Highly Confined Tunable Mid-Infrared Plasmonics in Graphene Nanoresonators

TLDR

Single‑layer graphene supports plasmonic modes with wavelengths roughly 20 times shorter than free‑space wavelengths in the 2–6 THz range, and active devices exploiting these modes have been demonstrated. The study aims to investigate graphene plasmon properties across the infrared spectrum, particularly at energies above the graphene optical phonon energy. Using infrared microscopy, the authors measured resonant frequency shifts of 15‑nm graphene nanoresonator arrays to map wave‑vector‑dependent dispersion, tuning resonator width and charge density to access plasmons with λp ≤ λ0/100 and resonances up to 310 meV. Electromagnetic simulations indicate that these highly confined modes exhibit a local density of optical states over a million times that of free space, suggesting strong enhancement of light‑matter interactions in the mid‑infrared.

Abstract

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 (λp ~ λ0/20) 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 λp ≤ λ0/100 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.