  The photoacoustic effect in liquids, generated by metal nanoparticles excited
with short laser pulses, offers high contrast imaging and promising medical
treatment techniques. Understanding the role of the thermal boundary resistance
(TBR) and the laser pulse duration in the generation mechanism of acoustic
waves is essential to implement efficient photoacoustic nanotransducers. This
work theoretically investigates, for the paradigmatic case of water-immersed
gold nanocylinders, the role of the TBR and of laser pulse duration in the
competition between the launching mechanisms: the thermophone and the
mechanophone. In the thermophone, the nanoparticle acts as a nanoheater and the
wave is launched by water thermal expansion. In the mechanophone, the
nanoparticle directly acts as a nanopiston. Specifically, for a gold-water
interface, the thermophone prevails under ns light pulse irradiation, while the
mechanophone dominates shortening the pulse to the 10 ps regime. For a
graphene-functionalized gold-water interface, instead, the mechanophone
dominates over the entire range of explored laser pulse durations. Results
point to high-TBR, liquid-immersed nanoparticles as potentially efficient
photoacoustic nanogenerators, with the advantage of keeping the liquid
environment temperature unaltered.
