Timing nature's fastest optical shutter

It's nature's fastest quick-change artist: In less than the time it takes a beam of light to travel a tenth of a millimeter, vanadium dioxide can switch from a transparent to a reflective, mirror-like state.

How this material (VO2) can turn from a transparent insulator into a reflective metal so rapidly has physicists scratching their heads, but a collaboration of researchers at Vanderbilt, Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory has clocked the transfiguration at one-tenth of a trillionth of a second.

"The change from insulator to metal is called a phase transition," explains Richard Haglund, the Vanderbilt physics professor who directed the study published in the March issue of Optics Letters. "Phase transitions in solids generally occur at the speed of sound in the material, but vanadium dioxide makes the switch 10 times faster. So far no one has succeeded in coming up with an excuse that allows it to occur with this kind of speed."

Vanadium dioxide's quick-change act isn't merely a matter of academic interest, although there's plenty of that:

· In 1982, a patent was filed on the idea of using a thin film of vanadium dioxide as the active ingredient in "thermochromic windows" as an energy saving device. When the material is colder than 68 degrees Celsius (154 degrees Fahrenheit) it is transparent. When it is heated a few degrees higher, however, it becomes reflective. So the basic idea is to create windows that are transparent at lower temperatures and then block out sunlight when the temperature soars, cutting down on air-conditioning bills.

· A more futuristic potential application is to use vanadium dioxide nanoparticles as microscopic thermometers. It is relatively easy to change the material's transition temperature to body temperature (98 degrees Fahrenheit; 37 degrees Celsius) by adding precise amounts of impurities. Such doped nanoparticles would be small enough to measure the temperature at different locations within an individual cell and, when injected into the body, could pinpoint hot spots by turning into microscopic mirrors.

· Other applications that have been suggested for this unusual material include chemical sensors, transparent electrical conductors and various kinds of ultra-fast electrical and optical switches.

Although vanadium dioxide's insulator/metal phase change has been known for some time, it is only recently that scientists have discovered how rapidly it can occur. In bulk quantities, the transition is dominated by the rate at which heat can spread through the material, which is a relatively slow process. Scientists only discovered the unusual speed with which vanadium dioxide makes this transition when researchers found ways to make it in layers so thin (100 nanometers) that it takes 1,000 of them to equal the thickness of a human hair. The discovery was made by co-author Andrea Cavalleri and his collaborators, first at UC San Diego and then in Robert Schoenlein's group at Lawrence Berkeley National Laboratory.

Schoenlein's group, working with thin films, and the Vanderbilt group, working with nanoparticles, reported these extremely fast switching speeds in the last year, but they weren't certain whether the transition they were measuring went from the insulator phase to an intermediate state - a phenomenon common in other materials with fast transition times - or directly to the metallic state.

In the latest paper, the researchers answered that question by detecting the appearance of a phenomenon called "surface plasmon resonance." This is a form of electron wave that only occurs on the surfaces of metals and is responsible for the glowing colors of stained glass. Detection of this effect confirmed that vanadium dioxide can switch all the way from transparent to reflective in less than 100 femtoseconds (a tenth of a trillionth of a second). Matteo Rini, a post-doctoral scholar in the Schoenlein group, carried out the ultra-fast optical measurements on nanomaterials prepared and characterized by René Lopez, research assistant professor of physics at Vanderbilt.

"We know that the reverse process - going from metallic to insulator - is somewhat slower," says Haglund, "But we don't know how much slower because we haven't been able to measure it."

Additional collaborators in the vanadium dioxide studies are Leonard C. Feldman, Stevenson Professor of Physics at Vanderbilt University, and Lynn A. Boatner and Tony E. Haynes from Oak Ridge National Laboratory.

Source: Vanderbilt University