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Abstract
As a two-dimensional semimetal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability, and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy EF, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl3-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations corresponding to EF > 1 eV. Near-field imaging of this highly doped form of graphene allows us to characterize plasmons, including their corresponding lifetimes, over a broad frequency range. For bilayer graphene, in contrast to the monolayer system, a phonon-induced dipole moment results in increased plasmon damping around the intrinsic phonon frequency. Strong coupling between intrinsic graphene phonons and plasmons is found, supported by ab initio calculations of the coupling strength, which are in good agreement with the experimental data.
Highlights
Doped graphene is of fundamental importance for a plethora of applications, including graphene plasmonic device technology development.[1−7] Control over the doping level enables surface plasmons (SPs), whose wavelength scales with the fourth root of the Fermi energy EF
We show that the phonon-induced dipole in bilayer graphene is distinctly different from that in monolayer graphene and leads to hybridization of the plasmon mode at the phonon frequency in the former, as evidenced by anticrossing behavior in the dispersion of the material
In order to investigate the effects of electron−phonon interactions, we performed plasmon nanoimaging measurements in graphene at unprecented high intrinsic carrier concentrations, achieved by intercalating graphene with FeCl3.14 This new material has been recently used in optoelectronic applications[15] and it showed an extraordinary linear dynamic range,[16] together with an unexpected resilience to environmental conditions.[17]
Summary
The frequency interval between measurements around the graphene phonon frequency was not larger than 4 cm−1, which is smaller than the energy width of the anticrossing (Δω ∼ 37 cm−1) This leads to a quantitative value for the electron−phonon coupling strength Δω/ω0 ∼ 2.3% in units of uncoupled frequency ω0.5 We attribute the splitting in the plasmon dispersion of the bilayer graphene to the breaking of inversion symmetry in this system, causing a finite dipole moment and rendering the graphene phonon mode IR-active. We have observed propagating plasmons in ultrahighly doped graphene with Fermi energy exceeding 1.2 eV and a splitting in the plasmon dispersion due to the interactions of plasmons with the finite dipolar moment of intrinsic optical phonons in bilayer graphene This effect is not observed in two-monolayer graphene because in such case there is no effective phonon dipole moment at the zone center due to inversion symmetry.
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