Ultrafast microscopy of the vibrational landscape of a single nanoparticle

Abstract

The vibrational landscape of an ∼250 nm-radius single gold nanoparticle dropped on a silica substrate is imaged with an ultrafast pump-probe experiment in a transient reflectivity configuration. A movie of the picosecond dynamics of the nanoparticle is recorded over 20 ns. A spatially resolved analysis of the spectrum of the transient reflectivity is also achieved. In addition to the axial oscillation of the nanoparticle driven by the normal contact stiffness and to the breathing mode of the nanoparticle, ultrafast microscopy allows us to reveal higher order acoustic eigenmodes otherwise hidden by the noise in single point measurements. These results are confirmed by calculations of the acoustic eigenfrequencies of the nanoparticle. The shear component of the particle surface displacement associated with the higher order modes is of strong interest for probing the elasticity of the surrounding medium in the GHz to THz range.