Graphene has become a focus of research for a variety of potential uses in photonics; now, researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have found a way to control how the material conducts electricity by using extremely short light pulses, which could enable its use as a broadband light detector.1
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 behavior 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, which is how a metal usually behaves. Therefore, by modulating graphene's electron concentration, the researchers could effectively alter graphene's photoconductive properties from semiconductorlike to metallike.
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," says Alex Frenzel, one of the researchers.
Optical-pump terahertz-probe spectroscopy
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 used optical-pump terahertz-probe spectroscopy, illuminating the graphene with a strong light pulse and measuring the change of electrical conduction by assessing the transmission of a second, terahertz light pulse.
The all-optical method avoids the need for adding extra electrical contacts to the graphene. Additionally, the short light pulses allow the researchers to change and reveal graphene's electrical response in only about a picosecond.
The researchers say 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. However, 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, says researcher Nuh Gedik.
Source: http://www.eurekalert.org/pub_releases/2014-08/miot-lpc073114.php
REFERENCE:
1. A. J. Frenzel et al., Physical Review Letters (2014); doi: http://dx.doi.org/10.1103/PhysRevLett.113.056602
