Fast Track

Nanoscale Switch Breaks Miniaturization Barrier

Industry and government are investing heavily in efforts to integrate optics and electronics as part of the evolution of information and communications technology. Tiny gears and cogs help form the basis of miniature devices, as seen in this highly magnified image. (VOLKER STEGER/SANDIA NATIONAL LAB/SCIENCE PHOTO LIBRARY)

 

An ultrafast, ultrasmall optical switch could advance the day when photons replace electrons in the innards of consumer products ranging from cellphones to automobiles. The new optical device can turn on and off trillions of times per second. It consists of individual switches that are only one five-hundredth the width of a human hair (200 nanometers) in diameter.

This size is much smaller than the current generation of optical switches and easily breaks one of the major technical barriers to the spread of electronic devices that detect and control light: miniaturizing the size of ultrafast optical switches.

The new device was developed by a team of scientists from Vanderbilt, the University of Alabama–Birmingham, and Los Alamos National Laboratory and is described in the March 12 issue of the journal Nano Letters.

The ultrafast switch is made of an artificial material engineered to have properties that are not found in nature. In this case, the “metamaterial” consists of nanoscale particles of vanadium dioxide (VO2)—a crystalline solid that can rapidly switch back and forth between an opaque, metallic phase and a transparent, semiconducting phase—which are deposited on a glass substrate and coated with a “nanomesh” of tiny gold nanoparticles.

The scientists report that bathing these gilded nanoparticles with brief pulses from an ultrafast laser generates hot electrons in the gold nanomesh that jump into the vanadium dioxide and cause it to undergo its phase change in a few trillionths of a second.

“We had previously triggered this transition in vanadium dioxide nanoparticles directly with lasers and wanted to see if we could do it with electrons as well,” says Richard Haglund, Stevenson Professor of Physics at Vanderbilt, who led the study. “Not only does it work, but the injection of hot electrons from the gold nanoparticles also triggers the transformation with one-fifth to one-tenth as much energy input required by shining the laser directly on the bare VO2.”

Industry and government are investing heavily in efforts to integrate optics and electronics because it is generally considered the next step in the evolution of information and communications technology. Intel, Hewlett-Packard and IBM have been building chips with increasing optical functionality that operate at gigahertz speeds—one-thousandth that of the VO2 switch.

“Vanadium dioxide switches have a number of characteristics that make them ideal for optoelectronics applications,” says Haglund. In addition to their speed and size, they:

• are completely compatible with current integrated circuit technology—both silicon-based chips and the new “high-K dielectric” materials being developed by the semiconductor industry;

• operate in the visible and near-infrared region of the spectrum that is optimal for telecommunications applications; and

• generate an amount of heat low enough that switches can be packed tightly enough to make practical devices.

Graduate students Kannatassen Appavoo, PhD’12, and Joyeeta Nag, MS’08, PhD’11, fabricated the metamaterial at Vanderbilt. The theoretical and computational studies that helped unravel the complex mechanism of the phase transition at the nanoscale were carried out by postdoctoral student Bin Wang and Sokrates Pantelides, University Distinguished Professor of Physics and Engineering and the William A. and Nancy F. McMinn Professor of Physics.