  Generation of ultra high frequency acoustic waves in water is key to nano
resolution sensing, acoustic imaging and theranostics. In this context water
immersed carbon nanotubes (CNTs) may act as an ideal optoacoustic source, due
to their nanometric radial dimensions, peculiar thermal properties and broad
band optical absorption. The generation mechanism of acoustic waves in water,
upon excitation of both a single-wall (SW) and a multi-wall (MW) CNT with laser
pulses of temporal width ranging from 5 ns down to ps, is theoretically
investigated via a multi-scale approach. We show that, depending on the
combination of CNT size and laser pulse duration, the CNT can act as a
thermophone or a mechanophone. As a thermophone, the CNT acts as a nanoheater
for the surrounding water, which, upon thermal expansion, launches the pressure
wave. As a mechanophone, the CNT acts as a nanopiston, its thermal expansion
directly triggering the pressure wave in water. Activation of the mechanophone
effect is sought to trigger few nanometers wavelength sound waves in water,
matching the CNT acoustic frequencies. This is at variance with respect to the
commonly addressed case of water-immersed single metallic nano-objects excited
with ns laser pulses, where only the thermophone effect significantly
contributes. The present findings might be of impact in fields ranging from
nanoscale non-destructive testing to water dynamics at the meso- to nano-scale.
