  Ultrasound is an important imaging modality for biological systems.
High-frequency ultrasound can also (e.g., via acoustical nonlinearities) be
used to provide deeply penetrating and high-resolution imaging of vascular
structure via catheterisation. The latter is an important diagnostic in
vascular health. Typically, ultrasound requires sources and transducers that
are greater than, or of order the same size as the wavelength of the acoustic
wave. Here we design and theoretically demonstrate that single silver nanorods,
acting as optical nonlinear dipole antennae, can be used to detect ultrasound
via Brillouin light scattering from linear and nonlinear acoustic waves
propagating in bulk water. The nanorods are tuned to operate on high-order
plasmon modes in contrast to the usual approach of using fundamental plasmon
resonances. The high-order operation also gives rise to enhanced optical
third-harmonic generation, which provides an important method for exciting the
higher-order Fabry-Perot modes of the dipole antenna.
