The ability to efficiently confine photons within nanoscale devices and circuits is a holy grail of contemporary nanophotonics. A practical approach to reach this goal is to form resonant modes—called plasmon polaritons—that can propagate through the material as hybridized light-matter waves. Atomically thin layers of graphene are one of the most promising candidates for this approach, as graphene is readily tunable and can be altered at ultrafast time scales. Although significant R&D effort has been poured into nanoscale light confinement without significant optical losses, the process has proven difficult for many optical materials, including monolayer graphene.
For the first time, researchers at Columbia University (New York, NY) and the University of California, San Diego (La Jolla, CA) have revealed fundamental limitations for the propagation of plasmon polariton waves in graphene. By exciting and imaging these waves within pristine graphene using an infrared (IR) scanning probe at liquid-nitrogen temperatures, the researchers found that graphene plasmons propagate ballistically, across tens of micrometers, throughout the tiny device. The performance of resonant modes like plasmons is measured by their quality factor and the new measurements demonstrate that plasmonic quality factors surpassing 100 are now attainable in graphene—an almost-tenfold improvement compared to previous results in graphene obtained at room temperature. The results also prove that graphene ranks among the best candidate materials for IR plasmonics, with applications in imaging, sensing, and nanoscale manipulation of light.
Researcher Guangxin Ni from Columbia University says, “It is very exciting to see that graphene plasmons travel ‘ballistically,’ even at the low temperatures accessible by using liquid nitrogen. Our discoveries will likely open new opportunities for future plasmonic control and switching, as well as investigation of nonlinear plasmonic phenomena.” This understanding of the fundamental physics of processes that limit propagation of plasmon waves in graphene helps remove the remaining roadblocks for long-range travel of versatile nanoconfined light within future optical devices. Reference: G. X. Ni et al., Nature, 557, 530–533 (May 24, 2018).
