  Plasmonic nanoantennas have proven to be efficient transducers of
electromagnetic to mechanical energy and vice versa. The sudden thermal
expansion of these structures after an ultrafast optical pulsed excitation
leads to the emission of hypersonic acoustic waves to the supporting substrate,
which can be detected by another antenna that acts as a high-sensitive
mechanical probe due to the strong modulation of its optical response.
Sophisticated fabrication techniques, together with the implementation of
numerical simulations, have allowed the engineering of nanostructures for the
controlled directional generation and detection of high-frequency acoustic
phonons at the nanoscale, with many potential applications in
telecommunications, sensing, mechanical switching, and energy transport. Here,
we propose and experimentally demonstrate a nanoscale acoustic lens comprised
of 11 gold nanodisks whose collective oscillation gives rise to an interference
pattern that results in a diffraction-limited surface acoustic beam of about
340 nm width, with an amplitude contrast of 60%. Via spatially decoupled
pump-probe experiments, we were able to map the radiated acoustic energy in the
proximity of the focal area, obtaining a very good agreement with the continuum
elastic theory.
