All-optical ultrafast spectroscopy of a single nanoparticle-substrate contact

We present an all-optical method to investigate the GHz dynamics of the elastic contact between a single metallic nanoparticle and a substrate. A resonant excitation mechanism driven by the 82-MHz Dirac comb of the femtosecond oscillator is associated with femtosecond pump-probe experiments performed in a transient reflectivity configuration. This scheme allows us not only to detect the known breathing mode of the nanoparticle but also to unravel the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer. We measured the eigenfrequency and the lifetime of this vertical motion, which are related to the contact stiffness and hysteresis, and to the acoustic leakage at the nanoparticle-substrate interface. A modeling of the axial oscillation in the framework of classical adhesion theories predicts a simple power law dependence of the axial eigenfrequency with respect to the breathing mode frequency. Measurements performed for single particles with radii ranging from 60 to 700 nm are in strong agreement with this prediction.