Oct 26, 2012
Nanocavity helps graphene absorb more light
As well as its unique electronic and mechanical properties, graphene also has remarkable optical properties and can absorb more than 2% of incoming visible light, which is astonishing since the material is only one atom thick. Now, researchers at Columbia University in New York are saying that the material could absorb more than 45% of light in the far infrared and visible frequency ranges if it is coupled to an optical resonator. The finding confirms, once again, that graphene could make the ideal base for a host of new-generation photonics devices, such as ultrafast modulators and switches, and detectors for optical communications.
Graphene, a 2D sheet of carbon just one atom thick, continues to amaze scientists with its growing list of unique electronic and mechanical properties. Some believe that it might even replace silicon as the electronic industry's material of choice. In part, this is thanks to the fact that electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with no rest mass.
Graphene could also be ideal for photonics applications – thanks again to its Dirac electrons – because it can absorb light over a very wide range of wavelengths, ranging from the visible to the infrared (a part of the electromagnetic spectrum that is important for optical telecommunications). Conventional III-V semiconductors do not absorb light over such a wide range.
However, although graphene absorbs 2.3% of the light falling on it, which is high considering its single atom thickness, this value is low in absolute terms. For real-world electro-optical and all-photonics applications, a much higher value is needed.
Coupling to a planar photonic crystal
The Colombia team, led by Tony Heinz and Dirk Englund, says that graphene’s interaction with light could be enhanced by placing it near a planar photonic crystal (PPC). Using detailed spectroscopic measurements, the researchers showed that the strong coupling between graphene and the light captured in the PPC’s nanocavity reduces the amount of reflected light by the cavity-graphene system by more than 100 times, turning it from translucent to nearly opaque.
Englund and colleagues explain their results using a so-called coupled mode theory that shows that the graphene-cavity absorbs more than 45% of light falling on it. “This enhanced light-graphene interaction is not only good news for practical applications, but it might also be used for high-precision Raman spectroscopy on sub-wavelength graphene regions,” says Englund.
The photonic crystal cavities studied by the Colombia team are sub-wavelength thickness semiconductor (gallium phosphide) membranes patterned with periodic holes. The cavity initially reflects light (it is "bright"), explains Englund, but after graphene has been coupled to it, the reflection all but vanishes and the hybrid structure becomes "dark", because it has absorbed the incoming light.
Towards even higher light absorption?
The story does not end here: "Our calculations show that if more efficient couplers, such as tapered fibres, on-chip waveguide couplers or 'ring resonators', were used in conjunction with graphene, the material might even be able to absorb 90% of incoming light," Englund told nanotechweb.org.
"Such cavity-enhanced optical absorption means that graphene could be used as the active material in compact photodetectors, ultrafast modulators and other novel opto-electronic devices that combine both the exceptional optical and electronic properties of graphene."
The Raman scattering studies made possible by this work might also allow graphene to be studied in more detail than is currently possible, so that its grain boundaries and edge states could be optically studied with high precision, he adds.
The researchers reveal that they have already made a head start in fabricating energy-efficient modulators and detectors for optical communications applications using their graphene-cavity system.
The present work is detailed in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.