Bright prospects for laboratory lasers.

Bright prospects for laboratory lasers

In physics, power is often the key tonew discoveries. That's why particle physicists who hunt for new kinds of matter want to build larger and more powerful accelerators. And it is also why atomic physicists have dreamed of making bright lasers with which to study the largely unexplored interaction between atoms and intense pulses of light.

Three groups of laser scientists are onthe verge of achieving that goal. Led by the work of Charles K. Rhodes at the University of Illinois in Chicago, researchers at Princeton (N.J.) University and Los Alamos (N.M.) National Laboratory have produced very intense laser beams by combining recent advances in making very short laser pulses, in focusing laser beams to small spots and in developing lasers that can handle high energies. At Los Alamos, for example, Gottfried Schappert and Robert Gibson announced last week that they have focused a 10(10)-watt beam from their krypton-fluoride gas laser to a spot less than 5 square microns in size. That gives them what they say is a record-setting intensity of greater than 10(17) watts/cm2 in the ultraviolet.

At this intensity, the electric field of alaser pulse that is shined on an atom is strong enough to compete for electrons with the nucleus's electric attraction, called the coulombic field. Ryszard Gajewski, director of the Division of Advanced Energy Projects at the U.S. Department of Energy in Germantown, Md., predicts that within half a year, the three groups will be producing pulses with electric fields that are much greater than the coulombic field.

"We are about to enter a new frontier inphysics,' he says. "If an electron's behavior is controlled by the laser field rather than the coulombic attraction of the nucleus, that's altogether a new regime. Lord knows what we'll find.'

As first suggested by Rhodes, veryintense ultraviolet beams from gas lasers will also open the door for making laboratory-scale X-ray lasers. The idea is to pump the X-ray laser with the ultraviolet laser by using its intense pulses to excite atoms to such a degree that they radiate X-rays. Laser physicists have long sought X-ray lasers because their short wavelengths would enable scientists to probe the structure of materials in remarkable detail.

The recent advances in powerful gaslasers have relied essentially on improving conventional technologies. At the University of Rochester in New York, Gerard Mourou and his colleagues have invented a new approach for producing intense, short laser pulses, which he says has given his group the most powerful tabletop laboratory laser in the world. Mourou's group has been working with a solid-state laser--the laser signal is amplified when it passes through a piece of glass doped with neodymium.

According to Mourou, the intensity ofbeams in these lasers has been limited because at a high intensity, the beam is distorted as it passes through the glass. To get around this problem, scientists have reduced the intensities in the glass by increasing the beam size. But this means that they have had to build very large and unwieldy amplifying systems to accommodate the larger beams.

With Mourou's technique, which hecalls "chirped pulse amplification,' researchers reduce the intensity by stretching out a laser pulse--making it last 1,000 times longer--before it enters the amplifying glass. After amplification, the researchers compress the pulse back to its original 1-picosecond duration. The same basic technique was used 40 years ago by radar scientists who were trying to use short radar pulses for accuracy while also using the high energies necessary for long range.

With chirped pulse amplification, saysMourou, "we can use a system that is 1,000 times smaller. This means that a system that was the size of a building becomes the size of a table. There's an enormous gain in compactness.'

Mourou's technique also means thatthe power from existing laser systems could increase by 1,000 times--and perhaps by 10,000 times in the near future. He says it could easily be applied to NOVA and other large lasers being developed for fusion, for X-ray laser work and for weapons simulation research (SN: 5/31/86, p.348).

Wayne Knox, at AT&T Bell Laboratoryin Holmdel, N.J., calls Mourou's technique "something very significant because it is a totally different approach.' The significance of Mourou$'s work, he says, is not only what he has already achieved, but how far he may be able to go in the future.