Polarization-resolved ultrafast dynamics of the complex polarizability in single gold nanoparticles.

Using a phase-sensitive degenerate four-wave mixing technique in heterodyne detection we measured the ultrafast changes of the complex polarizability in single gold nanoparticles at the surface plasmon resonance. Two components in the nanoparticle nonlinear response are distinguished, depending on the linear polarization direction of pump and probe pulses and particle geometry. One component is quantitatively modeled as the variation of the complex dielectric constant induced by the initial increase in the electron gas temperature following the absorption of the pump pulse, and the subsequent electron thermalization with the lattice and the surrounding. The dependence of the measured four-wave mixing on the probe field polarization direction is well reproduced by including a deviation from spherical symmetry in the nanoparticle dielectric constant. The second component manifests as a significant increase in the measured nonlinearity when the probe field is co-polarized with the pump field at pump-probe time overlap. We attribute this component to the nonlinear response from coherent surface plasmons before dephasing into single electron excitations.