CAMBRIDGE, Mass., Aug. 4, 2014 — A new way to control how graphene conducts electricity could prompt its use as a broadband light detector. A research team at MIT found that changing the concentration of electrons in a graphene sheet can change the way the material responds to short, intense light pulses. If the graphene sheet starts with a low electron concentration, it prompts the light pulse to increase the material's electrical conductivity, according to the study. However, if it begins with a high electron concentration, its conductivity is decreased by the light pulse. Regulating the graphene’s electron concentration alters the material’s photoconductive properties from those of a semiconductor to those of metal. This illustration shows a lattice of graphene with its bonds and connecting carbon atoms. By controlling the concentration of electrons in a graphene sheet, the material's electrical conductivity can be controlled. Courtesy of Jose-Luis Olivares/MIT. Part of the conductivity reduction at high electron concentration stemmed from a unique characteristic of graphene — similar to photons, graphene’s electrons travel at a constant speed, which causes the conductivity to decrease when the electron temperature increases under the illumination of the laser pulse. “Our experiment reveals that the cause of photoconductivity in graphene is very different from that in a normal metal or semiconductor,” said MIT graduate student Alex Frenzel. “We were able to tune the number of electrons in graphene, and get either response.” In the study, graphene was placed on top of an insulating layer with a thin metallic film beneath it. By applying a voltage between the two layers, the researchers tuned the graphene’s electron concentration. The graphene was illuminated with a strong light pulse; the researchers then measured the change of electrical conduction by assessing the transmission of an additional, lower-frequency light pulse. “We use two different light pulses: one to modify the material, and one to measure the electrical conduction,” said Dr. Nuh Gedik, an associate physics professor at MIT, adding that the pulses used to measure the conduction have a much lower frequency than the pulses used to modify the material’s behavior. To accomplish this, the researchers developed a device that was transparent, allowing the laser pulses to pass through it. This all-optical method avoided the need for extra electrical contacts with the graphene. “Normally, to measure conductivity you have to put leads on it,” Gedik said. “This approach, by contrast, has no contact at all.” This work could potentially lead to development of new light detectors with ultrafast response times and high sensitivity across many light frequencies, from the infrared to ultraviolet. The work was funded by the U.S. Department of Energy and the National Science Foundation. The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.113.056602). For more information, visit www.mit.edu.