Interband transition probing of coherent acoustic phonons of gold/metal oxide core-shell nanoparticles

We present ultrafast spectroscopic investigations of the coherent acoustic vibrations of Au/SiO2 and Au/TiO2 core-shell nanoparticles (NPs) upon excitation of the Au surface plasmon resonance. The oscillations are detected in the region of the interband transitions of Au in the deep-ultraviolet, where they appear in the form of intensity modulations with no changes in the spectra. For the Au/SiO2 NPs, the oscillation period (typically ∼10 ps) is similar to that of bare Au NPs having a size identical to that of the core, implying a negligible coupling of the core with the shell. For Au/TiO2 NPs, significantly slower (∼20 ps) oscillations appear, whose period is identical to that of a bare gold NP having the same total diameter, implying that the Au/TiO2 NPs can be treated as a single object. This may due to the strong chemical interaction at the gold/TiO2 interface. Finally, the amplitude modulations are a consequence of the modifications of the band structure of the Au NP, resulting from the strain due to the phonons, which may affect the joint density of states.