Figure 2: Hybrid plasmon–phonon modes generated through coupling of graphene plasmons and SiO₂ SP phonons.

(a) Extinction spectrum of a 160 nm GNR array on SiO₂ in the mid-infrared, measured with an Fourier transform infrared spectrometer under s-polarization (polarization perpendicular to the GNRs). Three hybrid plasmon–phonon modes can be identified. Their energies are determined by the interaction of the graphene plasmon mode with two SP phonons of the SiO₂ underlayer. (b,c) Extinction spectra in the region of the second peak (P2) for GNR arrays with GNR widths of (b) 150 nm and (c) 120 nm and varying chemical doping (Fermi) levels as indicated. The solid vertical line indicates the energy of the CO₂ laser used in subsequent photocurrent experiments. (d) Extinction as a function of doping, extracted from the two previous plots at the energy of the CO₂ laser, 943 cm⁻¹. Black squares: 150 nm GNRs; green circles: 120 nm GNRs. The solid lines are Lorentzian fits. (e) Calculated plasmon resonance frequency as a function of Fermi energy and gate-voltage tuning for GNR arrays of different widths. (f) Calculated plasmon loss function as a function of Fermi energy and gate-voltage tuning for GNR arrays of different widths at the experimental frequency of 943 cm⁻¹.