Coupling of a dipole emitter to doped homogeneous-graphene plasmons. (Top, left) Near-field produced by a perpendicular dipole situated 10 nm away from doped graphene. (Top, right) Poynting vector lines reveal strong coupling to the graphene surface plasmons. (Bottom) Tunable decay rate of an excited emitter in front of graphene with a carrier density corresponding to a Fermi energy EF.
Surface plasmons (SP)—the electromagnetic waves coupled to charge excitations at the surface of a metal—are the cornerstones of various applications, including ultra-sensitive optical biosensing, optical metamaterials and quantum information processing. However, even noble metals, which are widely regarded as the best available plasmonic materials, are hardly tunable. They also exhibit large ohmic losses that reduce the lifetime of these excitations to a few tens of optical cycles at wavelengths close to the SP threshold, where they produce maximum field confinement.
As a result, plasmon resonances in metals suffer large decoherence that limits their applicability to optical processing devices. In this context, graphene emerges as a unique alternative two-dimensional plasmonic material that displays a wide range of extraordinary properties. This atomically thick sheet of carbon has generated tremendous interest triggered by its superior electronic, mechanical and optical properties.
Recently, graphene has also been recognized as an optical material for novel photonic and optoelectronic applications such as solar cells, photodetectors, light-emitting devices and ultrafast lasers. All of these photonic and optoelectronic applications rely on the interaction of propagating far-field photons with graphene. In addition, surface plasmons bound to the surface of doped graphene1 exhibit a number of favorable properties that make graphene an attractive alternative to traditional metal plasmonics.
In particular, graphene plasmons are confined to volumes of the order of 106 times smaller than the excitation wavelength. Interestingly, this extreme field confinement is governed by the fine structure constant α3 and the ratio of the Fermi energy and photon energy.2 Furthermore, dramatic tuning of the plasmon spectrum is possible through electrically or chemically modifying the charge carrier density and the electronic structure of graphene. The ability to fabricate large, highly crystalline samples should give rise to SP lifetimes reaching hundreds of optical cycles, thereby circumventing one of the major bottlenecks facing noble-metal plasmonics.
We show that these remarkable plasmonic properties can be used to tailor extremely strong light-matter interactions2 associated to Purcell factors of up to 105 for just a graphene sheet. Based on analytical and numerical calculations, we predict observable effects of cavity quantum electrodynamics in our proposed graphene nanostructures.
Interestingly, graphene nano-structures can have resonant extinction cross-sections that greatly exceed their geometrical cross-sections, despite the small volume occupied by this thin material. This enables efficient and resonant excitation surface plasmons by far-field illumination. Moreover, using this mechanism, we show that it’s possible to boost the light absorption in graphene from 2.3 percent to 100 percent.3
These results pave the way to a wide variety of applications such as efficient light harvesting, nanoscale optical absorbers, tunable nanoscale metamaterials,4 waveguides and switches and quantum information processing.
F.H.L. Koppens and D.E. Chang are with the ICFO-Institut de Ciéncies Fotóniques, Mediterranean Technology Park, Barcelona, Spain. D.E. Chang is also with the California Institute of Technology, Pasadena, Calif., U.S.A. S. Thongrattanasiri and F.J.G. García de Abajo are with the Instituto de Óptica, Madrid, Spain.
References and Resources
1. M. Jablan and H. Buljan. “Plasmonics in graphene at infrared frequencies. Phys. Rev. B 80, 245435 (2009).
2. F.H.L. Koppens et al. “Graphene plasmonics: a platform for strong light–matter interactions,” Nano. Lett. 11, 3370 (2011).
3. S. Thongrattanasiri et al. “Total light absorption in graphene,” arXiv physics.optics (2011).
4. A. Vakil and N. Engheta. “Transformation optics using graphene,” Science 332, 1291 (2011).