Single-layer graphene has been shown to have intriguing prospects as a plasmonic material, as modes having plasmon wavelengths 20 times smaller than free space (λ_p ~ λ_0/20) have been observed in the 2–6 THz range, and active graphene plasmonic devices operating in that regime have been explored. However there is great interest in understanding the properties of graphene plasmons across the infrared spectrum, especially at energies exceeding the graphene optical phonon energy. We use infrared microscopy to observe the modes of tunable plasmonic graphene nanoresonator arrays as small as 15 nm. We map the wavevector-dependent dispersion relations for graphene plasmons at mid-infrared energies from measurements of resonant frequency changes with nanoresonator width. By tuning resonator width and charge density, we probe graphene plasmons with λ_p ≤ λ_0/100 and plasmon resonances as high as 310 meV (2500 cm^–1) for 15 nm nanoresonators. Electromagnetic calculations suggest that the confined plasmonic modes have a local density of optical states more than 10^6 larger than free space and thus could strongly increase light–matter interactions at infrared energies.
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