Lasers In A New LightNeil Gross
The pinprick pulses of light from Katsumi Tanigaki's tiny laser seem unlikely vehicles for landmark scientific discoveries. But as he peers at the faint green flashes, the 41-year-old NEC Corp. research manager expects nothing less. He has already pioneered ways to use beams of light to create pure silicon films by severing bonds between molecules--a key to packing more circuits on semiconductors. Now, Tanigaki and his colleagues at NEC's Exploratory Research Laboratory in Tsukuba expect lasers to deliver far grander breakthroughs. "People say that lasers are a mature technology," he muses. "But their most exciting applications are only now coming into view."
It's true: For 30-odd years, scientists have understood how lasers create intense, focused beams of light, and how they alter the characteristics of molecules and atoms. But never before have researchers from so many fields probed these effects. Chemists call it laser chemistry. Physicists try a different tack, called atom optics, to manipulate and study individual atoms. Whatever the name, "chemistry and physics are converging, and lasers are at the crossroads," says Tanigaki.
That convergence promises to create "new devices and new markets," predicts John Weiner, program director for atomic, molecular, and optical physics at the National Science Foundation. Within a decade, the ability of lasers to manipulate matter could lead to inexpensive flat-panel displays, ultraprecise measuring devices, computer storage systems with 1,000 times as much capacity as today's disk drives, and new materials with improved electrical characteristics and other yet-unimagined properties.
ATOM WIGGLING. From their birth in 1960, lasers have been ideal analytical tools: If you hit a molecule with a laser beam tuned to the right frequency, it will rescatter the light into a specific pattern, or change it in some way. This technique, called laser fluorescence, reveals much about the structure of molecules and what binds them. Researchers reasoned that the same process could alter or selectively break molecular bonds to produce chemicals--such as engineering plastics used in cars and other equipment. In the early 1980s, lured by new lasers that emitted ultraviolet light, big corporate labs in the U.S. and Europe flocked to the field as possibly a faster, cheaper alternative to chemical synthesis, the traditional way of altering molecular bonds to produce materials. But laser techniques were too crude, and the multinationals lost interest.
Scientists pressed on, though, and now they are on the verge of major advances. Over the past year, researchers at the University of Michigan's Center for Ultrafast Optical Sciences and Kent State University have split light beams into separate colors. Using liquid crystals, they vary the intensity and timing of the different-colored beams. Then they reassemble them into pulses of light with precisely engineered shapes. They hope that these shaped pulses will be far better than ordinary laser beams at teasing apart selected chemical bonds.
At Massachusetts Institute of Technology, meanwhile, chemistry professor Keith A. Nelson is training extremely short pulses of laser light on atoms in crystals. So far, he can wiggle individual atoms, observing their reactions using laser fluorescence. Soon, Nelson hopes to rearrange atoms in the crystal, which would let him construct entirely new materials. The trick will be making the changes stick, by adjusting the sequence of laser blasts. "It isn't obvious yet what sequence to use," Nelson says.
FULLERENES. Serendipity could provide an answer. It happened in 1984, when Richard E. Smalley, a chemistry professor at Rice University, trained lasers on carbon molecules. The laser vibrations set off a chemical reaction, producing clusters of molecules in unique geometric configurations called Fullerenes, for their resemblance to Buckminster Fuller's geodesic dome. Today, a dozen companies are making them in hopes of creating new superstrong plastics or catalysts.
The Japanese also are making strides. Over the years, Tokyo's Ministry of International Trade & Industry (MITI) has helped finance dozens of laser research projects, most for boosting the performance of lasers. But a few, such as one at Idemitsu Kosan Co., Japan's largest domestic oil company, chase important materials. There, team leader Nobuo Shimo uses a laser that emits ultraviolet light to irradiate a tank filled with organometallic gases. With a single burst of light, Shimo can produce a few grams
of magnetic materials called ferrites. These are used in electronic components, such as amplifiers, and command prices high enough to justify the cost of using lasers. "We don't have perfect selectivity," says Shimo, "but we have tricks to get around that."
More widely used products of laser chemistry could be novel data-processing and storage devices. In today's CD-ROMs and audio CDs, laser light is reflected off microscopic pits stamped into a plastic disk. Lasers inscribe and read data in these pits as digital information. Such disks can store up to 500 million bytes of data. But if instead of making little pits, you switched individual molecules on and off inside a block of solid material, then each square centimeter could hold roughly 10 billion bytes. That's enough to fit 1 trillion characters, or a stack of New York City's bulky Yellow Pages, on a sugar cube-sized block.
To that end, Sony, Hitachi, NEC, and a few others are working on molecular storage concepts first expounded by IBM in the late 1970s. Big Blue's scientists saw that the molecules of certain crystals and glass are filled with tiny imperfections, causing them to absorb light at slightly different frequencies, or colors. By hitting a molecule with a laser of the right frequency and intensity, you can change the frequency at which it absorbs light. The detectable change can then be read as a bit of information.
The next, more difficult step will be finding suitable materials for this approach. Already, the Japanese are amassing huge data bases on how different materials react to varying frequencies and intensities of light. "It won't happen quickly," predicts Wilfried Lenth, a research manager at IBM's Almaden Research Center in San Jose. But it's the kind of painstaking pursuit at which Japanese scientists excel.
Similar photochemical reactions may lead much sooner to improved flat-panel screens for computers and TVs. In today's active-matrix liquid-crystal displays, each dot is switched on and off by a separate transistor. Fabricating large screens with millions of transistors is a manufacturing nightmare. By mixing photosensitive molecules with a special liquid crystal, Tomiki Ikeda, a laser chemist at Tokyo Institute of Technology, showed in January that he can replace the electronic grid with a simpler configuration of lasers. They trigger chemical reactions in the materials sandwiched between the plates that make up the screen. The result could ultimately be sharper pictures, larger screens, and lower manufacturing costs, says Ikeda.
CROSS FIRE. Physicists, too, see payoffs from using lasers. Atoms normally are in constant motion. But by capturing them in a cross fire of laser beams, scientists at Stanford, MIT, the National Institute of Standards & Technology, and other labs "cool" atoms into slow motion, so they can be manipulated. In this way, they hope to create ultraprecise measuring equipment, such as gyroscopes used in aerial navigation or tools for sculpting miniature machines.
One leader in cooling atoms is Steven Chu, a physics professor at Stanford University. After other researchers had shown that a laser beam tuned to the right frequency can slow atoms down, Chu trained six beams on one atom, blocking it in every direction. In those conditions, the atom freezes to a temperature just a few millionths of one degree above absolute zero, -460F. Cooled atoms can be tossed in the air in a "fountain." Scientists measure their acceleration, and so determine the strength of the gravitational field tugging at them as they fall.
Chu's measurements are sensitive enough to detect differences in the gravitational field of just three centimeters' movement from the center of the earth. He can also take readings a few times per second, compared with one measurement every 10 seconds with older techniques. Although still only one-thirtieth as accurate as today's best current techniques, Chu's approach, say scientists, has the potential to produce the most accurate measurements. Within a decade, Chu thinks that gravitational sensors on trucks or aircraft could pinpoint oil deposits better than existing methods because they could best measure the differing densities between oil and the surrounding material. Dozens of labs are also building atomic clocks that could be 100 times as accurate as devices currently used in navigation, scientific testing, and communications systems: The best of these are accurate to one second in a million years.
DASHED DREAMS. At the National Science Foundation, meanwhile, physicist Weiner envisions beams of cooled atoms replacing some of today's semiconductor fabrication equipment. For years, engineers have used beams of atoms or molecules to construct minuscule electronic structures on the surface of silicon. The trouble is, focusing a beam of normal, "hot" atoms leaves the beam too weak to deposit the thin lines of atoms needed to build tiny structures. Using cooled atoms overcomes the problem, says Weiner. That would make it easier to craft tiny engines, pumps, and other "micromachines" that will repair damaged circuits or monitor key substances in the body, such as blood-sugar levels.
All this progress doesn't dazzle skeptics, who have watched the inadequacies of lasers dash one bright dream after another. Many lasers are still not as efficient as they need to be. And the lasers that are best suited to chemical or atomic manipulations can run up to $80,000. "Lasers still cost too much," says Koichi Tsukamoto, senior reseacher in photon processes at MITI's Electrotechnical Lab.
Still, lasers are improving fast. Manufacturers are boosting their power and life spans, and bringing down costs. As that continues, the painstaking work of 30 years should start to pay off in better living through physics and chemistry.