  Terahertz (THz) fields are widely applied for sensing, communication and
quality control. In future applications, they could be efficiently confined,
enhanced and manipulated - well below the classical diffraction limit - through
the excitation of graphene plasmons (GPs). These possibilities emerge from the
strongly reduced GP wavelength, lp, compared to the photon wavelength, l0,
which can be controlled by modulating the carrier density of graphene via
electrical gating. 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, analogous to plasmons in two-dimensional electron gases (2DEGs)
near conductive substrates. While infrared GPs have been visualised by
scattering-type scanning near-field optical microscopy (s-SNOM), the real-space
imaging of strongly confined THz plasmons in graphene and 2DEGs has been
elusive so far - only GPs with nearly free-space wavelength have been observed.
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 rather than optically. The on-chip
GP detection simplifies GP imaging, as sophisticated s-SNOM detection schemes
can be avoided. The photocurrent images reveal strongly reduced GP wavelengths
(lp = l0/66), 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. Acoustic GPs could thus
strongly benefit the development of deep subwavelength-scale THz devices.
