Coherent acoustic pulse emission by ensembles of plasmonic nanoparticles

The ability of a layer of silver nanoparticles with broad size dispersion to excite coherent acoustic phonon wave-packet in its supporting glass substrate following femtosecond laser excitation is tested using time-domain Brillouin scattering in transmission configuration. The generation and propagation over several micrometer distances of gigahertz acoustic phonons in the underlying glass is observed for all investigated samples, involving nanoparticle layers with different morphological properties. The phonon wave-packet exhibits frequency-dependent attenuation rates and amplitudes in the 40–50 GHz acoustic frequency region. The measured attenuation rates are consistent with previous experimental results on silica, indicating that anharmonic interaction of acoustic waves with the thermal phonon bath has dominating contribution to the phonon damping. The features of the acoustic waves emitted by a discontinuous nanoparticle layer are investigated using a simplified theoretical model, allowing to better understand at which condition they are similar to those of the waves emitted by a continuous thin film.