  Ultrasound detection is one of the most important nondestructive subsurface
characterization tools of materials, whose goal is to laterally resolve the
subsurface structure with nanometer or even atomic resolution. In recent years,
graphene resonators attracted attention as loudspeaker and ultrasound radio,
showing its potential to realize communication systems with air-carried
ultrasound. Here we show a graphene resonator that detects ultrasound
vibrations propagating through the substrate on which it was fabricated. We
achieve ultimately a resolution of $\approx7$~pm/$\mathrm{\sqrt Hz}$ in
ultrasound amplitude at frequencies up to 100~MHz. Thanks to an extremely high
nonlinearity in the mechanical restoring force, the resonance frequency itself
can also be used for ultrasound detection. We observe a shift of 120~kHz at a
resonance frequency of 65~MHz for an induced vibration amplitude of 100~pm with
a resolution of 25~pm. Remarkably, the nonlinearity also explains the generally
observed asymmetry in the resonance frequency tuning of the resonator when
pulled upon with an electrostatic gate. This work puts forward a sensor design
that fits onto an atomic force microscope cantilever and therefore promises
direct ultrasound detection at the nanoscale for nondestructive subsurface
characterization.
