  We demonstrate that in confined plasmonic metal structures subject to
ultra-fast laser excitation electron thermal diffusion can provide relaxation
faster than the energy transfer to the lattice. This relaxation occurs due to
excitation of nanometer-sized hot spots in the confined structure and the
sensitivity of its optical parameters to the perturbation in these regions.
Both factors become essential when the plasmonic resonance condition is met for
both excitation and detection. A pump-probe experiment on plasmonic gold
lattices shows sub-picosecond relaxation with the characteristic times
well-described by a two-temperature model. The results suggest that dynamical
optical response in plasmonic structures can be tuned by selection of the
structural geometry as well as the choice of wavelength and polarization of the
excitation and detection light.
