  We measure the transient absorption of single gold particles with a
common-path interferometer. The prompt electronic part of the signal provides
high-contrast images for diameters as small as 10 nm. Mechanical vibrations of
single particles appear on a longer timescale (period of 16 ps for 50 nm
diameter). They reveal the full heterogeneity of the ensemble, and the
intrinsic damping of the vibration. We also observe a lower-frequency mode
involving shear. Ultra-fast pump-probe spectroscopy of individual particles
opens new insight into mechanical properties of nanometer-sized objects.
