NEW ULTRAFAST LASER PROBES SECRETS OF SOLAR CELLS

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--The contents of a new laboratory at the University of California, Santa Cruz, are so sensitive that they require specially designed heating, air-conditioning, electrical-wiring, and water systems. Inside the lab, on top of tables that dampen all vibrations, underneath air filters that suck up all dust, and surrounded by thick black curtains that shield out all light, sits a glittering array of tiny mirrors, lenses, and prisms. Between them whizzes a narrow, often invisible, beam of light. This sophisticated apparatus, one of the first three in the nation, is a solid-state femtosecond laser.

Set up last spring by Jin Zhang, an assistant professor of chemistry at UCSC, the laser illuminates chemical reactions that occur so rapidly they are impossible to measure any other way. Like an ultrafast movie camera photographing many frames a second, the laser emits flashes of light that capture the very essence of these reactions: excited electrons plummeting from one chemical state to another. "The only goal we have for the whole laser system is to get very powerful, very short laser pulses so we can study very fast processes," says Zhang.

The pulses of light from the laser are extraordinarily brief: 100 femtoseconds, or 100 quadrillionths of a second. For comparison, 100 femtoseconds is to a second as a second is to 320,000 years. Appropriately enough, Zhang's research group nicknames the laser "Femticruizer."

Zhang aims Femticruizer at tiny beads of the semiconductor cadmium sulfide, CdS, with an ambitious goal in mind. He hopes to pave the way for highly efficient solar cells capable of harvesting sunlight in order to split water into oxygen and hydrogen--a clean, inexpensive, virtually unlimited fuel. Currently, solar cells convert only about 10 percent of the sun's energy into electricity; solar panels merely convert solar energy into heat, which cannot be stored. Three papers by Zhang's research group on the CdS project have been accepted for publication in scientific journals.

Zhang's system and the others like it in the U.S.--including one at UC San Diego--rely on a high-tech amplifier, alone worth $200,000. The amplifier enables the laser to put out 1,000 flashes of light every second--a rate 40 times faster than movie frames and 100 times faster than early femtosecond lasers. The amplifier also gives the new laser much more power than other femtosecond lasers, enabling it to send out pulses each energetic enough to light 50 million lightbulbs.

Two laser beams created by elements as exotic as argon, neodymium, and ytterbium activate and amplify the high-powered laser pulses. A lima bean-sized crystal of sapphire laced with titanium emits the final red light, making the laser a "solid-state" system, in contrast to the original femtosecond lasers that use liquid dyes to produce laser light.

After using dye lasers himself for several years, Zhang much prefers his solid-state setup. It is cheaper, less messy, and much more reliable. Dye lasers constantly require new dyes, whereas solid-state lasers run on a single, essentially unbreakable crystal. Many dye lasers and amplifiers also break down frequently. Zhang's titanium-sapphire laser, in contrast, works over 80 percent of the time. He completed an entire study in two days with the laser working 20 hours straight, unheard of with dye lasers, which rarely work for more than a few days a week.

One of the keys to the solid-state laser's impressive performance, Zhang says, is the $100,000 laboratory renovation he made to ensure an impeccably clean, temperature-controlled, vibration-free environment. Several labs are now using his scheme.

By providing details about chemical reactions responsible for solar energy conversion, the new laser technology may enable scientists harness the sun to split water--an accomplishment only dreamed about in the past. "In the '70s when there was an oil crisis, many research groups were studying how to dissociate water with sunlight," Zhang says. Certain materials, such as CdS, split water when the sun shines on them, but these solar cells are very inefficient.

Zhang induces the water-splitting process in his lab by zapping CdS with his laser. When the laser beam hits tiny clusters of CdS suspended in water, electrons in the CdS jump to higher energy levels, from which they can split water. Zhang monitors the high- energy electrons with flashes of a "probe" laser spaced every few femtoseconds. "It's like making a movie of the movement of electrons," he says. The reaction is complete when all the excited electrons tumble to their unexcited state, which takes about 3 trillionths of a second.

The key to improving the efficiency of solar cells is to force the electrons to stay longer in their high-energy state. Zhang's group has successfully accomplished this by partnering two semiconductors or by coating the solar cells with a light-absorbing substance similar to that found in green leaves. However, these techniques require much more research to become commercially viable.

Zhang is also using his laser to study reactions central to a new type of cancer treatment called photodynamic therapy. The technique, still considered experimental in the U.S. although approved for use in Canada, is a targeted form of chemotherapy with fewer side effects. Instead of injections of drugs that harm many cells of the body, photodynamic therapy uses drugs that naturally accumulate in tumor cells and are activated only when light shines on them. The wavelength of light necessary for the technique penetrates the skin easily and is the exact wavelength Zhang's laser emits.

Editor's note: You may reach Zhang at (408) 459-3776.

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