PORTLAND, Ore -- Graphene has been courted as the miraclematerial of the future, because different formulations have been fabricatedinto conductors, semiconductors, and insulators. Now IBM has added photonic tothe list by demonstrating a graphene/insulator superlattice that achieves aterahertz frequency notch filter and a linear polarizer, devices which could beuseful in future mid- and far-infrared photonic devices, including detectors,modulators, and three-dimensional metamaterials.
"In addition to its good electrical properties,graphene also has exceptional optical properties. In particular, it absorbslight from the far-infrared to the ultra-violet," said IBM Fellow PhaedonAvouris. "The terahertz range was of particular interest to IBM, becausethese frequencies can penetrate paper, wood and other solid objects forsecurity applications. Unfortunately, today there are very few ways ofmanipulating terahertz waves such as polarizing and filtering it, but becausegraphene operates well at terahertz frequencies we have concentrating oncreating these types of devices."
Teraherz frequency oscillations can be carried in grapheneby plasmons -- the collective oscillation of carriers -- to enable low-losstunable filters. But in single-layer graphene, the carrier concentration andresonant frequency was too weak for photonics applications, according to IBM.However, by going to a multi-layer graphene/instulator superlattice, transparentdevices can be patterned into photonic-like crystals that distribute thecarriers among the layers effectively enhancing both the carrier density andthe resonant frequency.
"We have foundthat graphene interaction with electromagnetic radiation is particularly strongin the terahertz range, however with a single layer of graphene the interactionwas still not strong enough," said Hugen Yan, a member of the NanoscaleScience and Technology Group at IBM's TJ Watson Research Lab (Yorktown Heights,NY). "But by using a multi-layer stack structure in microdisk arrays weachieved frequency selectivity in the terahertz range, allowing us to tune thedesired resonant frequency."
Scanning ElectronMicroscope image of, five-layer graphene/insulator superlattice array oftwo-micron diameter microdisks (purple).IBM found that by patterning the graphene/insulatormicrodisks in arrays, it was able to tune their resonant frequency by varyingthe size of the microdisks, the number of layers, their spacing, and the dopingof the graphene layers. Upon analysis, IBM discovered a unique carrier densityscaling law for its graphene/insulator superlattices that, unlike conventionalsemiconductor superlattices, is based on laws governing Dirac fermions (such asquarks, leptons, baryons, and hadrons).
As a result, IBM has been able to demonstrate patternedgraphene/insulator stacks implementing a widely tunable notch filters with an8.2-dB rejection ratio, and a terahertz linear polarizer with 9.5-dB extinctionratio. Implemented by laying down wafer scale alternating layers of grapheneand a polymer insulator, then patterning them into microdisks, IBM demonstratedthat these graphene/insulator superlattices shielded 97.5% of electromagneticradiation at frequencies below 1.2 terahertz.
For the future, the research group intends to tune itsgraphene/insulator superlattices for the infrared frequencies used by opticalcommunications equipment today. Fengnian Xia, a member of the Nanoscale Scienceand Technology Group at TJ Watson Research Lab, also contributed to the work.
This story was originally published on EETimes.
