Ultrafast resonant dynamics of surface plasmons in gold nanorods

Electron dynamics in Au nanorods are studied with femtosecond nonlinear spectroscopic techniques, by directly exciting and probing the longitudinal surface plasmon resonance. The dispersive and absorptive parts of the third-order signal are measured using optical heterodyne detected four-wave-mixing spectroscopy. These signals are used to describe dynamics in Au nanorods in terms of frequency shift and broadening of the plasmon resonance. Pump-probe experiments are performed with a series of pump intensities. The results are treated in two ways: (1) by calculating the temperature changes of electrons and phonons in the nanorods and the effects of these temperatures on the dielectric constant of Au; and (2) by a nonlinear least-squares fitting using a phenomenological response function. The first model agrees with the pump-probe experimental results for pump energies up to 2.0 nJ (2.5 GW/cm ²) and for delays in the range of 150 fs to 150 ps, but does not reproduce three additional features present in the data and the phenomenological model: (1) an "instantaneous" response, attributed to coherent plasmon oscillation; (2) a decaying component with an intensity-independent time constant of 170 fs, attributed to a nonthermal electron distribution or to two-photon-excited interband transitions; and (3) oscillations with a period of 71 ps, attributed to coherent vibration of the rods. Higher pump intensities yield substantial deviation at short delays from the lower-intensity response. Additional plasmon damping and higher-order nonlinear mechanisms are suggested to account for these deviations.