A team of researchers from Vanderbilt University, University of Alabama-Birmingham, and Los Alamos National Laboratory has developed an ultra-small and ultra-fast optical switch made from vanadium oxide (VO2). With the device's ability to switch at terahertz speeds, it is much faster than similar switches developed by electronic giants that operate at gigahertz speeds, the researchers say.
The use of VO2 as the basis of the optical switch comes as recent reports seemed to throw into question whether the material is really suitable as a replacement for silicon transistors. The appeal of VO2 has been that it can be switched from an insulator to a conductor by adding charge, which could make for a fundamentally different kind of transistor. But research at IBM seems to indicate that it isn’t going to be that easy. Its experiments showed that an unwanted and irreversible change occurs to the material that makes it inappropriate for solid-state transistors.
The joint team in this most recent research took a different approach with the material. They used the VO2 to make an artificial substance known as a metamaterial. A metamaterial can be broadly defined as an artificially structured material fabricated by assembling different, often nanoscale objects to take the place of the atoms and molecules that one would see in a conventional material. The resulting material has very different electromagnetic properties than those found in naturally occurring or chemically synthesized substances. The property of negative refraction is just one of the amazing characteristics of certain metamaterials.
In research, which was published in the journal Nano Letters ("Ultrafast Phase Transition via Catastrophic Phonon Collapse Driven by Plasmonic Hot-Electron Injection"), the metamaterial was made from nanoparticles of VO2 that were deposited on a glass substrate and coated with a “nanomesh” of tiny gold nanoparticles.
When brief pulses from an ultrafast laser strike the metamaterial, hot electrons from the gold nanomesh jump into the VO2 and cause its phase to change from an opaque, metallic phase to a transparent, semiconducting phase in a few trillionths of a second.
“We had previously triggered this transition in vanadium dioxide nanoparticles directly with lasers and we wanted to see if we could do it with electrons as well,” said 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.”
The optical switch is around 200 nanometers in diameter, which is significantly smaller than today's generation of optical switches, according to the researchers. This smaller size for ultrafast optical switches overcomes one of the technical barriers to their spread in systems that detect and control light.
When its reduced sized is combined with its terahertz speed, the VO2 could offer a solution that would change data-storage and telecommunications technologies.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.