GHz dynamics of a single nanoparticle-substrate contact probed by femtosecond intrinsic common-path interferometry

This paper reports on an all-optical and femtosecond time-resolved technique to investigate in the GHz range the adhesion between a single gold nanoparticle and a substrate, with particle radii ranging from 60 nm to 700 nm. Experiments rely on a usual femtosecond pump-probe method in a transient reflectivity configuration. This scheme allows not only to detect the known breathing mode of the nanoparticle but also to unravel for the first time in optical femtosecond spectroscopy the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer. 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, are measured. The particle radius dependence of the vertical oscillation frequency is in strong agreement with classical adhesion theories. Experiments have been performed with single gold particles deposited either on a silica substrate or on a sapphire substrate.