Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy

Near-field photocurrent nanoscopy is used for imaging strongly confined terahertz graphene plasmons with linear dispersion. Terahertz (THz) fields are widely used for sensing, communication and quality control 1 . In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs) 2 , 3 . These possibilities emerge from the strongly reduced GP wavelength, λ p , compared with the photon wavelength, λ 0 , which can be controlled by modulating the carrier density of graphene via electrical gating 4 , 5 , 6 , 7 , 8 . Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement 9 , 10 , 11 , analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates 12 . Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM) 6 , 7 , the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far—only GPs with nearly free-space wavelengths have been observed 13 . Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically 14 rather than optically 6 , 7 . This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths ( λ p ≈ λ 0 /66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.