Laser Switches Insulator to Conductor and Back in Femtoseconds

Ultrashort pulses of light could lead to petahertz-speed computing

11 December 2012—The best transistors today can switch on and off hundreds of billions of times a second. But laser pulses can be more than 10 000 times as quick. Now researchers in Germany have used such ultrashort laser pulses to induce current with the same kind of frequencies—creating an electric switch with laser speed. 

“Laser pulses are the shortest signal you can generate,” says Martin Schultze, a researcher at the Max Planck Institute of Quantum Optics, in Garching, Germany, who was involved in the work and is currently a visiting scientist at the University of California, Berkeley. “They are much faster than anything electronic.” 

Such switches could someday become the basis for circuits that operate in the petahertz range, far beyond the limits of today’s solid-state semiconductors, researchers say. The key was in using the laser to reversibly turn an insulator into a conductor.

Materials are sorted into three categories by their electrical properties. Conductors have free electrons that take off when exposed to even small electric fields. In semiconductors, the electrons need a well-defined amount of energy to spur the movement of electrons from the material’s valence band, across the bandgap, and into the conduction band where they can flow freely. Dielectric insulators do not allow electric charge to move easily at all but can be polarized by an electric field. 

The researchers wanted to know what would happen to dielectrics if they were exposed to ultrashort, intense laser pulses. Ordinarily, when insulators fail it is catastrophic and irreversible. In experiments, they irradiated a small silica-glass prism with gold electrodes on two sides, using laser pulses lasting a few femtoseconds—millionths of a billionth of a second. Light is an electromagnetic wave, so the pulses caused the amplitude of the electric field on the insulator to surge to more than 10 billion volts per meter in a matter of femtoseconds. This extreme electric field was able to mobilize the dielectrics, which are usually stationary charges.

While this result showed that the conductivity could be turned on within femtoseconds, the researchers had to turn to a second experiment to figure out whether it could be turned off just as quickly. Here, they added X-ray pulses to the mix, which are even shorter in duration—attoseconds. These helped make slow-motion “snapshots” of the electronic state of the insulator. The measurement confirmed that the material goes from an insulator to a conductor and back again, all in the femtoseconds length of the laser pulse. 

The two experiments are described in separate reports in last week’s edition of the journal Nature. Researchers at the Max Planck Institute of Quantum Optics, Ludwig-Maximilian University of Munich, and Technical University Munich carried out the physical experiments, and Mark Stockman’s group at Georgia State University, in Atlanta, did the computer modeling and theoretical work. 

The finding was, at first, a puzzle. “We had to understand a lot more than we thought in the beginning,” says Schultze, who led the “snapshot” experiment. “We had a far easier physical picture in mind. We had to replace it by a more sophisticated model.” 

Applications will certainly be far off. To achieve the effect, one needs a very strong electric field on the order of volts per angstrom, says Schultze. Ultrashort laser pulses may be the only way to achieve this effect, he says.

“To understand how far we are from turning this into a usable box,” he says, “is really the question of the availability of these strong pulses at a high repetition rate.” Usually, he says, these lasers have a repetition rate of only about a kilohertz. “The process [of switching] is very fast, but it happens only a thousand times a second.”