  Fermi liquids respond differently to perturbations depending on whether their
frequency is larger (collisionless regime) or smaller (hydrodynamic regime)
than the inter-particle collision rate. This results in a different phase
velocity between the collisionless zero sound and hydrodynamic first sound. We
performed terahertz photocurrent nanoscopy measurements on graphene devices,
with a metallic gate in close proximity to the sample, to probe the dispersion
of propagating acoustic plasmons, the counterpart of sound modes in electronic
Fermi liquids. We report the observation of a change in the plasmon phase
velocity when the excitation frequency approaches the electron-electron
collision rate. This first observation of the first sound mode in an electronic
Fermi liquid is of fundamental interest and can enable novel terahertz emitter
and detection implementations.
