Sculpting light to maneuver molecules.

Sculpting light to maneuver molecules

Ever since lasers started becoming labhold items in the 1960s, scientists have dreamed of using them to precisely control the chemical behavior of molecules. Each type of chemical bond can absorb only specific amounts of energy, which correspond to certain frequencies of light. So, by exposing molecules to laser light tuned to those frequencies, chemists suspected they might someday be able to feed enough energy into specific bonds of a molecule to make them especially susceptible to reacting with nearby molecules.

Can they realize their dream of directing molecules' various chemical bonds with light the way a conductor uses a baton to louden the woodwinds while silencing the strings?

The answer started out as "who knows?" and changed to "almost certainly not" in the 1980s after many frustrated attempts, but it has now taken an upswing to "maybe eventually." MIT chemists Keith A. Nelson and Gary P. Wiederrecht teamed up with laser experts at Bell Communications Research in Red Bank, N.J., to create sequences of ultrashort laser pulses. They have used these to drive specific vibrations in a crystal lattice, likening the process to "repetitively pushing a child on a swing" so that the child swings higher and higher.

In the March 16 SCIENCE, the researchers describe how they produced and "shaped" vibration-driving sequences of 75-femtosecond laser pulses. As the pulses pass through a fine grating, their several optical frequencies separate. A lens directs these through a specially designed mask that alters, or "shapes," their relative phases. Another grating-and-lens duo then recombines the components and shines the sculpted pulses into a crystalline sample of pyrelene. When relaxed, pairs of pyrelene molecules in the crystal stack together. When pumped with specific wavelengths of light, the pairs vibrate toward and away from one another, with a net result of snuggling closer.

The team has succeeded in sculpting trains of laser pulses that ping pyrelene pairs into vibrational action. The pulses move the molecules by roughly one-thousandth of an angstrom--far too little to inspire most chemical reactions. To realize the dreams of the 1960s, laser-wielding scientists must learn more about the dynamics between light and molecular motions and how to sculpt more effective, ultrashort light pulses with ever more finesse, the researchers say.