  The excitation of plasmonic nanoparticles by ultrashort laser pulses sets in
motion a complex ultrafast relaxation process involving the gradual
re-equilibration of the system's electron gas, lattice and environment. One of
the major hurdles in studying these processes is the lack of direct
measurements of the dynamic temperature evolution of the system subcomponents.
We measured the dynamic optical response of ensembles of plasmonic Au
nanoparticles following ultrashort-pulse excitation and we compared it with the
corresponding static optical response as a function of the increasing
temperature of the thermodynamic bath. Evaluating the two sets of data, the
optical fingerprints of equilibrium or off-equilibrium responses could be
clearly identified, allowing us to extract a dynamic thermometric calibration
scale of the relaxation process, yielding the experimental ultrafast
temperature evolution of the plasmonic particles as a function of time.
