  The selective optical detection of individual metallic nanoparticles (NPs)
with high spatial and temporal resolution is a challenging endeavour, yet is
key to the understanding of their optical response and their exploitation in
applications from miniaturised optoelectronics and sensors to medical
diagnostics and therapeutics. However, only few reports on ultrafast pump-probe
spectroscopy on single small metallic NPs are available to date. Here, we
demonstrate a novel phase-sensitive four-wave mixing (FWM) microscopy in
heterodyne detection to resolve for the first time the ultrafast changes of
real and imaginary part of the dielectric function of single small (<40nm)
spherical gold NPs. The results are quantitatively described via the transient
electron temperature and density in gold considering both intraband and
interband transitions at the surface plasmon resonance. This novel microscopy
technique enables background-free detection of the complex susceptibility
change even in highly scattering environments and can be readily applied to any
metal nanostructure.
