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Posted: Aug 29, 2011
Graphene's two-photon shuffle
(Nanowerk News) The absorption of light is a fundamental process used in many applications from solar cells to biological vision. Light absorption isn't always linear, however. In some cases, two photons combine together to initiate the absorption process — a phenomenon that offers unique advantages and which is used in a variety of technological applications from microfabrication to optical data storage. Wei Ji and co-workers from the National University of Singapore have now demonstrated that bilayer graphene shows a particularly strong two-photon absorption effect ("Giant Two-Photon Absorption in Bilayer Graphene").
Two-photon absorption is a nonlinear process that requires high light intensities, which often can only be achieved at the small focal point of light beams. The highly localized nature of two-photon absorption is of particular benefit for the fabrication of tiny structures, and has led to advances in three-dimensional microfabrication. The two-photon process has also been used for high-resolution microscopic imaging in biological research.
Schematic illustration of the measurements of two-photon absorption using an intense, ultrafast infrared laser pulse focused onto a bilayer graphene sample.
Single-layer graphene, although famous for its electronic properties, isn't a very appealing material for two-photon absorption. As it turns out, however, the absorption properties of bilayer graphene, which is made of two layers of carbon atoms, are much different. In their experiments, Ji and his team directed an ultrafast infrared laser at a sample of bilayer graphene and measured the transmittance as a function of laser intensity (see image).
To their surprise, the measured two-photon absorption coefficient was huge, about five orders of magnitude larger than for typical semiconductor materials in this wavelength region. According to Ji, the origin of this enhancement is the strong coupling between the two layers of graphene. "The two-photon absorption is particularly strong at wavelengths corresponding to the energy required for electrons to hop from one graphene layer to the other."
This observation is of particular relevance for potential applications in telecommunications, as the infrared wavelengths used here are similar to the wavelengths used for light transmission through optical fibers. Beyond such known applications, the use of graphene also offers new possibilities that additionally utilize the material's unique electronic properties. "Graphene's exotic quantum properties could be used to manipulate the two-photon absorption in bilayer graphene to produce novel absorption phenomena," says Ji.