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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films