The World's Fastest Nano-Optical Shutter

By Roland Piquepaille

Physicists from several U.S. labs have clocked the transition of vanadium dioxide nanoparticles from a transparent to a reflective, mirror-like state, at less than 100 femtoseconds (a tenth of a trillionth of a second). According to this Vanderbilt University report, this effect has a size limit: "it does not occur in particles that are smaller than about 20 atoms across (10 nanometers)." This opens the door -- if I can say so -- to windows that are transparent at low temperatures and block out sunlight when the temperature rises. But other applications are possible, such as nanosensors which could measure the temperature at different locations within human cells, or "ultrafast" optical switches which could be used in communications and optical computing. Read more...

Let's start with an image which probably took lots of work to its creator, René Lopez.

This is a rather surprising "nanoscale image of Don Quixote and Sancho Panza created in vanadium dioxide and shown in false color. When the temperature is below 68 degrees Celsius the material is transparent so the image isn't visible. When the temperature rises above this point, however, the material becomes reflective and the image appears." (Credit: René Lopez, Vanderbilt University)

[Note: the transition from visible to non-visible state of Don Quixote and Sancho Panza is viewable (no punt intended!) from the VU article or directly here (Macromedia Flash format).]

Now, let's go back to the facts as exposed by the Vanderbilt University (VU) online journal.

How this material (vanadium dioxide or VO2) can turn from a transparent insulator into a reflective metal so rapidly has physicists scratching their heads, but a collaboration among 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, a Vanderbilt physics professor. "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 a definitive explanation for that rapid a change."

But now, the researchers think they have one.

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).

The new Vanderbilt Institute of Nanoscale Science and Engineering (VINSE) allowed these researchers to go further and to discover that this switch effect had some size limits.

This has allowed them to verify that nanoparticles undergo the same phase transition as thin films. They also have determined that the effect has a size limit: It does not occur in particles that are smaller than about 20 atoms across (10 nanometers). The researchers have established that it is possible to raise and lower the temperature at which the insulator/metal transition takes place by as much as 35 degrees Celsius by adding small amounts of impurities.

And this temperature effect leads to new applications.

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.

Of course, there will be other applications for such a fast 'phase transition' effect.

For example, they are exploring whether they can create an "ultrafast" optical switch by putting a layer of vanadium dioxide nanoparticles on the end of an optical fiber. Such a switch could be useful in communications and optical computing.

This research work has been explained in a paper published by Optics Letters in its March 2005 issue under the name "Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance" (Volume 30, Issue 5, 558-560). Here is the text of the abstract.

We study the ultrafast insulator-to-metal transition in nanoparticles of VO2, obtained by ion implantation and self-assembly in silica. The nonmagnetic, strongly correlated compound VO2 undergoes a reversible phase transition, which can be photoinduced on an ultrafast time scale. In the nanoparticles, prompt formation of the metallic state results in the appearance of surface-plasmon resonance. We achieve large, ultrafast enhancement of optical absorption in the near-infrared spectral region that encompasses the wavelength range for optical-fiber communications. One can further tailor the response of the nanoparticles by controlling their shape.

Sources: David F. Salisbury, in Exploration, the online journal of Vanderbilt University, April 6, 2005; and various websites

Related stories can be found in the following categories.

• Materials
  
• Nanotechnology
  
• Optics
  
• Physics