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Light pulses key to graphene's electrical conductivity

Posted: 04 Aug 2014  Print Version  Bookmark and Share

Keywords:graphene  broadband light detector  Alex Frenzel 

MIT researchers have found a way to control how graphene conducts electricity with the help of short light pulses. The discovery could enable graphene to be used as a broadband light detector.

The new findings are published in the journal Physical Review Letters, in a paper by graduate student Alex Frenzel, Nuh Gedik, and three others.

The researchers found that by controlling the concentration of electrons in a graphene sheet, they could change the way the material responds to a short but intense light pulse. If the graphene sheet starts out with low electron concentration, the pulse increases the material's electrical conductivity. This behaviour is similar to that of traditional semiconductors, such as silicon and germanium.

But if the graphene starts out with high electron concentration, the pulse decreases its conductivity—the same way that a metal usually behaves. Therefore, by modulating graphene's electron concentration, the researchers found that they could effectively alter graphene's photoconductive properties from semiconductor-like to metal-like.

The finding also explains the photoresponse of graphene reported previously by different research groups, which studied graphene samples with differing concentration of electrons. "We were able to tune the number of electrons in graphene, and get either response," Frenzel says.

Light pulses control graphene's electrical behaviour

Light pulses control graphene's electrical behaviour. (Source: Jose-Luis Olivares/MIT)

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 the material with a strong light pulse and measured the change of electrical conduction by assessing the transmission of a second, low-frequency light pulse.

In this case, the laser performs dual functions. "We use two different light pulses: one to modify the material, and one to measure the electrical conduction," Gedik says, adding that the pulses used to measure the conduction are much lower frequency than the pulses used to modify the material behaviour. To accomplish this, the researchers developed a device that was transparent, Frenzel explains, to allow laser pulses to pass through it.

This all-optical method avoids the need for adding extra electrical contacts to the graphene. Gedik, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics, says the measurement method that Frenzel implemented is a "cool technique. Normally, to measure conductivity you have to put leads on it," he says. This approach, by contrast, "has no contact at all."

Additionally, the short light pulses allow the researchers to change and reveal graphene's electrical response in only a trillionth of a second.


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