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Graphene achieves terahertz frequency

Posted: 24 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:terahertz? graphene? photonic?

Dubbed as the miracle material of the future, graphene has since different formulations have been fabricated into conductors, semiconductors and insulators. Recently, IBM has added photonics to the list by showing a graphene/insulator superlattice that achieves a terahertz frequency notch filter and a linear polarizer, devices that could be useful in future mid- and far-infrared photonic devices such as detectors, modulators and 3D metamaterials.

"In addition to its good electrical properties, graphene also has exceptional optical properties. In particular, it absorbs light from the far-infrared to the ultraviolet," stated IBM fellow Phaedon Avouris. "The terahertz range was of particular interest to IBM, because these frequencies can penetrate paper, wood and other solid objects for security applications. Unfortunately, today there are very few ways of manipulating terahertz waves such as polarizing and filtering it, but because graphene operates well at terahertz frequencies we have concentrating on creating these types of devices."

Teraherz frequency oscillations can be carried in graphene by plasmonsthe collective oscillation of carriersto enable low-loss tunable filters. But in single-layer graphene, the carrier concentration and resonant frequency was too weak for photonics applications, according to IBM. However, by going to a multilayer graphene/insulator superlattice, transparent devices can be patterned into photonic-like crystals that distribute the carriers among the layers effectively enhancing both the carrier density and the resonant frequency.

"We have found that graphene interaction with electromagnetic radiation is particularly strong in the terahertz range, however with a single layer of graphene the interaction was still not strong enough," noted Hugen Yan, a member of the Nanoscale Science and Technology Group at IBM's T. J. Watson Research Lab. "But by using a multilayer stack structure in microdisk arrays, we achieved frequency selectivity in the terahertz range, allowing us to tune the desired resonant frequency."

graphene/insulator superlattice

Scanning electron microscope image of five-layer graphene/insulator superlattice array of 2? diameter microdisks (purple).

IBM found that by patterning the graphene/insulator microdisks in arrays, it was able to tune their resonant frequency by varying the size of the microdisks, the number of layers, their spacing, and the doping of the graphene layers. Upon analysis, IBM discovered a unique carrier density scaling law for its graphene/insulator superlattices that, unlike conventional semiconductor superlattices, is based on laws governing Dirac fermions (such as quarks, leptons, baryons and hadrons).

As a result, IBM has been able to demonstrate patterned graphene/insulator stacks implementing a widely tunable notch filters with 8.2dB rejection ratio and a terahertz linear polarizer with 9.5dB extinction ratio. Implemented by laying down wafer scale alternating layers of graphene and a polymer insulator, then patterning them into microdisks, IBM demonstrated that these graphene/insulator superlattices shielded 97.5 percent of electromagnetic radiation at frequencies below 1.2THz.

For the future, the research group intends to tune its graphene/insulator superlattices for the infrared frequencies used by optical communications equipment today. Fengnian Xia, a member of the Nanoscale Science and Technology Group at T. J. Watson Research Lab, also contributed to the work.

- R. Colin Johnson
??EE Times

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