Optical excitations, molecule by molecule.

In a feat of spectroscopic magic, two researchers have used a finely tuned laser to transform the optical properties of individual molecules, making them "disappear" and, possibly, rendering them useful for storing information.

Last year, physicist W.E. Moerner and his colleagues at the IBM Almaden Research Center in San Jose, Calif., demonstrated that they could monitor the light given off by single molecules lodged in a crystal (SN: 1/19/91, p.37). Now, Moerner and IBM physical chemist Thomas Basche have discovered that single-molecule spectroscopy works with materials widely dispersed in amorphous solids, revealing interesting quantum-mechanical properties about embedded molecules. They describe their new findings in the Jan. 23 NATURE.

For their experiment, Moerner and Basche made a clear, 10-micron-thick, polyethylene film that contained the organic compound perylene. They cooled the film to 1.5 kelvins and trained a tunable laser on it. Perylene molecules resonate and fluoresce when excited by laser light of a particular wavelength. The pressure exerted by the surrounding polyethylene alters this property, so that different perylene molecules resonate at slightly different frequencies.

Using a very efficient detector to monitor perylene's fluorescence spectrum, the IBM team discovered that an excited perylene molecule puts pressure on adjacent polymers, thereby changing its local environment. That change, in turn, slightly shifts the perylene's energy levels so that it no longer responds to the same wavelength laser. The perylene ceases to fluoresce, and its spectral signal disappears.

Other scientists have demonstrated this disappearing act, called "hole burning," but with thousands to millions of molecules at the same time. "We're modifying the optical properties of this impurity molecule by molecule," says Moerner. "It's the first time anyone has done this on a single molecule."

The IBM researchers know they are affecting one molecule at a time because of the abrupt disappearance of the spectral signal, Moerner says. If they were exciting several molecules, the fluorescence would fade little by little and the signal would change gradually.

Sometimes, a molecule recovers its original state; other times, it does not, Moerner and Basche report. The more powerful the laser, the faster the molecule's signal disappears.

"In being able to switch and read the molecular state, [Moerner and Basche] have the makings of a molecular memory element," says Dietrich Haarer, a physicist at the University of Bayreuth in Germany.

Although this kind of data storage could pack information more densely than other optical approaches, the technology remains speculative, Haarer and Moerner agree. "You'd want a system that is reversible, and you'd want to be able to control when it comes back," notes Moerner. "It's just a glimmer of an idea."