they cross. Sixty years later, James Clerk Maxwell proposed that light consists of oscillating electric and magnetic fields travelling through space.

Despite this understanding, no one has ever tried to capture the quivering wave motion of light because it is simply too fast. A beam of visible light completes a cycle of oscillation in about a millionth of a billionth (10^-15) of a second. This means the electromagnetic field imaged by Krausz鈥檚 group changes direction one million billion times each second.

It鈥檚 a bit like trying to take a picture of a friend sprinting on the football field. They come out blurred in the pictures because they move appreciably in the time that the shutter on your camera stays open. Krausz, however, has the ultimate in sports photography at his disposal: an attosecond photographic shutter, developed in 2003. It opens and closes every few hundred billion billionths of a second 鈥� one-tenth the length of a light cycle (快猫短视频, 6 November 2004, p 34).

Of course, Krausz鈥檚 shutter is no ordinary camera aperture. He uses a laser that creates identical short X-ray pulses that last just a few hundred attoseconds (10^-18 seconds). 鈥淎 couple of groups worldwide can make attosecond pulses, but we were the only ones to have them come out one pulse at a time,鈥� says Reinhard Kienberger who is a member of the team.

This was an important step in creating the picture, which was published in the journal Science last year (vol 305, p 1267). Eleftherios Goulielmakis and others in Krausz鈥檚 group at the Vienna University of Technology in Austria and the University of Bielefeld in Germany sent a light wave into a chamber of neon gas. Along with the light wave, they sent in an attosecond X-ray pulse to image it. To be sure which part of the wave they were imaging, the pulse was synchronised to the light.

The X-rays knock electrons from the atoms in the gas, which then move towards a detector. But the electric field of the passing light beam either slows or speeds up the motion of these electrons, depending on its direction and intensity (see Diagram). So each electron鈥檚 arrival time at the detector provides an instantaneous picture of the light beam on its journey through the chamber.

Many readings have to be added together to create an image of the whole light wave. So just as celebrities posing for photographs in the 19th century had to stand still for several minutes, Goulielmakis and Kienberger ran the laser pulses for as long as an hour. The final picture was made of about 6 million independent exposures of identical passing light waves.

鈥淲hen we first saw the trace it was a very special moment, because it had never been done before,鈥� says Kienberger. 鈥淚n basic physics class you learn about the waveform of light, but you only measure its intensity. You never actually capture the trace in such a beautiful way.鈥�

Sandu Popescu at the University of Bristol in the UK points out that such oscillations have been imaged for electromagnetic waves with longer wavelengths, such as radio waves, but this is a first for visible light. 鈥淚t鈥檚 a nice way of seeing a wave as a wave,鈥� says Tony Short, also at Bristol. 鈥淭o actually see it waving is as direct as we鈥檝e got so far.鈥�

Physicists, of course, knew in advance exactly what the shape of a light wave would be, but they still got a kick out of actually seeing it take shape. 鈥淚t鈥檚 like when you get a present and you know what鈥檚 inside the box, but then you open the box and hold it in your hands,鈥� says Kienberger.

It is that power that has researchers excited. Being able to capture the quivering field of light means they might also be able to capture atomic processes on the same timescale, many of which are not as well understood as light waves. By changing the conditions in the chamber 鈥� for example by filling it with atoms undergoing various physical processes 鈥� they can see the effect of those processes written onto their light pulse (see 鈥淔rom light to matter鈥�).

For now, though, imaging light beams is bringing other rewards. Krausz has co-founded a company based in Vienna called Femtolasers to sell his synchronised lasers, at around half a million dollars a pop. There is interest from scientists who want to take their own photos of fleeting light beams, and those who want to try their hand at imaging other quivering fields. 鈥淚t鈥檚 impressive,鈥� says Stephen Leone, a laser chemist at the University of California in Berkeley. 鈥淲e鈥檙e buying them.鈥�