Researchers from the Massachusetts Institute of Technology (MIT) have developed a method of using a pulsed laser to control the way in which graphene conducts electricity, opening up new applications for the material, such as use as a broadband light detector. The study was published in Physical Review Letters, and found that the cause of photoconductivity in graphene is very different to that of other materials such as semiconductors or metals.
Graphene, is an ultrathin form of carbon with electrical, optical, and mechanical properties. The researchers at MIT discovered that by controlling the concentration of electrons in a graphene sheet, they could change the way the material responded to short but intense laser pulses. If the graphene sheet started with low electron concentration, the laser pulse increased the material’s electrical conductivity. This behaviour is similar to that of traditional semiconductors, such as silicon and germanium.
But, if the graphene started with high electron concentration, the pulse decreased its conductivity — the same way that a metal usually behaves. Therefore, by modulating graphene’s electron concentration, the MIT team found that they could alter graphene’s photoconductive properties from semiconductor-like to metal-like.
To perform this study, the team deposited graphene on top of an insulating layer with a thin metallic film beneath it; by applying a voltage between graphene and the bottom electrode, the electron concentration of graphene could be tuned. The researchers then illuminated graphene with a strong laser pulse and measured the change of electrical conduction by assessing the transmission of a second, low-frequency light pulse.
The pulsed laser was used to perform dual functions, explained graduate student Nuh Gedik. ‘We use two different light pulses: one to modify the material, and one to measure the electrical conduction.’ The pulses used to measure the conduction were a much lower frequency than the pulses used to modify the material behaviour.
This all-optical method avoids the need for adding extra electrical contacts to the graphene. ‘Normally, to measure conductivity you have to put leads on it,’ Gedik added. This approach, by contrast, ‘has no contact at all.’
During the study, the team discovered that part of the conductivity reduction at high electron concentration stems from a unique characteristic of graphene: Its electrons travel at a constant speed, similar to photons, 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,' explained Alex Frenzel, graduate student at MIT who was also involved in the research.
In the future, the work could aid the development of new light detectors with ultrafast response times and high sensitivity across a wide range of light frequencies, from the infrared to ultraviolet. While the material is sensitive to a broad range of frequencies, the actual percentage of light absorbed is small. Practical application of such a detector would therefore require increasing absorption efficiency, such as by using multiple layers of graphene, Gedik noted.
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